Geological Survey of Denmark and Greenland Bulletin 1, 145-216 DENMARK, SOUTHERN SWEDEN AND THE NETHERLANDS Previous page: Jurassic sediments exposed in a clay-pit at Bagå on Bornholm, Denmark – see Michelsen et al. (2003, this volume). Photo: Peter K. Warna-Moors. 147 Jurassic lithostratigraphy and stratigraphic development onshore and offshore Denmark Olaf Michelsen, Lars H. Nielsen, Peter N. Johannessen, Jan Andsbjerg and Finn Surlyk A complete updated and revised lithostratigraphic scheme for the Jurassic succession of the onshore and offshore Danish areas is presented together with an overview of the geological evo- lution. The lithostratigraphies of Bornholm, the Danish Basin and the Danish Central Graben are described in ascending order, and a number of new units are defined. On Bornholm, the Lower–Middle Jurassic coal-bearing clays and sands that overlie the Lower Pliensbachian Hasle Formation are referred to the new Sorthat Formation (Lower Jurassic) and the revised Bagå Formation (Middle Jurassic). In the southern Danish Central Graben, the Middle Jurassic succession formerly referred to the Lower Graben Sand Formation is now included in the revised Bryne Formation. The Lulu Formation is erected to include the uppermost part of the Middle Jurassic succession, previously referred to the Bryne Formation in the northern Danish Central Graben. The Upper Jurassic Heno Formation is subdivided into two new members, the Gert Member (lower) and the Ravn Member (upper). The organic-rich part of the upper Farsund Formation, the for- mer informal ‘hot unit’, is established formally as the Bo Member. Dominantly shallow marine and paralic deposition in the Late Triassic was succeeded by wide- spread deposition of offshore marine clays in the Early Jurassic. On Bornholm, coastal and par- alic sedimentation prevailed. During maximum transgression in the Early Toarcian, sedimentation of organic-rich offshore clays took place in the Danish area. This depositional phase was termi- nated by a regional erosional event in early Middle Jurassic time, caused by uplift of the central North Sea area, including the Ringkøbing–Fyn High. In the Sorgenfrei–Tornquist Zone to the east, where slow subsidence continued, marine sandy sediments were deposited in response to the uplift. Uplift of the central North Sea area was followed by fault-controlled subsidence accom- panied by fluvial and floodplain deposition during Middle Jurassic time. On Bornholm, deposi- tion of lacustrine muds, fluvial sands and peats dominated. The late Middle Jurassic saw a gradual shift to shallow marine deposition in the Danish Central Graben, the Danish Basin and Skåne, southern Sweden. During the Late Jurassic, open marine shelf conditions prevailed with depo- sition of clay-dominated sediments while shallow marine sands were deposited on platform areas. The Central Graben received sand by means of sediment gravity flows. The clay sediments in the Central Graben became increasingly rich in organic matter at the Jurassic–Cretaceous transition, whilst shallow marine coarse-grained deposits prograded basinwards in the Sorgenfrei– Tornquist Zone. Keywords: Denmark, Danish Central Graben, Danish Basin, Sorgenfrei–Tornquist Zone, Bornholm, Jurassic, lithostratigraphy, basin development O.M., Geological Institute, University of Aarhus, C.F. Møllers Allé, DK-8000 Århus C, Denmark. L.H.N., P.N.J. & J.A., Geological Survey of Denmark and Greenland, Geocenter Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: lhn@geus.dk F.S., Geological Institute, University of Copenhagen, Geocenter Copenhagen, Øster Voldgade 10, DK-1350 Copen- hagen K, Denmark. Geological Survey of Denmark and Greenland Bulletin 1, 147–216 (2003) © GEUS, 2003 148 Jurassic deposits extend throughout most of the Danish area, from the island of Bornholm in the Baltic Sea in the east to the central part of the North Sea in the west (Fig. 1). They can be studied at out- crop on Bornholm and Skåne (southern Sweden), whereas they are only known from subsurface data over the remainder of the area. The Jurassic of Bornholm has been studied since the 19th century; the early literature includes Forchhammer (1837), Jespersen (1866, 1869), Bartholin (1892, 1894), Grönwall (1899), Hjort (1899), Møller (1902, 1903), Malling & Grönwall (1909), Malling (1911, 1914, 1920) and Höhne (1933). The stratigraphy and structural evolution was stud- ied by Gry (1969), and the sedimentary evolution of the Jurassic successions was discussed by Sellwood (1972), Rolle et al. (1979) and Gravesen et al. (1982); the last authors also erected the first formal lithostratigraphic scheme. Palynological studies have been undertaken by Hoelstad (1985), Koppelhus (1991), Koppelhus & Batten (1992), Batten et al. (1994) and Koppelhus & Nielsen (1994). Surlyk & Noe-Nygaard (1986) interpreted the depositional environments of the Pliensbachian Hasle Formation, and the rich ammonite fauna is described by Donovan & Surlyk (2003, this volume). Surlyk et al. (1995) provided a detailed sequence stratigraphic interpretation of the Hettangian– Sinemurian Sose Bugt Member (Rønne Formation). Koppelhus & Nielsen (1994) and Petersen & Nielsen (1995) interpreted the depositional environments of the Upper Pliensbachian – Bathonian Sorthat and Bagå Formations. The coal beds, burial depths and amount of uplift based on coal maturation were studied by Petersen (1993) and Petersen et al. (2003a, b, this volume). Jurassic sediments in the subsurface are known from a large number of deep wells drilled for hydrocarbon Contents Regional setting 149 Lithostratigraphy 154 Bornholm 155 Rønne Formation 155 Hasle Formation 161 Sorthat Formation 161 Bagå Formation 166 Danish Basin 168 Gassum Formation 168 Fjerritslev Formation 170 Haldager Sand Formation 173 Flyvbjerg Formation 174 Børglum Formation 174 Frederikshavn Formation 175 Danish Central Graben 176 Fjerritslev Formation 176 Bryne Formation 177 Lulu Formation 180 Middle Graben Formation 183 Lola Formation 185 Heno Formation 185 Gert Member 189 Ravn Member 190 Farsund Formation 191 Bo Member 195 Poul Formation 197 Stratigraphic evolution 197 Early Jurassic marine deposition 197 Middle Jurassic uplift 204 Middle Jurassic fluvial and paralic deposition 204 Late Jurassic marine deepening 208 Late Jurassic graben development and sedimentation 209 Acknowledgements 211 References 212 and geothermal exploration with supplementary infor- mation from seismic surveys. The presence of Jurassic rocks from the subsurface of onshore Denmark was described for the first time by Nørvang (1946) from a borehole situated in northern Jylland near the village of Vejrum. The Vinding-1 well, drilled in 1947, was the first deep onshore well to penetrate the Jurassic. Gregersen & Sorgenfrei (1951) presented a stratigraphic review of two well sections, Gassum-1 and Vinding-1. A lithological and stratigraphical summary of the Jurassic in a large number of wells was given by Sorgenfrei & Buch (1964). A lithostratigraphic subdivision of the entire Jurassic succession was undertaken by Larsen (1966), and later refined and partly revised by Bertelsen (1978) and Michelsen (1978a, 1989a). Larsen et al. (1968) described Jurassic strata encountered in wells drilled in the strait between Denmark and Sweden (Øresund); Petersen (1994) studied the genesis of the coals from these wells. Several biostratigraphic studies have been published including those of Nørvang (1957) who exam- ined the Early Jurassic foraminiferal faunas, Michelsen (1975) who established a zonation of the Lower Jurassic based on the ostracod faunas, Dybkjær (1988, 1991) and Poulsen (1992, 1994, 1996) who established a palyno- logical zonation of the Jurassic in the Danish Basin. The depositional environments and bivalve assemblages of the Lower Jurassic Fjerritslev Formation were inter- preted by Pedersen (1985, 1986). Koch (1983) discussed the depositional environments of the Middle–Upper Jurassic sandstone reservoirs, Thomsen et al. (1987) described the hydrocarbon plays, and Nielsen (1995, 2003, this volume) has described the Upper Triassic – Jurassic depositional sequences and the development of the Danish Basin and Fennoscandian Border Zone. In the Danish North Sea sector, the Jurassic was first penetrated by the A-2 well, drilled in 1967, and Sorgenfrei (1969) indicated the presence of Jurassic marine offshore deposits in the North Sea. The first lithological and stratigraphic summaries of drilled Jurassic successions were presented by Rasmussen (1974, 1978). A regional description and a preliminary lithostratigraphic subdi- vision of the Jurassic succession in the Central Graben were presented by Koch et al. (1982). The lithostrati- graphic subdivision defined in the Danish onshore well sections also applies to the easternmost North Sea wells located in the Norwegian–Danish Basin (Michelsen 1978a). A lithostratigraphic scheme for the deposits in the Central Graben was established by Jensen et al. (1986), and Jurassic basin development was discussed by Michelsen et al. (1987). The depositional environ- ments of the Middle Jurassic Bryne and Lulu Formations and the Upper Jurassic Heno Formation were inter- preted by Johannessen (1995, 1997, 2003, this volume), Johannessen et al. (1996), Petersen & Andsbjerg (1996) Andsbjerg (1997, 2003, this volume) and Andsbjerg & Dybkjær (2003, this volume). Biostratigraphic data from the offshore well sections were presented by Rasmussen (1974, 1978). The Early Jurassic ostracod faunas of the O-1 well were described by Michelsen (1978b), Hoelstad (1986a) studied the palynology of the Middle Jurassic in the U-1 well, and Poulsen (1986, 1991) presented a dinoflagellate cyst biostratigraphy of the Middle and Upper Jurassic in two wells from the Central Graben. Dybkjær (1998) and Andsbjerg & Dybkjær (2003, this volume) presented palynological data from the Middle and Upper Jurassic. The hydrocarbon plays were described by Damtoft et al. (1992); further details on source rocks were provided by Petersen et al. (1996, 1998, 2000), Petersen & Rosenberg (1998) and Ineson et al. (2003, this volume). The primary aim of the present paper is to present a complete and updated version of the lithostratigraphic scheme of the entire Danish onshore and offshore area (Fig. 2). In addition, an account is given of the strati- graphic development during the Jurassic Period in the Danish part of the North Sea Basin including the Danish Basin. Biostratigraphic and sequence stratigraphic aspects are dealt with in other papers in this volume and will be referred to where relevant. A summary of the dis- tribution and thickness of the lithostratigraphic units penetrated in well sections can be found in Nielsen & Japsen (1991). Regional setting The Danish area belongs to the eastern part of the North Sea Basin, which comprises a number of fault- bounded basins separated by structural highs (Figs 1, 3). The Danish Jurassic deposits represent a wide range of environments, from dominantly paralic and coastal in the east to deep marine in the west. Differential movements of individual structural features have con- trolled the distribution of Jurassic deposits. The most important structural features are therefore described briefly below. The North Sea Basin is separated from the Fenno- scandian Shield to the east by the strongly block-faulted Fennoscandian Border Zone (Sorgenfrei & Buch 1964; Baartman & Christensen 1975). The zone runs from northernmost Jylland, south-eastwards through Skåne to the island of Bornholm in the Baltic Sea. It comprises 149 150 Egersund Subbasin Southern Vestland Arch Fiskebank Basin North Sea Baltic Sea Fennoscandian Border Zone 10°E8°E 12°E 14°E 10°E8°E 12°E 14°E 16°E 58°N 57°N 56°N 55°N 58°N 57°N 56°N 55°N 6°E4°EA Sweden The Netherlands Germany Norway Denmark UK Skåne Bornholm Horn Graben Horsens-1 Rønde-1 Terne-1 Gassum-1 Mejrup-1 Rødding-1 Hyllebjerg-1 Vedsted-1 Flyvbjerg-1 Vinding-1 Nøvling-1 Oddesund-1 Skive-1, -2 Kvols-1 Børglum-1 Haldager-1 Frederikshavn-1, -2 Hans-1 Pernille-1 Stina-1 Fjerritslev-1,-2 F-1 Inez-1 K-1 Felicia-1 J-1 Well Fault High Well Fault 100 km Structural elements of southern Scandinavia B National boundaries Sorgenfrei–Tornquist Zone Børglum Fault Haldager Fault Fjerritslev Fault Danish Basin Skurup High Ringkøbing– Fyn High Skagerrak– Kattegat Platform Rø nn e G ra be n Bornholm 50 km Danish Basin Central Graben mainly marine and paralic Jurassic deposits, and was repeatedly overstepped during the Jurassic. In the Kattegat area and northernmost Jylland, the zone includes two different structural elements, the Sorgen- frei–Tornquist Zone and the Skagerrak–Kattegat Platform (Figs 1B, 3; EUGENO-S Working Group 1988). The Sorgenfrei–Tornquist Zone was established dur- ing Late Carboniferous – Early Permian times as a rift zone in conjunction with the Oslo Graben (Liboriussen et al. 1987; EUGENO-S Working Group 1988; Ro et al. 1990; Michelsen & Nielsen 1991, 1993; Mogensen & Korstgård 1993, 2003, this volume; Mogensen 1994, 1996; Vejbæk 1997). The zone was periodically active during the Mesozoic. Block faulting and increased sub- sidence rates prevailed in the Late Triassic – Early Jurassic and during latest Jurassic – earliest Cretaceous times, while inversion tectonism characterised Late Cretaceous – Early Cenozoic times. Slow subsidence occurred in 151 ■ ■ ■ ■ Gertrud Plateau/GrabenGert Ridge Late Jurassic Structural elements Normal fault Reverse fault Salt structures Well ▲ ■ 25 km ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ▲ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ▲ ▲ ■ ■ ■ ■ ■ ▲ ▲ ▲ ■ ■ ■ ■ ■ ■ ■ Ugle-1 Cleo-1 Amalie-1 Elin-1 West Lulu-1 West Lulu-3 Gert-1 Gert-4 Gert-2 Kim-1 Tordenskjold-1 P-1 2/8-3 Jeppe-1 Gwen-2 T-1 Ravn-1 Edna-1 Ravn-2 W-1 Elly-2 Elly-3 Ringkøbing–Fyn High East North Sea High Deep Gorm-1 G-1 M-8 A-2 O-1 E-1 L-1 U-1 Lulu-1 Nora-1 Poul Plateau V-1 Bo-1 Mid North Sea High Feda Graben Inge High Søgne Basin M ads H igh Tail End G raben R osa Basin Coffee Soil Fault Heno Plateau Baron-2 4°E 56°N Salt Dome Province Mandal High C Fig. 1. A: Outline structure map showing the three main Mesozoic tectonic elements: the Central Graben, the Danish Basin and the Fennoscandian Border Zone. The red line parallel to the Denmark–Norway border indicates the location of the geosection in Fig. 3. B: Simplified structural map of the Danish Basin with well locations. Modified from Nielsen (2003, this volume). C: Map of the Danish Central Graben showing the Late Jurassic structural elements and the location of wells mentioned in the text. Modified from Japsen et al. (2003, this volume). 152 Oddesund Fm K åg er öd Fo rm at io n System Series Stage Ju ra ss ic T ri as si c Åsgard Formation Leek Member Bo Member Heno Fm Lola Formation Bryne Formation Danish Basin SW NE Vedsted Formation Børglum Formation Flyvbjerg Formation Haldager Sand Formation Fj er ri ts le v Fo rm at io n Gassum Formation Vinding Formation Skagerrak Formation Jydegård Formation Robbedale Formation Rabekke Formation Bagå Formation Sorthat Formation Hasle Formation Galge- løkke Mb Sose Bugt Mb R øn ne F or m at io n F-IV mb F-III mb F-II mb F-Ib F-I mb F-Ia Fennoscandian Border Zone Risebæk Mb Munkerup Mb Frederikshavn Formation Danish Central Graben Ryazanian Volgian Kimmeridgian Oxfordian Callovian Bathonian Bajocian Aalenian Toarcian Pliensbachian Sinemurian Hettangian Rhaetian Norian Winterton Formation C re ta ce ou s Valanginian Fjerritslev Formation Vyl Fm Poul FmFarsund Formation Lo w er U pp er M id dl e Lo w er U pp er U L U L L U U M L L M U M L U L U U L L U M L U U L U M L Annero Fm A nn er o Fm M ar ie da l f m Vilhelmsfält Fm Röddinge fm ? ? ? Paralic and non-marine sandstones, siltstones, mudstones and coals Marine mudstones and siltstones Unconformity Offshore organic-rich marine shales Submarine fan sandstones and siltstones Shallow marine sandstones and siltstones Hiatus Fyle- dalen Clay Nytorp Sand Fortuna Marl Glass Sand Mb Fuglunda Mb Vitabäck Clay Helsing- borg Mb Döshult Mb Pankarp Mb Katslösa Mb Ryde- bäck Mb R ya F m H ög an äs F m Bjuv Mb Vallåkra Mb H öö r Ss t. Kågeröd Fm Skåne BornholmNW SE ?? Middle Graben Formation Lulu Formation Fig. 2. Lithostratigraphic scheme of the Jurassic (including the uppermost Triassic and lowermost Cretaceous) of the Danish Central Graben, the Danish Basin and the Fennoscandian Border Zone (Bornholm and Skåne, southern Sweden). Colours (Jurassic units pri- marily) indicate overall depositional environments and facies. 