Geological Survey of Denmark and Greenland Bulletin 12, pp. 77 +5 plates 1 GEOLOGICAL SURVEY OF DENMARK AND GREENLAND BULLETIN 12 · 2007 Lithostratigraphy of the Palaeogene – Lower Neogene succession of the Danish North Sea Poul Schiøler, Jan Andsbjerg, Ole R. Clausen, Gregers Dam, Karen Dybkjær, Lars Hamberg, Claus Heilmann-Clausen, Erik P. Johannessen, Lars E. Kristensen, Iain Prince and Jan A. Rasmussen GEOLOGICAL SURVEY OF DENMARK AND GREENLAND DANISH MINISTRY OF THE ENVIRONMENT 2 Geological Survey of Denmark and Greenland Bulletin 12 Keywords Lithostratigraphy, biostratigraphy, North Sea Basin, Palaeogene, Neogene. Cover Complex fabric created by multiple small-scale sand intrusions (light) into dark mudstones – such enigmatic fabrics are commonly associated with the sand-rich units of the Rogaland Group in the Siri Canyon area, offshore Denmark. The illustrated section of core is about 10 cm across and is from the lower Tyr Member (Lista Formation) in the Cecilie-1B well (2346.8 m). Photograph: Jakob Lautrup. Chief editor of this series: Adam A. Garde Editorial board of this series: John A. Korstgård, Geological Institute, University of Aarhus; Minik Rosing, Geological Museum, University of Copenhagen; Finn Surlyk, Department of Geography and Geology, University of Copenhagen Scientific editors of this volume: Jon R. Ineson and Martin Sønderholm Editorial secretaries: Jane Holst and Esben W. Glendal Referees: Paul van Veen (Norway) and Robert O’B. Knox (UK) Illustrations: Stefan Sølberg Digital photographic work: Benny M. Schark Graphic production: Knud Gr@phic Consult, Odense, Denmark Printers: Schultz Grafisk, Albertslund, Denmark Manuscript received: 29 August 2005 Final version approved: 8 September 2006 Printed: 29 June 2007 ISSN 1604-8156 ISBN 978-87-7871-196-0 Geological Survey of Denmark and Greenland Bulletin The series Geological Survey of Denmark and Greenland Bulletin replaces Geology of Denmark Survey Bulletin and Geology of Greenland Survey Bulletin. Citation of the name of this series It is recommended that the name of this series is cited in full, viz. Geological Survey of Denmark and Greenland Bulletin. If abbreviation of this volume is necessary, the following form is suggested: Geol. Surv. Den. Green. Bull. 12, 77 pp. Available from Geological Survey of Denmark and Greenland (GEUS) Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Phone: +45 38 14 20 00, fax: +45 38 14 20 50, e-mail: geus@geus.dk and Geografforlaget A/S Filosofgangen 24, 1., DK-5000 Odense C, Denmark Phone: +45 63 44 16 83, fax: +45 63 44 16 97, e-mail: go@geografforlaget.dk or at www.geus.dk/publications/bull © De Nationale Geologiske Undersøgelser for Danmark og Grønland (GEUS), 2007 For the full text of the GEUS copyright clause, please refer to www.geus.dk/publications/bull 3 Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Material and methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Offshore and onshore lithostratigraphic nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Chronostratigraphy and biostratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Paleocene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Eocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Oligocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Miocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Rogaland Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Våle Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Bor Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Lista Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Vile Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Tyr Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Ve Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Idun Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Bue Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Rind Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Sele Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Kolga Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Fur Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Balder Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Stronsay Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Horda Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Hefring Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Westray Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Lark Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Dufa Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Freja Member . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4 5 Abstract Schiøler, P., Andsbjerg, J., Clausen, O.R., Dam, G., Dybkjær, K., Hamberg, L., Heilmann-Clausen, C., Johannessen, E.P., Kristensen, L.E., Prince, I. & Rasmussen, J.A. 2007: Lithostratigraphy of the Palaeogene – Lower Neogene succession of the Danish North Sea. Geological Survey of Denmark and Greenland Bulletin 12, 77 pp. + 5 plates. As a result of a lithological, sedimentological and biostratigraphic study of well sections from the Danish sector of the North Sea, including some recently drilled exploration wells on the Ringkøbing– Fyn High, the lithostratigraphic framework for the siliciclastic Palaeogene to Lower Neogene sedi- ments of the Danish sector of the North Sea is revised. The sediment package from the top of the Chalk Group to the base of the Nordland Group is subdivided into seven formations containing eleven new members. The existing Våle, Lista, Sele, Fur, Balder, Horda and Lark Formations of previ- ously published lithostratigraphic schemes are adequate for a subdivision of the Danish sector at formation level. Bor is a new sandstone member of the Våle Formation. The Lista Formation is subdivided into three new mudstone members: Vile, Ve and Bue, and three new sandstone members: Tyr, Idun and Rind. Kolga is a new sandstone member of the Sele Formation. Hefring is a new sandstone member of the Horda Formation. Freja and Dufa are two new sandstone members of the Lark Formation. Danish reference sections are established for the formations, and the descriptions of their lithology, biostratigraphy, age and palaeoenvironmental setting are updated. __________________________________________________________________________________________________________ Authors’ addresses P.S.*, J.A., K.D. & L.E.K., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. * Present address: GNS Science, 1 Fairway Drive, Avalon, P.O. Box 30368, Lower Hutt, New Zealand. E-mail: p.schioler@gns.cri.nz O.R.C. & C.H.-C., Department of Earth Sciences, University of Aarhus, Høegh-Guldbergsgade 2, DK-8000 Århus C, Denmark. G.D. & L.H., DONG Energy, Agern Allé 24–26, DK-2970 Hørsholm, Denmark. I.P. & E.P.J., Statoil Norway, Forusbeen 50, N-4035 Stavanger, Norway. J.A.R., Geological Museum, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark. 6 Fig. 50 Fig. 56a Fig. 49 Fig. 56b Fig. 61 Fig. 58 Saxo-1 Wessel-1 Tordenskjold-1 Eg-1 Diamant-1 Bertel-1 Mona-1 Karl-1 W. Lulu-3,-1 Cleo-1 Augusta-1 Amalie-1 Tabita-1 Gulnare-1 Gwen-2 Iris-1 Baron-2 Nora-1 Elin-1W-1 Ravn-1 Falk-1 U-1 E-8 Tove-1 John-Flanke-1 Alma-1 Emma-1 Edna-1 Roxanne-1 Ugle-1 Frida-1 L-1 Francisca-1 Cecilie-1 Connie-1 Elna-1 Siri-2 Siri-3 Sofie-1 Floki-1 Sandra-1 Nolde-1 Nini-1 Nini-2 Nini-3 D-1 Vanessa-1 Ibenholt-1 Ida-1 R-1 C-1 K-1 F-1 Inez-1 S-1 Siri-1 V-1 G-1 Deep-Adda-1 Adda-2,-1Bo-1 North-Jens-1 Lulu-1 Lulu-2 Sten-1 Gert-1 Kim-1 Lone-1 57°00' 4°00' 6°00' 56°00' 25 km 250 km C offee Soil Fault North Sea Denmark N S UK G NL P Sir i C an yo n Mid North Sea High Central Graben a b Norwegian–Danish Basin C entral G raben 100 km North Polish Strait East Shetland Platform Fennoscandian Shield Scottish High V ik in g G ra be n Rhenish Massif Bohemian Massif Jylland Sjælland Storebælt Moray Firth Mid North Sea High Ringkøbing–FynHigh Fig. 1. Location maps showing the position of wells used in the study (a) and major structural elements in the greater North Sea area (b) mentioned in the text. On the well map (a) are indicated the locations of the seismic sections shown in Figs 49, 50, 56, 58 and 61. Grey shading on this map indicates the margins of the Siri Canyon; grey shading inside the canyon indicates an area of positive relief within the canyon. GGGGG, Germany; NNNNN, Norway; NLNLNLNLNL, Netherlands; PPPPP, Poland; SSSSS, Sweden; U KU KU KU KU K, United Kingdom. 7 Introduction Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding ofthesiliciclasticPalaeogenesedimentpackageintheDanish sector of the North Sea (Fig. 1). Many of the new wells were drilled in the search for oil reservoirs in sandstone bodies of Paleocene–Eocene age. The existing lithostrati- graphy was established on the basis of data from a gener- ation of wells that were drilled with deeper stratigraphic targets, with little or no interest in the overlying Palaeo- gene sedimentary succession. This means that this early scheme does not include Palaeogene sandstone units in the Danish sector. In order to improve the understanding of the distribution, morphology and age of the Palaeo- gene sediments, in particular the economically important sandstone bodies, a detailed study of this succession in the Danish sector has been carried out. The main aim was to update the lithostratigraphic framework of the succession on the basis of new data from recently drilled wells. All of the widespread Palaeogene mudstone units in the North Sea were established with Norwegian or Unit- ed Kingdom (UK) type wells. In the present work, these units have been maintained unchanged or with only slight modifications. Danish reference wells have been established for the units, however, and lithological descriptions have been expanded to cover the characteristics of these units in the Danish sector. Many of the sandstone bodies recently discovered in the Danish sector have a limited spatial distribution and are derived from sources different from those of most of the contemporaneous sandstone bodies in the Norwegian and UK sectors; furthermore, the Danish sandstone bodies probably neither overlap nor are in contact with the Nor- wegian/UK sandstones. These units have therefore been established as new in the Danish sector, and have been assigned Danish type and reference sections. The lithostratigraphy presented herein (Fig. 2) has its base at the top of the Early Paleocene (Danian) Ekofisk Formation (Chalk Group). The top of the study section is at the unconformity between the Late Eocene – Mid-Mio- cene Westray Group and the Mid-Miocene to Recent Nordland Group. Oil companies operating in the North Sea have collec- ted a substantial amount of lithostratigraphic data on the Palaeogene successions and a detailed lithostratigraphy has been developed for the Danish and Norwegian sectors (see e.g. Hamberg et al. 2005). A number of informal litho- stratigraphic units have been introduced that have subse- quently found their way into academia and geological survey organisations. It has been the aim of the present work formally to define these new units. This has been done maintaining their original (albeit informal) names whenever feasible. It has not been the aim of this work to provide a se- quence stratigraphic model for the Palaeogene sediments in the central and eastern North Sea; for this the reader is referred to Michelsen et al. (1992, 1995, 1998), Mudge & Bujak (1994, 1996a, b), Neal et al. (1994) and Dan- ielsen et al. (1997). The present contribution does not attempt to review the petroleum-related aspects of the Palaeogene succession. Information about this may be found elsewhere, for example in the annual reports from the Danish Energy Authority. Preliminary results from the present work, including a revised lithostratigraphic scheme, were previously pub- lished in a brief review paper (Schiøler et al. 2005). The present contribution formally describes the new strati- graphic units suggested in the review paper and further documents the Palaeogene – Lower Neogene lithostrati- graphy in the Danish sector of the North Sea. 8 Danian Selandian Thanetian Sparnacian Ypresian Lutetian Ma M id dl e Eo ce ne Lo w er E oc en e U pp er P al eo ce ne C ha lk G ro up R og al an d G ro up St ro ns ay G ro up W es tr ay G ro up St ro ns ay G ro up Lo w er P al eo ce ne Priabonian Rupelian Chattian Aquitanian Burdigalian Langhian Serravallian 15 20 25 30 35 45 50 55 60 65 M id dl e M io ce ne Lo w er M io ce ne Lo w er N eo ge ne Pa la eo ge ne Pa la eo ge ne U pp er O lig oc en e Lo w er O lig oc en e U pp er E oc en e M id dl e Eo ce ne Lillebælt Clay Fm Horda Fm Røsnæs Clay Fm Fur Fm F Ølst Fm Stolle Klint Clay Øster- rende Clay Holmehus Fm Ve Mb Bue Mb Rind Mb Æbelø Fm Danian Lime- stone Ekofisk Fm Våle Fm Li st a Fo rm at io n Vile Mb Sele Fm Balder Fm Bartonian Lutetian 40 Søvind Marl Fm Søvind Marl Fm Viborg Fm Linde Clay Branden Clay Vejle Fjord Fm Hodde Fm Gram Fm Lark Fm (undivided) Horda Fm Se ri es Sy st em St ag e D en m ar k on sh or e Danish North Sea Kolga Mb Hef- ring Mb Tyr Mb Bor Mb 49.0 55.5 54.5 57.9 60.0 41.3 37.0 33.7 28.5 23.8 20.5 16.4 14.8 Dufa Mb Odderup Fm Bastrup Sand Ribe Fm Freja Mb Idun Mb N or dl an d G ro up Arnum Fm Kerteminde Marl Lellinge Greensand 9 Geological setting The Danish sector of the North Sea is situated in the cen- tral and eastern North Sea and comprises three major struc- tural elements: the Central Graben, the Norwegian–Dan- ish Basin (the eastern part of the northern North Sea Ba- sin of Rhys 1974) and the Ringkøbing–Fyn High (Fig. 1; the geographic terminology and names of structural ele- ments in the North Sea used herein are adapted from Rhys 1974, Rønnevik et al. 1975, Deegan & Scull 1977 and Fyfe et al. 2003). The western boundary of the Danish sector largely coincides with the eastern boundary of the Mid North Sea High, the southern boundary largely co- incides with the southern limit of the Ringkøbing–Fyn High, and the northern boundary is in the Norwegian– Danish Basin. This basin as well as the Ringkøbing–Fyn High are Early Permian structures. Active rifting occurred in the Central Graben from the Middle to Late Jurassic along pre-established Palaeozoic fault trends. Major tec- tonic activity around the Palaeozoic and Jurassic struc- tures had largely ceased by Late Cretaceous time, and the sediment basin below the central North Sea was largely characterised by regional subsidence (Ziegler 1981). During the Late Cretaceous to Danian sea-level high, pe- lagic chalk sediments draped the structural highs and the northern and southern North Sea Basins became one North Sea Basin delimited by the Fennoscandian Shield to the north-east, the Rheinish–Bohemian Massif to the south and the British massifs, highs and platforms to the west (see Ziegler 1981 fig. 16 for details). Chalk sedimen- tation continued through to the end of the Danian Stage when it gave way to hemipelagic and siliciclastic sedimen- tation. This was probably caused by uplift of the basin margins to the west and east (Ahmadi et al. 2003). How- ever, most of the siliciclastic sediments were derived from the Scottish High and the East Shetland Platform, uplift- ed by the Iceland plume (Ahmadi et al. 2003). By the time of peak uplift, in the mid-Thanetian, large sand systems were building out towards the central North Sea. Most sediment came from the west, but the Siri Canyon system, a depression in the top chalk surface, was fed from the Fennoscandian Shield in the north-east and north (Fig 1; Ahmadi et al. 2003; Hamberg et al. 2005). Thermal subsidence centered above the Central Graben continued through the Eocene as sea level fell and the temperature decreased. Shallow-marine sediments char- acterised the margins of the North Sea Basin, especially its western margin, whereas basinal mudstone continued to accumulate in the basin centre and in the eastern part of the basin (Joy 1996). Inversions controlled by com- pression between the Atlantic spreading zone to the north- west and the orogenesis of the Alps to the south added to further uplift of the basin margins and submarine fans and turbidites were deposited near the centre of the basin (Jones et al. 2003). During the Oligocene, the North Sea Basin became part of a larger NW European basin. Connection with the North Atlantic broadened and enhanced communi- cation with the oceanic water mass to the north-west, whereas the connection to the south through the North Polish Strait became closed for the deep water (Fyfe et al. 2003). Glacio-eustatic sea-level changes became more fre- quent and controlled the sedimentary cycles. The east- ward progradation direction of the Paleocene and Eocene sediments gave way to sediment supply from the Euro- pean massifs to the far south (Fyfe et al. 2003). Continu- ed subsidence above the Mesozoic rift structures created accommodation space for thick sediment packages of basi- nal mudstones, and few sandstone units reached the basin depocentre above the Mesozoic rifts (Fyfe et al. 2003). In the Neogene Epoch, sediment started to be derived from the Fennoscandian Shield to the north, and the prograda- tion direction changed to the south-west and west in the Danish sector of the North Sea. Facing page: Fig. 2. Lithostratigraphic column for the Palaeogene and Lower Neo- gene of the Danish North Sea sector showing the approximate corre- lation with Danish onshore stratigraphic units. Timescale from Hard- enbol et al. (1998), except for the age of the Paleocene–Eocene bound- ary, which is adapted from Berggren & Aubry (1996) and the age of the Sparnacian–Ypresian boundary, which is from Aubry et al. (2003). Stratigraphy and ages of pre-Chatian onshore lithostratigraphic units are based on Heilmann-Clausen (1995) and Clemmensen & Thom- sen (2005). Post-Rupelian onshore stratigraphy and ages are from Dybkjær & Rasmussen (2000) and Rasmussen (2004a). F, Fur For- mation. 10 Previous work The Permian to Recent lithostratigraphy of the North Sea was described in two pioneering stratigraphic works. Rhys (1974) provided an overview of the structural elements of the North Sea and gave a brief description of the Palaeo- gene sediments. Deegan & Scull (1977) compiled a de- tailed lithostratigraphic subdivision and lithological de- scription for the central and northern North Sea (Figs 3, 4). They subdivided the siliciclastic Palaeogene, Neogene and Quaternary sediments into five major groups: the Montrose, Moray, Rogaland, Hordaland and Nordland Groups. The Montrose and Moray Groups established for the Outer Moray Firth – Forties area are proximal equiv- alents to the Rogaland Group and are not present in the Danish sector, whereas the Rogaland, Hordaland and Nordland Groups have widespread distribution in the Danish sector. The succession of major mudstone forma- tions contained within the three basinwide groups has formed the backbone of all subsequent lithostratigraphic schemes for the central and northern North Sea, includ- ing that of the present contribution. The post-Danian Cainozoic succession of the Danish Central Graben was divided into seven informal units by Kristoffersen & Bang (1982). The Palaeogene comprised five units: North Sea Marl and CEN-1–4 (Fig. 4). The ranks of the units were not stated. Although descriptions and interpretation of the CEN units were detailed, they are essentially informal and have been little used. A revised lithostratigraphy for the Palaeogene and Ne- ogene of the Norwegian North Sea sector was published by Hardt et al. (1989). Their lithostratigraphic scheme includes a number of new Palaeogene and Neogene sand- stone bodies observed in the Norwegian and British sec- tors of the North Sea (Fig. 4). Some of the names of the new sandstone units established by Hardt et al. (1989) were subsequently used informally for comparable sand- stone units discovered in the Danish sector. Mudge & Copestake (1992a, b) presented a revised Palaeogene stratigraphy for the Outer Moray Firth and northern North Sea Basins. In their papers they redefined the Moray and Montrose Groups of Deegan & Scull (1977) and abandoned the Rogaland Group. The authors also demoted the previously established sandstone forma- tions within the two former groups to the rank of mem- bers. Besides, in an innovative approach they allowed for a greater influence of biostratigraphic data on the charac- terisation of the various lithostratigraphic units, an ap- proach which is also followed herein. Knox & Holloway (1992) updated the lithostratigra- phic scheme for the Palaeogene in the British and Norwe- gian central and northern North Sea (Figs 3, 4). The au- thors followed Mudge & Copestake (1992a, b) in aban- doning the Rogaland Group of Deegan & Scull (1977), and used Mudge & Copestake’s revised definition of the Montrose and Moray Groups for the central North Sea as well.Furthermore, thethickand hitherto undivided Horda- land Group was subdivided into two new groups, the Stronsay Group succeeded by the Westray Group, each containing a distal and a proximal formation. The two distal formations of the two groups, the Horda and Lark Formations, together constitute the bulk of the Palaeo- gene sediments in the Danish sector of the North Sea and are adopted herein (Figs 2–4). Although sandstone units occur in both the Horda and Lark Formations in the Dan- ish sector, the two proximal sandstone formations of the Fig. 3. Correlation chart showing the approximate correlation between key lithostratigraphic schemes for the central and eastern North Sea at group and formation levels. Fu r Deegan & Scull (1977) Hardt et al. (1989) Knox & Holloway (1992) This study Chalk Group Chalk Group Chalk Group Hordaland Group Lista Unnamed unit/ Våle Sele Balder Balder Balder Lista Maureen Montrose Group Moray Group Lark M ou sa Sk ad e Horda Sele Rogaland Group Rogaland Group Stronsay Group Westray Group Stronsay Group Westray Group Lista Sele Horda Lark Våle Nordland Group Nordland Group Nordland Group 11 Stronsay and Westray Groups, the Mousa and Skade For- mations, are absent from the Danish sector. Following detailed analysis of new, high-resolution seismic surveys covering the succession in the eastern North Sea area, efforts were focused on establishing a sequence stratigraphic subdivision of the Palaeogene–Neogene sedi- ment package. The sedimentary succession was interpre- ted in a series of publications from a working group at the University of Aarhus (e.g. Michelsen et al. 1992, 1995, 1998; Michelsen 1993; Danielsen et al. 1997; Huuse & Clausen 2001). The result of that work was a subdivision of the Palaeogene to mid-Neogene sediment package covered by the present work into six genetic units (Fig. 4). The sequence stratigraphy of the upper Oligocene to Mi- ocene in the eastern North Sea was dealt with by Rasmus- sen (2004b). Further sequence stratigraphic contributions covering the larger North Sea Basin including the British and Norwegian sectors are given by Armentrout et al. (1993), Mudge & Bujak (1994, 1996a, b) and Neal et al. (1994). Coastal onlap Basinward 7 Michelsen et al. (1998) 6 6.3 6.2 6.1 5.4 5.3 5.2 5.1 4.4 4.3 4.2 4.1 1.2 1.1 5 4 3 2 1 Nordland Group Hordaland Group Balder Sele Lista Unnamed Unit Ekofisk CEN-5 CEN-4 CEN-3 CEN-2 CEN-1 North Sea Marl Chalk-6 Nordland Group Hordaland Group Balder Sele Sele Lista Lista Våle Ekofisk Nordland Group Lark Horda Balder Tay M o u sa S ka d e Fr ej a K ol ga F u r R in d Id un Ty r Bo r Sele Lista L is ta Fo rt ie s C ro - m ar ty M ey V ad e Fo rt ie s A n d re w H ei m d al H er m o d F is ke b an k F is ke b an k F ri gg R og al an d G ro u p Maureen M au re en Ekofisk Ekofisk Ekofisk Nordland Group Lark Horda Balder Sele Bue Ve Vile VåleVåle Deegan & Scull (1977) Northern North Sea Central North Sea Kristoffersen & Bang (1982) Hardt et al. (1989) Knox & Holloway (1992) This study Nordland Group Balder Ty Grid Ekofisk Frigg H ef - ri ng D uf a Skade Fig. 4. Correlation chart showing approximate correlation between key lithostratigraphic schemes for the central and eastern North Sea and the Norwegian part of the northern North Sea at formation and member levels. The sequence stratigraphic subdivision of Michelsen et al. (1998) is added for comparison. Sandstone-dominated units indicated in yellow. 12 Material and methods The present lithostratigraphic subdivision represents the combined results from studies of petrophysical logs, bio- stratigraphyandseismicprofiles,cuttings samples and cored sections. Petrophysical logs from c. 70 wells in the Danish sector have been scrutinised (see Fig. 1 for well locations). The wells have been correlated using petrophysical logs, predominantly gamma-ray and sonic logs. Five log panels form the basis for the log correlation (Plates 1–5). Lithostratigraphic well correlation has been supported by biostratigraphic data: biostratigraphic reports from 29 wells have been re-assessed with the aim of identifying key micropalaeontological and palynological events that occur consistently within the study area (taxa used are planktonic and benthic foraminifers, diatoms, radiolaria, sporomorphs and dinoflagellate cysts). Moreover, biostrati- graphic sample suites from 11 North Sea wells have been prepared at the Geological Survey of Denmark and Green- land in order to further determine the biostratigraphic event succession. The bulk of material studied for bio- stratigraphy is based on cuttings samples, and only few Table 1. Well data for the new type and reference wells in the Danish sector of the North Sea Augusta-1 Cecilie-1 Cleo-1 Connie-1 E-8 F-1 Floki-1 Francisca-1 Frida-1 Inez-1 K-1 Mona-1 Nini-3 Sandra-1 Siri-1 Siri-2 Siri-3 Tabita-1 Bor Mb(t), Bue Mb(t),Ve Mb(t) Bor Mb(r), Tyr Mb(r) Bue Mb(r), Lista Fm(r), Ve Mb(r), Vile Mb(r) Idun Mb(t), Rind Mb(t) Bue Mb(r), Lista Fm(r), Ve Mb(r), Vile Mb(r), Våle Fm(r) Dufa Mb(r) Hefring Mb(t) Freja Mb(t) Freja Mb(r) Dufa Mb(t), Fur Fm(r) Fur Fm(r) Balder Fm(r), Horda Fm(r), Lark Fm(r) Kolga Mb(r), Tyr Mb(t) Rind Mb(r) Horda Fm(r), Lark Fm(r), Sele Fm(r), Våle Fm(r) Idun Mb(r) Balder Fm(r), Kolga Mb(t), Vile Mb(t) Sele Fm(r) 56°17´57.40´́ N 04°24´04.64´́ E 56°24´23.73´́ N 04°45´42.00´́ E 56°23´23.54´́ N 04°25´22.70´́ E 56°24´28.34´́ N 04°42´30.36´́ E 55°38´13.42´́ N 04°59´11.96´́ E 57°01´53.4´́ N 06°54´28.6´́ E 56°27´48.58´́ N 05°16´47.11´́ E 56°22´27.95´́ N 04°48´05.30´́ E 56°17´14.15´́ N 05°01´50.20´́ E 56°50´28.39´́ N 06°57´41.62´́ E 57°07´37.74´́ N 07°09´43.11´́ E 56°16´35.94´́ N 04°00´15.81´́ E 56°41´31.96´́ N 05°24´12.35´́ E 56°35´13.33´́ N 05°01´35.19´́ E 56°29´11.10´́ N 04°54´57.49´́ E 56°29´40.53´́ N 04°52´13.26´́ E 56°30´34.92´́ N 05°03´48.27´́ E 56°13´37.50´́ N 04°23´47.56´́ E 04.03.2001 2991.0 MDRT 37.8 RT 65 15.10.2000 2361.0 MDRT 37.8 RT 59.4 06.02.1984 4866.1 MDKB 40.5 KB 63.1 02.02.2001 2351.8 MDRT 37.8 RT 61.5 08.04.1994 2527.4 MDKB 36.6 KB 43.6 06.10.1968 2421.6 MDKB 37.19 KB 40.8 29.08.2000 1878 MDRT 35.8 RT 53.2 20.07.1998 1888.5 MDRT 36.4 KB 60 26.07.1997 2274 MDRT 39.0 RT 54.3 11.09.1977 1983.9 MDKB 35.1 KB 35.4 22.01.1970 2292.4 MDKB 37.2 KB 56.4 03.10.1982 4241.6 MDKB 36.6 KB 65.5 12.01.2001 1851.2 MDRT 37.3 RT 58.2 18.06.1998 2139 MDRT 36 KB 65 28.11.1995 2220 MDKB 23 KB 60 03.08.1996 2297.5 MDRT 36.6 RT 60.6 30.08.1996 2171.5 MDRT 36.6 RT 60.1 10.09.1983 4353 MDKB 40 KB 65 DONG E&P a/s DONG E&P a/s Chevron Petroleum Co. DONG E&P a/s Maersk Oil & Gas a/s Gulf Oil Company Kerr-McGee Int. aps Dansk Operatørselskab i/s Dansk Operatørselskab i/s Chevron Petroleum Co. California Oil Co. Chevron Petroleum Co. DONG E&P a/s Statoil E&P a/s Statoil E&P a/s Statoil E&P a/s Statoil E&P a/s Statoil E&P a/s Type (t) or reference (r) well Coordinates Operator Spud date TD (logger’s KB/RT elevation Water for listed units: depth in m) (m above msl) depth (m)Well Fm: Formation. Mb: Member. MDRT: Measured Depth below Rotary Table. MDKB: Measured Depth below Kelly Bushing. 13 Fig. 5. Chronostratigraphy and biostratigraphy of the Paleocene – Middle Miocene. a: Paleocene–Eocene. b: Eocene–Oligocene. c: Oligocene – Middle Miocene. Calibration of chronostratigraphic units follows Hardenbol et al. (1998), Berggren & Aubry (1996) for the Paleocene–Eocene boundary and Aubry et al. (2003) for the Sparnacian–Ypresian boundary. Key dinoflagellate datums are calibrated mainly using age estimates from Hardenbol et al. (1998) and Williams et al. (2004). Key microfossil datums are calibrated via their correlation with calibrated dinoflagellate datums as suggested by Mudge & Bujak (1996b), using age estimates from Hardenbol et al. (1998) and Williams et al. (2004). The combined event succession is correlated with the North Sea microfossil zonation of King (1989) and lithostratigraphic units treated herein. In the microfos- sil event column, the planktonic foraminifer events appear in normal font, benthic foraminifers in italics; diatoms and radiolarians are under- lined. Senoniasphaera inornata Palynodinium grallator, Dinogymnium spp. Alisocysta reticulata abundant P. pyrophorum Isabelidinium? viborgense P. pyrophorum, P. australinum acme A. gippingensis Alisocysta margarita common Cerodinium wardenense Apectodinium augustum Apectodinium augustum, acme Apectodinium spp. acme D. oebisfeldensis, influx Inaperturopollenites spp., common H. tubiferum Deflandrea oebisfeldensis Dracodinium condylos NSP6 (pars) NSP5b NSP5a NSP4 NSP3 NSP2 NSP1 a b c NSB4 (pars) NSB3b NSB3a NSB2 NSB1 a b c Horda Balder Sele Ekofisk Tor Våle Bue Ve Vile Lista Planktonic foraminifers Benthic foraminifers Diatoms and radiolaria Planktonic microfossils North Sea Biozones (King 1989) Litho- stratigraphy Selected biostratigraphic events used in the present studyGeo- chronology Ma a Chronostratigraphy (Berggren et al. 1995) C re ta ce ou s (p ar s) Pa le oc en e Eo ce ne (p ar s) U pp er (p ar s) Lo w er U pp er Lo w er (p ar s) Fm MbDinoflagellate cysts Ypresian (pars) Maastrictian (pars) Thanetian Sparnacian Selandian Danian 54.5 55.5 57.9 60.0 65.0 50 55 60 65 Uvigerina batjesi Turrillina brevispira Gaudryina hiltermanni common Subbotina ex gr. linaperta Fenestrella antiqua, foraminifers very rare impoverished benthic agglutinated assemblage common Globoconusa daubjergensis Globanomalina cf. compressa, S. trivialis increasing diversity of calcareous foraminifers reappearance of planktonic foraminifers increasing diversity of cal- careous benthic foraminifers Cenodiscus spp., Cenosphaera spp. Cretaceous foraminifers common F. antiqua and Coscinodiscus morsianus Pseudotextularia elegans Benthic microfossilsStageSeries 14 35 40 50 45 Lark Formation Rupelian (pars) O lig oc en e (p ar s) Eo ce ne ( pa rs ) Priabonian 41.3 Bartonian Lutetian Ypresian (pars) NSB7a NSB6b NSB6a NSB5c NSB5b NSB5a NSB4 NSB3a NSB2 (pars) NSB3b Planulina costata Pseudohastigerina spp. abundant radiolaria (Cenosphaera spp.), Cyclammina amplectens Lenticulina gutticostata, Spiroplectammina spectabilis Balder Horda Eatonicysta ursulae Diphyes ficusoides Areosphaeridium michoudii Heteraulacacysta porosa Diphyes colligerum Areosphaeridium diktyoplokum common E. ursulae Phthanoperidinium clithridium Globigerinatheka index Cibicidoides truncanus Vaginulinopsis decorata 49.0 37.0 33.7 Uvigerina batjesi Turrillina brevispira Gaudryina hiltermanni common Subbotina patagonica Dracodinium condylos Deflandrea oebisfeldensis acme D. oebisfeldensis, influx Inaperturopollenites spp., common H. tubiferum Fenestrella antiqua, foraminifers very rare Cerebrocysta bartonensis Uvigerina germanica Karrulina conversa Corrudinium incompositum Sele (pars) NSP9b NSP9a NSP8c NSP8b NSP8a NSP7 NSP6 NSP5b NSP4 (pars) NSP5a Lo w er ( pa rs ) U pp er M id dl e Lo w er ( pa rs ) Planktonic foraminifers Benthic foraminifers Diatoms and radiolaria Planktonic microfossils North Sea Biozones (King 1989) Litho- stratigraphy Selected biostratigraphic events used in the present study Chronostratigraphy (Berggren et al. 1995) Fm MbDinoflagellate cysts Benthic microfossilsStageSeries Geo- chronology Ma b Fig. 5b. Chronostratigraphy and biostratigraphy of the Eocene–Oligocene. 15 Lark NSP9a (pars) NSB6b (pars) Uvigerina germanica Karrulina conversa NSP14b NSB13a NSP14a NSB12c NSP13 NSB12b NSB12a NSP12 NSB11 NSP11 NSP10 NSB10 NSB9 NSP9c NSB8c NSB8b NSB8a NSB7b NSB7a NSP9b Aulacodiscus allorgei Turrillina alsatica Bolboforma spiralis Asterigerina staeschei, Elphidium inflatum, Meonis pompilioides Uvigerina tenuipustulata Plectofrondicularia seminuda Aulacodiscus insignis quadrata (small), B. antiqua, G. girardana common Elphidium subnodosum, common Paragloborotalia nana Rotaliatina bulimoides “Turborotalia” ampliapertura common A. guerichi, Paragloborotalia opima s.s. Bolboforma metzmacheri Pararotalia canui Aulacodiscus insignis quadrata (large) Spirosigmoilinella compressa Cibicidoides mexicanus Gyroidina mamillata Wetzeliella gochtii Phthanoperidinium amoenum Chiropteridium spp. Membranophoridium aspinatum Distatodinium biffi Cordosphaeridium cantharellus Apteodinium spiridoides Caligodinium amiculum Thalassiphora pelagica Hystrichokolpoma cinctum Rhombodinium draco Corrudinium incompositum Achilleodinium biformoides Enneadocysta pectiniformis Burdigalian Aquitanian M io ce ne (p ar s) Lo w er Chattian U pp er Rupelian (pars) Lo w er (p ar s)O lig oc en e (p ar s) M id dl e Langhian Serravallian 28.5 23.8 20.5 16.4 14.8 11.2 Cousteaudinium aubryae Nordland Group Bulimina elongata Bolboforma clodiusi P. comatum 15 20 30 25 Cannosphaeropsis passio Tortonian (pars)U pp er (p ar s) Planktonic foraminifers Benthic foraminifers Diatoms and radiolaria Planktonic microfossils North Sea Biozones (King 1989) Litho- stratigraphy Selected biostratigraphic events used in the present study Chronostratigraphy (Berggren et al. 1995) Fm MbDinoflagellate cysts Benthic microfossilsStageSeries Geo- chronology Ma c Fig 5c. Chronostratigraphy and biostratigraphy of the Oligocene – Middle Miocene. 16 core samples have been available. As the use of stratigraphic lowest occurrences (LO) of taxa in cuttings samples may be hampered due to downhole caving, the event succes- sion comprises almost exclusively stratigraphic highest occurrences (HO) of taxa (a single significant LO is in- cluded in the succession). The event succession is shown in Fig. 5a–c; its correlation with international and North Sea biozones is shown in Fig. 6a–c. Seismic sections from the 2-D and 3-D seismic surveys CGD85, DK-1, RTD81–RE94, UCG96 and UCGE97 have been used to further support the well correlation and to map the stratigraphic units in areas with only scattered well coverage. The combined results from the correlation and mapping procedures are presented as isochore maps for individual stratigraphic units. Inspection of cuttings samples from 16 key wells sup- plemented with sedimentological studies of cored inter- vals from 23 wells have formed the basis for the litholog- ical and sedimentological descriptions of the units. The well depths mentioned in the lithostratigraphy sec- tion are loggers’ depths measured either from rotary table (MDRT) or kelly bushing (MDKB). Supplementary data for new type and reference wells are provided in Table 1. The names assigned to the new lithostratigraphic units definedhereinare derived from Nordic mythology and thus follow the nomenclatural tradition previously established for the Norwegian North Sea (Isaksen & Tonstad 1989). It should be noted that the micropalaeontology-based palaeoenvironmental terminology used herein was origi- nally developed for a passive margin situation (e.g. the terms ‘neritic’ and ‘bathyal’ to indicate the physiographic zones ‘shelf ’ and ‘shelf- slope’, respectively). Its application herein to the epicontinental North Sea Basin solely relates to depositional depth. Offshore and onshore lithostratigraphic nomenclature There is a high degree of lithological similarity between the Palaeogene–Neogene mudstone succession in Danish offshore boreholes and that in onshore exposures and bore- holes. However, the status of the Danish onshore units is quite varied since many units were named before a stan- dard for description of a lithostratigraphic unit was estab- lished; some fulfil these requirements, whereas others are still informal. If a previously established onshore unit and an offshore unit can be demonstrated to be identical (e.g. the Holmehus Formation and the new Ve Member pro- posed herein), the name of the onshore unit theoretically has priority over the name of the offshore unit (Salvador 1994). In other cases, names of offshore units can be ar- gued to have priority over onshore units (e.g. Sele and Balder Formations over Ølst Formation). However, in order to acknowledge the traditional distinction between offshore and onshore stratigraphic nomenclature, the two sets of nomenclature are kept separate herein. Whenever possible, comments are given in the text to explain the relationship between offshore and onshore Danish strati- graphic nomenclature. A correlation between the two sets of nomenclature is shown in Fig. 2. Chronostratigraphy and biostratigraphy Age assessment of the lithostratigraphic units in the North Sea sedimentary succession is based on correlation between key biostratigraphic events encountered in the units and the calibrated standard chronostratigraphy published by Berggren et al. (1995), with modification for the Pale- ocene–Eocene boundary following ratification of its posi- tion by the International Union of Geological Scientists (Aubry et al. 2002). The key events are from biostrati- graphic zonation schemes established for the North Sea area. Planktonic and benthic microfossils are covered by the zonation schemes of King (1983, 1989; Figs 5a–c, 6a–c). Dinoflagellates from the Paleocene and Eocene Epochs are covered by the zonation scheme of Mudge & Bujak (1996b; Fig. 6a, b); the Oligocene and Miocene Epochs are covered by the zonation schemes of Costa & Manum (1988) with modifications by Köthe (1990, 2003; Fig. 6b, c). Key events from these schemes used in this study are listed in Fig. 5a–c. For the dinoflagellate events, geochronological calibra- tion has been largely established using age estimates from Hardenbol et al. (1998), Munsterman & Brinkhuis (2004) and Williams et al. (2004). For events not mentioned in these three publications, the works of Mudge & Bujak 17 P2 P9 P7 P6 b a P5 P4 c P8 NP13 NP12 NP10 NP9 NP11 b a b a P3 c b a P1 Pα + P0 NP8 NP6 NP5 NP4 NP3 NP2 NP1 NP7 Abathom- phalus mayaroensis CC26 CC25 (pars) Pseudotextularia elegans P6 P5 P4 P3 P2 P1 E1a E2a E2b E2c E3a E3b L E1c E1bEo ce ne (p ar s) Pa le oc en e C re ta ce ou s (p ar s) NP14 (pars) E3c 50 60 55 65 NSP6 (pars) NSP5b NSP4 NSB4 (pars) NSB3a NSB2 NSP5a NSB3b NSP3 NSP2 NSP1 NSB1 65.0 Ypresian (pars) Lo w er (p ar s) U pp er Lo w er 55.5 60.0 Thanetian Selandian Danian U pp er (p ar s) Maastrichtian (pars) 57.9 54.5 Sparnacian c b a b c a Planktonic microfossils Benthic microfossils Dinoflagellate cystsPlanktonic microfossils Calcareous nannofossils North Sea biozonesStandard biozones Chronostratigraphy (Berggren et al. 1995) StageSeries Berggren & Miller (1988), Berggren et al. (1995) Mudge & Bujak (1996b) Martini (1971) King (1989) Geo- chronology Ma a Fig. 6. Biostratigraphic correlation charts showing approximate correlation of calibrated standard planktonic foraminifer and nannofossil bio- zones with North Sea microfossil and dinoflagellate biozones. Calibration of the standard biozones follows Hardenbol et al. (1998). Relationships between the North Sea biozones are approximate and their correlation with the standard zones may deviate from that of the original authors (for discussion, see text). a: Paleocene–Eocene biostratigraphic correlation chart. b: Eocene–Oligocene biostratigraphic correlation chart. c: Oli- gocene – Middle Miocene biostratigraphic correlation chart. P17 E6b NP14 P9 P7 P8 NP13 NP12 NP11 E2a E2b E2c E3a E3b E1c E1b E3c NSP6 NSP5b NSB4 NSB3aNSP5a NSB3b P18 P16 P15 Np23 (pars) NP22 NP21 NP19–20 NP18 P14 P12 P11 P10 P13 NP17 NP16 NP15 E3d E4a E4b E4c E4d E5a E5b E6a E6c E7a E7b E8b E8a D13 Mudge & Bujak (1996b) Costa & Manum (1988), Köthe (1990) Planktonic microfossils Benthic microfossils Dinoflagellate cystsPlanktonic microfossils Calcareous nannofossils North Sea biozonesStandard biozones P19 (pars) 35 40 50 45 O lig oc en e (p ar s) Lo w er ( pa rs ) Eo ce ne (p ar s) U pp er M id dl e Lo w er ( pa rs ) NSB7aNSP9b NSP9a NSB6b NSP8c NSB6a NSP8b NSB5c NSP8a NSP7 NSB5b NSB5a P6 b a Np10 (pars) NSP4 (pars) NSB2 (pars) E1a (pars) Geo- chronology Ma Chronostratigraphy (Berggren et al. 1995) StageSeries Berggren & Miller (1988), Berggren et al. (1995) Costa & Manum (1988), Köthe (1990), Mudge & Bujak (1996b) Martini (1971) King (1989) Rupelian (pars) Priabonian 41.3 Lutetian Ypresian (pars) 49.0 37.0 33.7 Bartonian b Fig. 6b. Eocene–Oligocene biostratigraphic correlation chart. 18 M7 M9 M12 M8 M11M10 M6 M5 M3 M2 M4 M1 b a P22 P21 b a P20 P19 P18 NN5 NN6 NN9A– NN7 NN4 NN3 NN2 NN1 NP25 NP24 NP23 NP22 NP21 (pars) D13 D14 D15 D16 D17 D18 D19 Tortonian (pars) Burdigalian Aquitanian M io ce ne (p ar s) Lo w er Chattian U pp er Rupelian (pars) Lo w er ( pa rs ) O lig oc en e (p ar s) M id dl e Langhian Serravallian 28.5 23.8 20.5 16.4 14.8 11.2 15 20 30 25 U pp er (p ar s) NSP9a (pars) NSB6b (pars) NSP14b NSB13a NSP14a NSB12c NSP13 NSB12b NSB12a NSP12 NSB11 NSP11 NSP10 NSB10 NSB9 NSP9c NSB8c NSB8b NSB8a NSB7b NSB7a NSP9b M13a (pars) NN9b (pars) Planktonic microfossils Benthic microfossils Dinoflagellate cystsPlanktonic microfossils Calcareous nannofossils North Sea biozonesStandard biozones Chronostratigraphy (Berggren et al. 1995) StageSeries Berggren & Miller (1988), Berggren et al. (1995) Costa & Manum (1988), Köthe (1990), Martini (1971) King (1989) Geo- chronology Ma c Fig. 6c. Oligocene – Middle Miocene biostratigraphic correlation chart. 19 20 (1996b), Dybkjær (2004), Piasecki (2005) and Schiøler (2005) have been consulted. However, whereas Harden- bol et al. (1998) and Williams et al. (2004) used the time- scale of Berggren et al. (1995), Mudge & Bujak used the slightly older timescale from Haq et al. (1987) for cali- bration of their events. Therefore, the ages of events only listed by Mudge & Bujak have been recalibrated herein to conform to the timescale of Berggren et al. (1995). King (1989) calibrated his planktonic and benthic mi- crofossil zone markers with the standard chronostrati- graphic scale of Berggren et al. (1985a, b). However, King noted that only a few first-order correlations were possi- ble; most of the calibrations were made using dinoflagel- lates, planktonic foraminifers and nannoplankton from onshore sections in the North Sea Basin (King 1989 p. 420); the correlation of the Lower Miocene is particularly uncertain (King 1989 p. 446). Paleocene and Eocene key planktonic and benthic microfossil events from King (1989) were subsequently correlated with the North Sea dinoflagellate events by Mudge & Bujak (1996b). By using the above-mentioned recalibration of key dinoflagellate events from Mudge & Bujak (1996b), it is feasible to in- directly correlate King’s North Sea microfossil events with the timescale of Berggren et al. (1995). This has been at- tempted in Fig. 5a–c. Figure 6a–c shows the relationships between the North Sea biozones and their correlation with the standard plank- tonic foraminifer and calcareous nannofossil zones. How- ever, it should be noticed that in a few cases the correla- tion of the North Sea microfossil and dinoflagellate zones with the standard zones in Fig. 6a–c is at variance with that of the authors of the same zones. This is an effect of improved age determinations of the standard zones and the dinoflagellate events used to calibrate the North Sea microfossil zones. The section below outlines the current status for the PalaeogeneandNeogenechronostratigraphicunitscovered bythestudiedsuccessionandlistskeybiostratigraphic events used for chronostratigraphic correlation of the succession. Paleocene The bases of the Selandian and Thanetian Stages, which together constitute the Upper Paleocene Series, have yet to be formally defined. However, ongoing work in the International Subcommission on Palaeogene Stratigraphy indicates that the Global Standard Stratotype-section and Point (GSSP) of the base of the Selandian Stage will prob- ably be close to the P2–P3a or the P3a–P3b standard planktonic foraminifer zone boundary, while the GSSP for the Thanetian Stage will probably be at the base of Magnetochron C26n (Gradstein & Ogg 2002). Harden- bol et al. (1998) followed Berggren et al. (1995) in plac- ing the base of the Selandian Stage at the base of Zone P3a, at the lowest occurrence of the planktonic foramini- fer Morozovella angulata. However, many of the micro- fossil species that characterise the Danian–Selandian boun- dary interval in the international zonation schemes, in- cluding M. angulata, are extremely rare or absent in the North Sea Basin thereby hampering chronostratigraphic correlation of the boundary. Based on a study of core material from the type area for the Danian and Selandian Stages, Clemmensen & Thomsen (2005) concluded that the Danian–Selandian stage boundary is located in the upper part of the NP4 standard nannofossil zone, close to the NP4–NP5 zone