153 0. 0 1. 0 2. 0 3. 0 4. 0 5. 0 6. 0 0. 0 1. 0 2. 0 3. 0 4. 0 5. 0 J- 1 K -1 F- 1 P- 1 T- 1 C en tr al G ra be n SW SW N E Sk ag er ra k– K at te ga t Pl at fo rm N E 25 k m To rd en sk jo ld -1N or w eg ia n– D an is h Ba si n So rg en fr ei –T or nq ui st Z on e N or w eg ia n– D an is h Ba si n TWT sec TWT sec C en oz oi c U pp er C re ta ce ou s Lo w er C re ta ce ou s U pp er Ju ra ss ic Lo w er –M id dl e Ju ra ss ic T ri as si c Z ec hs te in R ot lie ge nd es Fi g. 3 . G eo se ct io n f ro m t h e C en tr al G ra b en i n t h e w es t to t h e Sk ag er ra k– K at te ga t P la tf o rm i n t h e ea st ; p o si tio n i n d ic at ed i n F ig . 1A . M o d if ie d f ro m V ej b æ k (1 99 7) , b as ed o n se is m ic l in e R T D -8 1- 22 . Middle Jurassic time (Nielsen 1995, 2003, this volume; Andsbjerg et al. 2001). The Skagerrak–Kattegat Platform was a stable area that was transgressed during the Jurassic and is characterised by eastwards thinning of the relatively undisturbed Mesozoic succession. The Danish Basin, constituting the eastern part of the Norwegian–Danish Basin, is situated south-west of the Sorgenfrei–Tornquist Zone. The basin is bordered to the south by the ESE–WNW-trending Ringkøbing–Fyn High, which consists of a series of shallow fault blocks of Precambrian basement (Sorgenfrei & Buch 1964). The high is of pre-Permian age and is separated from the Mid North Sea High by the north–south-oriented Central Graben (Fig. 1). The Ringkøbing–Fyn High acted as a submarine intra-basinal high during the Early Jurassic. Middle Jurassic uplift of the central North Sea affected the Ringkøbing–Fyn High, which was emergent through- out Middle and Late Jurassic times (Michelsen 1978a; Koch 1983; Ziegler 1990; Nielsen 2003, this volume). The high separates the Norwegian–Danish Basin in the north from the North German Basin in the south. The Danish Basin, formerly described as the Danish Embayment (Sorgenfrei & Buch 1964), and its exten- sion to the west, the Norwegian–Danish Basin, began to subside in Permian times (Vejbæk 1989, 1997). It contains a thick succession of Jurassic deposits, gener- ally comprising marine Lower Jurassic claystones, flu- vial and paralic Middle Jurassic sandstones, and marine Upper Jurassic claystones and sandy claystones. There seems to have been open marine connections with basinal areas to the south, west and north during the Early Jurassic (Michelsen 1978a). After the mid-Jurassic uplift of the Ringkøbing–Fyn High, the connection to the mid-European basinal areas was mainly restricted to the Central Graben area in the west. South of the Ringkøbing–Fyn High, in the northern part of the North German Basin, marine deposition probably only occurred prior to the mid-Jurassic uplift of the high. Subsequent erosion seems to have removed much of the Lower Jurassic deposits, which in the Danish area are only known from wells in the north- eastern part of the basin. The Central Graben is a complex structural feature, including a number of fault-bounded basins, highs and platforms (Fig. 1C), which possibly started to subside in the Early Permian (Ziegler 1975; Gowers & Sæbøe 1985). However, it remains uncertain if there was a connection in the Late Permian between the northern and southern Zechstein basins through the graben (Vejbæk 1992). A minor angular unconformity sepa- rates the Triassic from the Lower Jurassic, indicating a Late Triassic tectonic event. The Early Jurassic was char- acterised by regional subsidence and relative tectonic quiescence, and the Central Graben was a part of the larger North Sea Basin. Marine Lower Jurassic deposits equivalent to those known from the Danish Basin are present in the southern part of the graben. They were probably widely distributed in the North Sea Basin prior to Middle Jurassic uplift and erosion of the central North Sea area; the Lower Jurassic deposits, as preserved today, represent erosional remnants (Gowers & Sæbøe 1985; Michelsen et al. 1992). The main rifting of the Central Graben took place in Middle and Late Jurassic times. Major fault-controlled subsidence occurred in the eastern part of the graben, along the bounding Coffee Soil Fault zone, and fluvial-deltaic Middle Jurassic sediments were deposited along the fault zone, prob- ably representing the initial syn-rift deposits. Subsidence rates increased significantly during the Late Jurassic, and more than 4000 m of clay-dominated sediments were deposited in the eastern part of the graben. Wrenching led to considerable lateral variation in subsidence rates, and major depocentres developed such as the Tail End Graben and the Feda Graben (Møller 1986; Vejbæk 1992; Japsen et al. 2003, this volume). The depositional area gradually expanded to the west, and marine Upper Jurassic deposits cover the entire graben area (Michelsen et al. 1987; Damtoft et al. 1992; Johannessen et al. 1996). Lithostratigraphy Within the last forty years, a large number of lithostrati- graphic units have been defined within the Jurassic suc- cessions of the Danish onshore and offshore areas (Larsen 1966; Bertelsen 1978; Michelsen 1978a, 1989a; Gravesen et al. 1982; Jensen et al. 1986). An overview of the lithostratigraphic units encountered in wells in the Danish onshore and offshore areas was presented by Nielsen & Japsen (1991). All Jurassic formations are reviewed here, some formations are revised, and two new formations and two new members are defined (Fig. 2). The overall lithology, depositional environ- ments, distribution, thickness and age are described. The formations of Bornholm are primarily known from outcrops, and the definitions thus include sedi- mentary facies; information on gamma-log characteris- tics from shallow cored wells are also included to provide a basis for comparison with subsurface and offshore sections (Fig. 4). The formations in the Danish Basin and the Central Graben are only known from wells, mainly drilled for hydrocarbon exploration, and 154 from reflection seismic data. The successions pene- trated by wells are mainly represented by cuttings sam- ples, and petrophysical logs are therefore used in the identification, delimitation and description of the sub- surface formations. The descriptions including the lithol- ogy and log features (predominantly gamma-ray and sonic logs) are shown on figures with subsurface type sections or reference sections. Formal and informal members of certain formations are discussed briefly under the ‘remarks’ given to the respective formations. The formations from Bornholm are described first, fol- lowed by formations from the Danish Basin, and the lithostratigraphic section is concluded with the forma- tions from the Danish Central Graben. Within each area, the formations are described in ascending stratigraphic order; with the exception of the uppermost formation in each area, only the lower boundary is described for each formation, the upper boundary being described under the succeeding formation. In borehole/well sec- tions, the depths are given in relation to the land sur- face (Bornholm), the well reference level (typically Kelly Bushing (KB)) and mean sea level (MSL). Metric (SI) units are preferred, but wells drilled in feet are listed as such, relative to the well reference level, to ensure the accuracy of primary borehole data. Bornholm The Jurassic succession of Bornholm is referred to four formations, the Rønne, Hasle, Sorthat and Bagå For- mations (Fig. 2), of which the Sorthat Formation is new and the Bagå Formation is revised. The four formations are included in the Bornholm Group of Gravesen et al. (1982). The Lower Cretaceous Rabekke Formation is not dealt with here, although it may reach down into the uppermost Jurassic. Rønne Formation History. The Rønne Formation was defined by Gravesen et al. (1982). Type area. The cliffs on the south-west coast of Bornholm between Sønderborg and Næbbe Odde where the upper parts of the formation are exposed (Fig. 4; Gravesen et al. 1982). Type sections are defined for the two upper members only, the Sose Bugt Member and the Galgeløkke Member (Gravesen et al. 1982). Reference section. The combined cored section of the Galgeløkke-1 and -2 wells is designated as a reference section to complement the discontinuous exposures and to facilitate correlation to other subsurface and off- 155 Rønne– Hasle Fault Block Nyker Block Rønne Graben Arnager–Sose Fault Block Bornholm High 3 7 4 5 2 1 5 km Cretaceous Lower–Middle Jurassic Sorthat Formation and Middle Jurassic Bagå Fm Lower Jurassic Hasle Formation Fault Lower Jurassic Rønne Formation Upper Triassic Kågeröd Formation Lower Palaeozoic Precambrian crystalline basement N 9 6Sorthat Næbbe Odde Rønne Sønderborg 8 Vellensby Fig. 4. Geological map of south-west Bornholm (see inset) show- ing the position of type localities, and type and reference sec- tions. Localities marked with the red lines indicate extensive coastal exposures. 1, Munkerup coastal section, type locality of the Munkerup Member, Rønne Formation; 2, Sose Bugt coastal section, type section of the Sose Bugt Member, Rønne Formation; 3, Galgeløkke coastal section, type section of the Galgeløkke Member, Rønne Formation; 4, Hasle coastal section, type local- ity of the Hasle Formation; 5, Korsodde coastal section, reference section of the Sorthat and Bagå Formations; 6, Hasle Klinkerfabrik clay pit, type section of the Bagå Formation; 7, position of the Galgeløkke-1 and -2 core wells, reference section of the Rønne Formation; 8, position of the Hasle-1 core well, reference sec- tion of the Hasle Formation; 9, position of the Levka-1 well, ref- erence section of the Hasle Formation and type section of the Sorthat Formation. Modified from Gravesen et al. (1982). 156 390 400 350 360 370 380 330 340 230 240 250 260 270 280 290 300 310 320 Depth in m Depth in m ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ 2Clay Si Sand 4 8 mm 2Clay Si Sand 4 8 mm Galgeløkke-1, -2 composite core log Reference section: Rønne Formation 0 225 GR API 0 225 GR API 157 120 130 140 160 170 180 190 200 210 220 ✲✲✲ ✩ ✲✲✲✩ 10 20 30 40 50 60 70 80 90 100 110 ✩ ✩ ✩ ✩ ✩ ✩ ✩ ✩ ? Depth in m Depth in m 150 2Clay Si Sand 4 8 mm 2Clay Si Sand 4 8 mm 0 225 GR API 0 225 GR API shore well sections. The wells were drilled at the same locality, south of the town of Rønne and c. 300 m ENE of the coastal cliff at Galgeløkke where the type sec- tion of the Galgeløkke Member is exposed (Gravesen et al. 1982; Nielsen 1995). The cored wells encountered almost 400 m of the Rønne Formation (3.2–400.7 m b. surface) and provided continuous cores and a gamma- ray log (Fig. 5). The type section of the Galgeløkke Member overlies the cored well sections with a strati- graphic gap of some tens of metres, as interpreted from map evidence and the dip of the exposed strata (Gry 1969; Jensen & Hamann 1989; Nielsen 1995). The base of the Rønne Formation was not reached by the wells; the upper boundary with the overlying Hasle Formation was cored in the Hasle-1 well (Fig. 6). Lithology and environment. The Rønne Formation con- sists of a wide variety of lithologies dominated by lam- inated or massive mud, heteroliths of mud and fine- grained sand, fine- to medium-grained sand, and thin coal beds with rootlets. Deposition took place in par- alic and marine environments including coastal plains, 158 Lithology Mudstone Siltstone Heterolith, mud/sand (50%) Sandstone Conglomerate Pebble/granule lags Coal Calcareous cement Pyritic concretions Carbonaceous detritus Mudstone chips Sedimentary structures Erosional surface Parallel bedding/lamination Planar cross-bedding Cross-bedding with mudstone drapes Cross-bedding with double mudstone drapes Trough cross-bedding Low-angle cross-bedding Hummocky cross-stratification Gutter casts Cross-lamination and climbing ripples Bimodal current-ripple lamination Wave ripples Flaser bedding Wavy bedding Lenticular and silt-streaked bedding Disturbed bedding Load structures Water escape structures Synaeresis cracks clast clast clast Basement clast clast Fragments Biogenic structures Bioturbation Degree of bioturbation Chondrites isp. Diplocraterion isp. Equilibrichnion isp. Rhizocorallium isp. Skolithos isp. Teichichnus isp. Thalassinoides isp. Horizontal burrows Large burrows Rootlets Fossils Bivalve Plant Wood Stem Claystone Siltstone Clayey Sandstone Silty sandstone Sandstone Conglomerate Scattered gravel clasts Coal bed and clasts Limestone bands Limestone Marlstone Dolomite bands Volcanic rocks Cored section Legend: Sedimentological logs Stratigraphic logs Preceding pages and above: Fig. 5. Composite core log of the Rønne Formation in the Galgeløkke-1 and -2 wells, located in the Rønne–Hasle Fault Block just south of the town of Rønne on the west coast of Bornholm, c. 300 m ENE of the coastal cliff at Galgeløkke and just west of the eastern bounding fault of the Rønne Graben; reference section for the Rønne Formation. No formal subdivision into members is attempted for this section although it is probably largely within the Sose Bugt Member. The formation was cored from 400.7–3.2 m b. surface; the base of the formation was not penetrated. The Galgeløkke-1 well was terminated at 123.75 m b. surface due to technical problems; coring was resumed from 120 m to 400.7 m in the Galgeløkke-2 well drilled 3 m from Galgeløkke-1 (Nielsen 1987). Note that to facilitate comparison of the shallow cored well sections with outcrop sections on Bornholm, composite logs are organised with base to the lower left and top to the upper right. This is the reverse of that typically adopted for deep wells (e.g. Figs 11, 16, 26). Modified from Nielsen (1995). The accompanying legend is applicable to all subsequent sedimen- tological and stratigraphic logs. lakes, fluvial channels, tidal channels, tidal flats, marine shoreface and restricted offshore. Log characteristics. The gamma-ray logs from the Galge- løkke-1, -2 and Hasle-1 wells show a highly variable pattern reflecting the wide variety of lithologies, with sand displaying blocky, coarsening-upwards (i.e. val- ues decrease upwards) and fining-upwards (values increase upwards) trends. Mud is indicated by rela- tively uniform high values, while the common het- eroliths are shown by intermediate, variable values. Lower boundary. Onshore, the formation rests uncon- formably on Triassic or Cambrian strata. The lower bound- ary is defined by a change from red, green and variegated clay and greenish sandstones lacking carbonaceous detri- tus of the Triassic Risebæk Member (Kågeröd Formation) or from Cambrian quartzitic sandstones to light and dark grey to black clay with abundant carbonaceous detritus and subordinate sand of the Rønne Formation. Offshore, in the Pernille-1 and Stina-1 wells, the Rønne Formation (Munkerup Member) overlies Upper Triassic strata, prob- ably of Rhaetian age, mainly consisting of slightly cal- careous to marly brownish claystones; the boundary is tentatively placed at the change to non-calcareous clay- stones with organic detritus. Distribution and thickness. The Rønne Formation may be up to 500 m thick in the eastern part of the Rønne Graben near the town of Rønne (Nielsen 1995). The formation extends offshore in the Rønne Graben as shown by the Pernille-1 and Stine-1 wells (Fig. 1C, see also Fig. 31). Age. The Rønne Formation is essentially of Hettangian– Sinemurian age but may extend into the earliest Pliensbachian (Gry 1969; Koppelhus 1991; Surlyk et al. 1995; Donovan & Surlyk 2003, this volume). The Munkerup Member is of Early Hettangian age. The Sose Bugt Member is of Late Hettangian – Late Sinemurian or earliest Pliensbachian age in the Arnager–Sose Block, while it may be limited to the Late Hettangian – Early Sinemurian in the Rønne–Hasle Fault Block, where the Galgeløkke Member probably spans the Late Sinemurian. Subdivisions. Gravesen et al. (1982) subdivided the for- mation, in ascending stratigraphic order, into the Munkerup Member, the Sose Bugt Member and the Galgeløkke Member (Fig. 2). The Munkerup Member is known from numerous small clay pits, long since infilled. The description of the member is therefore based on old literature, and a type section has not been measured (Gravesen et al. 1982). The member is dominated by grey to black clay with subordinate thin beds of red, brown and white clay, and sandstone. Carbonaceous detritus and plant fossils are abundant in the clays, locally forming coal laminae or thin coal beds. The sandstones are very fine-grained, white to yellowish. In addition to the occurrences at the type locality on the coast at Munkerup and at Vellensby, where a reference section has been defined in a shal- low borehole, the member seems to be present in the Pernille-1 and Stina-1 wells in the Rønne Graben (Fig. 1C, see also Fig. 31; Nielsen 1995). In these wells, c. 50 m of mainly grey to olive grey and light grey clay is interbedded with fine- to medium-grained sand con- taining a mixed assemblage of Early Jurassic and reworked Triassic spores and pollen comparable to that of the onshore Munkerup Member (Koppelhus 1991; Nielsen 1995). According to Gry (1969) the thickness of the Munkerup Clay onshore is 20 m although the upper boundary is not seen. The Sose Bugt Member is recognised onshore Bornholm in both the Arnager–Sose and Rønne–Hasle Fault Blocks, and a type section has been defined from the outcrop at Sose Bugt (Fig. 4; Gravesen et al. 1982; Surlyk et al. 1995). The member appears to be thickly developed off- shore in the Rønne Graben (Nielsen 1995). It is domi- nated by alternating thin beds of fine-grained, cross- laminated sand, commonly with abundant organic detri- tus, and thin beds of grey laminated clay or heteroliths of sand and clay. Rootlet horizons and thin coal beds are common. Fine-grained and medium- to coarse-grained sand units, up to 12 m thick, with cross-lamination and cross-bedding are common in the Rønne–Hasle Fault Block as shown by the Galgeløkke-2 well (Fig. 5). The Galgeløkke Member is dominated by light grey to white or yellowish, cross-laminated, fine-grained sand or heteroliths showing wavy and flaser-laminated sand and clay, particularly in the lower part. Large-scale cross-bedded medium- to coarse-grained sand beds with tidal bundles and mud couplets (Sellwood 1972; Tyge 1990) dominate the middle part of the member. Thin coal beds and carbonaceous clays with rootlets occur, as well as organic detritus. The type section of the member is defined in the Rønne–Hasle Fault Block (Fig. 4), and the member is also present offshore in the Rønne Graben (Gravesen et al. 1982; Nielsen 1995). It has, however, not been possible on the basis of the avail- able data to define the boundary between the Sose Bugt and Galgeløkke Members with confidence in the Galgeløkke-1, -2, Pernille-1 and Stina-1 wells. The mem- ber is probably not present in the Arnager–Sose Block 159 160 Clay Si Sand 2 4 8 mm Depth in m 70 80 90 100 110 120 118.3 130 140 150 R øn ne F or m at io n H as le F or m at io n 0 700 GR API 0 700 GR API Depth in m 10 20 30 40 50 60 H as le F or m at io n Clay Si Sand 2 4 8 mm Hasle-1, core log Reference section: Hasle Formation GR GR (Nielsen 1995; Surlyk et al. 1995), as originally sup- posed (Gravesen et al. 1982). Hasle Formation History. The Hasle Formation was defined by Gravesen et al. (1982). Type area. The coastal cliff, south of Hasle town, Bornholm (Fig. 4). A type section has not been mea- sured, but representative sections of the formation may be found in Surlyk & Noe-Nygaard (1986). Reference section. The cored Hasle-1 well (2.6–155.0 m b. surface), drilled c. 1.1 km south of the Hasle harbour (Fig. 4), provides continuous cores and a gamma-ray log from the lower–middle part of the Hasle Formation and illustrates the lower boundary with the Rønne Formation (Fig. 6). The cored Levka-1 well drilled between the type area of the Hasle Formation and the type section of the Bagå Formation in the Hasle Klinkerfabrik clay pit, c. 2.5 km south of the Hasle har- bour and c. 1 km north of the clay pit, provides cores from the middle–upper part of the Hasle Formation (Fig. 7). The two wells are designated as reference sec- tions to facilitate correlation to other subsurface and off- shore well-sections; together they illustrate the entire formation with some overlap. Lithology and environment. At outcrop, the Hasle For- mation consists of rusty yellow to brownish (dark green- ish when unweathered), coarse-grained siltstones and very fine-grained sandstones with swaley to hummocky cross-stratification and marked erosion surfaces draped with small pebble lags. Thin poorly sorted gravels, greenish grey and brown clays and clay-ironstones occur locally (Gravesen et al. 1982; Surlyk & Noe- Nygaard 1986). The sandstones are intercalated with fos- siliferous clays in the area of the town of Rønne and the south coast at Stampe Å (northern end of the Korsodde section, see Fig. 4), and a diverse marine fauna including numerous ammonites has been described from the clays (Malling & Grönwall 1909; Malling 1911, 1914, 1920; Höhne 1933; Donovan & Surlyk 2003, this volume). Clay beds seem to be absent in the type area to the north as clays were not encoun- tered in the Hasle-1 and Levka-1 wells which in combi- nation illustrate the entire Hasle Formation (Figs 6, 7). Deposition took place in a storm-dominated shoreface to offshore environment (Surlyk & Noe-Nygaard 1986). Log characteristics. The gamma-ray log from the Has- le-1 well (Fig. 6) shows intervals with relatively uniform low readings interrupted by log spikes that locally show extremely high values; such high values in the sandstones are caused by concentrations of uranium and thorium (Nielsen 1995). Lower boundary. The formation conformably overlies the Rønne Formation and is marked by a change from largely unconsolidated heterolithic sand and clay with carbonaceous detritus to indurated limonitic brownish (weathered) or dark greenish (unweathered) sandstones or thick clay beds containing marine macrofossils. In the Hasle-1 reference section, the base of the Hasle Formation is defined at a marked pebble-strewn sur- face interpreted as a ravinement surface capping cross- bedded and flaser-laminated sandstones with coal particles of the underlying Rønne Formation (Fig. 6, 118.3 m; Nielsen 1995). Distribution and thickness. The formation is probably up to 140 m thick in the type area and seems to be pre- sent offshore in the Stina-1 well in the southern part of the Rønne Graben in a slightly more fine-grained, clayey facies (see Fig. 31; Nielsen 1995). Age. The formation is of Early Pliensbachian age (Donovan & Surlyk 2003, this volume). Sorthat Formation new formation History. The coal-bearing clays and sands that overlie the Hasle Formation in the Rønne–Hasle Fault Block have 161 Facing page: Fig. 6. Hasle-1 well, located in the Rønne–Hasle Fault Block, north of the town of Rønne on the west coast of Bornholm, just west of the eastern bounding fault of the Rønne Graben (Nielsen 1987); reference section for the Hasle Formation. The lower c. 37 m of the cored section is referred to the Rønne Formation; the Hasle Formation was cored from 118.3–2.6 m b. surface. The upper part of the Hasle Formation is not repre- sented in Hasle-1, but is cored in Levka-1 (see Fig. 7). Note that the scale on the gamma-ray log differs from the Galge- løkke-1, -2 and Levka-1 wells. Modified from Nielsen (1995); for legend, see Fig. 5. 162 So rt ha t Fo rm at io n Clay SandSi 2 4 8 mm Depth in m Levka-1, core log Reference section: Hasle Formation Type section: Sorthat Formation 0 GR 225 GR API Depth in m Clay SandSi 2 4 8 mm 0 225 GR API GR H as le F or m at io n So rt ha t Fo rm at io n 229.20 120 130 140 150 160 170 180 190 200 210 220 225 13.4 20 30 40 50 60 70 80 90 100 110 been referred to as the Levka, Sorthat and Bagå beds in older literature (reviewed by Gry 1969). A detailed strati- graphic interpretation of the beds has been difficult to achieve owing to complicated block faulting and fold- ing, scattered outcrops and the absence of marine fos- sils and distinct marker beds. Based on the content of megaspores, Gry (1969) referred all three units to the Middle Jurassic with the Levka and Sorthat beds being roughly contemporaneous, and the Bagå beds being possibly slightly younger. Gravesen et al. (1982) defined the Bagå Formation to include the coal-bearing clays and sands of the Levka, Sorthat and Bagå beds and the coal-bearing strata at Korsodde and Onsbæk (northern end of the Korsodde section as indicated on Fig. 4). Preliminary palynological investigations of samples from the Levka-1 core-well and the Korsodde section sug- gested the presence of Upper Pliensbachian strata (E.B. Koppelhus in: Nielsen 1987; Koppelhus 1988; Nielsen & Koppelhus 1989). A reassessment of the megaspore col- lections of H. Gry demonstrated the presence of several megaspore species from the Bagå Formation (sensu Gravesen et al. 1982) that suggest the presence of Toarcian–Aalenian strata, although their long range pre- cluded a definite dating (Koppelhus & Batten 1992). A thorough palynological–sedimentological study of all available exposures and cores from the Lower–Middle Jurassic has revealed that the marine Lower Pliensbachian sandstones of the Hasle Formation are overlain by a suc- cession referable to the Levka and Sorthat beds that com- prises bioturbated sands, heteroliths and clays with thin coal seams containing relatively diverse brackish-marine dinoflagellate assemblages indicative of the Upper Pliensbachian, Toarcian and possibly Lower Aalenian (Koppelhus & Nielsen 1994). These paralic deposits are overlain by a succession equivalent to the Bagå beds com- prising crevasse and fluvial gravels and sands, lacustrine clays, carbonaceous clays and coals belonging to the Upper Aalenian? and Bajocian–Bathonian. These new data confirm that the Levka and Sorthat beds are litho- logically different from the Bagå beds. Thus the Levka and Sorthat beds are here included in the new Sorthat Formation, and the Bagå Formation is revised to include the Bagå beds only. Both formations are included in the Bornholm Group of Gravesen et al. (1982). The section measured from the Sorthat beds by Gravesen et al. (1982, fig. 24) covers part of the new Sorthat Formation. Name. After the locality of Sorthat on the west coast of Bornholm, between the towns of Hasle and Rønne, where the formation was previously exposed (Fig. 4; Gry 1969; Gravesen et al. 1982; Nielsen 1995). Type section. The shallow Levka-1 core-well is selected as the type section as no exposures are currently avail- able in the Sorthat area (Fig. 4); this well was drilled between the type area of the Hasle Formation and the type section of the Bagå Formation in the Hasle Klinkerfabrik clay pit, c. 2.5 km south of the Hasle har- bour and c. 1 km north of the clay pit. The section from 13.4–164.6 m b. surface is represented by both cores and a gamma-ray log (Fig. 7). Reference section. An intermittently exposed succes- sion, 130–140 m thick at Korsodde on the south-west coast of Bornholm is selected as the reference section (Fig. 8). Lithology and environment. The lithology of the Sorthat Formation is highly variable. The cores and gamma log from the lower c. 112 m of the formation in the Levka-1 well reveal sharp-based, fining-upwards units, 3–14 m thick, consisting of coarse-grained, occasionally pebbly sand, overlain by muddy, carbonaceous, micaceous, fine- to medium-grained sand, laminated to homoge- neous clay and coal seams with rootlets (Fig. 7). Most of the cored sand shows parallel lamination with sub- ordinate thin beds with cross-bedding, cross-lamina- tion and flaser lamination. Large plant fragments and small quartz pebbles are common. Occurring between the fining-upwards units are thinly interbedded sand and clay with rootlets and thin coal seams. Marine paly- nomorphs are not found and the interval is interpreted as having been deposited on a coastal or delta plain with fluvial channels, lakes and swamps (Koppelhus & Nielsen 1994). The upper c. 40 m of the well-section consist of cross-bedded, cross-laminated, wave-rippled and bioturbated sand and heteroliths with sporadic synaeresis cracks, pyrite nodules, Planolites isp. and Teichichnus isp. burrows and brackish-marine paly- nomorphs. Fining-upwards successions of sand, clay and coal seams are also represented and are characterised 163 Facing page: Fig. 7. Levka-1 well, located in the Rønne–Hasle Fault Block north of the town of Rønne on the west coast of Bornholm, just west of the eastern bounding fault of the Rønne Graben (Fig. 4; Nielsen 1987); type section for the new Sorthat Formation. The formation was cored from 164.6–13.4 m b. surface. A gamma- ray log was not recorded from the Hasle Formation. Modified from Nielsen (1995); for legend, see Fig. 5. 164 Clay Si Sand 2 4 8 mm So rt ha t Fo rm at io n So rt ha t Fo rm at io n So rt ha t Fo rm at io n So rt ha t Fo rm at io n H as le F or m at io n 3.2 m not exposed m 92.8 m 32 34 36 37.6 60 72 72.5 76 84.6 86 88 40°S 56° 90 92 8 m not exposed 3 m not exposed 11 m not exposed 21.4 m not exposed 8.4 m not exposed 116 114 112 100 102 104 106 108 110 96 98 95 94.7 m 140 138 136 134 130 132 128 126 122 124 120 118 36°S 48° 22°E 30° 49°S 76° 58°S 68° 57°S 67° 62°S 64° 22 24 26 28 20 16 18 12 14 3.6 0 2 m 28.8 Clay Si Sand 2 4 8 mm Clay Si Sand 2 4 8 mm Clay Si Sand 2 4 8 mm Korsodde coastal section Reference section: Sorthat Formation Reference section: Bagå Formation Fig. 8. Coastal section from south-west Bornholm, Rønne–Hasle Fault Block, close to the eastern bounding fault of the Rønne Graben; reference section of the new Sorthat Formation. The formation overlies the Hasle Formation, but the formation boundary occurs within an unexposed interval of 8.4 m. Modified from Koppelhus & Nielsen (1994); for legend, see Fig. 5. 