boundary, approximately at the P3a– P3b zone boundary, at c. 60 Ma on the timescale of Hard- enbol et al. (1998). They further concluded that there is a hiatus between the Danian and Selandian Stages in the Danish area outside the Central Graben due to trunca- tion of the Danian limestones of the Ekofisk Formation (Fig. 5a; Clemmensen & Thomsen 2005). Hence, the Danian–Selandian stage boundary is herein placed just below the downhole reappearance (provisional HO) of planktonic foraminifers and the HO of the dinoflagellate Alisocysta reticulata, but above the closely spaced events marked by the HO of the planktonic foraminifers Sub- botina trivialis and Globanomalina cf. compressa (e.g. Jones 1999; Mudge & Bujak 2001). The Selandian–Thanetian stage boundary is herein approximated by the HO of the dinoflagellate Palaeope- ridinium pyrophorum, at the base of the P5 dinoflagellate Zone of Mudge & Bujak (1996b). This level is close to the base of Magnetochron C26n, according to Harden- bol et al. (1998). Eocene The base of the Eocene is at the base of the negative car- bon isotope excursion (CIE) at 55.5 Ma (Berggren & Aubry 1996; Aubry et al. 2002). This position is below the base of the Ypresian Stage, the lowermost Eocene Stage. Therefore it has been proposed to reintroduce the Spar- nacian Stage as the new basal Eocene Stage between the CIE and the base of the Ypresian (Aubry et al. 2003). The CIE has been correlated with the proliferation of the dino- flagellate genus Apectodinium, an event recognised glo- bally (e.g. Knox 1996; Crouch et al. 2001). Onshore Den- mark, the CIE and the proliferation of Apectodinium coin- cides precisely with the laminated Stolle Klint Clay in the 21 2900 m 3000 2900 m 3000 2000 m 2100 2700 m 2900 m Horda Fm Balder Fm Sele Fm Lista Fm Bue Mb R og al an d G ro up St ro ns ay G ro up Ve Mb Vile Mb Våle Fm Chalk Group 3000 2800 Kim-1 GR Sonic GR Sonic GR Sonic GR Sonic GR Sonic Kim-1 Mona-1 Cleo-1 Gulnare-1 E-8 E-8 lowermost part of the Haslund Member of the Ølst For- mation (Heilmann-Clausen & Schmitz 2000; Willum- sen 2004). In the North Sea Basin, the acme of Apectodin- ium is located in the lowermost, laminated part of the Sele Formation (sensu Deegan & Scull 1977, see below) according to Knox (1996). As the event is a LO, its posi- tion cannot be determined with certainty in wells in which this interval is covered only by cuttings samples. In the North Sea Basin, however, this stratigraphic level is char- acterised by a prominent excursion on the gamma-ray log near the base of the Sele Formation which therefore can be used as an approximation for the base of the Eocene Series. The remaining stages of the Eocene Series, the Ypre- sian, Lutetian, Bartonian and Priabonian Stages, lack ba- sal boundary GSSPs for the present. In this paper, we fol- low Mudge & Bujak (1996b) and approximate the bases of the three latter stages by using three key dinoflagellate events: the base of the Lutetian Stage is at the HO of common Eatonicysta ursulae, the base of the Bartonian Stage is close to the HO of Diphyes colligerum, and the base of the Priabonian Stage is close to the HO of Heter- aulacacysta porosa. The base of the classic Ypresian Stage is at the LO of the calcareous nannoplankton species Tri- brachiatus digitalis. As yet, there is no commonly recog- nised HO index event at that level in the North Sea Ba- sin, but the boundary between the Sparnacian and the Ypresian Stages may be placed below the HOs of com- mon Cerodinium wardenense and Apectodinium augustum (Fig. 5a), both dinoflagellate species. Oligocene The GSSP for the Eocene–Oligocene boundary is in the Massignano section (central Italy), at the highest occur- rence of the planktonic foraminifer genera Hantkenina and Cribrohantkenina, immediately above the P17–P18 plank- tonic foraminifer zone boundary (Premoli Silva & Jenkins 1993). However, hantkeninids have not been observed from the North Sea Basin and alternative zone markers have therefore been used here. In the North Sea Basin, the planktonic foraminifer Globigerinatheka index and the benthic foraminifer Cibicidoides truncanus have their HOs in the uppermost Eocene (King 1989), and the two events may be used to approximate the Eocene–Oligocene bound- ary. A palynological marker of the lowermost Oligocene is the HO of the dinoflagellate Areosphaeridium diktyo- plokum (Brinkhuis & Biffi 1993; Brinkhuis & Visscher 1995), which is widespread in the North Sea Basin. The three latter events in combination serve as useful markers for bracketing the Eocene–Oligocene boundary in the North Sea Basin. The principal criterion for the Rupelian–Chattian (Lower–Upper Oligocene) boundary has not yet been de- Fig. 7. Log panel illustrating the thickness variation of the Rogaland Group formations in the Danish Central Graben. 22 cided by the Subcommission on Palaeogene Stratigraphy. Indications are that the boundary may be positioned at the base of the P21b planktonic foraminifer zone (Premoli Silva 2005), at 28.5 Ma (Hardenbol et al. 1998). How- ever, the defining boundary event cannot be recognised in the North Sea Basin and its exact correlation with the North Sea biostratigraphic event succession remains un- certain. Instead, most North Sea biostratigraphers recog- nise the Rupelian–Chattian stage boundary at the HO of the benthic foraminifer Rotaliatina bulimoides. This event marks the top of the NSB7 Zone of King (1983, 1989; Fig. 5c) and the NSR7 Zone of Gradstein et al. (1994). The HO of R. bulimoides is at 29 Ma in the northern North Sea according to Gradstein & Bäckström (1996), slightly older than the 28.5 Ma for the Rupelian–Chat- tian stage boundary quoted by Hardenbol et al. (1998). The Rupelian–Chattian stage boundary may also be ap- proximated by the HO of the dinoflagellate Rhombodin- ium draco. In the North Sea wells reported herein, where both the HOs of R. bulimoides and R. draco have been recorded, these events are largely contemporaneous. How- ever, in the type area of the Rupelian and Chattian Stages, R. draco has its HO above R. bulimoides in the type Chat- tian (van Simaeys et al. 2004). Therefore, it may be infer- red that the two latter events probably bracket the Rupe- lian–Chattian boundary (Fig. 5c). Ve Mb Tyr Mb Bue Mb Log depth Core depth Våle Fm Lista Fm Bor Mb GR SonicCecilie-1 clay si. vf. f. m. Sand c. vc. P P P P P P P P P ? S 5o 2240 2250 2260 2270 2280 2240 2250 2260 2270 2280 Vile Mb Fig. 8. Core log showing intrusive sand- stones in the Våle and Lista Formations in the Cecilie-1 well. For legend, see Fig. 9. The two intervals marked by grey bars in the core depth column are shown as core photographs in Fig. 10. 23 Lithology Sedimentary structures Sandstone intrusions Mudstone clasts Chert Siderite Calcite concretions Trace fossils Sandstone Mudstone Marl Chalk Carbonate cement (non-calcitic) Calcite cement Pyrite Glaucony Parallel lamination Faint parallel lamination Water-escape pipes (large) Load cast Dish structures and pipes Deformed/slumped bedding Fractures/faults Bed boundary Cross-lamination Sandstone intrusions Flow structures Stylolites Zoophycos Helminthopsis Planolites Thalassinoides Chondrites Low Moderate Intense Degree of bioturbation P G S C Miocene The Oligocene–Miocene Series boundary is bracketed by a number of HOs at its type section (Lemme-Carosio, north-west Italy). Unfortunately, none of the foraminifer events are believed to be true stratigraphic tops (facies de- pendent), and reworking in the section hampers the use of nannofossil tops (Steininger et al. 1997). However, the dinoflagellate succession from the Lemme-Carosio section has been documented in detail by Powell (1986), Brinkhuis et al. (1992) and Zevenboom (1995, 1996), and provides a means for direct correlation to the North Sea Basin (Munsterman & Brinkhuis 2004). The HO of Distato- dinium biffii is below the Chattian–Aquitanian boundary in its type section and the HO of Chiropteridium spp. is above. This succession of events can be recognised in many North Sea wells, and the Chattian–Aquitanian boundary is positioned between the two. Supporting microfossil events that characterise the lowermost Miocene include the HO of the diatom Aulacodiscus insignis quadrata (small morphotype, same as diatom sp. 3 of King 1983, 1989), a widespread event in the North Sea Basin, and the HO of the benthic foraminifer Brizalina antiqua (King 1989). The HO of the planktonic foraminifer Paragloborotalia nana marks uppermost Chattian strata. The principal criteria for the Aquitanian–Burdigalian, Burdigalian–Langhian and Langhian–Serravallian stage boundaries are as yet undecided. Most authors place the three boundaries at microfossil zone boundaries or mag- netochron boundaries at 20.5, 16.4 and 14.8 Ma, respec- tively (Hardenbol et al. 1998; Williams et al. 2004). The correlation of the three boundaries to the North Sea Ba- sin is feasible using the dinoflagellate zonation scheme of De Verteuil & Norris (1996), established for US East Coast sections and the review of dinoflagellate index events pub- lished by Williams et al. (2004). The former zonation scheme is correlated directly with the zonation schemes of Berggren et al. (1995) and the Miocene timescale by means of calcareous nannofossils and foraminifers. The Aquitanian–Burdigalian boundary is positioned just above the HO of the dinoflagellate Caligodinium amiculum. The Burdigalian–Langhian boundary is placed between the HO of the dinoflagellates Hystrichokolpoma cinctum and Pyxidinopsis fairhavenensis, two events that bracket the boundary level. The Langhian–Serravallian boundary is slightly above the HO of the dinoflagellate Cousteaudi- nium aubryae. In this study, these four events have been used to approximate the three stage boundaries. Fig. 9. Legend for core logs (Figs. 8, 11, 18, 27, 30 and 39); the lithological colour scheme is also adopted on well sections (e.g. Fig. 13). 24 0 10 20 30 40 50 60 70 80 90 100 cm 2239 m 2240 m 2245 m 2246 m Cecilie-1 Fig. 10. Core photographs showing dark grey, largely structureless, discordant, intrusive sandstones within the lighter grey mudstones of the upper Tyr Member in the Cecilie-1 well. Depths are core depths. Stratigraphic positions of the figured intervals are shown on Fig. 8. Lithostratigraphy Rogaland Group The Rogaland Group was established by Deegan & Scull (1977) and comprises the Paleocene to Lower Eocene marl- stone and mudstone succession between the top of the Ekofisk Formation of the Chalk Group (Deegan & Scull 1977) and the glaucony-rich mudstones of the Horda- land Group (now Stronsay and Westray Groups) in the central North Sea. In most of the Danish sector, the Ro- galand Group has a relatively uniform thickness and com- prises the Våle, Lista, Sele and Balder Formations (Fig. 7). The Fur Formation is a part of the Rogaland Group and is present in a limited area in the north-eastern part of the Danish sector of the North Sea stretching into the Norwegian sector. Hardt et al. (1989) added three new sandstone units to the Rogaland Group in the southern Viking Graben (Norwegian sector): Ty Formation, Heim- dal Formation and Hermod Formation (Fig. 4). Although these sandstone units are broadly comparable to coeval sandstone units encountered in the Siri Canyon System (Figs 1, 4) in the Danish sector, the Norwegian units and the Siri Canyon sandstones have different provenances and are not contiguous with each other. Therefore, the sand- stone units in the Danish sector are described herein as new members. In the Siri Canyon (Fig. 1), the mudstones of the Ro- galand Group contain concordant or discordant postdepo- sitional sandstone intrusions (Hamberg et al. 2005). In some wells (e.g. Cecilie-1 and Nini-3) sandstone intru- sions are very common (Figs 8–11). The sands have in- truded most levels in the Rogaland Group, but the Ve Member (new member of the Lista Formation, see below) in the middle part of the group is particularly rich in in- trusions. Most of the intrusive sandstones are only a few millimetres thick, but they may reach a thickness of 5 m. In some wells they constitute up to 30% of the total sand- stone thickness. The intrusions are usually massive, but faint lamination is locally present, especially at the top of the beds. Most of the intrusive sandstone bodies are sepa- rated by in situ mudstones, but they may also occur in intervals showing multiple intrusions. The boundaries with the host rock are slightly to very irregular or wavy, and in places discordant. Minor intrusive offshoots (apophyses) into the host rock are common. The petrography of the intrusive sandstones is similar to that of the in situ sand- stones and they are therefore most likely sourced from the 25 latter. The intrusion of sand was mainly subhorizontal, parallel to the bedding, and most of the intrusions can thus be classified as sills. The sandstone intrusions are either unconsolidated or cemented by calcite or other carbonate minerals. In some wells the intrusive sandstones are chlo- rite-cemented. In the Nini-3 cores, the Vile and Ve Members of the Lista Formation (new members, see below) are particu- larly rich in intrusive sandstones (Figs 11, 12); the intru- sions increase in number and thickness upward through the Vile Member to terminate in a large intrusion com- plex in the Ve Member. Commonly, mudstone clasts are abundant in the sandstones and in the intrusion complex in the Nini-3 well. Where present, they constitute from a few percent up to 90% of the volume of the host sand- stones. They are most abundant in the upper parts of the beds. The clasts range in size from a few millimetres to wider than the core diameter. Most of the clasts are angu- lar, often with delicate protrusions, and aligned parallel to the bounding planes of the intrusive sandstone body. In thick intrusions, flow banding and dewatering struc- tures are occasionally present. Top-bed rip-down mudstone clasts (Stow & Johansson 2000) are common (Fig. 12). Fossil wood fragments are present, but rare. As the distribution of intrusive sandstones is the result of postdepositional rather than synsedimentary processes, they may cross lithostratigraphic boundaries. When an injected sandstone body occurs in direct contact with an in situ sandstone unit (e.g. as seen in the higher parts of the new Tyr Member in Fig. 8), it is impossible to distin- guish between the two genetically different units on the basis of petrophysical logs and cuttings samples alone; only a sedimentological study of core material may reveal the different nature of the two sandstones. Våle Formation History. The Våle Formation was established by Hardt et al. (1989) for the marls with interbedded claystones, lime- stones and silt- and sandstone stringers that overlie the Chalk Group in the central and northern North Sea. The Lista Fm Vile Mb Tyr Mb Idun Mb Nini-3 clay si. vf. f. m. Sand c. vc. P C C Log depth Core depthGR Sonic 1760 1750 1740 1730 1760 1750 1740 1730 Fig. 11. Core log showing intrusive sandstones in the Vile and Idun Members in the Nini-3 well. For legend, see Fig. 9. The two intervals marked by grey bars in the core depth column are shown as core photographs in Fig. 12. 26 Fig. 12. Core photographs of the Vile and Idun Members in the Nini- 3 well showing sandstone intrusions, weak flow banding (1), injec- tion breccia with abundant irregular and angular clasts (2), mudstone clasts in injected sand (3) and top bed rip-down clasts (4). Depths are core depths. Stratigraphic positions of the figured intervals are shown on Fig. 11. Fig. 13. E-8, Danish reference well for the Våle and Lista Formations, and reference well for the Vile, Ve and Bue Members. Black bar shows cored section. 1726 m 1727 m Nini-3 1739 m 0 10 20 30 40 50 60 70 80 90 100 cm 4 3 1 2 2060.3 2044.0 2030.3 2027.6 2057.0 Horda Fm Balder Fm Sele Fm Lista Fm Bue Mb Ve Mb Vile Mb Våle Fm Chalk Gp 2100 2000 m E-8 GR Sonic presence of a marly succession on top of the Chalk Group was previously noted by Deegan & Scull (1977) and treat- ed informally as an equivalent to the more coarse-grained Maureen Formation in the UK sector of the North Sea. Kristoffersen & Bang (1982) established the North Sea Marl for an exclusively marly and calcareous unit corre- sponding to the Maureen Formation-equivalent unit of Deegan & Scull (1977). Although the description of the North Sea Marl fulfils the requirements for a formal de- scription of a lithostratigraphic unit (with the exception of lacking indication of the rank of the unit), they speci- fically stated that their unit was only informally estab- lished. The name North Sea Marl has only rarely been used outside the Danish sector of the North Sea; the sed- iments are instead referred to the Våle Formation, which covers most national sectors of the North Sea Basin. As doubt may be raised about the formal status of the North Sea Marl unit, and in order to promote communication between North Sea stratigraphers, it is considered by the present authors that the Våle Formation of Hardt et al. (1989) serves as the better name for the marlstone unit. Type well. Norwegian sector well 1/3-1, 3258–3209 m MDKB. Danish reference wells. E-8, 2060.3–2057.0 m MDKB (Fig. 13);Siri-1,2186.5–2156.3mMDKB(Fig.14;Plates1, 4). Distribution and thickness. The Våle Formation and its equivalents are present throughout the North Sea Basin, except in a few areas where their absence is due to non- deposition or erosion. The Våle Formation is absent on 27 pyrite-bearing marlstones dominate the formation (Fig. 16). Thin sandstone intrusions are present locally. In the Siri Canyon, the marls are interbedded with turbidite sand- stones; where sandstone-dominated, the succession is re- ferred to a new member (Bor Member, defined below). Log characteristics. From its base to its top, the Våle For- mation is characterised by an overall steady increase in gamma-ray response, combined with an overall steady decrease in sonic readings. When the Bor Member sand- stones are present, blocky log signatures with higher gam- ma-ray values and lower sonic readings interrupt this gene- ral trend (Fig. 17). Boundaries. In most wells in the Danish sector, the change from the chalks of the Chalk Group to the marlstones of the Våle Formation is gradational and the boundary can be difficult to position (Fig. 16). In the Siri Canyon, how- ever, most wells show an erosional contact between the Chalk Group and the Våle Formation and the formation boundary is sharp. On the petrophysical logs, the boun- dary is placed where the stable, low gamma-ray response characteristic of the Ekofisk Formation starts to increase upwards and the high sonic readings (also characteristic of the latter formation) start to decrease upwards. The change in the log pattern may be stepwise with each step represented by a small increase in gamma-ray values and an accompanying decrease in sonic readings. The Våle Formation is overlain by the Lista Formation. intrabasinal highs (Hardt et al. 1989) and in parts of the Siri Canyon where the Rogaland Group overlies the Chalk Group with an erosionalunconformity. Its thickness varies from 0 to 48 m in the Danish sector of the North Sea (Fig. 15). Lithology. Light grey to greenish grey, heavily bioturbated Fig. 15. Isochore map of the Våle Formation in the study area. The positions of the two Danish reference wells, E-8 and Siri-1, are indi- cated on the figure. 2047.5 2072.6 2186.5 2156.3 Horda Fm Balder Fm Sele Fm Lista Fm Rind Mb Idun Mb Tyr Mb Vile Mb Ve Mb Bue Mb Bue Mb Våle Fm Chalk Gp 2000 2100 2200 m Siri-1 GR Sonic Neutron/DensityFig. 14. Siri-1, Danish reference well for the Våle and Sele Formations. Black bars show cored sections. E-8 Siri-1 0 10 20 30 40 Thickness (m) Våle Formation 25 km 28 cm 2398 m 2399 m 2400 m Cecilie-1B 2401 m 2402 m 0 10 20 30 40 50 60 70 80 90 100 cm underlying Ekofisk Formation is characterised by the HO of the dinoflagellate Senoniasphaera inornata followed uphole by the HO of the planktonic foraminifer Globo- conusa daubjergensis. There is a hiatus at the contact bet- ween the Ekofisk and Våle Formation in many sections and wells (Clemmensen & Thomsen 2005). The basal part of the Våle Formation, just above the top of the Ekofisk Formation, is marked by the downhole increase in calca- reous foraminifer diversity and the HO of the planktonic foraminifers Globanomalina cf. compressa and Subbotina Subdivision. The Våle Formation includes a sandstone unit (Bor Member, new) in the Danish North Sea sector. Macro- and ichnofossils. Fragments of shelly macrofossils are present, but rare. The Våle Formation is heavily bio- turbated. Trace fossils in the formation include Chondrites ispp., Phycosiphon ispp., Planolites ispp. and Zoophycos ispp. Thalassinoides ispp. burrows are only present locally. Microfossils and palynomorphs. The uppermost part of the Fig. 16. Core photographs of the Ekofisk–Våle formation boundary in the Cecilie-1B well. The shift from chalk to marlstones is gradational and placing the boundary can be difficult; it is positioned in the middle part of the core interval 2400.00–2401 m, at 2400.35 m (arrow), where light grey marls become dominant. Depths are core depths. 29 trivialis. Thus, the boundary between the two formations may in practice be located by reference to these three events (Fig. 5a). The HO of the dinoflagellate Alisocysta reticula- ta marks a level in the lower part of the Våle Formation. Calcareous microfossil events near the top of the Våle Formation in the Danish sector include the provisional HO of planktonic foraminifers. Depositional environment. Over most of the Danish sec- tor, the marlstones of the Våle Formation comprise hemi- pelagic deposits and deposits from dilute turbidity cur- rents. The marlstones are probably largely of turbiditic origin. The foraminifer fauna of the Våle Formation is characterised by common calcareous taxa. Taxa belong- ing to the neritic ‘Midway-type’ fauna (Berggren & Au- bert 1975) are especially common. The plankton/benthos ratio varies from approximately 1:1 in some areas to a total dominance of calcareous benthic foraminifers in other areas. The microfaunal composition indicates that the Våle Formation was deposited in an open marine, outer neritic environment that periodically reached upper bathyal depths. The bottom conditions were predominantly oxic with periods of dysoxia. The indications from the micro- fauna are supported by the trace fossil assemblage, which indicates water depths of at least 200 m combined with oxic to dysoxic bottom conditions. In the Siri Canyon, where thin turbidites are common in the Våle Formation, gravity flows played a major role during the deposition of the formation. Age. Selandian. Correlation. The Våle Formation is equivalent to the Lel- linge Greensand and the Kerteminde Marl onshore Den- mark and lithologically most closely resembles the latter. The oldest part of the Kerteminde Marl and the Lellinge Greensand are coeval, but the latter has a more restricted distribution (Sjælland and Storebælt regions only, Fig. 1; Thomsen 1994; Clemmensen & Thomsen 2005). Aliso- cysta reticulata is consistently present in the lowest part of the Kerteminde Marl (Clemmensen & Thomsen 2005). The HO of A. reticulata is therefore an important intra- Våle as well as intra-Kerteminde Marl marker that may be used to correlate the two formations. The Våle Forma- tion correlates with the marly facies of the Maureen For- mation in the UK and Norwegian sectors of the Central and Viking Grabens (Knox & Holloway 1992). 