165 Ba gå F or m at io n Ba gå F or m at io n Ba gå F or m at io n So rt ha t Fo rm at io n m 200 198 196 194 192 190 188 186 46°S 66° 40°S 70° 68°E 44° 56°S 74° 60°S 38° 5.2 m not exposed Clay Si Sand 2 4 8 mm Clay Si Sand 2 4 8 mm Clay Si Sand 2 4 8 mm m 186 184 182.4 177.2 176 166 168 170 172 174 162 164 m 160 158 156 154 152 150 148 144 146 140 142 by non-marine palynomorphs. This upper c. 40 m inter- val is interpreted as having been deposited in lagoons, coastal lakes and fluvial channels with the clean sand at the top probably representing a marine shoreface. The reference section at Korsodde is also lithologi- cally highly variable (Fig. 8). The lower c. 93 m thick unit consists of medium- to coarse-grained, pebbly, cross-bedded and parallel-laminated sand units, up to 6 m thick. Thin, fine-grained and cross-laminated sand beds occur. The sand is locally almost black due to abundant organic debris. Interbedded with the sand are laminated to almost homogeneous clay beds and fining-upwards heteroliths overlain by laminated dark clay and coaly beds with rootlets. Brackish paly- nomorphs are present. This part of the section is inter- preted to have been deposited in fluvial channels, coastal lakes and lagoons. Above a pronounced ero- sion surface (Fig. 8, 105.5 m) is a fining-upwards unit, c. 19 m thick, of cross-bedded, coarse- to fine-grained sand overlain by heteroliths and clays capped by a coal seam. Some cross-beds show tidal structures such as reactivation surfaces, bundles and mud-drapes that yield brackish-marine dinoflagellates. Small burrows are also common. Larger burrows, including Diplocraterion isp., occur in the wavy to flaser bedded heteroliths. The unit is interpreted as the fill of an estuarine channel. It is overlain by bioturbated heteroliths with dinoflagellates and burrows of Diplocraterion isp., Teichichnus isp., Skolithos isp. and Planolites isp. Well-sorted, fine-grained sand beds with wave-ripples and swaley cross-stratifi- cation also occur. These inferred lagoonal deposits are followed by fluvial sands and lacustrine clays. The uppermost part of the formation in the Korsodde sec- tion consists of very fine- to fine-grained, swaley cross- stratified and parallel-laminated yellowish–brown sands and sandstones with thin bioturbated and wave-rip- pled heterolithic beds deposited in the shoreface to offshore transition zone (Koppelhus & Nielsen 1994). The coals of the Levka-1 and Korsodde sections were primarily formed in anoxic, water-saturated inter-channel environments on a coastal plain (Petersen et al. 2003, this volume). Log characteristics. The gamma-ray log shows a very variable pattern reflecting the interbedded sands, het- eroliths, muds and coals (Fig. 7). Both fining-upwards and coarsening-upwards trends are represented, as tes- tified by gamma log values that increase and decrease upwards, respectively. Lower boundary. The lower boundary is defined at the change from dark green (unweathered), yellow to indurated brownish (weathered) limonitic marine sand- stones of the Hasle Formation to unconsolidated light grey- ish fluvial sands, lacustrine grey to dark grey clays, and coals (Figs 7, 8). In the type section, the boundary is marked by a change from laminated and cross-bedded, medium- to coarse-grained beach sand of the upper- most Hasle Formation to coal-bearing sand and clay with roots. In the Korsodde reference section, the boundary with the Hasle Formation occurs within a non-exposed interval of c. 8.4 m, covered by a concrete construction. Distribution and thickness. The formation occurs in the Rønne–Hasle Fault block, where it is up to 200 m thick. The formation is also present offshore in the Rønne Graben, as indicated by seismic data and the Stina-1 well (see Fig. 31; Hamann 1994; Nielsen 1995). Age. Gry (1969) proposed a Middle Jurassic age for the Sorthat, Levka and Bagå beds based on the megaspore content. However, a reassessment of the megaspores revealed the possible presence of Toarcian–Aalenian strata (Koppelhus & Batten 1992). Based on the recog- nition of several dinoflagellate assemblages supported by spores and pollen, Koppelhus & Nielsen (1994) pro- posed that the age of the Sorthat Formation is Late Pliensbachian – Toarcian, possibly extending up into the Early Aalenian. Bagå Formation revised formation History. The Bagå Formation was defined by Gravesen et al. (1982) to include the coal-bearing clays and sands in the Rønne–Hasle Fault Block traditionally named the Levka, Sorthat and Bagå beds (reviewed by Gry 1969). The coal-bearing strata at Korsodde and Onsbæk were also included in the Bagå Formation. As discussed above (Sorthat Formation), the Bagå Formation as revised here 166 Facing page: Fig. 9. Hasle Klinkerfabrik clay pit, type section of the Bagå Formation; re-measured by Koppelhus & Nielsen (1994) to cover the full exposure. Modified from Koppelhus & Nielsen (1994); for legend, see Fig. 5. 167 Clay SandSi 2 4 8 mm Clay SandSi 2 4 8 mm Clay SandSi 2 4 8 mm Clay SandSi 2 4 8 mm m 24 20 22 16 18 12 10 14 8 6 4 2 0 m 72 70 68 64 66 60 58 62 56 54 52 50 48 m 100 88 90 92 94 96 98 84 82 86 80 78 76 74 72 m 48 46 44 40 42 36 34 38 32 30 28 26 24 ✩ ✩ ✩ ✩ ✩ ✩ Hasle Klinkerfabrik clay pit Type section: Bagå Formation is adopted in a restricted sense to only include the Middle Jurassic Bagå beds of Gry (1969). Type section. The succession exposed in the Hasle Klinkerfabrik clay pit, immediately south of the Bagå stream, is selected as the type section in accordance with Gravesen et al. (1982), who presented 23 m of section. The exposed section has been re-measured to present the full section of the pit (Fig. 9). Reference section. The upper c. 65 m of the section exposed at Korsodde is proposed as a new reference section (Fig. 8), as the former reference section used by Gravesen et al. (1982, fig. 24) is now referred to the Sorthat Formation. Lithology and environment. The Bagå Formation in- cludes thick laminated to homogeneous, grey clay units, up to 10 m thick, dark to black coaly clays with rootlets and coal beds, up to 2.5 m thick, and medium- to fine- grained, cross-bedded or poorly laminated sand beds forming units up to 5.5 m thick. Poorly sorted, muddy and pebbly sand beds, locally with boulders of weath- ered granite, occur in the upper part (Fig. 9). The lower part of the reference section at Korsodde comprises poorly parallel-stratified, coarse- to very coarse-grained pebbly sand with boulders of Cambrian quartzites and Lower Palaeozoic kaolinised mudstones (Fig. 8). The mid- dle part comprises laminated grey clay and dark highly carbonaceous clay. The upper part consists of poorly sorted fine- to medium-grained, poorly laminated sand beds, 1.5–3.0 m thick, interbedded with muddy and carbonaceous sand. Deposition took place in lakes and swamps, small crevasse channels and lacustrine deltas, and fluvial channels (Gravesen et al. 1982; Koppelhus & Nielsen 1994; Nielsen 1995). The coaly clays and coal beds were primarily deposited in open freshwater mires subjected to some siliciclastic deposition. Coal beds in the upper part of the formation were deposited on an alluvial fan with a fluctuating watertable. Boundaries. The lower boundary is not exposed in the Sorthat area and was not encountered in the cored wells. In the Korsodde reference section, the lower boundary is defined at a significant pebble-strewn ero- sion surface separating swaley to hummocky cross- stratified, weakly cemented, yellowish brown, fine- grained shoreface sandstones of the upper Sorthat Formation from overlying coarse-grained, pebbly, light grey to white, largely unconsolidated fluvial sands with pebbles and boulders (Fig. 8, 145.2 m). Onshore, the upper boundary is an erosion surface defining the present-day land surface or the base of the Quaternary. The Korsodde section is faulted against the Upper Cretaceous Bavnodde Greensand. Distribution and thickness. The Bagå Formation is pre- sent in the Rønne–Hasle Fault Block, where it is esti- mated to be more than 190 m thick (Gry 1969), and is also present offshore in the Rønne Graben with a sim- ilar thickness as indicated by seismic data (Jensen & Hamann 1989; Hamann 1994). Age. The Bagå beds were referred to the Middle Jurassic by Gry (1969) based on megaspores. Hoelstad (1985) referred the Bagå beds as exposed in the Hasle Klinkerfabrik clay pit to the uppermost Toarcian – Aalenian and Bajocian–Bathonian. However, the paly- noflora used by Hoelstad (1985) for the identification of the uppermost Toarcian – Aalenian in the Bagå beds appear to have a longer range than originally supposed (T. Hoelstad, personal communication 1994). Based on the content of spores and pollen in the Bagå beds in the Hasle Klinkerfabrik clay pit and the reference sec- tion from Korsodde, the age of the re-defined Bagå Formation is Middle Jurassic, probably including the Late Aalenian, Bajocian and Bathonian (Koppelhus & Nielsen 1994). Danish Basin The Upper Triassic – lowermost Cretaceous succession of the Danish Basin is referred to six formations, the Gassum, Fjerritslev, Haldager Sand, Flyvbjerg, Børglum and Frederikshavn Formations (Fig. 2). New reference sections are proposed here for the Gassum and Fjerritslev Formations. Gassum Formation History. The Gassum Formation was defined by Larsen (1966), and redefined by Bertelsen (1978). Type section. The Gassum-1 well, 5406–4980 ft b. KB, 1590–1460 m b. MSL, in the Danish Basin (Larsen 1966; Bertelsen 1978). Reference section. The Hyllebjerg-1 well, 2751–2582 m b. KB, 2723–2554 m b. MSL, drilled centrally in the Danish Basin is here selected as a reference section in 168 order to present a modern log suite as the petrophys- ical logs available from the Gassum-1 well are of poor quality (Fig. 10). Lithology and environment. The formation consists pre- dominantly of light grey to whitish fine- to medium- grained, and in places coarse-grained sandstones, interbedded with grey and greenish grey heteroliths, dark-coloured claystones and a few thin coal beds. An overall deltaic origin was suggested by Larsen (1966) and Bertelsen (1978); the sandstones are both of flu- vial channel and marine shoreface origin, however, and occur as widespread sheets that represent several progra- dational events. The intervening claystones are domi- nantly marine with some of lacustrine and lagoonal origin in the south-eastern and north-eastern parts of the basin (Nielsen et al. 1989; Hamberg 1994; Hamberg & Nielsen 2000; Nielsen 2003, this volume). Log characteristics. The formation is characterised by changes between high and low gamma-ray values and rather constant and high sonic readings, reflecting interbedded sandstones and claystones. Thin intervals with low sonic values probably reflect coal beds. The gamma-ray and SP logs show different styles in different locations within the Danish Basin. In the cen- tral area of the basin, the lower part of the formation includes two intervals showing low, decreasing-upwards gamma-ray values (Fig. 10), while more blocky motifs occur in the northern part of the basin; in the south, the gamma-ray log shows increasing-upwards trends. Above is a thin interval with high gamma-ray values, succeeded by an overall decreasing-upwards trend, but interrupted by gamma spikes attaining high values. The top of the formation is commonly characterised by a blocky gamma-ray low. To the north and north-west, the upper levels of the formation are characterised by rather high gamma-ray values, decreasing slightly upwards. Lower boundary. Decreasing-upwards gamma-ray val- ues and increasing sonic values reflect the transition from the underlying Vinding Formation to the Gassum For- mation. The boundary is located at the base of the low- ermost significant sandstone bed within this transition. The Skagerrak Formation – Gassum Formation bound- ary, in the northern and eastern parts of the basin, is similarly defined at the base of the first significant sand- stone bed overlying claystones of the uppermost Skagerrak Formation. 169 Hyllebjerg-1 Reference section: Gassum Formation Reference section: Fjerritslev Formation C hr on os tr at ig ra ph y Fo rm at io n M em be r H.S. m b. MSL Bø rg l. Fl . M.J. U . J ur . F- IV F- III F- II F- I F- Ib F- Ia IIa IIb IIc Fj er ri ts le v G as su m Tr ia ss ic Lo w er Ju ra ss ic V i. Li th ol og y 0 150 140 40 m b. KB GR API 1894 1913 1922 1977 2100 2000 2400 2600 2700 2205 2293 2502 2751 2582 2554 2723 1866 1894 1949 2177 2265 2474 1885 Sonic msec/ft Fig. 10. Hyllebjerg-1 well, located in the Danish Basin; new ref- erence section for the Gassum Formation, 2723–2554 m b. MSL, and the Fjerritslev Formation, 2554–1894 m b. MSL. Børgl., Børglum; Fl., Flyvbjerg; H.S., Haldager Sand; Vi., Vinding; M.J., Middle Jurassic; U. Jur., Upper Jurassic. Modified from Michelsen (1989a); for legend, see Fig. 5. Distribution and thickness. The formation is present in the Danish Basin and locally in the North German Basin. Maximum thicknesses of more than 300 m occur in the Sorgenfrei–Tornquist Zone. Age. Late Norian – Rhaetian over most of the basin, younging towards the north-eastern margin, where the formation is of Hettangian – Early Sinemurian age (Michelsen 1975; Bertelsen 1978, 1980; L.H. Nielsen, L. Hamberg and E.B. Koppelhus in: Nielsen 1995). Remarks. The formation was subdivided informally into three members by Bertelsen (1978), but new well sec- tions suggest that they are not consistently identifiable. Fjerritslev Formation History. The Fjerritslev Formation was defined by Larsen (1966), and amended by Michelsen (1978a). Type section. The Fjerritslev-2 well, 2233–1322 m b. KB, 2225–1314 m b. MSL, in the Danish Basin (Fig. 11; Larsen 1966; Michelsen 1978a). Reference section. Petrophysical logs are not available from the entire section in the Fjerritslev-2 well, and the section in Hyllebjerg-1, 2582–1922 m b. KB, 2554–1894 m b. MSL, is selected as reference section (Fig. 10). This section was described in detail by Michelsen (1989a). Lithology and environment. The formation consists of a relatively uniform succession of marine, dark grey to black, slightly calcareous claystones, with a varying content of silt and siltstone laminae. Clay-ironstone con- cretions are common. Siltstones and fine-grained sand- stones form a minor proportion of the succession, being most common on the Skagerrak–Kattegat Platform. Deposition took place in a deep offshore to lower shoreface environment (Michelsen 1975, 1978a; Pedersen 1985; Nielsen 2003, this volume). Log characteristics. The formation is characterised by rather uniform high gamma-ray values, but subordi- nate lows are present in three of the five lithostrati- graphic units described below (Fig. 10; F-Ia, F-Ib, F-II, F-III, F-IV). The log-motifs of the SP curve closely con- form to those of the gamma-ray curve (Fig. 11). The sonic and resistivity values are higher in the three units characterised by gamma-ray lows than in the two inter- vening units (Figs 10, 11). Lower boundary. In the Danish Basin, the lower bound- ary is defined at an abrupt change from low to high gamma-ray values, indicating the shift from the sand- stones of the Gassum Formation to the claystones of the Fjerritslev Formation (Fig. 10). In some cases, the boundary is characterised by a more gradual transition from low to high gamma-ray values (see Figs 30, 32). Distribution and thickness. The formation is present over most of the Danish Basin, in the southern part of the Danish Central Graben, and locally within the Norwegian part of the Norwegian–Danish Basin. The thickness is very variable due to mid-Jurassic erosion. The maximum thickness recorded in well sections is 911 m in the type section. Seismic data indicate a thickness of more than 1000 m in the Fjerritslev Trough. Age. Mainly Early Jurassic (Michelsen 1975, 1989a), although locally extending down into the Late Rhaetian (Dybkjær 1991) and up into the Early Aalenian (Michelsen & Nielsen 1991; Nielsen 1992, 2003, this volume; Poulsen 1996). The lower boundary of the for- mation is diachronous, younging towards the north- east where it is of Early Sinemurian age. The upper boundary is erosional except for within the Sorgenfrei– Tornquist Zone. Subdivision. Michelsen (1978a, 1989a) subdivided the formation into four informal members (F-I – F-IV), the lowermost of which can be subdivided into two units, F-Ia and F-Ib, over much of the Danish Basin. The Upper Rhaetian – lowermost Sinemurian F-Ia unit consists of uniform claystones and claystones interbed- ded with siltstones, which are locally carbonate- cemented. The unit is characterised by decreasing- upwards gamma-ray values and increasing sonic values. The lower boundary coincides with the lower bound- ary of the formation. The unit is recognised in most wells in the Danish Basin and in a few wells in the Danish Central Graben. The Sinemurian – Lower Pliensbachian 170 Facing page: Fig. 11. Fjerritslev-2 well, located in the Sorgenfrei–Tornquist Zone; type section for the Fjerritslev Formation, 2225–1314 m b. MSL. M. Jur., Middle Jurassic. Modified from Michelsen (1978a); for legend, see Fig. 5. 