2900 2800 2940.5 2903.0 2894.4 2913.3 2963.2 Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Bor Mb Vile Mb Ve Mb Bue Mb Chalk Gp m Augusta-1 GR Sonic Neutron/DensityFig. 17. Augusta-1, type well for the Bor, Ve and Bue Members. Black bars show cored sections. 30 Våle Fm Chalk Group Vile Mb Bor Mb Lista Fm Augusta-1 clay si. vf. f. m. Sand c. vc. C C C 45° 27° 2920 2930 2940 2950 2960 2920 2930 2940 2950 2960 Log depth Core depthGR Sonic Bor Member new member History. The Bor Member encompasses sandstone bodies enveloped in the marlstones of the Våle Formation in the Danish North Sea sector. Hardt et al. (1989) recognised a pure sandstone unit, the Ty Formation, located between the Ekofisk and Lista Formations in the southern Viking Graben. The Ty Formation replaces the Våle Formation in its occurrence area and may be contemporaneous with the Bor Member, but it is not contiguous with it and it has a different source area. The presence of sandstone bodies in the Våle Formation in the Danish sector was recognised by a stratigraphic working group at Statoil Norway in the mid-1990s and the sandstones were infor- mally named the ‘Borr Member’. Derivation of name. After Bor (Danish spelling), the fa- ther of Odin. Type well. Danish sector well Augusta-1, 2963.2–2940.5 m MDRT (Figs 17, 18). Reference well. Danish sector well Cecilie-1, 2319.6–2284.6 m MDRT (Fig. 19). Distribution and thickness. The Bor Member has been en- countered at the mouth of the Siri Canyon as well as in Fig. 18. Core log of Bor Member sandstones in the Augusta-1 well. Legend in Fig. 9. The interval around the Våle–Lista formation boundary marked by a grey bar in the core depth column is shown as core photographs in Fig. 23. 31 the nearby wells Tabita-1, Augusta-1 and Cleo-1 (Fig. 20a; Plate 1). It reaches a thickness of up to 23 m. Lithology. The Bor Member consists of olive-green, partly calcite-cemented sandstones. The sandstones are very fine grained to fine grained and well sorted (Fig. 18). Round- ed and translucent quartz grains dominate, but the con- tent of glaucony grains is high (20–25%). Mica and py- rite concretions are present in small amounts. Angular chalk and claystone clasts occur locally. Although com- posed exclusively of sandstone in the type well (Fig. 18), the member may also include subordinate interbedded marlstones (e.g. Cecilie-1, Fig. 19). Log characteristics. The Bor Member sandstones are best identified on the density log where they produce a blocky pattern with density values significantly lower than those of the marlstones beneath and above. The sandstones may also be identified from a combination of the density and neutron logs, as the presence of pure sandstone results in a ‘cross-over’ of the two log curves (Figs 17, 19). On the gamma-ray log,theBorMember ischaracterised by a blocky log signature with only small-scale increasing or decreas- ing trends and with values clearly higher than those of the subjacent, suprajacentand locally interbeddedmarlstones. Boundaries. The boundaries with the marlstones of the Våle Formation, the chalks of the Ekofisk Formation and the mudstones of the Lista Formation are sharp and char- acterised by prominent shifts on the gamma-ray, sonic and density logs (Figs 17–19, 21). Depositional environment. The sandstones of the Bor Mem- ber were deposited from highly concentrated gravity flows at bathyal depths. Age. Selandian. Correlation. The Bor Member is contemporaneous with parts of the Kerteminde Marl onshore Denmark. The Lellinge Greensand, which appears between the top chalk surface and the Kerteminde Marl in some areas in eastern Denmark, may also be broadly contemporaneous with the Bor Member, but differs from it lithologically in being predominantly a glaucony-rich calcilutite, rich in bryo- zoan fragments. The Bor Member may be compared with the Ty Formation (Hardt et al. 1989) and with sandstones in the Maureen Formation (Deegan & Scull 1977) in the Norwegian and UK sectors of the southern Viking Graben and the Central Graben. However, it is not contiguous with these units and it has a different source area. 2100 2200 2300 Bue Mb Ve Mb Vile Mb Tyr Mb Bor Mb Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp m Cecilie-1 GR Sonic Neutron/Density 2241.7 2276.6 2284.6 2319.6 Fig. 19. Cecilie-1, reference well for the Bor and Tyr Members. Black bar shows cored section. 32 Fig. 20. Location map showing the distribution of the Rogaland Group sandstones in the Siri Canyon (the outline of the canyon is indicated by grey shading, the grey shading inside the canyon indicates an area of positive relief within the canyon). a: Bor and Tyr Members. b: Idun Member. c: Rind and Kolga Members. Cleo-1 Francisca-1 Frida-1 D-1 Nolde-1 Siri-3Siri-2 Connie-1 Siri-1 Augusta-1 Elna-1 Tabita-1 Amalie-1 Cecilie-1 10 km Augusta-1 Tabita-1 Nini-3 Amalie-1 Cleo-1 Elna-1 Frida-1 D-1 Nolde-1 Cecilie-1 Siri-3 Sandra-1 Sandra-1 Siri-2 Siri-1 Connie-1 10 km Augusta-1 Tabita-1 Amalie-1 Cleo-1 Elna-1 Frida-1 D-1 Nolde-1 Cecilie-1 Siri-3 Sandra-1 Siri-2 Siri-1 Connie-1 10 km Tyr Mb Bor Mb Both mbs Idun Mb Rind Mb Kolga Mb Both mbs Sir i C an yo n Sir i C an yo n Sir i C an yo n a b c Nini-3 Francisca-1 Francisca-1 Nini-3 Lista Formation History. Deegan & Scull (1977) established the Lista For- mation for the widespread, non-laminated mudstones that overlie the marls of the unit equivalent to the Maureen Formation (Våle Formation). Kristoffersen & Bang (1982) established the non-calcareous clay and shale unit CEN-1 between the top of their North Sea Marl (Våle Forma- tion) and the base of the beds with volcanic tuff. They noted that the CEN-1 unit corresponds to the Lista For- mation. For reasons of seniority and the informal nature of the CEN units, we maintain the name Lista Formation for this stratigraphic unit. Type well. Norwegian sector well 2/7–1, 2917.5–2872.5 m MDKB. Danish reference wells. E-8, 2057.0–2027.6 m MDKB (Fig. 13); Cleo-1, 2812.0–2765.5 m MDKB (Fig. 21; Plate 1). Distribution and thickness. The Lista Formation is present throughout the North Sea Basin, except in a few areas where it has been removed by erosion. In the Danish sec- tor, its thickness varies from 0 to 108 m (Fig. 22). Lithology. The formation is characterised by dark coloured, predominantly greyish, greenish or brownish, non-lami- nated to faintly laminated, non-calcareous mudstones. The Lista Formation is predominantly non-tuffaceous but be- comes tuffaceous towards its top. In the Siri Canyon, glau- cony-rich, massive sandstone layers and injected sandstone bodies occur in the Lista Formation. Log characteristics. Although fluctuating, both the gam- ma-ray and sonic log readings in the Lista Formation have higher mean values than those of the underlying Våle Formation and lower mean values than those of the over- lying Sele Formation. In wells where mudstone facies dom- inate in the Lista Formation, the gamma-ray and sonic log patterns can be subdivided into three. The tripartite log pattern reflects the succession of three different mud- stone units, established as new members herein (see below). Boundaries. In most wells where the transition has been cored, the boundary is sharp between the light-coloured marlstones of the Våle Formation and the dark-coloured, non-calcareous mudstones of the lower Lista Formation (Vile Member, see below; Fig. 23). On the gamma-ray log, the boundary is picked at an abrupt upward shift to higher values than in the underlying Våle Formation. This level can typically be identifiedon the sonic log at a velocity 33 minimum. Above this minimum, the sonic readings in- crease slightly upwards. The Lista Formation is overlain by the Sele Formation. The base of the Sele Formation was defined by Deegan & Scull (1977 p. 34) at the contact between “non-laminated, non-tuffaceous shales” (Lista Formation) and “laminated tuffaceous shales” (Sele Formation). This boundary defi- nition was followed by Mudge & Copestake (1992a, b). On the other hand, Knox & Holloway (1992 p. 46) fol- lowed O’Connor &Walker (1993) and placed the boun- dary somewhat lower, at the contact between “grey-green and green-grey, blocky, bioturbated claystones” of the Lista Formation and “dark grey fissile mudstones” of the Sele Formation. The boundary concept of Knox & Holloway implies that the “non-laminated, non-tuffaceous shales” of Deegan & Scull are incorporated in the Sele Forma- tion where these, together with overlying laminated in- disputable Sele mudstones, constitute the basal Sele unit S1a (Knox & Holloway 1992). In the present paper, the boundary concept of Deegan & Scull (1977) is followed, and the lower part of the unit of “non-laminated, non- tuffaceous shales” (the “dark grey, fissile mudstones” of Knox & Holloway) that overlies the grey-green mudstones is retained in the Lista Formation as its topmost unit. This unit is formalised as a new member of Lista Formation herein (Bue Member, see below). Subdivision. The Lista Formation is subdivided into six new members. Three of these, the Vile, Ve and Bue Mem- bers, are mudstone units that have widespread distribu- tion in the North Sea Basin and can be correlated with Danish onshore units. In the Siri Canyon, fine-grained Fig. 22. Isochore map of the Lista Formation in the study area. The positions of the two Danish reference wells, Cleo-1 and E-8, are indi- cated. 2700 2800 Bue Mb Ve Mb Vile Mb Bor Mb Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp m Cleo-1 2765.5 2812.0 2792.4 2777.6 GR Sonic DensityFig. 21. Cleo-1, Danish reference well for the Lista Formation and the Vile, Ve and Bue Members. Lista Formation Cleo-1 E-8 20 40 60 80 100 Thickness (m) 25 km 34 2918 m 2919 m Augusta-1 2920 m 2921 m 0 10 20 30 40 50 60 70 80 90 100 cm sandstone bodies occur within each of the three mudstone units. These sandstone-dominated units are proposed here as three new members: the Tyr Member for the sandstones in the Vile Member, the Idun Member for the sandstones in the Ve Member, and the Rind Member for the sand- stones in the Bue Member. Knox & Holloway (1992) recognised a threefold sub- division of the Lista Formation exclusively based on bio- stratigraphy. Their L1 and L2 units are separated by the HO of the dinoflagellate Palaeoperidinium pyrophorum, and the L2 and L3 units are separated by the HO of Are- oligera gippingense. It is noticeable that these two bioevents occur close to the boundaries between the three mudstone members of the Lista Formation proposed herein on the basis of lithology. Macro- and ichnofossils. Macrofossils have not been reported fromtheListaFormation;theformationismoderatelytoheavily bioturbated(for ichnotaxa, see individualmembersbelow). Microfossils and palynomorphs. The Lista Formation dif- fers from the underlying Våle Formation by lacking com- mon planktonic foraminifers and from the overlying Sele Formation by having an impoverished agglutinated ben- thic foraminifer assemblage. The Lista Formation contains a characteristic sequence of palynomorph datums that can aid separation of its members. These datums are treated under the individual Lista members below. Depositional environment. The Lista Formation consists predominantly of hemipelagic mudstones and was proba- bly deposited from very dilute turbidity currents and from suspension. The composition of the microfaunal assemblage indi- cates a relatively open marine depositional setting in upper to possibly middle bathyal depths with oxic to dysoxic bottom conditions. This is based on the presence of an impoverished agglutinated foraminifer assemblage domi- nated by tubular suspension feeders (especially Rhabdam- mina spp.) together with epifaunal and infaunal detriti- vores (e.g. Haplophragmoides spp. and Spiroplectammina spectabilis). The relative abundance of tubular suspension feeders is higher in wells in the Siri Canyon than in wells outside the canyon. This probably indicates slightly deep- er water within the canyon area during deposition of the Lista Formation. Age. Selandian–Thanetian (Upper Paleocene). The Selan- dian–Thanetian boundary may be placed in the middle part of the Lista Formation (in the lower part of the Ve Member, see below), at the HO of the dinoflagellate Pal- aeoperidinium pyrophorum. Correlation. The Lista Formation corresponds to the fol- lowing succession of upper Paleocene units from onshore Fig. 23. Core photographs of the Våle–Lista formation boundary in- terval in the Augusta-1 well. The boundary is at 2919.46 m where dark grey non-calcareous mudstones of the Vile Member overlie green- ish grey marls of the Våle Formation. In most cores the boundary is sharp, as illustrated here. Depths are core depths. Stratigraphic posi- tion of the figured interval is shown in Fig. 18. 35 Fig. 25. Correlation diagram showing possible diachronism of the Våle–Vile boundary in an east–west transect extending into the Norwegian sector of the North Sea (1/3-1 and 2/7-1 are Norwegian sector wells). The distribution of biostratigraphic events shows that the Våle–Vile boundary youngs in a westerly direction (HO, highest occurrence). Alternatively, the event distribution could be explained as a result of rework- ing of older strata into the Lista Formation in the Danish sector. The figure also shows an example of a well (2/7-1) with a relatively large separation between the base of the Sele Formation and the lowest and most conspicuous gamma-ray peak in the formation (see text for further explanation). Æbelø Fm Holmehus Fm Østerrende Clay Ølst Fm Kerteminde Marl Danian Limestone 1/3-1 GR Sonic GR Sonic GR Sonic GR Sonic 1/3-1 2/7-1 Mona-1 E-8 Viborg-1 Viborg-1 Sele Fm Lista Fm Bue Mb Ve Mb Vile Mb Våle Fm Chalk Group HO abundant Palaeoperidinium pyrophorum HO Isabelidinium? viborgense HO diverse calcareous benthic foraminifers HO (provisional) planktonic foraminifers 2050 2075 450 400 3050 3000 2900 2950 3150 3200 3250 Kolga Mb S3 S2b S2a S1b Bue Mb Ve Mb Vile Mb Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp 2000 2100 m Siri-3 1998.8 2016.8 2036.1 2066.4 2072.8 2102.6 GR Sonic DensityFig. 24. Siri-3, type well for the Vile and Kolga Members, and Danish reference well for the Balder Formation. The figure also shows the subdivision of the Sele Formation used by Knox & Holloway (1992); in this well, the Bue Member is equivalent to the S1a subunit of these authors. Black bar shows cored section. 36 Denmark: Æbelø Formation (informal mudstone unit described by Bøggild 1918 and Heilmann-Clausen 1995), Holmehus Formation (Heilmann-Clausen et al. 1985) and Østerrende Clay (informal mudstone unit described by Nielsen et al. 1986 and Heilmann-Clausen 1995). Vile Member new member History. The Vile Member comprises the widespread, dark olive-grey to dark grey, non-calcareous, fissile mudstones that constitute the lower part of the Lista Formation. The unit was recognised by a stratigraphic working group at Statoil Norway in the mid-1990s and informally named the ‘Vile Formation’. Derivation of name. After Vile, the brother of Odin. Type well. Danish sector well Siri-3, 2102.6–2072.8 m MBRT (Fig. 24; Plate 4). Reference wells. Danish sector wells E-8, 2057.0–2044.0 m MDKB (Fig. 13); Cleo-1, 2812.0–2792.4 m MDKB (Fig. 21; Plate 1). Distribution and thickness. The Vile Member has been rec- ognised in a large number of North Sea wells, and the unit probably has a basinwide distribution. However, it is apparently lacking in the Siri Canyon wells Connie-1 and Siri-2 (Figs 29, 31), probably due to erosion. Its thickness varies between 0 and 30 m over most of the Danish sec- tor. It greatest thickness is reached in the Siri Canyon. Lithology. The member consists of dark olive-grey to dark grey, non-calcareous, swelling, smectitic, fissile mudstones (Fig. 23). Thin silicified layers occur in the member. Cal- cite is common and occurs as small nodules and larger concretions. In the Siri Canyon, small pyrite concretions and less than 1 cm thick, silty, very fine-grained glaucony-rich sand- or siltstone laminae are locally present in the Vile Member. The laminae are parallel to the bedding of the mudstones; they have sharp bases and are normally grad- 1803.7 1763.9 1700.4 1717.2 Horda Fm Lark Fm Balder Fm Sele Fm Kolga Mb Bue Mb Vile Mb Idun Mb Tyr Mb Lista Fm Chalk Gp 1600 1700 1800 m Nini-3 GR Sonic Neutron/Density S3 S2b S2a S1b Fig. 26. Nini-3, type well for the Tyr Member and reference well for the Kolga Member. The figure shows the subdivision of the Sele Formation used by Knox & Holloway (1992); in this well, the Bue Member is equivalent to the S1a subunit of these authors. Black bar shows cored section. 37 ed. Thin concordant or discordant, postdepositional sand- stone intrusions are locally present. In the lower part of the Vile Member, the intrusions are only a few millime- tres thick, but they often increase in number and thick- ness towards the top of the member (Fig. 11). Log characteristics. In most wells there is a gradual increase in gamma-ray response up through the Vile Member, ac- companied by a slight decrease in sonic readings. Boundaries. The lower boundary of the Vile Member is that of the Lista Formation. The upper boundary is defined by the base of the Ve Member; boundaries with the sand- stone-dominated Tyr Member are described under that member. Macro- and ichnofossils. The Vile Member is moderately to intensely bioturbated. Ichnogenera in the member include Chondrites ispp., Phycosiphon ispp., Planolites ispp. and Zoophycos ispp. Microfossils and palynomorphs. The Vile Member is char- acterised by a general decrease in the diversity of benthic foraminifers and radiolaria from its base to its top. In the Danish sector, the transition from the underlying Våle Formation to the Vile Member is marked by the provi- sional HO of planktonic foraminifers. A conspicuous drop in diversity of benthic foraminifers takes place in the mid- dle of the Vile Member. The HO of the dinoflagellate Isabelidinium? viborgense is an important intra-Vile marker located in the upper part of the member. Above it, a sud- den decrease in the abundance of radiolaria further char- Lista Fm Chalk Group Nini-3 clay si. vf. f. m. Sand c. vc. P P P 1800 1790 1780 1770 1760 1800 1790 1780 1770 1760 Log depth Core depthGR Sonic Vile Mb Tyr Mb Fig. 27. Core log of the Tyr Member in the Nini-3 well. For legend, see Fig. 9. 38 2908 m2907 m 2909 m Augusta-1 2910 m 2911 m 2912 m 0 10 20 30 40 50 60 70 80 90 100 cm acterises a level within the uppermost part of the Vile Member. The transition from the Vile Member to the overlying Ve Member is marked by a conspicuous drop in the abundance of the dinoflagellate Palaeoperidinium py- rophorum(whichhas itsHOataslightlyhigherstratigraphic level, within the lower part of the Ve Member, see below). Depositional environment. The mudstones of the Vile Member are hemipelagic deposits, whereas the thin sand- stone and siltstone laminae are interpreted as the deposits of low-density turbidity currents. The presence of Zoo- phycos ispp. suggests depositional water depths of at least 200 m (Bottjer & Droser 1992). Age. Selandian. Correlation.The Vile Member corresponds to theÆbelø For- mation, onshore Denmark (informal mudstone unit descri- bedbyBøggild1918andHeilmann-Clausen1995). It corre- sponds to the Lista L1 subunit of Knox & Holloway (1992). Fig. 28. Core photographs of mudstones of the Vile and Ve Members in the Augusta-1 well. Depths are core depths. The boundary between the two members is placed where greenish and reddish grey mudstones become dominant, at 2910.4 m (arrow). This depth corresponds to log depth 2913.3 m on Fig. 17. 