171 Haldager Sand Fm G as su m Tr ia ss ic M .Ju r. Li th ol og y 0 150 Resistivity Ohm m2/m 100 Fjerritslev-2 Type section: Fjerritslev Formation F- III F- IV F- II C hr on os tr at ig ra ph y Fo rm at io n M em be r Li th ol og y C hr on os tr at ig ra ph y Fo rm at io n M em be r 50 100 SP Millivolt SP Millivolt Fj er ri ts le v Lo w er Ju ra ss ic F- Ia F -I F -I I F -I Ib F- Ib F -I Ia F- IIb F- IIc Fj er ri ts le v Lo w er Ju ra ss ic m b. KB m b. KB m b. MSLm b. MSL 1399 1322 1678 1400 1500 1600 1700 1314 1391 1670 1800 1857 1900 2000 2100 2200 2233 1849 2225 155 Resistivity Ohm m2/m F-Ib unit is dominated by uniform claystones showing constantly high gamma-ray values. The lower bound- ary is characterised by a distinct log-break to higher gamma-ray values and lower sonic values. The unit is recognised in most wells in the Danish Basin and also in the Danish Central Graben. The Lower–Upper Pliensbachian F-II member differs from the deposits below and above in that it consists of faintly laminated claystones interbedded with silt- stones and sandstones. Towards the north-east, on the Skagerrak–Kattegat Platform, the member is dominated by fine-grained sandstones. The member was subdivided into three units (F-IIa to F-IIc) by Michelsen (1978a, 1989a) in a few wells located in the central part of the Danish Basin. The member typically shows decreas- ing-upwards gamma-ray and increasing sonic values, with a distinct log-break on the sonic curve at the base of the member. The member varies laterally in thick- ness, but is recognised in most wells in the Danish Basin. The Upper Pliensbachian – Lower Toarcian F-III member is dominated by uniform claystones, which in places are slightly silty. Consistently high gamma-ray values typify the member, and a distinct shift to lower sonic values defines the lower boundary. The member is recognised in most wells in the Danish Basin, except for the southernmost part. 172 C hr on os tr at ig ra ph y Fo rm at io n Li th ol og y -65 90 Haldager-1 Type section: Haldager Sand Formation Reference section: Flyvbjerg Formation m b. MSL 1120 1068 1275 H al da ge r Sa nd M id dl e Ju ra ss ic Fl yv bj er g U pp er Ju ra ss ic Bø rg lu m Fj er ri ts le v Lo w er Ju ra ss ic ft b.KB 4300 4198.5 4100 4000 3900 3800 3693 3600 3520 3500 SP Millivolt -10 Resistivity Ohm m2/m 15 Fig. 12. Haldager-1 well, located in the Sorgenfrei–Tornquist Zone; type section for the Haldager Sand Formation, 1275–1120 m b. MSL; reference section for the Flyvbjerg Formation, 1120–1068 m b. MSL. Modified from Michelsen (1978a); for legend, see Fig. 5. Flyvbjerg-1 Type section: Flyvbjerg Formation Reference section: Haldager Sand Formation Fj er ri ts le v H al da ge r Sa nd M id dl e Ju ra ss ic L. Ju ra ss ic U pp er Ju ra ss ic Fl yv bj er g Bø rg lu m Li th ol og y 10 150 -5 10 904 944 998 m b. MSL 951 991 1045 1050 1000 m b. KB C hr on os tr at ig ra ph y Fo rm at io n Resistivity Ohm m2/m SP Millivolt Fig. 13. Flyvbjerg-1 well, located in the Sorgenfrei–Tornquist Zone; type section for the Flyvbjerg Formation, 944–904 m b. MSL; ref- erence section for the Haldager Sand Formation, 998–944 m b. MSL. Modified from Michelsen (1978a); for legend, see Fig. 5. The Upper Toarcian – Lower Aalenian F-IV member consists of uniform claystones with varying silt con- tent, interbedded with sandstones that are locally car- bonate-cemented. The proportion of sandstone, locally with organic debris and coal fragments, increases in the Fjerritslev Trough and on the Skagerrak–Kattegat Platform. The member is characterised by a distinct shift to higher sonic values at the base. The presence of sandstone intercalations is identified by gamma-ray lows, and carbonate-cemented levels are reflected by high sonic peaks. The top of the member coincides with the top of the formation. The member is recog- nised in the north-eastern part of the Danish Basin, the Sorgenfrei–Tornquist Zone and the Skagerrak–Kattegat Platform. Haldager Sand Formation History. The Haldager Formation was defined by Larsen (1966), and amended by Michelsen (1978a) who sub- divided the formation into two members, the lower of which was defined as the Haldager Sand Member. The member was later given the rank of formation, and the Haldager Formation was abandoned (Michelsen 1989a). Type section. The Haldager-1 well, 4198.5–3693 ft b. KB, 1275–1120 m b. MSL in the Sorgenfrei–Tornquist Zone is defined as the type section (Fig. 12; Larsen 1966; Michelsen 1978a). Reference section. The Flyvbjerg-1 well, 1045–991 m b. KB, 998–944 m b. MSL in the Sorgenfrei–Tornquist Zone (Fig. 13; Michelsen 1978a). Lithology and environment. The formation consists of thick beds of light olive-grey, fine- to coarse-grained occasionally pebbly sandstones and siltstones interbed- ded with claystones and thin coal beds. The south- western part of the basin and the Skagerrak–Kattegat Platform is dominated by sandstones deposited in braided rivers and lacustrine mudstones (Koch 1983). Shallow marine sandstones, estuarine sandstones and mudstones, and marine mudstones occur in the Sorgen- frei–Tornquist Zone (Nielsen 2003, this volume). Log characteristics. The dominance of thick sandstones (low gamma-ray values) interbedded with thin clay- stones (high gamma-ray spikes) yields a characteristic blocky pattern which is replicated on the SP and resis- tivity curves (Figs 12, 13). Lower boundary. The lower boundary is defined at an abrupt change from high to low gamma-ray values and the corresponding shift of the SP log, indicating the abrupt change from the claystones of the Fjerritslev Formation to the sandstones of the Haldager Sand Formation (Figs 12, 13). A regional unconformity sep- arates the two formations. Distribution and thickness. The formation is distributed in the central and northern part of the Danish Basin, 173 C hr on os tr at ig ra ph y Fo rm at io n Li th ol og y 10 12 -5 22 ft b. KB 1665 1800 1900 1700 2000 2104 2130 2200 2302 Frederikshavn-1 Type section: Frederikshavn Formation Reference section: Flyvbjerg Formation Børglum Formation Fl yv bj er g Ve ds . L. C r. Fr ed er ik sh av n U pp er Ju ra ss ic – L ow er C re ta ce ou s B. 495 636 689 629 m b. MSL M .J. H .S . Resistivity Ohm m2/m SP Millivolt Fig. 14. Frederikshavn-1 well, located on the Skagerrak–Kattegat Platform; type section for the Frederikshavn Formation, 629–495 m b. MSL; reference section of the Flyvbjerg Formation, 689–636 m b. MSL and Børglum Formation, 636–629 m b. MSL. B., Børglum; Veds., Vedsted; H.S., Haldager Sand; M.J., Middle Jurassic; L.Cr., Lower Cretaceous. Modified from Michelsen (1978a); for legend, see Fig. 5. in the Sorgenfrei–Tornquist Zone and on the Skagerrak– Kattegat Platform. Maximum thicknesses of more than 150 m are found in the Sorgenfrei–Tornquist Zone (including the type section), and a significant thinning is seen south-west and north-east of this zone. Age. The formation is generally referred to the Bajocian– Bathonian although the age is poorly constrained; in the Sorgenfrei–Tornquist Zone, the formation is proba- bly of Aalenian–Callovian age although may extend into the Early Oxfordian (Michelsen & Nielsen 1991; Poulsen 1996; see discussion in Nielsen 2003, this volume). Flyvbjerg Formation History. The Haldager Formation was defined by Larsen (1966). Michelsen (1978a) amended and subdivided the formation into two members, the upper of which was defined as the Flyvbjerg Member. The member was later elevated to formation rank, and the Haldager Formation was abandoned (Michelsen 1989a). Type section. The Flyvbjerg-1 well, 991–951 m b. KB, 944–904 m b. MSL, in the Sorgenfrei–Tornquist Zone (Fig. 13; Michelsen 1978a). Reference sections. The Haldager-1 well in the Sorgenfrei– Tornquist Zone, 3693–3520 ft b. KB, 1120–1068 m b. MSL, and the Frederikshavn-1 well on the Skagerrak– Kattegat Platform, 2302–2130 ft b. KB, 689–636 m b. MSL (Figs 12, 14; Michelsen 1978a). Lithology and environment. The basal part of the for- mation consists of light olive-grey sandstones and silt- stones with traces of coals and locally abundant roots. It is overlain by olive-grey and greyish to olive-black claystones, interbedded with fossiliferous, calcareous sandstones. The upper part of the formation consists of fine-grained sandstones, which are highly calcareous and contain glauconite and traces of pyrite. The sed- iments were mainly deposited in a shallow marine to offshore marine environment. Log characteristics. The formation is characterised by relatively high gamma-ray values and corresponding SP values at the base, decreasing to low values at the top, and by variable sonic velocities. The overall decrease upwards in gamma-ray values is interrupted by thin inter- vals with low values, reflecting interbedded claystones and thin sandstones. In most wells, a thick sandstone bed represented by a blocky gamma-ray low and a corre- sponding SP value occurs at the top of the formation. Lower boundary. The lower boundary is defined at an abrupt change from low to higher gamma-ray values and a corresponding shift of the SP curve, indicating the change from the sandstones of the Haldager Sand Formation to the more fine-grained clay-rich deposits lowermost in the Flyvbjerg Formation (Fig. 13). In some wells, a more gradual increase in gamma-ray values characterises the uppermost Haldager Sand Formation, while the lower part of the Flyvbjerg Formation shows a few gamma-ray lows representing thin sandstone beds (Haldager-1 and Børglum-1; Fig. 12, see also Fig. 30). Distribution and thickness. The formation occurs in the central part and along the northern margin of the Danish Basin, in the Sorgenfrei–Tornquist Zone and on the Skagerrak–Kattegat Platform forming a north-eastwards thickening wedge. Maximum thicknesses of over 50 m are found in the Sorgenfrei–Tornquist Zone and on the Skagerrak–Kattegat Platform. Age. Late Early or early Middle Oxfordian to Late Kimmeridgian. Both the base and top of the Flyvbjerg Formation are diachronous, younging towards the north- eastern margin of the Danish Basin (Poulsen 1996). Børglum Formation History. The Børglum Formation was defined by Larsen (1966) but was subsequently demoted to the Børglum Member of the Bream Formation by Michelsen (1978a). The Bream Formation was abandoned by Vollset & Doré (1984), and the Børglum Member was later restored to the rank of formation (Michelsen 1989a). Type section. The Børglum-1 well in the Sorgenfrei– Tornquist Zone, 3323–3213 ft b. KB, 990–957 m b. MSL (Fig. 15; Larsen 1966; Michelsen 1978a). Reference section. The Frederikshavn-1 well on the Skagerrak–Kattegat Platform, 2130–2104 ft b. KB, 636–629 m b. MSL (Fig. 14; Michelsen 1978a). Lithology and environment. The formation consists of a relatively uniform succession of olive to blackish grey, slightly calcareous, homogeneous claystones with vary- ing contents of silt, mica and pyrite. The sediments were mainly deposited in an offshore marine environment. 174 Log characteristics. The formation is characterised by rather uniform high gamma-ray values and low sonic velocities. The sonic curve is rather constant in the lower part of the formation, but becomes serrated in the upper part. Lower boundary. The lower boundary is located at the top of the thick sandstone beds uppermost in the Flyvbjerg Formation (Fig. 14) or slightly above (Fig. 15). The bound- ary is defined by an abrupt shift from high resistivities and high sonic velocities to lower resistivities and sonic velocities above the boundary (see Fig. 30). Distribution and thickness. The formation is distributed over most of the Danish Basin. Maximum thicknesses, up to 300 m, are found in wells located close to the Fjerritslev Fault (e.g. J-1, see Fig. 32); a pronounced thin- ning occurs towards the north-east, south and south- west. Age. Earliest Early Kimmeridgian – Early Volgian over most of the basin (Poulsen 1996). The base and top of the formation are diachronous; the base youngs towards the north-eastern margin of the basin (Poulsen 1996), and towards the west the upper part of the formation is contemporaneous with the Frederikshavn Formation and extends into the Ryazanian (Michelsen 1989a). Frederikshavn Formation History. The Frederikshavn and Skagen Formations were defined by Larsen (1966) but were subsequently combined and reduced in rank to the Frederikshavn Member of the Bream Formation by Michelsen (1978a). The Bream Formation was abandoned by Vollset & Doré (1984), and the Frederikshavn Member was sub- sequently elevated to the rank of formation (Michelsen 1989a). Type section. The Frederikshavn-1 well, 2104–1665 ft b. KB, 629–495 m b. MSL, on the Skagerrak–Kattegat Platform (Fig. 14; Larsen 1966; Michelsen 1978a). Reference section. The Børglum-1 well, 3213–2478.5 ft b. KB, 957–733 m b. MSL, in the Sorgenfrei–Tornquist Zone (Fig. 15; Michelsen 1978a). Lithology and environment. The formation consists of siltstones and fine-grained sandstones, greenish grey, light olive-grey and olive-black, slightly calcareous, in places with micro-lignite. The siltstones and sandstones are interbedded with thin silty, greyish black, and cal- careous claystones and form 2–3 generally coarsening- upwards units separated by claystone-dominated intervals. Glauconite and fragments of bivalves and 175 Li th ol og y Børglum-1 Type section: Børglum Formation Reference section: Frederikshavn Formation -10 15 Fr ed er ik sh av n U pp er Ju ra ss ic – L ow er C re ta ce ou s 10 SP Millivolt 990 957 733 C hr on os tr at ig ra ph y Fo rm at io n m b. MSLft b. KB 150 Bø rg lu m Fl yv bj . Ve ds . L. C r. 2600 2478.5 2700 2800 2900 3000 3100 3200 3213 3300 3323 Resistivity Ohm m2/m Fig. 15. Børglum-1 well, located in the Sorgenfrei–Tornquist Zone; type section for the Børglum Formation, 957–990 m b. MSL. Flybj., Flyvbjerg; Veds., Vedsted; L.Cr., Lower Cretaceous. Modified from Michelsen (1978a); for legend, see Fig. 5. ammonites are common in the lower and upper parts of the formation (Sorgenfrei & Buch 1964; Larsen 1966; Birkelund & Pedersen 1980). In the Sorgenfrei–Tornquist Zone, sandstones are particularly common in the mid- dle part of the formation. Close to the north-eastern basin margin, coal beds a few centimetres thick occur in the upper part of the formation. Deposition mainly took place in a shallow marine to offshore environment. Non-marine conditions prevailed on the Skagerrak- Kattegat Platform and locally in the Sorgenfrei–Tornquist Zone during deposition of parts of the middle coars- ening-upwards unit and the lower part of the upper unit (Larsen 1966; Michelsen 1978a). Log characteristics. The 2–3 coarsening-upwards suc- cessions of siltstones and sandstones are reflected by repeated decreasing-upwards trends on the gamma-ray curve, and corresponding features on the SP curve (Figs 14, 15). The uppermost part of the formation is char- acterised by a gradual increase in gamma-ray values. The sonic velocities are high and rather constant, except for a slight decrease in values at the two clayey inter- vals that separate the coarsening-upwards successions (Hyllebjerg-1; see Fig. 30). Boundaries. The lower boundary is defined by a decrease upwards in gamma-ray values and corre- sponding features of the SP curve, reflecting the tran- sition from the claystones of the underlying Børglum Formation to the more coarse-grained Frederikshavn Formation. The boundary may be best defined by a minor shift from low to higher sonic velocities (Hylle- bjerg-1, see Fig. 30). The upper boundary with the claystones of the Lower Cretaceous Vedsted Formation is also a gradual transi- tion. An increase upwards in gamma-ray values and a shift to lower sonic velocities characterises the bound- ary in the central part of the Danish Basin (e.g. Hylle- bjerg-1). In the Sorgenfrei–Tornquist Zone, a more abrupt boundary is indicated by the SP and resistivity logs (Figs 14, 15). Distribution and thickness. The formation occurs in the eastern part of the Danish Basin. Maximum thicknesses of more than 230 m are found in wells from the Sor- genfrei–Tornquist Zone (e.g. Haldager-1, Terne-1). Relatively thick and coarse-grained successions are also found in the Børglum-1, Frederikshavn-1, Rønde-1 and Horsens-1 wells. The thinner and more fine-grained successions found in the Hyllebjerg-1, Skive-1, Kvols-1, Rødding-1 and Oddesund-1 wells, located in the cen- tral part of the basin, seem to mark the westernmost extent of the Frederikshavn Formation. The succession previously assigned to the formation by Michelsen (1978a) in the Nøvling-1, J-1, F-1 and K-1 wells further to the west is fine-grained, mainly clay-dominated, and is best referred to the Børglum Formation (Michelsen 1989a). Age. Volgian–Ryazanian. The formation is time-equiv- alent with the upper part of the Børglum Formation in the central and western parts of the Norwegian–Danish Basin (Michelsen 1989a; Poulsen 1996). Danish Central Graben The Jurassic succession of the Danish Central Graben is referred to seven formations, the Fjerritslev, Bryne, Lulu, Middle Graben, Lola, Heno and Farsund Formations, of which the Bryne Formation is revised and the Lulu Formation is new (Fig. 2). Two new mem- bers are defined in the Heno Formation and the infor- mal ‘hot unit’ in the Farsund Formation is defined as the Bo Member. Fjerritslev Formation History, type and reference sections. The formation was defined from the Danish Basin (Larsen, 1966; Michelsen 1978a) and a full description is given above. Jensen et al. (1986) defined a reference section (O-1 well) in the Danish Central Graben. Lithology and environment. The formation is domi- nated by dark grey, slightly calcareous claystones, as seen in the Danish Basin (see above). In contrast to the latter area, however, the lower levels of the formation in the Central Graben are notably more carbonate-rich, with interbeds of calcareous silty claystone and soft marlstone (Jensen et al. 1986). Log characteristics. Uniform high gamma-ray values and a uniform sonic velocity pattern characterise the formation; a more erratic sonic log in the lower levels reflects the presence of calcite-cemented silty claystones and thin soft marlstone beds. Lower boundary. In the Danish Central Graben, the for- mation overlies the Triassic clay-dominated Winterton Formation, which is characterised by very low sonic 176 velocities and high gamma-ray values. The boundary is defined by a marked change from high to lower gamma-ray values and very low to higher sonic values. Distribution and thickness. The formation has only been found in wells in the southern part of the Danish Central Graben, where it forms an erosional remnant due to extensive mid-Jurassic erosion. The thickness of the formation is thus very variable; the maximum mea- sured thickness of 257 m occurs in the Edna-1 well. Age. In the M-8, O-1 and U-1 wells, the formation is dated to the Hettangian–Sinemurian (Michelsen 1978a); in the Edna-1 and Deep Gorm-1 wells, the upper part of the formation may extend up into the Lower Pliens- bachian (Andsbjerg & Dybkjær 2003, this volume). Bryne Formation revised formation History. The Bryne Formation was defined in the Norwegian Central Graben (Vollset & Doré 1984) and was described from the Danish Central Graben by Jensen et al. (1986). The deposits in the southern Danish Central Graben referred to the Lower Graben Sand Formation by Jensen et al. (1986) are here included in the Bryne Formation. The upper part of the Bryne Formation as adopted by Jensen et al. (1986) is here included in the new Lulu Formation. With this revision, usage of the Bryne Formation is in accordance with the original definition of the Bryne Formation by Vollset & Doré (1984). Type section. The Norwegian 9/4-3 well, 2613–2507.5 m b. KB (Vollset & Doré 1984, fig. 29). Reference section. The Danish Lulu-1 well, 12134–11712 ft b. KB, 3669–3540 m b. MSL, was selected as the Danish reference section by Jensen et al. (1986). However, new correlations show that much of the lower part of the formation is missing in Lulu-1, probably due to the location of the well on a salt structure that was active in Jurassic times. The more complete succession in the Danish West Lulu-1 well, 12602–11795 ft b. KB, 3801– 3555 m b. MSL, is selected as the new reference sec- tion (Fig. 16). Lithology and environment. The Bryne Formation is characterised by laterally extensive sandstone units sep- arated by thick successions of interbedded sandstones and mudstones (Andsbjerg 1997). The laterally exten- sive sandstone bodies vary from fining-upwards or blocky units, 2–10 m thick, to more than 20 m thick amalgamated units (e.g. uppermost 20 m of Bryne Formation, Fig. 16B). The sandstones are light to medium grey and yellowish brown, very fine- to medium-grained. They are characterised by trough and planar cross-bed- ding, and ripple cross-lamination. Contorted bedding and other soft sediment deformation structures are also common. Associations of sedimentary structures that may indicate tidal influence occur in the uppermost laterally extensive sandstones, suggesting an origin as estuarine channel sands. Sandstones in the lower and middle parts of the formation were deposited in fluvial channels (Andsbjerg 2003, this volume). The successions that separate major sandstone units are fining-upwards or fining- to coarsening-upwards and commonly 20–60 m thick. They consist of grey to brown very fine- to medium-grained sandstones interbedded with siltstones, claystones and occasional thin coals. The sandstone beds are generally less than 2 m thick and show cross- bedding, current ripple cross-lamination, climbing rip- ple cross-lamination, parallel lamination and chaotic bedding. Dark to light grey mudstone and siltstone beds may be up to 5 m thick, and most commonly are structureless or show parallel lamination. The fine- grained deposits represent a variety of overbank envi- ronments ranging from crevasse splays and crevasse channels through distal levee to ponds and lakes. Abundant root traces, thin coals and strongly mottled silt- and mudstones testify to vegetated substrates and soil development. Log characteristics. Most of the lithologies are easily dis- tinguished on gamma-ray logs. However, coals are best recognised on the sonic log as distinct low-velocity peaks while high-velocity peaks reflect carbonate-cemented 177 Following two pages: Fig. 16. A: West Lulu-1 well, located in the Søgne Basin, Danish Central Graben; new reference section for the Bryne Formation, 3801–3555 m b. MSL. Cored section indicated by solid bar and illustrated in B. Note that the boundary between the Lulu and Lola Formations in this well is marked by the change from a coal bed to marine mudstones and is thought to be a faulted contact (Fig. 16B; Andsbjerg 2003, this volume). For legend, see Fig. 5. 178 M id dl e Ju ra ss ic U pp er Ju ra ss ic Br yn e Lu lu Lo la Br yn e M id dl e Ju ra ss ic Tr ia ss ic West Lulu-1 Reference section: Bryne Formation 0 180 0 180 160 0160 0 11600 11687 11700 3523 11795 11800 11900 12000 12100 3555 C hr on os tr at . Fo rm at io n 3801 ft b. KB m b. MSL ft b. KB m b. MSL C hr on os tr at . Fo rm at io n 12200 12300 12400 12500 12602 12600 GR API Sonic msec/ft GR API Sonic msec/ft A 179 Br yn e Fo rm at io n M id dl e Ju ra ss ic Br yn e Fo rm at io n Lo la F m Lu lu F or m at io n 0 180 West Lulu-1 0 180 Normal fault SiClay Sand Gr SiClay Sand Gr GR API GR API M id dl e Ju ra ss ic 11800 11900 11795 11700 ft b. KB 12000 12100 ft b. KB B deposits. The gamma-ray logs typically show low val- ues for the sandstones. Channel sandstones may show gamma-ray values that increase upwards, indicative of fining-upwards units (e.g. Fig. 16B, 12035–12016 ft b. KB), or box-like, uniformly low values (e.g. Fig. 16B, 11840–11797 ft b. KB). The intermediate to high gamma- ray values of floodplain deposits commonly show increasing-upwards (e.g. Fig. 16A, 12512–12437 ft b. KB) or increasing-upwards to decreasing-upwards (e.g. Fig. 16A, 12200–12165 ft b. KB) gamma-ray patterns. Lower boundary. The Bryne Formation unconformably overlies claystones of the Lower Jurassic Fjerritslev Formation in the southern part and Triassic and Permian rocks in the northern part of the Danish Central Graben. The boundary is marked by a distinct increase in gamma- ray values where the Bryne Formation overlies Permian salt, whereas the boundary elsewhere may vary from highly distinct to indistinct depending upon the under- lying lithology. Distribution and thickness. The formation is found in the Søgne Basin, in the Tail End Graben, in the Salt Dome Province and along the south-eastern fringe of the Heno Plateau. The thickness of the Bryne Formation is very variable, varying from a few tens of metres in marginal parts of the basin to commonly more than 200 m. The thickest section, 289 m, was recorded in the Amalie-1 well in the southernmost Søgne Basin (base not pene- trated). Age. The relatively sparse palynological data suggest that the lower part of the Bryne Formation is Aalenian or earliest Bajocian in age. Based on the occurrence of the dinoflagellate cyst Impletosphaeridium varispinosum in the uppermost estuarine sandstones of the Bryne Formation in the West Lulu-1 and West Lulu-3 wells, the top of the Bryne Formation is dated to the latest Bath- onian – earliest Callovian (Andsbjerg & Dybkjær 2003, this volume). Remarks. The Middle Jurassic deposits of the Søgne Basin in the northern part of the Danish Central Graben were previously referred to the Bryne Formation (Jensen et al. 1986). The uppermost part of the succession con- sists of interfingering shallow marine and paralic sand- stones and mudstones; this uppermost unit differs from the remainder of the Middle Jurassic succession both in terms of lithology and depositional environment. In accordance with the original definition of the Bryne Formation (Vollset & Doré 1984), these deposits are no longer assigned to the Bryne Formation but are referred to the new Lulu Formation (defined below), equivalent to the Sandnes Formation in the Norwegian sector. The Middle Jurassic succession in the southern part of the Danish Central Graben was referred to the Central Graben Group (the Central Graben Subgroup of van Adrichem Boogaert & Kouwe 1994) by Jensen et al. (1986) as well control was missing in the central part of the graben. However, with more well data available it has become clear that the lower part of the Middle Jurassic succession in the southern Danish Central Graben shows greater affinity to the Bryne Formation than to the Lower Graben Formation as originally defined by NAM & RGD (1980) and modified by van Adrichem Boogaert & Kouwe (1994). In core sections, it is evi- dent that with respect to both lithology and deposi- tional environment this part of the succession cannot be distinguished from that of the Bryne Formation of the northern Danish Central Graben. It is thus proposed to include the Middle Jurassic deposits from the south- ern Central Graben in the Bryne Formation. Lulu Formation new formation History. The succession that makes up the new Lulu Formation was included in the upper part of the Bryne Formation by Jensen et al. (1986). Vollset & Doré (1984) defined the equivalent succession in the Norwegian sector, penetrated in the Fiskebank Basin and the Egersund Subbasin, as a separate formation, the Sandnes Formation. This unit is missing from most of the more than 100 km wide area that separates the southern part of the Søgne Basin from the central Fiskebank Basin. It is thus preferred not to adopt the term Sandnes Formation for the unit in the southern Søgne Basin and northern Tail End Graben. Name. From the Lulu structure in the Søgne Basin which was the target for the Lulu-1 well (Fig. 1C). Type section. The Danish Lulu-1 well, 11813–11715 ft b. KB, 3571–3541 m b. MSL (Fig. 17). Reference section. The Danish West Lulu-3 well, 12035– 11865 ft b. KB, 3633–3581 m b. MSL (Fig. 18). Lithology and environment. The Lulu Formation is char- acterised by coarsening-upwards wedges of shallow 180 marine sandstones, which interfinger with paralic sand- stone successions towards the western basin margin. Laterally extensive coaly claystones with coal seams, up to 5 m thick, are characteristic of the formation, in particular the lower part (e.g. Fig. 18, West Lulu-3, 12035–12009 ft b. KB). The shallow marine wedges in the deeper parts of the Søgne Basin and northern Tail End Graben each show an 8–12 m thick coarsening- upwards succession (Fig. 17, Lulu-1, 11794–11773 and 11757–11729 ft b. KB). These successions are domi- nated by hummocky cross-stratified sandstones with interbeds of mudstone and siltstone, overlain by grad- ually thicker and more coarse-grained sandstone beds showing swaley cross-stratification and planar cross- bedding. The sandstones are light grey to greyish brown and most commonly well-sorted, very fine- to 181 ft b. KB ft b. KB m b. MSL C hr on os tr at ig ra ph y Fo rm at io n Li th ol og y Lulu-1 Type section: Lulu Formation Clay Si Sand Gr 160 Sonic msec/ft 40 U .J. Lu lu Lo la Br yn e M id dl e Ju ra ss ic 0 GR API 125 0 GR API 180 11700 11715 11800 11813 11900 11750 11800 11850 3541 3571 Fig. 17. Lulu-1 well, located in the Søgne Basin, Danish Central Graben; type section for the new Lulu Formation, 3571–3541 m b. MSL. Cored section indicated by solid bar and illustrated in the core log. U.J., Upper Jurassic. For legend, see Fig. 5. 182 ft b. KB m b. MSL ft b. KB M id dl e Ju ra ss ic Br yn e Lu lu Lo la Clay Si Sand Gr C hr on os tr at ig ra ph y Fo rm at io n GR API GR API Sonic msec/ft Li th ol og y West Lulu-3 Reference section: Lulu Formation 0 150 160 40 0 75 3633 358111865 11900 11800 12000 12035 12100 11900 12000 Fig. 18. West Lulu-3, located in the Søgne Basin, Danish Central Graben; reference section for the new Lulu Formation, 3633–3581 m b. MSL. Cored section indicated by solid bar and illustrated in the core log. For legend, see Fig. 5. fine-grained. The coarsening-upwards successions are interpreted as prograding shoreface deposits. Along the western margin of the Søgne Basin and Tail End Graben, the succession is dominated by back- barrier deposits, consisting mainly of sandstones with evidence for strong tidal influence. The sandstones are commonly moderately to well-sorted, dark and brown- ish grey to light brown, and show cross-bedding, current and climbing ripple-lamination and abundant double mud-drapes and bundled foresets. Bioturbated intervals occur commonly (Fig. 18, 11980 ft b. KB in West Lulu-3). Only thin units of shallow marine sand- stone and mudstone are present in the paralic wedges (Fig. 18, 11904 ft b. KB. in West Lulu-3; Andsbjerg 2003, this volume). The coal seams, 0.1 – > 1 m thick with a cumulative thickness of up to 5 m, were de- posited on a coastal plain during relative sea-level rise (Petersen & Andsbjerg 1996). Log characteristics. The heterogeneous lithologies are reflected by variable log patterns. Two 8–12 m thick units showing gamma-ray values that decrease upwards in the deeper parts of the Søgne Basin and Tail End Graben represent coarsening-upwards successions of shoreface and beach sandstones. In the western parts of the Søgne Basin and Tail End Graben, 0.5–5 m thick units show gamma-ray values that are consistently low or increase slightly upwards; these units represent channel fills. Units with high or decreasing-upwards gamma-ray val- ues between the channel-fills represent lagoonal, tidal flat and bay-head delta environments (Andsbjerg 2003, this volume). Sonic log intervals 0.5–5 m thick that show very low sonic velocities (usually corresponding to low gamma-ray readings) represent coal deposits. Lower boundary. The formation conformably overlies the Bryne Formation over most of the Danish Central Graben. The lower boundary is placed at the base of the lowermost laterally persistent major coal bed, or at the base of a correlative unit of interbedded sandstones and organic-rich mudstones with thin coals, as seen in Amalie-1 and Lulu-1. Distribution and thickness. The Lulu Formation is widely distributed in the Søgne Basin and the northern Tail End Graben. In the southern part of the Tail End Graben and in the northern part of the Salt Dome Province, the Lulu Formation is replaced by the partly time-equiva- lent, mudstone-dominated Middle Graben Formation (NAM & RGD 1980; Jensen et al. 1986; van Adrichem Boogaert & Kouwe 1994). The thickness of the Lulu Formation varies from 30–60 m. Age. The formation is of Early–Late Callovian age. This is indicated by the occurrence of the latest Bathonian – earliest Callovian dinoflagellate cyst Impletosphaeri- dium varispinosum in the uppermost estuarine sand- stones of the Bryne Formation in the West Lulu-3 well and the appearance of the Late Callovian – Middle Oxfordian dinoflagellate cyst Liesbergia scarburghen- sis in the basal part of the Lola Formation in several wells (Andsbjerg & Dybkjær 2003, this volume). Middle Graben Formation History. The Middle Graben Shale Formation was defined in the Dutch North Sea sector by NAM & RGD (1980), and extended to the Danish Central Graben by Jensen et al. (1986). The name was modified to the Middle Graben Formation by van Adrichem Boogaert & Kouwe (1994). 