39 Biostratigraphic correlation with well sections in the Norwegian North Sea sector may indicate that the lower boundaryof theVileMember isdiachronous (Fig. 25). In the type well for the Lista Formation (Norwegian well 2/7-1; Fig. 25), its base (i.e. its contact with the marlstones of the underlying Våle Formation) is above the HO of a di- verse calcareous benthic foraminifer assemblage. In wells in the Danish sector, this event occurs within the Vile Member. Similarly, the provisional HO of planktonic fo- raminifers, an event that is close to the boundary between the Våle and Lista Formations in Denmark, is found well within the Våle Formation in well 2/7-1. In the type well for the Våle Formation (Norwegian well 1/3-1; Fig. 25), the HO of I.? viborgense coincides with the Våle–Lista boundary (I. Prince, unpublished biostratigraphic data). This event occurs above the Våle Formation in the Dan- ish sector, in the middle to upper part of the Vile Mem- ber. In the Danish onshore well Viborg-1, the latter event and the HO of the diverse calcareous benthic foraminifer assemblage occur above the Kerteminde Marl in the upper part of the Æbelø Formation, a correlative of the Vile Member (Fig. 25; Heilmann-Clausen 1985). The distri- bution pattern of the biostratigraphic events indicates that sedimentation of marls continued in the Norwegian sec- tor some time after marl sedimentation was replaced by sedimentation of non-calcareous mudstones in the Dan- ish sector. Alternatively, calcareous foraminifer assemblages and associated lithologies have been reworked into higher levels of the Våle Formation or even into the Lista Forma- tion in the Danish sector. Tyr Member new member History. The Tyr Member consists of glaucony-rich, sand- stone-dominated deposits that are laterally equivalent to, and commonly underlain and overlain by, mudstones of the Vile Member. These sandstones were previously rec- ognised by a stratigraphic working group at Statoil Nor- way in the mid-1990s and informally referred to the Ty Formation of Hardt et al. (1989). Derivation of name. After Tyr, the son of Odin. Type well. Danish sector well Nini-3, 1803.7–1763.9 m MDRT (Figs 26, 27; Plate 4). Reference well. Danish sector well Cecilie-1, 2276.6–2241.7 m MDRT (Figs 8, 19). Distribution and thickness. The Tyr Member has only been encountered in the Siri Canyon and it may be restricted to thatarea. It reaches a thickness of up to 40 m (Fig. 20a). Lithology. The Tyr Member is characterised by thick beds of olive-green to greenish grey, very fine-grained to fine- grained and well-sorted sandstone (Fig. 27). Rounded and translucent quartz grains dominate, but the content of glaucony grains is high (15–20%), hence the greenish colour of the sandstones. Mica and small pyrite concre- tions are present in small amounts throughout the mem- ber. Angular chalk and claystone clasts occur locally in the sandstones. The sandstones are partly calcite-cemen- ted. Intrusive sandstones are common, particularly towards the top of the member where they may be several metres thick (Fig. 8). Subordinate interbedded dark grey non- calcareous mudstones resemble those of the laterally equiv- alent Vile Member. Log characteristics. The Tyr Member is best identified on the density log where the sandstones are characterised by a conspicuously lower density than the associated mud- stones. The sandstones may also be identified from a combination of the density and neutron logs, as the pres- ence of pure sandstone results in a ‘cross-over’ of the two log curves (Figs 19, 26). The gamma-ray response resem- bles that of the underlying Våle Formation, but is slightly lower than the response of the Vile Member (Figs 8, 26). This log pattern makes it feasible to differentiate even minor sandunits from mudstone beds in the Tyr Member. Thicker sand units may show decreasing- or increasing- upwards gamma-ray values. These trends do not seem to berelatedtograin-sizevariations, judgingfromcore studies. Boundaries. The boundaries to the mudstones of the Vile Member, the marlstones of the Våle Formation and the chalks of the Ekofisk Formation are sharp and character- ised by prominent shifts in gamma, sonic and density log readings (Figs 19, 26, 27). In some wells, the Tyr Member overlies the Våle Formation or the Ekofisk Formation with an erosional contact (e.g. Nini-3; Figs 26, 27). Depositional environment. Although the sandstones of the Tyr Member were deposited from highly concentrated gravity flows, their present appearance is dominated by theeffectsofpostdepositional liquefaction and fluidisation. Age. Selandian. Correlation. The Tyr Member is contemporaneous with parts of the lithologically dissimilar Æbelø Formation in 40 onshore Denmark and with the lower part of the Heim- dal Formation of Deegan & Scull (1977) as well as the Andrew Sandstone and the Mey Sandstone Member of Knox & Holloway (1992) in the Norwegian and UK sec- tors of the southern Viking Graben. However, it is not contiguous with the latter three sandstone units and has a different source area. Ve Member new member History. The Ve Member consists of variegated mudstones that have previously been recognised from North Sea wells as the Holmehus Formation by Heilmann-Clausen et al. (1985), who gave no further details, and by Danielsen & Thomsen (1997), who indicated its presence in several wells. The unit was also recognised by a stratigraphic work- ing group at Statoil Norway in the mid-1990s and infor- mally named the ‘Ve Formation’. Derivation of name. After Ve, the brother of Odin. Type well. Danish sector well Augusta-1, 2913.3–2903.0 m MDRT (Fig. 17). Reference wells. Danish sector wells E-8, 2044.0–2030.3 m MDKB (Fig. 13); Cleo-1, 2792.4–2777.6 m MDKB (Fig. 21; Plate 1). Distribution and thickness. The sediments of the Ve Mem- ber have been recognised from a large number of North Sea wells, and the unit probably has an almost basinwide distribution. Its thickness varies from 0 to 21 m in the Danish sector. Lithology. The Ve Member consists of mottled green, blu- ish green, reddish brown and brown mudstones (Fig. 28). Mottled, purple coloured intervals are also present locally. The middle part of the member is often characterised by a thick dark reddish brown to chocolate brown interval. TheVeMembermudstones are non-calcareous and richin smectite. Pyrite and carbonate concretions occur through- out the member. A weak biogenic lamination is some- times observed in cores. Only very little organic material is present in the member. In the Siri Canyon, thin intru- sive sandstones are common in the Ve Member. Log characteristics. In general, the gamma-ray log shows a decreasing-upwards trend through the Ve Member, as opposed to the increasing trend through the underlying Vile Member. In the uppermost part of the Ve Member, the gamma-ray response increases over a short interval before reaching the base of the overlying Bue Member. The sonic log pattern throughout the Ve Member is smooth and relatively stable compared with the sonic pattern of the Vile Member. It also differs from the latter in having an increasing-upwards trend. The log pattern of the Ve Member differs from that of the overlying Bue Member in having a lower gamma-ray response level. Boundaries. The lower boundary, with the Vile Member, is placed at the first appearance of greenish, bluish or red- dish brown mudstones above the dark olive-grey to dark grey mudstones of the Vile Member. The colour change from Vile to Ve mudstones is often gradational and the boundary may be difficult to define precisely (Fig. 28), especially when only cuttings samples are available. How- ever, in the colour transition interval, a gamma-ray spike separates an interval with an increasing-upwards gamma- ray trend below from an interval with a decreasing gam- ma-ray trend above (compare Figs 17 and 28). In the ab- sence of a clear indication of the boundary level from sedi- ment colour change, the gamma spike at the shift from increasing to decreasing gamma-ray values may be used as a marker for the boundary. The Vile Member is absent from the Connie-1 well, and in this well the lower con- tact of the Ve Member is with the marlstones of the Våle Formation (Fig. 31). The upper boundary is at the base of the Bue Member (see below). Boundaries with the Idun Member are described under that member. Macro- and ichnofossils. The mudstones of the Ve Mem- ber are normally heavily bioturbated. The most common trace fossils are Phycosiphon ispp. and Zoophycos ispp. Chon- drites ispp. and Planolites ispp. are present, but rare. Microfossils and palynomorphs. In the Danish sector of the North Sea, the HO of abundant Palaeoperidinium pyro- phorum is located at or close to the Vile–Ve boundary. The HOs of P. pyrophorum and Palaeocystodinium austral- inum are in the lower part of the Ve Member. In general, the dinoflagellate assemblage from the upper part of the Ve Member is sparse and is characterised by specimens of Areoligera gippingensis. An acme of the latter species marks a level in the upper part of the Ve Member. The highest in situ occurrence of the dinoflagellate Alisocysta margarita is located close to the top of the Ve Member. Depositional environment. Deposition of the mudstones of the Ve Member was controlled by hemipelagic sedi- mentation and sedimentation from dilute turbidites. The 41 Idun Member new member History. The Idun Member consists of sandstone-domi- nated deposits that are laterally equivalent to, and com- monly underlain by, mudstones of the Ve Member. This sandstone unit was previously recognised by a stratigraphic working group at Statoil Norway in the mid-1990s and was informally referred to the Heimdal Formation of Deegan & Scull (1977). Derivation of name. After Idun, the goddess of youth. Type well. Danish sector well Connie-1, 2368.3–2332.0 m MDRT (Figs 29, 30). 2200 2300 2368.3 2332.0 2329.6 2292.2 Idun Mb Rind Mb Bue Mb Bue Mb Ve Mb Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp m Connie-1 GR Sonic Neutron/DensityFig. 29. Connie-1, type well for the Idun and Rind Members. In this well, the Rind Member may be divided into three major sandstone intervals. The Idun Member consists of two thick sandstone intervals, separated by a thick mudstone unit. Black bars show cored sections. occurrence of the trace fossil Zoophycos ispp. indicates a water depth of at least 200 m (Bottjer & Droser 1992). The overall high degree of bioturbation, the lack of or- ganic material and the greenish, bluish and reddish brown colours together suggest oxygenated bottom conditions. Age.Selandian–Thanetian.TheSelandian–Thanetianboun- daryisplacedattheHOofthedinoflagellatePalaeoperidinium pyrophorum, in the lower part of theVe Member. Correlation. The Ve Member correlates with the Holme- hus Formation (Heilmann-Clausen et al. 1985) onshore Denmark and is lithologically indistinguishable from that formation. 42 Lista Fm Våle Fm Connie-1 clay si. vf. f. m. Sand c. vc. 45o S v G G G PP PPP 45o 2290 2300 2310 2320 2340 2350 2360 2300 2290 2310 2320 2330 2340 2350 2360 2370 Log depth Core depthGR Sonic P P P P P P P P P P P P Change of core depth scale Bue Mb Idun Mb Bue Mb Rind Mb Ve Mb Fig. 30. Core log of the Idun and Rind Members in the Connie-1 well. For legend, see Fig. 9. Minor mudstone beds separate the three major sandstone intervals of the Rind Member. Minor mudstone layers are also intercalated with the Idun Member sandstone intervals. The core depth scale of the lower core is offset by c. 1.6 m relative to the scale of the upper core in the figure. For reasons of consistency with core data from this well, the original (albeit erroneous) core depths of the lower core are maintained in the figure. This does not affect the depths of the top and base of the Idun Member given in the text, as these are based on log depths. 43 Reference well. Danish sector well Siri-2, 2205.5–2127.0 m MDRT (Fig. 31). Distribution and thickness. The Idun Member is only known from the Siri Canyon, and it may be restricted to that area (Fig. 20b). It reaches a thickness of up to 179 m in the Siri-2 well. Lithology. The Idun Member is dominated by very fine- grained to fine-grained, well-sorted sandstones (Fig. 30). Rounded and translucent quartz grains dominate, but the content of glaucony grains is high (15–25%). The sand- stones are olive green to greenish grey due to the high content of glaucony. Mica and small pyrite concretions are present in small amounts. Angular chalk and claystone clasts occur locally. Intrusive sandstones are also repre- sented (Figs 11, 12, 30). The sandstones and adjacent mudstones are partly calcite-cemented. In the Nini and Siri wells, the sandstones occur in thick amalgamated suc- cessions with only rare, thin mudstone interbeds; the latter are lithologically comparable to the laterally equivalent Ve Member mudstones (see above). In the Connie-1 well, however, the sand-rich succession is interrupted by a di- screte 6 m thick mudstone unit (Fig. 30). Log characteristics. The sandstone-dominated Idun Mem- ber is best identified on the density log where it is charac- terised by a conspicuously lower density than the under- lying and overlying mudstones (Figs 29, 31). The sand- stone component may also be identified from a combina- tion of the density and neutron logs, as the presence of pure sandstones results in a ‘cross-over’ of the two log curves (Figs 29, 31). The Idun Member is characterised by a blocky, decreasing-upwards gamma-ray and density log pattern. Intervals with an overall constant gamma-ray pattern may be characterised by many small-scale increas- ing- or decreasing-upwards gamma-ray cycles. Boundaries. In sections where the sandstones of the Idun Member are enveloped by mudstones of the Ve Member, the boundaries are sharp and characterised by prominent shifts on the gamma-ray, sonic and density logs (Figs 29, 30). Where the Ve Member is absent, comparable, sharp boundaries are observed with the mudstones of the Vile Member beneath and the Bue Member above (Figs 11, 26, 30). In the Siri-2 well, the lower Lista Formation is absent and an erosive unconformity separates the Idun Member sandstones from the marlstones of the Våle For- mation (Fig. 31). Depositional environment. Although the sandstones of the Idun Member were deposited from highly concentrated gravity flows, their present appearance largely records post- depositional liquefaction and fluidisation processes. Age. Selandian–Thanetian. Bue Mb Bue Mb Rind Mb Idun Mb Balder Fm Horda Fm Sele Fm Lista Fm Chalk Gp Våle Fm 2100 2200 m Siri-2 GR Sonic Neutron/Density 2127.0 2205.5 Fig. 31. Siri-2, reference well for the Idun Member. Black bar shows cored section. 44 with these sandstone units, however, and has a different source area. Bue Member new member History. The Bue Member encompasses the light to dark grey and greyish black mudstones that occur between the top of the Ve Member and the base of the Sele Formation. These mudstones have not previously been recognised as a separate unit in the Danish sector. Derivation of name. After Bue, the son of Odin and Rind. Typewell. DanishsectorwellAugusta-1,2903.0–2894.4m MDRT (Fig. 17). Referencewells.Danishsector wells E-8, 2030.3–2027.6 m below MDKB (Fig. 13); Cleo-1, 2777.6–2765.5 m MD- KB (Fig. 21; Plate 1). Distribution and thickness. The sediments of the Bue Mem- ber have been recognised from a largenumberof North Sea wells, and the unit probably has a basinwide distribution. Its thickness varies from 0 to 18 m in the Danish sector. Lithology. The Bue Member consists of light to dark grey and greyish black mudstones. The mudstones are gene- rally rich in smectite. In the Siri Canyon, the upper part of the member sometimes contains laminae of very fine- grained to fine-grained sandstones or siltstones, mimick- ing the laminated mudstones of the overlying Sele For- mation (Figs 32, 39). The laminae are less than 2 cm thick, have sharp bases, are normally graded and show parallel lamination. Concordant or discordant sandstone intru- sions are locally present in the member (Fig. 8). Small calcite and siderite concretions are occasionally present. Moderately to intensely bioturbated intervals are inter- bedded with non-bioturbated intervals. Tuff layers may be present in the member. Log characteristics. The gamma-ray response of the Bue Member is generally higher than that of the underlying Ve Member, but lower than that of the overlying Sele For- mation. In some Siri Canyon wells, minor coarsening- upwards cycles are indicated by the gamma-ray log of the Bue Member. Boundaries. The transition from typical lithologies of the Ve Member to those of the Bue Member is often grada- Fig. 32. Core photographs showing the Bue Member mudstones with numerous sandstone laminae in the Augusta-1 well. Depths are core depths. 0 10 20 30 40 50 60 70 80 90 100 cm 2891 m 2892 m Augusta-1 2893 m Correlation. The Idun Member is contemporaneous with parts of the lithologically dissimilar Holmehus Formation onshore Denmark and with parts of the Heimdal Forma- tion of Hardt et al. (1989) and the Lower Balmoral Sand- stone and Tuffite of the Mey Sandstone Member of Knox & Holloway (1992). The Idun Member is not contiguous 45 Rind Member new member History. The Rind Member consists of sandstone-domi- nated deposits that are laterally equivalent to, and com- monly underlain and overlain by, mudstones of the Bue Member. Sandstone bodies at this stratigraphic level were previously recognised by a stratigraphic working group at Statoil Norway in the mid-1990s and were informally re- ferredtotheHeimdalFormationofDeegan&Scull (1977). Derivation of name. After the giantess Rind. Type well. Danish sector well Connie-1, 2329.6–2292.2 m MDRT (Figs 29, 30). Reference well. Danish sector well Sandra-1, 2066.3– 2004.8 m MDRT (Fig. 33). Distributionand thickness. The Rind Member has only been encountered in the Siri Canyon, and it may be restricted to that area where it reaches a thickness of 62 m (Fig. 20c). Lithology. The Rind Member consists of very fine-grained, well-sorted sandstones interbedded with thin mudstone beds that typically form less than 15% of the member (Fig. 30). Rounded and translucent quartz grains domi- nate in the sandstones, but the content of glaucony grains in the very fine-grained to fine-grained size fraction is high (15–25%). The sandstones are olive green to greenish grey due to the high content of glaucony. Mica and small py- rite concretions are present in small amounts. Angular chalk and claystone clasts occur locally in the sandstones, which are partly calcite-cemented. The interbedded mud- stones are lithologically comparable to the Bue Member mudstones (see above). Log characteristics. The Rind Member is best recognised on the density log where it shows either a blocky or a serrate pattern created by the alternation of sandstone beds or amalgamated units (low density) with thin mudstone beds (high density; Figs 29, 33). The sandstones may also be identified from a combination of the density and neu- tron logs, since the presence of pure sandstones results in a ‘cross-over’ of the two log curves (Figs 29, 33). The gam- ma-ray log shows a low-amplitude serrate pattern. This pattern does not reflect alternating sand or mudstones, judging from core inspection. Boundaries. The boundary between the sandstones of the Rind Member and the mudstones of the Bue Member is tional and the boundary may therefore be difficult to po- sition precisely. It is placed where mottled green, bluish green, reddish brown and brown mudstones pass upwards into grey mudstones with sandstone and siltstone lami- nae. On the petrophysical logs, this transition is reflected by a shift from decreasing- to increasing-upwards gam- ma-ray values or at an abrupt increase in the gamma-ray response. The upper boundary of the Bue Member is at the base of the Sele Formation. Macro- and ichnofossils. Trace fossils recognised in the Bue Member are Phycosiphon ispp., Planolites ispp., Thalas- sinoides ispp. and rare Zoophycos ispp. Microfossils and palynomorphs. The Ve–Bue boundary is bracketed by the stratigraphic succession of the HO of in situ Alisocysta margarita (occurring in the upper Ve Mem- ber) followed by the HO of an impoverished assemblage of benthic agglutinated foraminifers (in the lower part of the Bue Member). The upper part of the Bue Member is characterised by common spores and pollen, in particular bisaccate pollen and Inaperturopollenites spp. The Bue– Sele boundary is marked by the base of an acme of the dinoflagellate genus Apectodinium and the LO of Apecto- dinium augustum. Depositional environment. The normally graded sandstone to siltstone laminae in the upper part of the member indi- cate that deposition of the Bue Member took place from dilute, low-density, turbidity currents in a generally sedi- ment-starved environment at this level. The minor coars- ening-upwards cycles observed on petrophysical logs from some Siri Canyon wells probably indicate either small distal lobes of deep-water channel-sandstones or levee deposits. Age. Thanetian. Correlation. The Bue Member corresponds to the Øster- rende Clay (informal mudstone unit described by Nielsen et al. 1986 and Heilmann-Clausen 1995) onshore Den- mark. The level here defined as the boundary between the Ve and theBueMemberswascorrelatedbyKnox (1997 fig. 3; the Lista–Sele boundary of this worker) with the boun- dary between the Holmehus Formation and the Øster- rende Clay (as ‘Grey Clay’) onshore Denmark. The Bue Member further correlates with the lower part of the S1a subunit of the Sele Formation established by Knox & Holloway (1992; see correlation section under the Lista Formation for further details). 46 sharp and characterised by prominent shifts on both the sonic and density logs (Figs 29, 33). It is often difficult to identify the boundaries on the gamma-ray log alone. Depositional environment. Although the sandstones of the Rind Member were deposited from highly concentrated gravity flows, their present appearance largely records post- depositional liquefaction and fluidisation processes. Age. Thanetian. Correlation. The Rind Member may be contemporaneous with parts of the lithologically dissimilar Østerrende Clay encountered in the Storebælt region (Fig. 1), with sand- stones in the higher parts of the Heimdal Formation (Hardt et al. 1989) and with the Upper Balmoral Sand- stone of the Mey Sandstone Member of Knox & Hollo- way(1992).However, theRindMember is not contiguous with those sandstone units and has a different source area. Sele Formation History. The Sele Formation was established by Deegan & Scull (1977) for the dark grey to greenish grey, laminated and carbonaceous, tuffaceous, montmorillonite-rich shales and siltstones that overlie the non-laminated and non- tuffaceous shales of the Lista Formation in some areas, or arenaceous sediments belonging to a variety of different units in other areas. The original definition of the Sele boundary is followed herein. This implies that the base of the Sele Formation is located at the base of the “laminat- ed tuffaceous shales” that overlie the “non-laminated, non- tuffaceous shales” of the Lista Formation (Deegan & Scull 1977; see Boundaries section under the Lista Formation for further details). Sandstones occur in the Sele Forma- tion in the Danish sector; these are established as a new member, the Kolga Member. Typewell.British sectorwell21/10-1,2131–2100m MDKB. Bue Mb Rind Mb Vile Mb Ve Mb Balder Fm Horda Fm Lark Fm Sele Fm Lista Fm Våle Fm 1800 1900 2000 2004.8 2066.3 2100 m Sandra-1 GR Sonic Neutron/Density Fig. 33. Sandra-1, reference well for the Rind Mem- ber. Black bar shows cored section. 