183 C hr on os tr at ig ra ph y Fo rm at io n GR API Sonic msec/ft Li th ol og y m b. MSLft b. KB U-1 Reference section: Middle Graben Formation 0 180 180 40 10853 10668 10700 10800C al lo vi an M id dl e G ra be n Br yn e Lo la C al lo v. (? ) 3223 3279 Fig. 19. U-1 well, located in the southern part of the Danish Central Graben; reference section for the Middle Graben Formation, 3279–3223 m b. MSL. Callov., Callovian. Modified from Jensen et al. (1986); for legend, see Fig. 5. Type section. The Dutch F03-03 well, 3090–2670 m b. KB (NAM & RGD 1980). Reference section. The U-1 well, 10853–10668 ft b. KB, 3279–3223 m b. MSL, is used as a reference section in the Danish sector (Fig. 19; Jensen et al. 1986). Lithology and environment. The formation consists of dark brownish grey claystones interbedded with silt- stones and rare sandstone beds. Coal beds, locally sev- eral metres thick, may occur in the lower part of the formation. Deposition took place in swampy environ- ments, probably interdistributary bays or coastal lagoons. Log characteristics. The gamma-ray log commonly shows a response slightly lower than the overlying Lola Formation but considerably higher than that of the underlying Bryne Formation. However, some coal beds may be characterised by a very low gamma-ray response and a low sonic velocity. Lower boundary. The Middle Graben Formation in the Danish Central Graben overlies the Bryne Formation. The boundary is placed at the base of a prominent coal bed, usually the lowermost of a series of coal beds and commonly characterised by a low gamma-ray and a low sonic velocity log response. Distribution and thickness. The formation extends from the southern North Sea into the Salt Dome Province of the Danish Central Graben. It is not recognised north of the Salt Dome Province, where the Bryne Formation is overlain by the Lulu Formation in the Danish sector. The thickness of the Middle Graben Formation ranges from 15–56 m in the Danish sector. Age. Based on datings of the underlying deposits belong- ing to the Bryne Formation by Hoelstad (1986a), the formation is not older than Late Callovian. There is no good data to constrain the youngest age of the forma- tion, although it probably ranges into the Early or Middle Oxfordian, based on the occurrence of the dinoflagel- late cyst Liesbergia scarburghensis. The formation may be slightly diachronous, younging southwards to the 184 L ol a C hr on os tr at . Fo rm at io n Li th ol og y 0 180 180 40 U-1 Type section: Lola Formation ft b. KB 2884 O xf or di an C al lo vi an M . G ra be n 3223 m b. MSL 10600 10500 10400 10300 10200 10100 10000 9900 9800 9700 9600 9500 9555 9400 10668 10700 K im m er id gi an Fa rs un d GR API Sonic msec/ft Fig. 20. U-1 well, located in the southern part of the Danish Central Graben; type section for the Lola Formation, 3223–2884 m b. MSL. M. Graben, Middle Graben. Modified from Jensen et al. (1986); for legend, see Fig. 5. Dutch North Sea area, where it has an Early–Middle Oxfordian age. Lola Formation History. The Lola Formation was defined by Jensen et al. (1986). Type section. The U-1 well, 10668–9555 ft b. KB, 3223–2884 m b. MSL, in the Danish Central Graben (Fig. 20; Jensen et al. 1986). Lithology and environment. The formation consists of dark olive-grey to grey claystones, with organic mate- rial, mainly of terrestrial origin. Deposition took place in a low energy, offshore open marine environment. Log characteristics. The formation is characterised by fairly constant, high gamma-ray readings and relatively low sonic velocities. A few high velocity peaks corre- sponding to dolomite or limestone beds may occur (Fig. 20). Lower boundary. The formation overlies the Lulu For- mation in the northern part of the Danish Central Graben and the Middle Graben Formation in the southern part of the Danish Central Graben. On structurally high areas along the western margin of the Danish Central Graben and on local highs, the formation rests unconformably on Triassic or older strata. The lower boundary with the Lulu Formation is placed at an abrupt, fairly large increase in gamma-ray values and a minor decrease in sonic velocity (Figs 17, 18). The boundary with the Middle Graben Formation shows a sudden and fairly large drop in sonic velocity (Figs 19, 20). Distribution and thickness. The formation is present in the eastern and southern parts of the Danish Central Graben. It thins west of the Tail End Graben and prob- ably continues southwards and grades into the lower part of the Kimmeridge Clay Formation in the Dutch North Sea area (Michelsen & Wong 1991). Maximum thicknesses of c. 1000 m occur in the Tail End Graben. Age. Late Callovian/Early Oxfordian – Late Kimmerid- gian. The base of the formation is considered diachro- nous, younging southwards. The uppermost part is not younger than Early Kimmeridgian in the Elly-2 well in the Tail End Graben (Andsbjerg & Dybkjær 2003, this volume). Heno Formation revised formation History. The Heno Formation was defined by Jensen et al. (1986) to include Upper Jurassic shallow marine sandstones in the Danish Central Graben. Wells drilled in recent years have demonstrated that sandstones of the Heno Formation interfinger with claystones of the Lola Formation. In the southern part of the Feda Graben, two sandstone units are separated by claystones and south and east of the Feda Graben, on the Heno and Gertrud Plateaus, clayey sandstones separate two dis- crete sandstone units. The type section of the Heno Formation, as defined by Jensen et al. (1986), com- prises only the upper sandstone unit (the new Ravn Member). The informal terms ‘basal sandstone unit’ and ‘Heno Equivalent’ have been used for the lower sand- stone unit by Bergan et al. (1989), Michelsen & Wong (1991), Nielsen & Japsen (1991), Söderström et al. (1991), 185 C hr on os tr at ig ra ph y Fo rm at io n M em be r GR API Sonic msec/ftLi th ol og y Fa rs un d m b. MSLft b. KB 13520 13500 13400 13820 13800 13700 13600 13839 4184 4178 4087 W-1 Type section: Heno Formation K im m er id gi an Permian H en o R av n Lola 0 150 140 40 Fig. 21. W-1 well, located on the Heno Plateau, Danish Central Graben; type section for the Heno Formation, 4178–4087 m b. MSL. Modified from Jensen et al. (1986); for legend, see Fig. 5. 186 C hr on os tr at ig ra ph y Fo rm at io n M em be r GR API GR API Sonic msec/ft Li th ol og y Fa rs un d 15500 15400 m b. MSLft b. KB ft b. KB 15526 4694 15600 15756 16038 15700 15800 15900 16000 16100 16200 16300 16200 16300 16328 4764 4850 4938 Gert-1 Reference section: Heno Formation Type section: Gert Member K im m er id gi an Permian Lo la H en o G er t H en o R av n 0 150 140 40 Clay Si Sand Gr 0 75 Fig. 22. Gert-1 well, located in the northern part of the Danish Central Graben; new reference section for the Heno Formation and type section for the new Gert Member (4938–4850 m b. MSL). Cored section indicated by solid bar and illustrated by core log. Note that this well also encountered the new Ravn Member (4764–4694 m b. MSL) separated from the Gert Member by 86 m of Lola Formation mudstones. For legend, see Fig. 5. Damtoft et al. (1992), Michelsen et al. (1992), Johan- nessen & Andsbjerg (1993) and Johannessen et al. (1996). The revised Heno Formation, as proposed here, includes both sandstone units that are given the rank of members, the Gert and Ravn Members. Type section. The W-1 well, 13820–13520 ft b. KB, 4178–4087 m b. MSL, in the Central Graben, Heno Plateau, was selected as type section by Jensen et al. (1986; Fig. 21). Reference section. The Gert-1 well in the easternmost part of the Feda Graben bordering the Gertrud Plateau, and the Ravn-1 well on the Heno Plateau are here selected as reference sections for the Heno Formation (Figs 22, 23). In the Gert-1 well, the lower sandstones, 16328–16038 ft b. KB, 4938–4850 m b. MSL, are referred to the Gert Member and the upper sandstones, 15756–15526 ft b. KB, 4764–4694 m b. MSL, are referred to the Ravn Member. Claystones of the Lola Formation lie between the two members. In the Ravn-1 well, the sandstone interval 4173–4067 m b. KB, 4132–4026 m b. MSL, is referred to the Ravn Member. Lithology and environment. The formation consists of pre- dominantly grey to white very fine- to fine-grained sand- stones and minor siltstones interbedded with claystones and coal beds. Conglomerate beds, 0.08–2 m thick, are present on plateau areas. The Gert Member was deposited in a back-barrier and marine shoreface environment while 187 C hr on os tr at ig ra ph y Fo rm at io n M em be r Li th ol og y m b. MSLm b. KB m b. KB 4067 4026 4173 4200 4100 4100 4150 4132 GR API GR API Sonic msec/ft Ravn-1 Reference section: Heno Formation Type section: Ravn Member K im m er id gi an O xf ? R av n Fa rs un d Lo la H en o 0 150 140 40 500 Clay Si Sand Gr Fig. 23. Ravn-1 well, located on the Heno Plateau, Danish Central Graben; type section for the new Ravn Member (Heno Formation), 4132–4026 m b. MSL. Cored section indicated by solid bar and illustrated by core log. Oxf., Oxfordian. For legend, see Fig. 5. the Ravn Member was deposited in a marine shoreface environment (Johannessen & Andsbjerg 1993; Johannessen et al. 1996; Johannessen 1997, 2003, this volume). Log characteristics. The formation is characterised by low gamma-ray and high sonic readings representing sandstones. Some higher gamma-ray and lower sonic readings represent siltstone and claystone beds. Lower boundary. In the type section, the formation rests on Upper Oxfordian – Lower Kimmeridgian clay- stones of the Lola Formation (Andsbjerg & Dybkjær 2003, this volume). In the Gert-1 well, the base of the formation is defined by the base of the lowermost sand- stones and claystones of the Gert Member uncon- formably overlying Lower Permian volcanic rocks. 188 C hr on os tr at ig ra ph y Fo rm at io n M em be r GR API Sonic msec/ftLi th ol og y m b. MSLm b. KB m b. KB 4940 5000 5014 4902 5041 5003 4976 4970 4980 4950 4960 0 60 GR APIJeppe-1 Reference section: Gert Member K im m er id gi an Permian R av n G er t Fa rs un d H en o 0 150 140 40 Clay Si Sand Gr Fig. 24. Jeppe-1 well, located in the northern part of the Danish Central Graben; reference section for the new Gert Member (Heno Formation), 5003–4976 m b. MSL. For legend, see Fig. 5. Distribution and thickness. The formation is present in the Feda Graben and on the Gertrud and Heno Plateaus in the northern part of the Danish Central Graben. The maximum thickness of the formation on the Heno Plateau is 106 m in the Ravn-1 well. In the Feda Graben, the total thickness of the Heno Formation is 158 m in the Gert-1 well (Gert Member 88 m, and Ravn Member 70 m), where the formation interfingers with 86 m of claystones of the Lola Formation. Age. The formation was assigned a Middle–Late Oxfordian to Kimmeridgian age (Jensen et al. 1986), but new bio- stratigraphic data indicate a latest Early – Late Kimmerid- gian age (Andsbjerg & Dybkjær 2003, this volume). Subdivision. In the eastern part of the Feda Graben, the Gertrud Plateau and the western part of the Heno Plateau, the lower part of the formation is characterised by a serrated gamma-ray and sonic log pattern includ- ing very high gamma-ray peaks corresponding to very low sonic values, recording the presence of thin coal and clay beds within the sand-dominated section. This part of the formation is defined as the new Gert Member, described below. In the same area, the upper part of the formation is characterised by moderate to low gamma-ray and moderate to high sonic readings rep- resenting sandstone facies seen in the type section of the W-1 well. This part of the formation is defined as the new Ravn Member, described below. The type sec- tion of the Heno Formation in the W-1 well comprises only the Ravn Member (Fig. 21). Remarks. Other shallow marine sandstones somewhat younger than the Heno Formation may be present in the western part of the Danish area (Damtoft et al. 1992). A sandy siltstone succession in the Kim-1 well at the eastern margin of the Grensen Nose Basin is thought to be a distal equivalent of the Early–Middle Volgian shallow marine ‘Fife sandstones’ in the UK sec- tor (Mackertich 1996; Andsbjerg & Dybkjær 2003, this volume). Similar sandstones are expected to be present in the intervening area of the Ål and Grensen Nose Basins. Gert Member new member Name. The Gert Member is named after the Gert Ridge situated at the eastern margin of the Feda Graben. Type section. The Gert-1 well, 16328–16038 ft b. KB, 4938–4850 m b. MSL, is proposed as the type section (Fig. 22). Reference section. The Jeppe-1 well, 5041–5014 m b. KB, 5003–4976 m b. MSL, on the Gertrud Plateau, is selected as the reference section (Fig. 24). Lithology and environment. The member consists pre- dominantly of grey to white very fine- to fine-grained sandstones and minor siltstones interbedded with clay- stones and coal beds (Fig. 22). The sediments were de- posited in back-barrier and shoreface environments. Log characteristics. The lower part of the member in the type section is characterised by serrated gamma-ray and sonic log motifs. Log readings from the upper part are more uniform and smooth. The very high gamma- ray values and very low sonic values in the serrated sec- tion represent thin claystone and coal beds interbedded with sandstones. The upper part with low gamma-ray and high sonic values consists of clean sandstones with- out coal and clay beds. The uppermost part of the mem- ber is characterised by a step-wise increase in gamma-ray values and decrease in sonic values, representing a fin- ing-upwards succession from sandstones to the clay- stones of the overlying Lola Formation (Fig. 22). Boundaries. In the type and reference sections, the mem- ber rests unconformably on Lower Permian volcanic rocks. In the Gert-1 well, the upper boundary is defined at the change to the clay-dominated Lola Formation marked by a significant log shift to higher gamma-ray values and lower sonic velocities. In the Jeppe-1 well ref- erence section (Fig. 24) and the Gert-2 well, the Gert Member is directly overlain by the Ravn Member, as off- shore claystones of the Lola Formation are absent. In these wells, the top of the Gert Member is defined at the high- est gamma-ray and lowest sonic readings, defining the base of the coarsening-upwards Ravn Member. Distribution and thickness. The member is present in the northern part of the Danish Central Graben with a maximum thickness of 88 m in the Gert-1 well at the eastern margin of the Feda Graben. On the adjacent Ger- trud Plateau, the thickness of the member is reduced to 27 m in the Jeppe-1 well and to 23 m in the Gert-2 well. Age. The member is of latest Early – Late Kimmeridgian age (Johannessen et al. 1996; Andsbjerg & Dybkjær 2003, this volume). 189 Ravn Member new member Name. The member is named after the Ravn-1 and Ravn-2 wells situated on the south-eastern part of the Heno Plateau (Fig. 1C). Type section. The Ravn-1 well, 4173–4067 m b. KB, 4132– 4026 m b. MSL, is selected as the type section (Fig. 23). Reference section. The Gwen-2 well, 14084–13857 ft b. KB, 4256–4187 m b. MSL, on the Gertrud Plateau, is selected as the reference section (Fig. 25). 