47 Danish reference wells. Siri-1, 2072.6–2047.5 m MDKB (Fig. 14; Plates 1, 4); Tabita-1, 2958.8–2941.4 m MD- KB (Fig. 34; Plate 2). Distribution and thickness. The Sele Formation is recog- nised from a large number of North Sea wells and it has a basinwide distribution. In the Danish sector, the thick- ness varies from 5 to 54 m (Fig. 35). Lithology. The Sele Formation consists of medium to dark grey, brownish or black laminated mudstones. Thin tuff layers occur in the upper part of the formation. It con- tains three or more well-laminated intervals where dark mudstone beds alternate with lighter coloured mudstone beds. The well-laminated intervals are enriched in organ- ic material resulting in a high gamma-ray response, pri- marily due to increased uranium content. The most or- ganic-rich, and often darkest, most well-laminated inter- val is found in the basal part of the formation (Fig. 36). The mudstones of the Sele Formation show an overall upward increase in the silt fraction. In the upper half of the formation, the mudstones may be interbedded with thin, very fine-grained sandstone laminae and thin sand- stone beds (Fig. 37). The sandstone beds are up to 12 cm thick, normally graded and display parallel lamination. Locally, and dominantly in the upper part of the forma- tion, graded tuff laminae less than 1 cm thick are present. In cores, the tuff laminae have a light purple colour. Small calcite concretions are present, but rare. In the Siri Can- yon, the Sele Formation is interbedded with sandstones or it grades upwards into a succession of thinly interbed- ded sandstones and mudstones. Thin sandstone intrusions occur, but only in the lower part of the formation. Log characteristics. The Sele Formation is characterised by high gamma-ray readings throughout, with a number of gamma-ray peaks. On the gamma-ray log, the base of the Sele Formation is generally marked by a conspicuous up- ward shift to consistently higher gamma-ray readings than those of the underlying Bue Member (Figs 38, 39). In most wells, a pronounced gamma-ray peak follows a short distance above the base of the Sele Formation (e.g. Au- gusta-1 and E-8; Fig. 38). In wells to the north and west of the Danish sector, the stratigraphic distance between the shift to higher gamma-ray readings at the base of the Sele Formation and the gamma-ray peak is considerably greater (e.g. in the Norwegian well 2/7-1; Fig. 25). In some wells in the Danish sector (and in most Siri Canyon wells), the basal high gamma-ray interval is missing and the base of the Sele Formation is marked by the pronounced gam- ma-ray peak (e.g. Cleo-1 and Nini-3; Figs 38, 39). Boundaries. The lower boundary is characterised by a change from the light to dark grey and greyish black mud- stones with thin sandstone laminae of the Bue Member (Lista Formation), to dark grey to black well-laminated mudstones without sandstone laminae of the Sele Forma- tion (Fig. 36). The upper boundary is at the base of the Balder Formation. Subdivision. Knox & Holloway (1992) suggested an in- formal threefold subdivision of the Sele Formation based Fig. 35. Isochore map of the Sele Formation in the study area. The positions of the two Danish reference wells, Siri-1 and Tabita-1, are indicated in the figure. Siri-1 Tabita-1 10 20 30 40 50 Thickness (m) Sele Formation 25 km Fig. 34. Tabita-1, Danish reference well for the Sele Formation. Horda Fm Balder Fm Sele Fm Lista Fm Vile Mb Ve Mb Bue Mb Våle Fm Chalk Gp 2900 3000 m Tabita-1 2941.4 2958.8 GR Sonic 48 in Figs 24, 26 and 38), whereas other lithological changes are more subtle. The subdivision is outlined below. Unit S1 This unit comprises a lower subdivision S1a and an overlying subdivision S1b. Subdivision S1a is identi- cal to the Bue Member of the Lista Formation and the stratigraphic interval from the base of the Sele Forma- tion (sensu Deegan & Scull 1977) up to the base of the lowermost conspicuous gamma-ray peak within that formation (Fig. 38). That peak is associated with relatively low sonic values. Subdivision S1b has its base at the gamma-ray peak and its top at the base of the next gamma-ray peak. The gamma-ray response de- creases up through S1b. Unit S1 consists of brownish grey to dark grey and black, well-laminated mud- stones. Unit S2 The base of the unit is at the base of the second gam- ma-ray peak. This peak can be differentiated from the gamma-ray peak at the base of S1b by its association with high sonic values. Unit S2 can be divided into a lower subdivision (S2a) characterised by a relatively high gamma-ray response level and an overlying sub- division (S2b) with a lower gamma-ray response (Fig. 38). The two subdivisions are separated by a gamma- ray low. Unit S2 consists of light grey to brownish dark grey, laminated to well-laminated mudstones. Both the lamination and the colour of the two subdivisions are very similar in the cores encountered in this study and it is almost impossible to distinguish the two subdivi- sions on lithology alone. Knox & Holloway (1992) observed tuff layers in the basal part of unit S2b in the British wells; tuffs were not observed in the Danish wells.. Unit S3 This unit is characterised by high and increasing- upwards gamma-ray values. The base of the unit is defined by a sharp increase in gamma-ray readings (Fig. 38). There is no significant colour difference between the mudstones of units S2 and S3 in the Danish wells, but lamination seems to be better developed in unit S3 than in unit S2. Tuff layers were observed in unit S3 in the British wells studied by Knox & Holloway (1992). Similar tuff layers have been observed in wells from the Siri Canyon, and may further be used to dis- tinguish unit S3 from the upper part of unit S2. 0 10 20 30 40 50 60 70 80 90 100 cm 2888 m 2889 m Augusta-1 2890 m Sele Fm Bue M b Fig. 36. Core photographs of the Lista–Sele formation boundary in- terval in the Augusta-1 well. The upper part of the Bue Member (Lis- ta Formation) consists of mudstones with thin sand- and siltstone laminae superficially resembling mudstone-in-mudstone lamination. At 2890.35 m, the Bue Member is overlain sharply by the laminated mudstones of the Sele Formation. Depths are core depths. on the gamma-ray log signature. This subdivision can also be recognised on petrophysical logs from most wells in the Danish sector (the subdivision is shown in five wells 49 0 10 20 30 40 50 60 70 80 90 100 cm 1689 m 1690 m Nini-3 1692 m 1693 m The Sele Formation includes a sandstone unit (Kolga Member, new) in the Danish North Sea sector. Macro- and ichnofossils. Fish scales and skeletal fragments are common in cores from the Sele Formation. Bioturba- tion is very rare, but Chondrites ispp. has been observed locally. Microfossils and palynomorphs. Benthic foraminifers are rare in the Sele Formation. The LO of an acme of the dinofla- gellate genus Apectodinium and the coeval LO of the short- ranged A. augustum mark a level at, or a few centimetres above the base of the Sele Formation. The HO of A. au- gustum is located in the lower part of the Sele Formation. The HO of an influx of the dinoflagellate Cerodinium war- denense marks a level in the upper part of the Sele Forma- tion. The HO of an influx of the diatoms Fenestrella anti- qua and Coscinodiscus morsianus is located in the upper- most part of the Sele Formation. Throughout, the forma- tion contains abundant spores and pollen, in particular pollen of the genus Inaperturopollenites. Depositional environment. The mudstones of the Sele For- mation represent a mixture of pelagic fallout and dilute, low-density mud turbidites. The well-laminated charac- ter of the sediment, the high content of organic material and uranium, and the general lack of trace fossils and ben- thic foraminifers indicate starved sedimentation under dysoxic to anoxic bottom conditions. Common diatoms indicate a high nutrient level in the water mass. The tuffs of the Sele Formation are evidence of exten- sive volcanism in the region. The significant depaupera- tion of the benthic microfaunas during the deposition of the Sele Formation was most likely caused by isolation of the North Sea Basin (Schmitz et al. 1996). The restric- tion and isolation of the basin was the result of a sea-level fall, possibly combined with (or caused by) tectonic uplift to the north-west (Knox et al. 1981). Based on microfos- sils, the palaeoenvironment has been suggested to repre- sent an upper bathyal setting with a palaeodepth estimate of around 300 m (Mitlehner 1996). The palynomorph assemblage indicates a marine environment characterised by a massive influx of terrestrial palynomorphs. Age. Sparnacian (sensu Aubry et al. 2003) – early Ypres- ian, with the lowermost level possibly of Thanetian age. Correlation. The Sele Formation corresponds to the Has- lund Member of the Ølst Formation (Heilmann-Clausen et al. 1985). Its upper part correlates with the Haslund Member-equivalent diatomitic Knudeklint Member of the Fig. 37. Core photographs from the Nini-3 well showing the upper, arenaceous part of the Sele Formation. The interval belongs to the S2a subunit of the Sele Formation (see text for further explanation). Stratigraphic position of the figured interval is shown in Fig. 39. Depths are core depths. Fur Formation in north-west Jylland (Danielsen & Thom- sen 1997). The lower boundary of the Sele Formation correlates with the boundary between the Østerrende Clay and the Haslund Member onshore Denmark. 50 2760 2740 m 2780 S3 S2b S2a S1b S1a 2900 Cleo-1 Augusta-1 E-8Horda Fm Balder Fm Sele Fm Lista Fm Bue Mb Ve Mb 2880 2040 2860 2020 m m GR Sonic GR Sonic GR Sonic Cleo-1 E-8 The lower part of the Sele Formation, consistingof lam- inated, dark grey to black mudstones, correlates with the lithologically very similar, 15 m thick informal unit Stolle Klint Clay that constitutes the lower part of the Haslund Member, onshore Denmark (Heilmann-Clausen 1995). This unit is known from throughout the North Sea (Hardt et al. 1989; Knox & Holloway 1992) and constitutes most or all of the Sele S1b unit of Knox & Holloway (1992). Kolga Member new member History. The Kolga Member consists of sandstone depo- sits within the Sele Formation. These sandstones were pre- viously recognised by a stratigraphic working group at Statoil Norway in the mid-1990s and were informally re- ferred to the Hermod Formation of Hardt et al. (1989). Derivation of name. After the goddess Kolga. Type well. Danish sector well Siri-3, 2066.4–2036.1 m MDRT (Fig. 24; Plate 4). Reference well. Danish sector well Nini-3, 1717.2–1700.4 m MDRT (Figs 26, 39; Plate 4). Distribution and thickness. The Kolga Member has a restric- ted distribution in the Siri Canyon in the northern part of the Danish sector (Fig. 20c). It reaches a thickness of up to 30 m. Lithology. The member consists primarily of fine-grained to very fine-grained, olive-green to greenish grey, well-sort- ed, quartz-rich sandstones (Fig. 39). Rounded and trans- lucent quartz grains dominate the mineralogical assem- blage, but the content of glaucony grains is high (15– 25%). Mica and small pyrite concretions are present in small amounts. Locally, the sandstones are partly cemen- ted by calcite and chlorite. The member usually includes one thick unit composed of amalgamated sandstone beds and a number of thinner sandstone beds interbedded with mudstones that are lithologically comparable to the Sele Formation mudstones described above. Log characteristics. The Kolga Member is clearly defined on the gamma-ray log by a blocky pattern with inter- mediate values. This pattern differs from the high gam- ma-ray readings that normally characterise the lower Sele Formation. The Kolga Member may show a gradual up- ward decrease in gamma-ray response in its lower part (e.g. in the well Nini-3; Figs 26, 39). However, this does not reflect grain-size change, judging from core examination. The density log shows a blocky pattern with low density Fig. 38. Correlation diagram of the Sele Formation showing the Sele units S1–3 of Knox & Holloway (1992). 51 v v v Sele Fm Lista Fm Kolga Mb Bue Mb GRSonic 1700 1690 1710 1720 1700 1710 1720 Nini-3 clay si. vf. f. m. Sand c. vc. Log depth Core depthFig. 39. Core log of the sandstone- dominated Kolga Member encased in the Sele Formation mudstones in the Nini-3 well. For legend, see Fig. 9. Intervals marked by grey bars in the core depth column are shown as core photos in Fig. 37. values for the Kolga Member sandstones and relative high values for the interbedded mudstones (Fig. 24). Boundaries. The boundaries between the sandstones of the Kolga Member and the mudstones of the Sele Formation are sharp and characterised by prominent shifts on the gamma-ray and sonic logs (Figs 24, 26). Microfossils and palynomorphs. The Kolga Member is char- acterised by an abundance of Apectodinium spp., including A. augustum. Depositional environment. The sandstones of the Kolga Member were deposited from highly concentrated gravity flows, although the present character of the Kolga Mem- ber mainly reflects postdepositional liquefaction and flu- idisation processes. Primary sedimentary structures are common in the Kolga Member in some wells (e.g. San- dra-1),however, indicatingthat thememberhasexperienced less postdepositional remobilisation in certain areas. Age. Sparnacian (sensu Aubry et al. 2003) possibly includ- ing the latest Thanetian. Correlation. The Kolga Member is contemporaneous with parts of the Sele Formation onshore Denmark. It possibly correlates with the Forties Sandstone Member (Knox & Holloway 1992) in the Central Graben and with the Teal and Skadan Sandstone Members (Knox & Holloway 1992) in the southern Viking Graben. However, the Kolga Mem- ber is not contiguous with those units and has another source area. Fur Formation History. The Fur Formation is a marine diatomite with numerous ash layers. It was formally established by Peder- sen & Surlyk (1983) with a type section in the coastal cliff Knudeklint on the island of Fur, Denmark. Its lower boundary was revised by Heilmann-Clausen et al. (1985). The characteristic lithology of the Fur Formation was sub- sequently recognised by Thomsen & Danielsen (1995; Danielsen & Thomsen 1997) in cuttings samples from three offshore wells located in the north-eastern part of the Danish sector of the North Sea, as well as in one well in the Norwegian sector. 52 350 450 m K-1 GR Sonic Balder Fm Horda Fm Fur Fm Sele Fm Lista Fm Våle Fm Chalk Group 398.9 402.2 400 m 485.5 697.1 795.3 810.7 500 600 700 800 900 Dufa Mb Horda Fm Balder Fm Lark Fm Sele Fm Fur Fm Lista Fm Våle Fm Chalk Gp Inez-1 GR Sonic Type section. The coastal cliff Knudeklint, the island of Fur, onshore Denmark (for location map, see Pedersen & Surlyk 1983). Reference sections. Silstrup south cliff, Skarrehage, Fegge- klit, Harhøj, Stolleklint (for location maps, see Pedersen & Surlyk 1983). Danish reference wells. Danish sector wells K-1, 402.2– 398.9 m MDKB (Fig. 40; Plate 5); Inez-1, 810.7–795.3 m MDKB (Fig. 41; Plates 1, 5). Distribution and thickness. Onshore Denmark, the Fur Formation is distributed in a limited area in north-west Jutland. It is c. 61 m thick in its type section. Offshore, the formation occurs in a belt stretching from the north- western coast of Jutland, continuing into the Norwegian sector parallel to the southern coast of Norway (Thomsen & Danielsen 1995 text-fig. 6; Fig. 42). It reaches a thick- ness of 15.4 m in the Inez-1 well, 7.9 m in the C-1 well and 3.3 m in the K-1 well (Figs 40–42). Lithology. The lithology of the Fur Formation was descri- bed from its onshore exposures by Pedersen (1981) and Pedersen & Surlyk (1983). It is a clayey, porous, dark grey diatomite with numerous volcanic ash layers. Diatom frus- tules constitute 65 wt% of the rock, clay particles consti- tute 35 wt% (Pedersen 1981). Tests of Coscinodiscus spp. and Stephanopyxis are the major constituent of the dia- tomite fraction (Thomsen & Danielsen 1995). Fine lam- ination is the primary sedimentary structure, but at some Fig. 41. Inez-1, reference well for the Fur Formation and type well for the Dufa Member. Fig. 40. K-1, reference well for the Fur Formation. 53 25 km K-1x 3.3 15.4 7.9 Inez-1 C-1 57°00' 6°00' 8°00' 56°00' 55°00' ? ? ? ? Fur Formation levels the lamination has been destroyed by bioturbation. The ash layers are black, graded and consist of volcanic glass particles. Individual layers range from 1–20 cm in thickness but are fairly uniform in thickness over a limi- ted area (Pedersen & Surlyk 1983). The diatomite recog- nised in cuttings samples from North Sea wells by Thom- sen & Danielsen (1995) is a similar lithology to that of the onshore sections studied by Pedersen (1981) and Peder- sen & Surlyk (1983). Log characteristics. The Fur Formation is identified by the combination of a low gamma-ray response and low sonic read- ingswithinan interval of higher sonic readings characterising mudstones below and above (Thomsen & Danielsen 1995; Danielsen& Thomsen1997;Figs40,41;Plates 1, 5). Boundaries. In the C-1 and K-1 wells, the Fur Formation is enveloped by the Balder Formation. In Inez-1, the for- mation is bounded by the Sele and Balder Formations (Figs 40, 41; Plates 1, 5). It should be noted, however, that Thomsen & Danielsen (1995 text-fig. 4) and Danielsen & Thomsen (1997 fig. 5) placed the Fur Formation en- tirely within the Sele Formation in the K-1 well. The dis- crepancy between the interpretation herein and that of the former authors is due to different interpretations of the position of the Balder–Sele boundary in the well. The boundary of the Fur Formation with the Balder and Sele Formations is characterised by a change from laminated or structureless diatomite with ash layers to the dark mud- stones of the Balder and Sele Formations. This lithologi- cal change is reflectedonthe sonic logbyan abrupt increase in velocity (Figs 40, 41; Plates 1, 5). Subdivision. Onshore Denmark, the Fur Formation is di- vided into the lower, laminated Knudeklint Member that contains relatively few, widely spaced ash layers and the upper, mainly structureless Silstrup Member with nume- rous ash layers (Pedersen & Surlyk 1983). Macro- and ichnofossils.Themacrofossil assemblage descri- bed from onshore exposures of the Fur Formation encom- passes fish, birds, turtles, snakes, starfish, shellfish, snails, mussels, crabs, pteropods, insects, fossil wood, leaves and fructifications (Bonde 1966, 1979, 1987, 2003; Pedersen 1981; Pedersen & Surlyk 1983; Kristoffersen 2001). Ichnofossils from onshore exposures include Planolites ispp., Teichichnus ispp., Chondrites ispp. and Taenidium ispp. (Pedersen & Surlyk 1983). Microfossils and palynomorphs. Diatom frustules are rock- forming in the Fur Formation (Pedersen 1981). Silico- Fig. 42. Distribution map of the Fur Formation with formation thick- ness (m) indicated for three wells. flagellates are present (Perch-Nielsen 1976) whereas cal- careous microfossils are absent. The formation contains abundant dinoflagellates and sporomorphs (Hansen 1979; Heilmann-Clausen 1982; Willumsen 2004). Depositional environment. The deposition of the Fur Forma- tion diatomites took place in a long, narrow zone under up- welling conditions (Bonde 1974, 1979). The upwellingwas controlled by northerly winds (Bonde 1974, 1979) or it may have been created by a combination of bottom currents and bottomtopography (Pedersen& Surlyk 1983). Age. Early Ypresian. Correlation. Onshore Denmark, the Knudeklint and Sil- strup Members are largely contemporaneous with the upper part of the Haslund Member and the overlying Værum Member of the Ølst Formation (Heilmann- Clausen et al. 1985), respectively, and correlate with the 54 upper part of the Sele Formation and the lower unit B1 (see below) of the Balder Formation (Heilmann-Clausen 1995; Knox 1997 fig. 3). Balder Formation History. Deegan & Scull (1977) established the Balder Formation for the succession of variegated, fissile and lam- inated shales with interbedded tuff layers that lie between the Sele and Horda Formations. Type well. Norwegian sector well 25/11-1, 1780–1705 m MDKB. Danish reference wells. Mona-1, 2945.0–2930.8 m MDKB (Fig. 43; Plate 1). Siri-3, 2016.8–1998.8 m MDRT (Fig. 24; Plate 4). Distribution and thickness. The Balder Formation extends over most of the central and northern North Sea. In the Danish sector, it reaches a thickness of more than 20 m in the Siri-3 and Frida-1 wells on the western part of the Ringkøbing–Fyn High (Fig. 1) and 20 m in Gwen-2 in the northern part of the Danish sector of the Central Graben. The Balder Formation thins to less than 5 m towards the south-west and to less than 10 m in the east- ern part of the Danish sector of the North Sea. The Bal- der Formation is lacking in the Danish well S-1 (Michelsen et al. 1998). An isochore map of the Balder Formation is shown in Fig. 44. Lithology.TheBalderFormationis composedof laminated, dominantly grey, fissile shales with interbedded dark and light grey, purple, buff and green sandy tuffs (Fig. 45). The tuffs are normally graded and less than 5 cm thick. Locally the tuff beds are slumped. The tuff layers may be cut by irregular, vertical, calcite-filled cracks up to 20 cm long (Fig. 45). Similar cracks have been reported from the Balder Formation in the Grane Field, Norwegian sec- tor of the North Sea (Haaland et al. 2000). Sandstone beds, interpreted as intrusive sandstone bodies, occur locally in the Balder Formation. Log characteristics. The Balder Formation is characterised by a relatively high gamma-ray values in its lower and high- er parts, but shows low values in its middle part. The change in gamma-ray response is normally gradual, but relatively steep. The gamma-ray motif is mirrored by a gradual increase in sonic readings commencing at the for- mation base, culminating at or slightly below the level of minimum gamma-ray values in the middle part of the Fig. 44. Isochore map of the Balder Formation in the study area. The positions of the two Danish reference wells, Mona-1 and Siri-3, are indicated in the figure. Siri-3 Mona-1 Balder Formation 5 10 15 20 Thickness (m) 25 km Fig. 43. Mona-1, Danish reference well for the Balder Formation. 3000 2900 m 2930.8 2945.0 Horda Fm Balder Fm Sele Fm Lista Fm Våle Fm Vile Mb Ve Mb Bue Mb Chalk Gp Mona-1 GR Sonic 55 2013 m Siri-3 2012 m 2014 m 2015 m 0 10 20 30 40 50 60 70 80 90 100 cmformation, followed by a gradual decrease towards the top of the formation where the lowest sonic reading is reached. The gamma and sonic motifs together create a characte- ristic barrel-shaped log pattern (e.g. Figs 24, 33, 38, 43; Plates 1–5). Boundaries. In general, the boundary with the underlying Sele Formation is gradational, although it can be sharp in some wells. Where gradational, it is placed where the tuff layers become prominent (e.g. Fig. 45). On petrophysical logs, the lower boundary is identified at a significant up- ward decrease in gamma-ray response accompanied by an increase in sonic readings (e.g. Figs 24, 33, 38, 43). The upper boundary is at the base of the Horda Formation. Subdivision. Knox & Holloway (1992) subdivided the Balder Formation into a lower, laminated and tuff-rich unit (B1) and a poorly laminated upper unit (B2). This subdivision can be recognised in a number of Danish wells west of Cecilie-1, but the B2 unit seems to be absent from Danish North Sea wells north-east of, and including, the Cecilie-1 well. Macro- and ichnofossils. Macrofossils have not been obser- ved. The Balder Formation is non-bioturbated to mode- rately bioturbated. Ichnofossils comprise Chondrites ispp., Phycosiphon ispp., Planolites ispp. and Thalassinoides ispp. Microfossils and palynomorphs. The Sele–Balder boundary interval is characterised by the HO of common Fenestrella antiqua and Coscinodiscus morsianus (both diatoms). This event is located in the uppermost part of the Sele Forma- tion but may be used as a biostratigraphic guide to locate the boundary. The diatom Fenestrella antiqua characterises the Balder Formation and has its HO at the formation top. The dinoflagellate Deflandrea oebisfeldensis shows an acme at the top of the Balder Formation. As observed in the underlying Sele Formation, the Balder Formation con- tains high numbers of spores and pollen, in particular pollen of the genus Inaperturopollenites spp., and the top of the Balder Formation is marked by the HO of com- mon representatives of that genus. In contrast to the over- lying Horda Formation, calcareous benthic foraminifers are virtually absent in the Balder Formation. Depositional environment. A restricted marine palaeoenvi- ronment at upper bathyal depths with dysoxic to anoxic bottom conditions is suggested for the Balder Formation. This is based on the scarcity of calcareous microfossils and agglutinated foraminifers combined with common to abundant siliceous microfossils, especially diatoms. The Fig. 45. Core photographs of tuffaceous Balder Formation mudstones from the Siri-3 well. The tuff layers are seen as light coloured, graded intervals (e.g. at 2012.42–2012.40 m). The boundary with the under- lying Sele Formation is placed where tuffs become common, at 2015.5 m (large arrow); two tuff layers may be seen in the uppermost Sele Formation, at 2015.66 and 2015.90 m (small arrows). Two small, lightning-shaped cracks are seen at 2015.4 m. Depths are core depths. 56 2363.5 2930.8 1598.3 Horda Fm H2 H1 H3 L2 L3 L4 Lark Fm Nordland Gp Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 m Mona-1 GR Sonic Fig. 46. Mona-1, Danish reference well for the Horda and Lark For- mations. The figure shows the tripartite subdivision of the Horda For- mation and the L2–4 units of the Lark Formation. The L1 unit is absent in the Mona-1 area. presence of abundant terrestrial palynomorphs further supportsa restricted,marginalmarinedepositional setting. A petrographic and geochemical study of the Balder Formation in the Grane Field, Norwegian North Sea sec- tor, shows that the tuffs can be classified as representing sub-alkaline basalts and basaltic andesites of intra-plate origin (Haaland et al. 2000). The tuffs are similar to the contemporaneous Lower Basalts in East Greenland, the Rockall Trough and the Middle Series of the Faeroe Is- lands, all linked to the opening of the North Atlantic (Haaland et al. 2000). The volcanic phase took place at 55–52 Ma. Age. Early Ypresian. Correlation. Although unit B2 of Knox & Holloway (1992) is apparently lacking in wells in the north-eastern part of the Danish sector, both units B1 and B2 can be correlat- ed with strata onshore Denmark, although unit B2 is very thin. Unit B1 corresponds to the lithologically similar Værum Member of the widespread Ølst Formation on- shore Denmark and with the diatomaceous Silstrup Mem- ber of the Fur Formation in north-west Jylland (Knox 1997 fig. 3). The lower boundary of the Værum and Sil- strup Members is placed at ash layer no. +1 in the tephra- chronology of Bøggild (1918). The ash chronology has not been identified in the type section of the Balder For- mation and precise correlation with the lower boundary of the Værum and Silstrup members is therefore uncer- tain. However, judging from the abundance of thick ash layers in the Balder Formation and the scarcity of ash lay- ers in the underlying Sele Formation, it is likely that the base of the Balder Formation approximately correlates with ash layer no. +1, i.e. with the base of the Værum and Silstrup Members. According to Knox (1997 fig. 3), unit B2 probably correlates with the Knudshoved Member of the Røsnæs Clay Formation (Heilmann-Clausen et al. 1985). This member has a very restricted distribution in north-west Jylland where it overlies the Silstrup Member of the Fur Formation. The Knudshoved Member consists of a lower dark grey, pyritic clay unit rich in pyritised diatoms, and an upper greenish clay unit (Heilmann-Clausen et al. 1985). Only a few, thin volcanic ash layers are present in the member (Håkansson & Sjørring 1982). Based on litho- logical comparison, it is suggested that at least the lower, 57 pyritic part of the Knudshoved Member may correlate with theupper, tuff-poorunitB2 of the Balder Formation. Stronsay Group Knox & Holloway (1992) replaced the Hordaland Group of Deegan & Scull (1977) with two new groups: the Stron- say Group succeeded by the Westray Group (Fig. 3). The two groups together comprise the light grey, green and brown coloured, soft, fissile, marine shales with thin lime- stone streaks that overlie the Rogaland Group and under- lie the Nordland Group. These groups each contain two formations, one representing sandy shelf lithofacies and the other representing basinal mudstone lithofacies. In the central North Sea, and in the Danish sector, the Stronsay Group is represented by its mudstone facies, the Horda Formation (Knox & Holloway 1992). Sandstone units of varying thickness occur at many levels in the Stronsay and Westray Groups along the basin margin in the Norwe- gian and British sectors, and many of these have been defined as formations or members (Deegan & Scull 1977; Hardt et al. 1989; Knox & Holloway 1992). A sandstone unit also occurs in the Horda Formation on the Ringkø- bing–Fyn High in the Danish sector and is described here as a new member (Hefring Member). Horda Formation History. Knox & Holloway (1992) established the Horda Formation for the greenish grey basinal mudstone facies of their Stronsay Group that overlies the grey tuffaceous mudstones of the Balder Formation and underlies the greenish grey to brown mudstones of the Lark Formation (Knox & Holloway 1992). Type well. British sector well 22/1-1A, 2379.5–1992 m MDKB. Danish reference wells. Mona-1, 2930.8–2363.5 m MD- KB (Fig. 46); Siri-1, 2037.9–1916.5 m MDKB (Fig. 47). Distribution and thickness. The Horda Formation extends over the central and northern North Sea and is present in 1916.5 2037.9 Horda Fm Chalk Gp Lark Fm Balder Fm Sele Fm Lista Fm Våle Fm 1900 2000 2100 2200 m Siri-1 GR Sonic Rind Mb Idun Mb Tyr Mb Vile Mb Ve Mb Bue Mb Bue Mb Horda Formation Siri-1 Mona-1 Floki-1 100 200 300 400 500 600 700 800 900 Thickness (m) 25 km Fig. 47. Siri-1, Danish reference well for the Horda Formation. Black bars show cored sections. Fig. 48. Isochore map of the Horda Formation in the study area. The positions of the two Danish reference wells for the Horda Formation, Mona-1 and Siri-1, are indicated on the map. The position of Floki-1, the type well for the Hefring Member, is also indicated. 58 Gulnare-1 Adda-2 Alma-1 NNW SSE 10 km Horda FmHorda FmHorda Fm L4L4L4 L2L2L2 0 500 1000 1500 2000 2500 3000 3500 TWT (msec) TL TF UOU TL2 TH TB TC L3L3 Lark FmLark FmLark Fm 1000 1500 2000 2500 3000 3500 Gert-1 SW NE Mona-1 Elna-1 Sandra-1 Horda FmHorda FmHorda Fm H2H2 H3H3TH2TH2 H2 H3H3TH2 10 km Lark FmLark FmLark Fm TL UOU TH TB TC TWT (msec) TH1TH1TH1 H1H1H1 Fig. 49. NNW–SSE-trending seismic section (RTD81-RE94-17A) in the Central Graben showing south- and eastward thinning of the Horda Formation. The location of the seismic section is shown in Fig. 1. The L2, L3 and L4 subunits of the Lark Formation are indicated, as well as the mudstone-equivalent of the Freja Member. TL, Top Lark; TF, Top Freja; UOU, Upper Oligocene Unconformity; TL2, Top L2; TH, Top Horda; TB, Top Balder; TC, Top Chalk. Fig. 50. SW–NE-trending seismic section (RTD81-RTD94-19A) showing the tripartite subdivision (H1–3) of the Horda Formation in the eastern part of the Danish Central Graben (Gert-1 and Mona-1) and pronounced thinning of the Horda Formation east of the Central Graben (Elna-1 and Sandra-1). The location of the seismic section is shown in Fig. 1. TH1, Top H1 marker; TH2, Top H2 marker; other abbreviations as in Fig. 49. 59 all wells in the Danish sector of the North Sea. However, the lower part of the Horda Formation (Fig. 4; equivalent to Sequence 2 of Michelsen et al. 1998) is lacking in the eastern wells R-1 and S-1 and in the eastern part of the Ringkøbing–Fyn High. The upper part of the Horda For- mation (Fig. 4; equivalent to Sequence 3 of Michelsen et al. 1998) is thin or absent in the same area (Michelsen et al. 1998). The Horda Formation reaches a thickness of 906 m in the Central Graben well Tordenskjold-1, but thins towards the east and south-east to less than 100 m, with minimum recorded thicknesses of 9 m in the Ida-1 well and 4 m in the S-1 well. An isochore map of the Horda Formation is shown in Fig. 48. The overall thin- ning of the Horda Formation towards the south-east, east and north-east is also shown on the seismic sections in Figs 49 and 50 and on the log panel in Fig. 51. Lithology. The Horda Formation is characterised by green- ish grey to greyish green fissile mudstone. Subordinate limestone benches and thin layers of black mudstones occur at some levels in the formation. In many wells, par- ticularly in the Central Graben, the lowermost 20–50 m of the Horda Formation consists of red-brown mudstones (Fig. 52). This lithology is apparently lacking in the east- ern wells of the Danish sector. Log characteristics. The Horda Formation is characterised by an overall stable gamma-ray and sonic log motif with a lower gamma-ray response than that displayed by the underlying Balder Formation and the overlying Lark For- mation. In a few wells, the base of the Horda Formation shows relatively high gamma-ray values, which decrease to lower and more stable values over a short interval. The sonic readings decrease slightly upwards from the base to the top of the Horda Formation. Boundaries. The base of the Horda Formation is placed at the change from the laminated, predominantly grey mud- stones with interbedded sandy tuffs of the Balder Forma- tion to the predominantly non-laminated, fissile, green- ish grey or red-brown massive mudstones that form the basal part of the Horda Formation. The Balder–Horda boundary may be conformable or marked by a hiatus. The boundary is often difficult to pick on petrophysical logs. In basinal settings, Knox & Holloway (1992) advocated placing the lower boundary of the Horda Formation at the base of a marked gamma-ray peak believed to repre- sent a glaucony-rich condensed layer in the basal part of the Horda Formation. However, in many sections in the Danish sector there are two or more gamma-ray peaks in the Balder–Horda boundary interval. As the glaucony- rich layer has not been identified with certainty in the few cores taken across the boundary in the Danish sector, it is not possible to identify the key gamma-ray peak unam- biguously. Therefore, it is suggested that the lower bound- ary of the Horda Formation is placed on the basis of the sonic log where a gradual decrease in values in the upper part of the Balder Formation is succeeded by relatively stable, but somewhat lower readings in the Horda Forma- tion (Figs 46, 47). The upper boundary is at the base of the Lark Formation. Subdivision. Knox & Holloway (1992) suggested a three- fold subdivision of the Horda Formation (H1–3), based on lithology and biostratigraphy. A threefold subdivision can also be seen on seismic sections in the Danish sector of the North Sea (Fig. 50). In some Central Graben wells, the subdivision may also be recognised on shifts in log patterns on both gamma-ray and sonic logs (Fig. 51). In these wells, subtle peaks separate the three units on the gamma-ray log and coincide with the top H1 and top H2 seismic markers. Based on analysis of cuttings samples, the subdivision apparently lacks lithological expression in the Danish sector. Knox & Holloway (1992) noticed that the top of unit H1 is close to the HO of the dinoflagellate Eatonicysta ursulae, and that the top of unit H2 is close to the HO of the foraminifer Spiroplectammina spectabilis. This observation is supported by biostratigraphical data from the present study. A sandstone body within the Horda Formation has been encountered in the well Floki-1 on the Ringkøbing–Fyn High) in the Danish sector of the North Sea. This sandstone is defined herein as the new Hefring Member (see below). Macro- and ichnofossils. The Horda Formation is mode- rately to intensely bioturbated. Ichnofossils comprise Chon- drites ispp., Phycosiphon ispp. and Planolites ispp. Microfossils and palynomorphs. In wells where the Horda Formation rests conformably on the Balder Formation, the dinoflagellate events HO Deflandrea oebisfeldensis and HO Dracodinium condylos occur in the lowermost part of the Horda Formation. Characteristic microfossil datums from the lowermost part of the Horda Formation are the HO of the planktonic foraminifer Subbotina ex gr. linap- erta, which occurs abundantly, followed upwards by the almost coeval HOs of the benthic foraminifers Uvigerina batjesi, Turrilina brevispira and Gaudryina hiltermanni. A hiatus between the Horda and Balder Formations is indi- cated in wells in the northern and eastern part of the Dan- ish sector by the absence of D. oebisfeldensis and D. condy- los from the lower part of the Horda Formation. Central 60 1700 1800 1900 1400 GR Sonic GR Sonic Density GR Sonic Density GR Sonic Density 1500 1600 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 m 1000 1100 1200 700 800 900 1300 1400 1500 1600 1700 1800 1900 2100 2200 m 1700 1800 1900 1400 1500 1600 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 m 1800 2000 1500 2100 2200 2300 2400 2500 2600 2700 2800 3000 m Kim-1 Kim-1 Mona-1 West Lulu-3 Siri-1 Siri-1 1600 1700 1900 2900 2000 Horda H1 H3 H2 Lark L4 TL UOU TL2 TL1 TH TH2 TH1 TB L3 L2 L1 Balder Sele Lista Våle Chalk Grp SW NE 61 above the top of the lower, oxic part of the formation. The occasional influxes of radiolaria recorded through- out the upper part of the formation suggest that deeper marine conditions prevailed periodically. The palynofacies of the Horda Formation is character- ised by a rich and dominant dinoflagellate assemblage with dispersed terrestrial matter (phytoclasts, spores and pol- Facing page: Fig. 51. SW–NE-trending log panel showing eastward thinning of the Horda Formation. The figure also shows the variation in architec- ture and distribution of the Horda units H1–H3 and Lark units L1– 4 between the Central Graben (Kim-1, Mona-1 and West Lulu-3) and the Ringkøbing–Fyn High (Siri-1). Seismic ties: TL1, Top L1; TH2, Top H2; TH1, Top H1; other abbreviations as in Fig. 49. Fig. 52. Core photographs showing red to reddish grey mudstones of the lowermost part of the Horda Formation in the Sofie-1 well. Depths are core depths. cm 1887 m 1888 m Sofie-1 1901 m 1902 m 0 10 20 30 40 50 60 70 80 90 100 Graben wells contain the downhole succession of the dino- flagellate cyst events HO Areosphaeridium diktyoplokum, HO Areosphaeridium michoudii, HO Heteraulacacysta po- rosa and HO Cerebrocysta bartonensis from the top of the Horda Formation, indicating an age as young as earliest Rupelian (Fig. 5b). In wells to the north and east, the top of the Horda Formation seems to be slightly older since A. michoudii is recorded from the top of the formation, indicating a mid-Priabonian age (Fig. 5b). Significant dino- flagellate events from the middle to upper part of the Horda Formation are the succession of the HOs of Eatoni- cysta ursulae, Diphyes ficosoides and Phthanoperidinium clit- hridium in the middle part of the formation, and the HOs of Diphyes colligerum and C. bartonensis in the upper part of the formation. In Central Graben wells, significant microfossil events in the lower part of the Horda Forma- tion include the HO of abundant radiolaria of the genus Cenosphaera and the HO of the planktonic foraminifer Cyclammina amplectens. Key events in the middle and upper parts of the formation are the HOs of Pseudohasti- gerina spp. (planktonic foraminifers), Lenticulina gutticos- tata, Spiroplectammina amplectens and Planulina costata (benthic foraminifers). The top of the Horda Formation contains the HOs of Cibicidoides truncanus and Vaginuli- nopsis decorata. Depositional environment. The lower part of the Horda Formation contains a microfauna that is significantly diffe- rent from that of the underlying Balder Formation. The basal 5–40 m of the Horda Formation are characterised by a diverse fauna of both benthic and planktonic calca- reous foraminifers together with agglutinated foramini- fers. This indicates that the depositional setting was open marine, bathyal and with oxic bottom conditions. The upper part of the Horda Formation is character- ised by an abundant and diverse agglutinated foraminifer fauna. Calcareous foraminifers are very sparse or absent in this interval. The assemblage of Rhabdammina discre- ta, Haplophragmoides spp., Recurvoides spp. and Usbekista- nia charoides indicate that the upper part of the Horda Formation was deposited at upper bathyal depths with dysoxic bottom conditions. Radiolaria occur commonly in several narrow intervals, the lowest of which is slightly 62 Horda Fm Lark Fm Hefring Mb Våle Fm Balder Fm Sele Fm Lista Fm Chalk Gp 1500 1600 1700 1800 m Floki-1 1731.3 1793.4 GR Sonic Neutron/Density Fig. 53. Floki-1, type well for the Hefring Member. len) as a minor component, indicating an open marine environment with only limited influx from surrounding terrestrial areas. Age. In the Central Graben, where the Horda Formation is most complete, the formation spans from the middle Ypresian (Early Eocene) at its base to earliest Rupelian (earliest Oligocene) at its top. In wells to the east and north, the top is as old as middle Priabonian (see also biostrati- graphic section above). This indicates that the top of the Horda Formation is diachronous, younging in a south- westerly direction. This is possibly due to increased ero- sion or longer intervals of non-deposition towards the north-east in the basin, or both. Correlation. The Horda Formation can be correlated with the onshore Danish succession of the Røsnæs Clay For- mation, the Lillebælt Clay Formation and the Søvind Marl Formation (Heilmann-Clausen et al. 1985), and the Vi- borg Formation (Christensen & Ulleberg 1973). The red-brown mudstones near the base of the Horda Formation in the central North Sea can be correlated litho- logically with the Røsnæs Clay Formation and the lower part of the Lillebælt Clay Formation. 63 The lower part of the overlying main body of greenish and greyish mudstones in the offshore succession can be correlated with the coeval and lithologically similar upper part of the Lillebælt Clay Formation. The upper part of the Horda Formation can be correlated with the Søvind Marl Formation, which consists of grey marls. The high- est part of the Horda Formation, only observed in Cen- tral Graben wells, may be correlated with the Viborg For- mation on biostratigraphic evidence. Hefring Member new member History. The Hefring Member consists of sandstone depo- sits within the Horda Formation. These sandstones have not previously been recognised as a separate unit in the Danish sector. Derivation of name. After the goddess Hefring. Type well. Danish sector well Floki-1, 1793.4–1731.3 m MDRT (Fig. 53). Distribution and thickness. The Hefring Member is only known from the Floki-1 well located in the northern part of the Danish sector. As the unit currently cannot be iden- tified on seismic sections, its further distribution is un- known. In the Floki-1 well, the member is 62 m thick. Lithology. The Hefring Member consists of greenish grey, fine-grained, immature sandstones with glaucony grains. Logcharacteristics.TheHefring Member ischaracterisedby aconspicuousblockysignatureonthegamma-ray, sonicand density logs (Fig. 53). Gamma-ray responses are lower than those of the enveloping Horda Formation mudstones. The Hefring Member can also be recognised from a combination of the density and neutron logs as the presence of pure sand- stones results ina ‘cross-over’of the two logcurves (Fig. 53). Boundaries. The boundaries with the mudstones of the Horda Formation are sharp and characterised by promi- nent shifts on the gamma-ray and sonic logs (Fig. 53). Depositional environment. No cores have been taken in the Hefring Member, but the sandstones were probably depo- sited from concentrated gravity flows, based on log simi- larity with the other fine-grained sandstone bodies in the nearby Siri Canyon. Age. Lutetian (Middle Eocene) based on the age of the associated Horda Formation mudstones. Correlation. Based on biostratigraphic data, the Hefring Member may be contemporaneous in part with the Lille- bælt Clay Formation onshore Denmark, with the lower part of the Grid Sandstone Member (Knox & Holloway 1992) in the Viking Graben and with the upper part of the Tay Sandstone Member (Knox & Holloway 1992) in the northern part of the Central Graben. Westray Group The Westray Group is the upper of the two groups estab- lished by Knox & Holloway (1992) to replace the Horda- land Group of Deegan & Scull (1977; Fig. 3). In the cen- tral North Sea and in the Danish sector of the North Sea, the Westray Group is represented by the Lark Formation. Lark Formation History. The Lark Formation was established by Knox & Holloway (1992) for the brownish grey mudstone-domi- nated lithofacies of the Westray Group that overlies the more variable association of red and green-grey mudstones, silty mudstones and sandstones of the Horda Formation and underlies the grey, sandy and shelly mudstones, silt- stones and sandstones of the Nordland Group of Deegan & Scull (1977; Fig. 3). The Lark Formation is also recog- nised in the Danish sector although its lithology is more variable than that given in the original description. Type well. British sector well 21/10-4, 1867–1217 m MDKB. Danish reference wells. Mona-1, 2363.5–1598.3 m MD- KB (Fig. 46); Siri-1, 1916.5–819.3 m MDKB (Fig. 54). Distribution and thickness. The Lark Formation extends over the central and northern North Sea and is probably present in the entire Danish sector of the North Sea. Its depocentre is in the central and northern part of the Dan- ish sector, along the eastern boundary of the Danish Cen- tral Graben, where it reaches a thickness of 1194 m in the Siri-3 well. The Lark Formation thins west to a thickness of 389 m in the Tordenskjold-1 well in the Central Graben, and east to a thickness of 240 m in the S-1 well on the Ringkøbing–Fyn High (Fig. 55). 64 Lithology. The lower Lark Formation (L1–3, see below) is dominated by dark, greenish grey, non-fissile mudstones in most wells; in some wells subordinate intervals of brown- ish grey mudstones are also present. Thin layers of white or reddish brown carbonate are also recorded in the upper levels of the lower Lark Formation. The upper Lark Formation (L4, see below) is domi- nated by pale to dark brownish grey mudstones with sub- ordinate intervals of greenish grey mudstones in its lower levels. The uppermost 50–100 m of the formation consist of yellowish grey to light brown mudstones. In eastern and northern parts of the Danish sector, discrete sand- stoneinterbeds and thin sandstone stringers occur through- out the formation. Log characteristics. The lower part of the Lark Formation is characterised by an overall stable gamma-ray log signa- ture, whereas the upper part of the formation has a more unstable signature (Figs 46, 54). This change in gamma- ray log signature coincides approximately with the change from lithologies dominated by greenish grey mudstones to lithologies dominated by dark to light brownish grey mudstones at the base of unit L4 (see below). Boundaries. The base of the Lark Formation is marked by a change from fissile, greenish grey mudstones of the Horda Formation to non-fissile, greenish grey mudstones of the Lark Formation. This change in lithology coincides with an abrupt increase in gamma-ray values to a consistently higher level than that displayed by the Horda Formation (Figs 46, 51, 54). Wells in the eastern part of the Danish Central Graben and on the Ringkøbing–Fyn High show a conspicuous log break on the gamma-ray log at the for- mation boundary, whereas the log break is less pronounced in wells from the central and western parts of the Danish Central Graben (Fig. 51). Although the actual increase in gamma-ray response may be limited in the latter wells, the offset is usually sharp and well defined. On the sonic log, the boundary between the Horda Formation and the Lark Formation is characterised by a transition from a stable sonic signature to one characterised by numerous fluctuations. The Lark Formation is overlain by the undifferentiated Nordland Group of Deegan & Scull (1977). Over most of the area, the boundary seems conformable and is repre- sented by a change from yellowish grey and light brown mudstones to medium to dark grey mudstones characte- 1916.5 819.3 Horda Fm Lark Fm Nordland Gp L4 L3 L2 L1 Balder Fm Sele Fm Lista Fm Våle Fm Chalk Gp 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 m Siri-1 GR Sonic No log data Fig. 54. Siri-1, Danish reference well for the Lark Formation. The units L1–4 are all present in this well. Black bars show cored sections. 65 rised by intervals with shell-hash and coarse-grained sands. This boundary is marked by a conspicuous gamma-ray peak at the base of a 20–40 m thick interval with elevated gamma-ray values in the lowermost Nordland Group (Figs 46, 51). This interval is further characterised by a marked double peak on the gamma-ray log. In thenorth-easternparts of the Danish sector (Nini-1, Vanessa-1, Cecilie-1 and Siri-1; Figs 1, 51, 54), sediments of the Nordland Group rest unconformably on the Lark Formation. In this area the uppermost Lark Formation and the lowermost Nordland Group are missing, proba- bly due to erosion and/or non-deposition. The distinct gamma-ray peak that marks the top of the Lark Forma- tion as well as the double gamma-ray peak in the lower- most Nordland Group are lacking in these wells, and there- fore the top of the Lark Formation is more difficult to identify on petrophysical logs. Subdivision. The Lark Formation can be subdivided into four major mudstone packages, L1–L4, based on seismic and log evidence (Figs 46, 49, 51, 54, 56; Plates 1–5). These units are described below; isochore maps of the units are shown in Fig. 57a–d. L1 (Figs 51, 56a, b, 57a; Plates 1, 4, 5) This unit has been recognised in the north-eastern part of the Danish sector only (Fig. 57a). It is bounded beneath by the TH marker and above by the TL1 marker (Fig. 56a, b). It is characterised by downlapping reflectors and represents a south-westwards prograding mudstone succession. On the gamma-ray log, the L1 unit is characterised by a relatively high and relatively stable response. In most wells, it shows a weakly concave pattern, going from a relatively high gam- ma-ray response at its base, over a gamma-ray low halfway through the unit to a level close to starting level at the top of the unit (e.g. Ida-1, Inez-1, K-1, F-1 and Sandra-1; Plates 1, 4, 5). In the Siri-1 and Siri-3 wells, near the south-western limit of the L1 unit, the gamma-ray log motif instead ap- pears slightly convex (Figs 51, 54; Plates 1, 4). The L1 unit consists predominantly of greenish grey mudstones but also includes yellowish brown and dark grey mudstones. L2 (Figs 46, 51, 56a, b, 57b; Plates 1–5) The unit is recognised over the entire study area. On the gamma-ray and sonic logs the unit is characterised by a sta- ble log signature. The gamma-ray log shows two to three slightly concave patterns with signatures similar to that of the L1 unit (Fig. 51; Plates 1, 4). The lithology is characte- rised by dark beige-grey to greenish grey mudstones, green- ish colours becoming dominant towards the top of the unit. L3 (Figs 46, 51, 56a, b, 57c; Plates 1–5) This unit is encountered in the northern and eastern parts of the Danish sector, east of the Central Graben (Fig. 57c) but is not recognised on logs or seismic sections in the Central Graben area. It is characterised by stable gamma-ray and sonic log signatures (Figs 46, 51; Plates 1–4). The unit con- sists almost invariably of dark, greenish grey mudstones. L4 (Figs 46, 51, 56a, b, 57d; Plates 1–5) The unit is recognised over the entire study area (Fig. 57d). The interval is characterised by a slightly more unstable gam- F-1 Inez-1 Siri-1 Francisca-1 Frida-1 Torden- skjold-1 S-1 Mona-1 25 km 200 300 400 500 600 700 800 900 1000 1100 1200 1300 Freja Member distribution Dufa Member distribution Lark Formation Thickness (m) Fig. 55. Isochore map of the Lark Formation in the study area. The positions of the two Danish reference wells, Mona-1 and Siri-1, are indicated in the figure. The map also shows the distribution of the sandstones of the Dufa and Freja Members and the location of their type and reference wells, Inez-1 and F-1, and Francisca-1 and Frida-1, respectively. 66 Fig. 56. a: SW–NE-trending seismic section (RTD81-RE94-22A) showing the complex architecture of the Lark Formation and its subdivision into L1–4 units. The vertical white line indicates change in section direction. The locations of the two seismic sections are shown on Fig. 1; abbreviations as in Figs 49 and 51. 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Tordenskjold-1 F-1 Tabita-1 Cecilie-1 Siri-3 SW NE Horda FmHorda Fm L2L2 Lark FmLark Fm L4 Horda Fm L2 aa L2L2 L1 Lark FmLark Fm L4 L3 L1 10 km Lark Fm Lark Fm L4 L3L3L3 L4 L3 UOU TL2 TH TB TC TL TF UOU TL2 TL UOU TL2 TL1 TH TB TC TWT (msec) TWT (msec) 67 25 50 75 100 125 Thickness (m) L1 25 km 25 km 100 200 300 400 Thickness (m) L2 100 200 300 400 500 600 700 800 Thickness (m) 100 200 300 400 500 600 700 800 Thickness (m) L3 L4 a b c d Fig. 57. Isochore maps of Lark Formation subunits. a: L1. b: L2. c: L3. d: L4. a and d are at the same scale, b and c are at the same scale. 1500 2000 2500 3000 3500 TWT (msec) 20 km Nini-2 D-1 Nolde-1 NW SE TL UOU TL2 TL1 TB TC L1 L2L2 L3 L4 L1 L2 L3 L4 bb Fig. 56. b: NW–SE-trending seismic section (RTD81-RE94-14A) showing subdivision of the Lark Formation and marked thinning of this formation towards the south-east. The locations of the two seismic sections are shown on Fig. 1; abbreviations as in Figs 49 and 51. 68 ma-ray and sonic log signature than that of the underlying units (Figs 46, 51; Plates 1–5). It is dominated by brown to yellowish brown mudstones, but in some wells an interval of greenish grey mudstones occurs in its lower part. In wells to the east and north, thin sandstones are interbedded with the mudstones and become more frequent towards the top of the unit. Two thick sandstone units occur in the Lark Formation on the Ringkøbing–Fyn High and are described here as two new members (Dufa and Freja Members). Macro- and ichnofossils. Only observed in cores taken in the Freja Member (see below). Microfossils and palynomorphs. Farthest to the north and east the basal part of the formation includes the down- hole succession of HO Areosphaeridium diktyoplokum and HO A. michoudii indicating a late Priabonian (Late Eocene) age for the base of the formation in this area. In the Central Graben area, the base of the Lark Formation is significantly younger. Here it contains an event succes- sion characteristic of the middle and lower Rupelian (Lower Oligocene) Stage (HOs of Phthanoperidinium amoenum, Achilleodinium biformoides and Phthanoperi- dinium comatum). The top of the Lark Formation is bracketed by a number of conspicuous biostratigraphic events: the uppermost part contains the HOs of the benthic foraminifers Asterigerina staeschei and Elphidium inflatum followed downhole by the HO of Uvigerina tenuipustulata. Dinoflagellate events near the top of the Lark Formation include the HOs of Apteodinium spiridoides and Cousteaudinium aubryae. The lowermost part of the overlying Nordland Group contains the HOs of the calcareous microfossils Bolboforma clodiu- si, Bolboforma spiralis and Bolboforma metzmacheri, the HO of the benthic foraminifer Bulimina elongate, and the HO of the dinoflagellate cyst Cannosphaeropsis passio. A large number of HOs characterise the Lark Formation; key events are listed in Fig. 5c. Depositional environment. The L1 unit is characterised by abundant agglutinated foraminifers dominated by Rhab- dammina discreta and similar tubular taxa, together with Haplophragmoides spp. and Recurvoides spp. The micro- faunal assemblage indicates that the unit was predomi- nantly deposited in an open marine, dysoxic palaeoenvi- ronment at upper bathyal depths. The L2 unit and most of the L3 unit are characterised by an increasing abundance and diversity of calcareous benthic and planktonic foraminifers. The relative propor- tions of agglutinated, calcareous planktonic and benthic foraminifers vary considerably from well to well, indicat- ing pronounced lateral changes in the depositional envi- ronment. The calcareous plankton/benthos ratio is usually low, indicating a neritic setting for most of the succes- sion, but in a few restricted intervals it may reach 1:2 or even 1:1 indicating an outer neritic setting. Thus, the fora- minifer assemblage indicates an open marine, neritic to outer neritic setting with well-oxygenated bottom condi- tions for the lower to middle part of the Lark Formation. The microfossil assemblage in the uppermost part of the L3 unit as well as the L4 unit is dominated by calca- reous benthic foraminifers, and agglutinated foraminifers are generally rare. Epifaunal and shallow infaunal foramini- fers are more common than deep infaunal taxa, indicat- ing oxic bottom conditions during this interval. In gene- ral, the microfaunal assemblage in this part of the Lark Formation suggests that it was deposited in a neritic, prob- ably middle neritic, palaeoenvironment over most of the study area. The palynofacies assemblage in the Lark Formation is characterised by a rich dinoflagellate assemblage and abun- dant dispersed terrestrial matter (phytoclasts, spores and pollen), indicating an open marine environment with con- siderable influx from nearby land areas. Stratigraphic var- iations in the relative abundance of terrestrial palyno- morphs in the Lark Formation suggest successive pulses of progradation and backstepping of the palaeocoastline. Age. The Lark Formation is of Priabonian to Serravallian (Late Eocene to Middle Miocene) age with Eocene sedi- ments being present in the L1 unit only. The L1 unit is Priabonian to early Rupelian (Early Oligocene) in age, the base of the unit being oldest farthest to the north and east and younging towards the south and west. The age of the L2 unit is Rupelian; the L3 unit is Rupelian in age in its lower part and Chattian (Late Oligocene) in its upper part. The Rupelian–Chattian boundary is located in the lower part of the unit. The Chattian–Aquitanian (Oligocene– Miocene) boundary is located just above the top of the L3 unit. In some wells, a hiatus is indicated at this level by the clustering of HOs. The Chattian–Aquitanian, Aqui- tanian–Burdigalian, Burdigalian–Langhian and Langhi- an–Serravallian stage boundaries are all located in the L4 unit. The uppermost part of the Lark Formation is of mid- Serravallian age. Correlation. Based on biostratigraphic correlation, the lowermost L1 unit is probably largely coeval with the Vi- borg Formation onshore Denmark, and with Sequence 4.1 of Michelsen et al. (1998). The L2 unit may be corre- 69 lated with the Linde Clay onshore Denmark (informal mudstone unit described by Heilmann-Clausen 1995). Intervals in the L3 unit may be correlated with the Branden Clay (Ravn 1906) onshore Denmark, based on lithologi- cal similarities and biostratigraphy. Intervals in the Lark Formation around the L3–L4 boundary (around the Chat- tian–Aquitanian boundary) may be correlated with the two lowermost, clay-rich units of the onshore Vejle Fjord Formation(theBrejningClayandVejleFjordClayofLarsen & Dinesen1959).Theuppermostpart of the Lark Forma- tion possibly correlates with the onshore Arnum Forma- tion (Sorgenfrei 1958) and the Hodde Formation onshore Denmark (Rasmussen 1961), based on biostratigraphy. Dufa Member new member History. The Dufa Member comprises a thick sandstone- dominated unit that occurs within unit L3 of the Lark Formation in the northern and eastern part of the Danish sector of the North Sea. The unit has not been previously described. Derivation of name. After the goddess Dufa. Type well. Danish sector well Inez-1, 697.1–485.5 m MDKB (Figs 41, 58; Plates 1, 5). Reference well. Danish sector well F-1, 337.5–324.3 m MDKB (Figs 58, 59; Plate 5). Distribution and thickness. The Dufa Member is present in the north-eastern part of the Danish sector of the North Sea (Fig. 55). In its type well, the Dufa Member is 210 m thick and consists of three major sandstone units with thicknesses 30–120 m (Fig. 41). The sandstone units are separated by mudstone intervals up to 20 m thick. Towards the north, in the F-1 well, the lower sandstone units are Fig. 58. N–S-trending composite seismic section (RTD81-RE94-45/RTD81-RE94-09). Blue-coloured lines indicate the outline of the Dufa Member. The gamma-ray logs from the Inez-1 and F-1 wells are inserted (see Figs 55 and 59 for depth-converted gamma-ray logs for the two wells). The Horda Formation is thin in this area and the Top Horda reflector (TH) is therefore indistinguishable from the Top Balder reflector (TB). The location of the seismic section is shown in Fig. 1; abbreviations as in Fig. 49. Inez-1F-1 0 500 1000 TWT (msec) TF TL2 TB/TH N S Dufa Mb Dufa MbDufa Mb 5 km 70 well, the lower sandstone intervals of the Dufa Member show the presence of a number of 5–10 m thick sand- stone packets showing blocky, decreasing-upwards gam- ma-ray log signatures suggesting coarsening-upwards sand bodies. These sandstones are separated by intervals of Fig. 60. Francisca-1, type well for the Freja Member. Black bars show cored sections. 1400 1500 1600 1700 1800 m 1562.8 1840.7 Freja Mb Lark Fm Francisca-1 GR Sonic missing (Figs 58, 59). The Dufa Member is absent in wells west of F-1. Lithology. The lower sandstone units predominantly con- sist of coarsening-upwards successions of very fine-grained to fine-grained, greenish brown, muddy sandstones. The upper sandstone unit fines upwards and consists of medium- to coarse-grained, quartzitic, relatively pure sand with intervals rich in glaucony. Lignite has been observed in cuttings samples. Log characteristics. The member is characterised by an over- all blocky signature on the gamma-ray log. In the type Fig. 59. F-1, reference well for the Dufa Member. Dufa Mb Lark Fm 300 400 500 600 m F-1 324.3 337.5 GR Sonic 71 mudstones with higher gamma-ray response. The upper unit is characterised by an overall blocky signature with minor gamma-ray peaks and trends suggesting a number of fining-upwards intervals, 10–20 m thick, and a few coarsening-upwards intervals, 5–10 m thick (Fig. 41). Boundaries. The upper and lower boundaries of the mem- ber with the mudstones of the Lark Formation are sharp and characterised by prominent shifts on the gamma-ray log (Figs 41, 59). Depositional environment. Judging from seismic evidence, the Dufa Member is positioned partly on the offlap break, partly seaward of it (Fig. 58). Based on this palaeosetting and the presence of lignite in cuttings samples, the Dufa Member sandstones are interpreted to represent deltaic, shallow-marine sediments, probably deposited in pulses during an overall relative sea-level low. Age. Rupelian, based on the age of the enveloping mud- stones. Correlation. There are no Danish onshore correlatives to the Dufa Member. The correlation with the Norwegian offshore successions is currently uncertain. Freja Member new member History. The Freja Member is a conspicuous sandstone- dominated unit that occurs within the upper levels (L4) of the Lark Formation in the northern and central parts of the Danish sector of the North Sea. The unit has not previously been described. Derivation of name. After the goddess Freja. Type well. Danish sector well Francisca-1, 1840.7–1562.8 m MDRT (Figs 60, 61). Reference well. Danish sector well Frida-1, 1623.5–1487.7 m MDRT (Fig. 62; Plate 4). Distribution and thickness. The Freja Member is present in the northern and central parts of the Danish sector of the North Sea (Fig. 55). In its type well, the Freja Mem- ber spans a stratigraphic interval of c. 280 m and includes major sandstone units separated by subordinate intervals of mudstones (Fig. 60). In the Cecilie-1 well, the member is c. 150 m thick whereas in the Frida-1 well the member attains c. 130 m (Fig. 62). Lithology. In its type well, the lower half of the Freja Mem- ber consists of very fine-grained to fine-grained quarzitic sandstones with many thin mudstone interbeds. The mem- ber becomes less muddy in the upper third of this inter- val. The upper third of the Freja Member consists largely of relatively pure quarzitic, very fine-grained sandstones with mudstone interbeds becoming frequent towards the top (Fig. 60). Between these two major sandstone units is a c. 60 m thick interval dominated by mudstones but with carbonate-cemented, sandstone-dominated packets in its upper part. Log characteristics. The Freja Member has an overall blocky gamma-ray log signature. In the type well, its lowermost part (1840.7–1750 m) can be split into a number of small- er units with blocky or increasing-upwards gamma-ray log signatures separated by gamma-ray peaks. In comparison, the overlying sandstones (1750–1720 m) display a more stable, low gamma-ray log pattern with few gamma-ray log spikes (Fig. 60). The mudstone-dominated interval (1720–1660 m) separating the two sandstone-dominated units in the type well generally shows high gamma-ray values: Thin calcite-cemented sandstone packets are in- tercalated with the mudstones in this interval (e.g. 1680– 1670m)and show decreasing-upwards gamma-ray values. Boundaries. The lower boundary of the Freja Member with the Lark Formation mudstones is sharp and characterised by prominent shifts on the gamma-ray and sonic logs. In the type well, where the upper levels of the Freja Member are characterised by interbedded mudstones and sand- stones, the upper boundary of the member is less promi- nent. In this well, it is placed at the top of the uppermost discrete sandstone bed, at 1562.8 m (Fig. 60). Macro- and ichnofossils. Intervals with shell debris have been observed in core sections of the Freja Member in the Francisca-1 well. Ichnofossil genera from the Freja Mem- ber comprise Chondrites ispp., Phycosiphon ispp., Plano- lites ispp., Terebellina ispp., Thalassinoides ispp. and Zoo- phycos ispp. Depositional environment. The Freja Member represents stacked successions of thick- and thin-bedded turbidite sands deposited in submarine channels and proximal levee environments (Figs 60, 62). The upper parts of the tur- bidite successions show transitions from normally graded turbidites, deposited in slightly more distal levee environ- 72 Francisca-1 W E N S 2 km 0 500 1000 1500 2000 2500 3000 TWT (msec) Freja MemberFreja MemberFreja Member Horda FmHorda Fm Lark FmLark Fm Lark FmLark Fm Horda Fm Lark Fm L3L3 L2L2 L4L4 Lark Fm TL TF UOU TL2 TH TB ments and minor turbidite channels, to mainly silty tur- bidite deposits that represent distal levee and fan fringe environments and the transition to the open slope. The source of the sand was probably a marginal marine shelf environment, judging by the abundance of the marginal marine acritarch Paralecaniella indentata. Age. In the type well, the Freja Member is Chattian to Aquitanian in age, based on the age of mudstones within and bounding the member. In the Frida-1 well, the Freja Member is entirely Chattian in age. Correlation. The Freja Member is broadly contemporane- ous with the Vejle Fjord Formation onshore Denmark, with the Vade Formation (Hardt et al. 1989) in the Norwe- gian Central Graben and with the Skade Formation (Hardt et al. 1989) in the Viking Graben. Thick, coarsening-up- ward sandstone bodies are present above the Dufa Mem- Fig. 61. Composite seismic section (DK1–5623A RE94/DK1–0448B RE94) with the Freja Member indicated between the reflectors UOU and TF. The gamma-ray log from the Francisca-1 well is indicated on the figure (see Fig. 60 for depth-converted gamma-ray log). The vertical white bar indicates where the section changes direction. The location of the seismic section is shown on Fig. 1; abbreviations as in Fig. 49. 73 1487.7 1623.5 Freja Mb Lark Fm 1400 1500 1600 1700 m Frida-1 GR Sonic ber in the Inez-1 well (shown as unnamed sandstones in Fig. 2); these sandstones may be contemporaneous or even contiguous with those of the Freja Member. 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