190 Gwen-2 Reference section: Ravn Member K im m er id gi an Triassic Lo la H en o R av n 0 140 C hr on os tr at ig ra ph y Fo rm at io n M em be r Li th ol og y ft b. KB ft b. KB 13857 14290 14300 14200 14084 14100 14000 13900 13800 13700 13600 14400 m b. MSL 4187 4256 4319 Fa rs un d Vo lg ia n 14100 14000 Clay Si Sand Gr 0 160 140 40 GR API GR API Sonic msec/ft Fig. 25. Gwen-2 well, located on the Gertrud Plateau, Danish Central Graben; reference section for the new Ravn Member (Heno Formation), 4256–4187 m b. MSL. Modified from Jensen et al. (1986); for legend, see Fig. 5. Lithology and environment. The Ravn Member consists predominantly of grey to white, very fine- to fine-grained sandstones, muddy sandstones and minor siltstones that are bioturbated to such a degree that almost no primary sedimentary structures are preserved (Fig. 23). Conglome- rate beds, 0.08–2 m thick, locally associated with medium- grained sandstones, are present on plateau areas. In the graben areas, the member is dominated by clayey, very fine-grained sandstones and siltstones. Calcareous beds and/or concretions occur. Deposition took place in a marine shoreface environment. The conglomerates rep- resent river or beach deposits that were later winnowed during transgressive erosion (Johannessen et al. 1996; Johannessen 2003, this volume). Log characteristics. In the Ravn-1 and Gwen-2 wells (Figs 23, 25), the lower and upper parts of the mem- ber include clayey sandstones characterised by rela- tively high gamma-ray and low sonic readings. The middle part of the member consists of sandstones and conglomerates characterised by low gamma-ray and high sonic readings. The overall pattern is thus char- acterised by decreasing-upwards gamma-ray values and increasing-upwards sonic values followed by increas- ing gamma-ray values and decreasing sonic values. In the Gert-1 and Jeppe-1 wells, the member does not include clayey sandstones in the upper part (Figs 22, 24). The gamma-ray and sonic logs are strongly serrated in the Ravn-1 well, less serrated in Gwen-2, and rela- tively smooth in Gert-1 and Jeppe-1 (Figs 22–25). This may be due to variation in the development of cal- careous beds and/or concretions. Lower boundary. The member overlies offshore clay- stones of the Lola Formation in the Ravn-1 well type section. The base of the member is defined by a shift to lower gamma-ray and higher sonic readings mark- ing the base of an overall coarsening-upwards succes- sion beginning with clayey sandstones (Figs 23, 25). In the Gwen-2 reference section and the Gert-1 well, the member overlies sandy offshore claystones of the Lola Formation (Figs 22, 25), whereas in the Jeppe-1 well, the Ravn Member directly overlies the Gert Member (Fig. 24). Distribution and thickness. The member is present in the Feda Graben and on the Gertrud and Heno Plateaus in the northern part of the Danish Central Graben. A maximum thickness of 106 m is found in the Ravn-1 well on the southern part of the Heno Plateau. Age. The member is of Late Kimmeridgian age (Ands- bjerg & Dybkjær 2003, this volume). Farsund Formation History. The Farsund Formation was defined by Vollset & Doré (1984) in the Norwegian North Sea area and extended to the Danish Central Graben by Jensen et al. (1986). New correlations have shown that Jensen et al. (1986) placed the lower boundary of the formation at a lower stratigraphic level than that defined by Vollset & Doré (1984). In addition, Forsberg et al. (1993) pointed out a discrepancy between the base of the Farsund Formation as picked by Vollset & Doré (1984) in their type well (2/7-3) and that picked in their reference well (2/8-3); neither of these picks correspond with that sug- gested by Jensen et al. (1986). The practice of Jensen et al. (1986) is followed here as they defined the boundary at a distinct and easily recognisable log change that marks a significant change in depositional style, which is probably related to changes in subsidence patterns (Andsbjerg & Dybkjær 2003, this volume). However, the upper part of the for- mation is missing from the U-1 well that was chosen as the Danish reference well for the Farsund Formation by Jensen et al. (1986). A new reference well, the G-1 well, is thus introduced here. Type and reference sections. The Norwegian 2/7-3 well, 3626–3414 m b. KB, is the type section; the G-1 well in the Danish Central Graben, 12036–7863 ft b. KB, 3631–2359 m b. MSL, is designated as a reference sec- tion in the Danish sector (Fig. 26). A correlation line showing the stratigraphic relationship between the Norwegian reference well 2/8-3 and three Danish wells including the reference well G-1 and the old reference well U-1 is depicted in Figure 27. Lithology and environment. The Farsund Formation consists of medium to dark grey claystones; they are carbonaceous and variably calcareous, and are inter- calated with numerous thin beds of brownish dolomite. The organic matter is mainly liptinitic, and deposition took place in a relatively deep marine environment. Thin units of turbidite sandstones occur locally in the deeper parts of the basins. Towards the eastern part of the Danish Central Graben, close to the Coffee Soil Fault, the proportion of sandstones increases and there appears to be a transition locally to the sandy Poul Formation. 191 192 Fa rs un d Vo lg ia n R ya za ni an Bo M b 7863 8000 8073 8121 2423 2438 8500 9000 2359 9500 10000 K im m er id gi an Vo lg ia n Å sg ar d C hr on os tr at ig ra ph y Fo rm at io n m b. MSLft b. KB G-1 Reference section: Farsund Formation Li th ol og y 0 150 80 40 C hr on os tr at ig ra ph y Fo rm at io n Li th ol og y GR API Sonic msec/ft m b. MSLft b. KB 0 150 80 40 GR API Sonic msec/ft Fig. 26. The Farsund Formation in the G-1 well in the southern part of the Danish Central Graben, 3631–2359 m b. MSL, designated as the new reference section in the Danish sector. For legend, see Fig. 5. 193 Fa rs un d Lo la K im m er id gi an Fa rs un d K im m er id gi an 10500 11000 11500 12000 12036 3631 C hr on os tr at ig ra ph y Fo rm at io n m b. MSLft b. KB Li th ol og y 0 150 80 40 GR API Sonic msec/ft C hr on os tr at ig ra ph y Fo rm at io n m b. MSLft b. KB Li th ol og y 0 150 80 40 GR API Sonic msec/ft 194 N2/8-3 Baron-2 G-1 U-1 DenmarkNorway ? Je ns en e t a l. 19 86 Fa rs un d Fo rm at io n Lo la F m Fa rs un d Fm Mandal Fm Vo lls et & D or é 19 84 H au ge su nd F or m at io n Fa rs un d Fo rm at io n Lo la F m GR Sonic GR Sonic GR Sonic GR Sonic 200 m 50 km N 2/8-3 Baron-2 G-1 U-1 Log characteristics. The formation is characterised by relatively high gamma-ray values and relatively low sonic velocities. High velocity peaks and correspond- ing low gamma-ray readings reflecting dolomite or lime- stone beds or carbonate-cemented sandstone and silt- stone beds are a common feature of the formation. Boundaries. The Farsund Formation overlies the Lola Formation in most of the Danish Central Graben (Figs 26, 27). On plateau areas in the western and north- western part of the Danish Central Graben, the Farsund Formation overlies the Heno Formation (Figs 21–25). The boundary with the Lola Formation is defined above a velocity minimum and a corresponding gamma-ray maximum. Above the boundary, there is commonly a general increase in velocity, a decrease in gamma-ray values and a much higher frequency of sonic velocity peaks. The boundary with the Heno Formation is char- acterised by an abrupt or gradual increase in gamma- ray readings and a corresponding decrease in velocity. The upper boundary with the Åsgard Formation (Cromer Knoll Group) is placed at the first significant change from high gamma-ray and low sonic velocity readings of the Farsund Formation to the low gamma- ray values and higher sonic velocities of the Åsgard Formation (Figs 26–28). Distribution and thickness. The formation extends from the Norwegian North Sea area, through the Danish Central Graben. Southwards it correlates with the upper part of the Kimmeridge Clay Formation in the Dutch North Sea area. Eastwards, the formation correlates with the Børglum Formation of the Norwegian–Danish Basin, although the transition zone has not yet been located. Maximum thicknesses of more than 3000 m occur in the Tail End Graben. Age. The Farsund Formation is of Late Kimmeridgian – Ryazanian age (Poulsen 1986; Heilmann-Clausen 1987; Andsbjerg & Dybkjær 2003, this volume). Subdivision. Organic-rich mudstones in the upper lev- els of the Farsund Formation, previously referred to infor- mally as the ‘hot unit’, are herein defined as the Bo Member of the Farsund Formation. The remainder of the formation is not subdivided at the level of members. Bo Member new member History. Jensen et al. (1986) described the ‘hot unit’ as an informal member of the Farsund Formation charac- terised by mudstones that display unusually high gam- ma-ray values. Correlatives at least in part are known from the Norwegian sector where they are referred to the Mandal Formation (Vollset & Doré 1984), and from the Dutch sector where they form the Clay Deep Member of the Kimmeridge Clay Formation (van Adrichem Boogaert & Kouwe 1994). However, both the Mandal Formation and the Clay Deep Member span a greater stratigraphic interval and include mudstones showing intermediate gamma-ray values beneath and above the ‘hot unit’ as described by Jensen et al. (1986). Use of the Mandal Formation in the Danish sector is consid- ered impractical due to difficulties in recognising the lower boundary in many of the Danish wells. The ‘hot unit’ of Jensen et al. (1986) is thus retained and given formal status as the Bo Member of the Farsund For- mation. Name. The Bo Member is named after the Bo-1 well, located in the southern Tail End Graben where the member is well-developed. Type section. The Danish Bo-1 well, 8561–8434 ft b. KB, 2576–2537 m b. MSL (Fig. 28). Reference section. The Danish E-1 well, 9853–9771 ft b. KB, 2966–2940 m b. MSL (Ineson et al. 2003, this vol- ume, fig. 6). Lithology and environment. The member is dominated by black to dark grey-brown, laminated claystones, which are carbonaceous and slightly calcareous to non- calcareous. The total organic carbon content ranges from 3–8%, locally attaining values of more than 15% (Ineson et al. 2003, this volume). The organic matter is mainly liptinitic. Thin beds of dolomite may occur. In the Jeppe-1 core, the unit also includes thin sand- stone–mudstone couplets up to 5 cm thick; log data 195 Facing page: Fig. 27. Well-log panel from the Norwegian 2/8-3 well (Nor- wegian reference well of the Farsund Formation) through the Baron-2 well, the G-1 well (new Danish reference well of the Farsund Formation) and the U-1 well (previous Danish reference well of the Farsund Formation). Note the position of the basal boundary of the Farsund Formation as picked in the Norwegian reference well and in the Danish wells. suggest that thicker sandstone-rich intervals are also present in the member (Ineson et al. 2003, this volume). Deposition occurred in a low-energy, oxygen-deficient deep marine environment. The sandstone–mudstone couplets were deposited from dilute turbidity currents (Ineson et al. 2003, this volume). Log characteristics. The member is primarily identified by its high gamma-ray values, which are significantly higher than those of the underlying and overlying clay- stones of the Farsund Formation. The sonic velocity is low. The gamma-ray values may show significant vari- ation within the member with decreasing-upwards trends, 3–5 m thick, separated by intervals with more consistently high values (e.g. Bo-1). Boundaries. The lower and upper boundaries are placed at shifts to the significantly lower gamma-ray values of the Farsund Formation. Distribution and thickness. The member is recognised widely in the Danish Central Graben, where this por- tion of the Farsund Formation is preserved. On struc- tural highs, the upper part of the Farsund Formation is commonly truncated and the member may be absent or reduced in thickness. The thickness varies greatly from less than 10 m in the southern Salt Dome Province to more than 100 m in the western part of the Danish Central Graben, probably controlled by local factors such as structural position and sediment supply (Ineson et al. 2003, this volume). Age. Late Volgian – Late Ryazanian, mainly occurring within the Lower Ryazanian H. kochi Chronozone with 196 8700 8800 8900 ft b. KB 8561 8434 8348 8400 8500 8600 Li th ol og y C hr on os tr at ig ra ph y Fo rm at io n M em be r 0 180 ?? R ya za ni an –V al an gi ni an 140 60 Vo lg ia n Fa rs un d 2576 m b. MSL 2537 2511 Å s. Bo Bo-1 Type section: Bo Member GR API Sonic msec/ft Fig. 28. Bo-1 well, located in the Danish Central Graben; type section of the new Bo Member (Farsund Formation), 2576–2537 m b. MSL. Ås., Åsgard. Modified from Jensen et al. (1986); for legend, see Fig. 5. Fa rs un d Fa rs un d Po ul 28189356 9400 9300 9463 9500 9600 2851 m b. MSLft b. KB Vo lg ia n C hr on os tr at ig ra ph y Fo rm at io n GR API Sonic msec/ftLi th ol og y 0 140 140 40 V-1 Type section: Poul Formation Fig. 29. V-1 well, located in the eastern part of the Danish Central Graben; type section for the Poul Formation, 2851–2818 m b. MSL. Modified from Jensen et al. (1986); for legend, see Fig. 5. a maximum age range from the C. preplicomphalus to the C. stenomphalus Chronozones (Birkelund et al. 1983; Poulsen 1992; Dybkjær 1998; Ineson et al. 2003, this volume). Poul Formation History. The Poul Formation was defined by Jensen et al. (1986). Type section. The V-1 well, 9463–9356 ft b. KB, 2851–2818 m b. MSL, in the eastern part of the Danish Central Graben was selected by Jensen et al. (1986; Fig. 29). Lithology and environment. The Poul Formation typi- cally consists of dark grey siltstones and light grey, very fine-grained sandstones. The siltstones are partly argilla- ceous and grade into claystones. In the Ugle-1 well, located at the eastern margin of the Danish Central Graben close to the Coffee Soil Fault zone, the forma- tion includes pebble conglomerates, coarse-grained sandstones and fine- to very fine-grained, silty sand- stones. Shelly debris and lithic clasts are common, and the conglomerates may be polymictic. The clasts rep- resent a wide range of lithologies suggesting a meta- morphic source terrain with phyllites and other meta- sediments. Deposition was probably from turbidity currents on fault-bounded plateaus along the Coffee Soil Fault. The exceptionally thick and immature succes- sion in the Ugle-1 well was probably deposited as a fault- attached, localised slope apron. Log characteristics. The Poul Formation is characterised by low gamma-ray values and intermediate sonic val- ues representing silt- and sandstones, with a few peaks showing higher sonic velocities representing carbonate- cemented beds. Boundaries. The formation interfingers with the Farsund Formation, and the lower and upper boundaries are clearly defined at abrupt changes to lower gamma-ray values and higher sonic velocities than those that char- acterise the Farsund Formation. Distribution and thickness. The formation is present in the Danish Central Graben close to the eastern faulted margin of the Central Graben. The thickest sandstone units occur on down-faulted plateaus along the Coffee Soil Fault zone. The maximum thickness of 175 m is observed in the Ugle-1 well (2591–2416 m b. MSL). Age. The Poul Formation is of Volgian age (Hoelstad 1986b; Poulsen 1986). Stratigraphic evolution The Jurassic succession of Denmark, from Bornholm in the east to the Central Graben in the west, provides a detailed record of major basin evolutionary events that were experienced along the length of the Jurassic rift system. A summary of the Jurassic stratigraphic evolu- tion of Denmark is given below; detailed accounts are presented elsewhere in this volume (Andsbjerg 2003, this volume; Andsbjerg & Dybkjær 2003, this volume; Johannessen 2003, this volume; Nielsen 2003, this volume). Early Jurassic marine deposition The Early Jurassic transgression of the Danish Basin was initiated in the Norian (Late Triassic) and recorded by deposition of restricted marine claystones of the Vinding Formation and paralic – shallow marine sand- stones and mudstones of the overlying and partly interfingering Gassum Formation. Mudstones of the Lower Jurassic Fjerritslev Formation conformably over- lie the Gassum Formation over most of the Danish Basin and Sorgenfrei–Tornquist Zone, indicating a fur- ther deepening of the basin (Figs 30–32). The Fjerritslev Formation is subdivided into four members, all of which are present in the Danish Basin. The two sub-units of the lower member (F-Ia, F-Ib) are also recognised in the Danish Central Graben (Michelsen 1978b). The for- mation is patchily preserved in the southern part of the Norwegian North Sea sector and has been drilled in the Egersund Subbasin and in the area around the Southern Vestland Arch (Vollset & Doré 1984). To the east, com- parable mudstones have been exposed at Gantofta, Skåne, where they are referred to the Pankarp Member of the Rya Formation (Fig. 2; Frandsen & Surlyk 2003, this volume). The Fjerritslev Formation was probably deposited over most of the south-eastern North Sea. The isolated occurrences of the formation are erosional out- liers of a previously widely distributed Lower Jurassic cover, which existed prior to early Middle Jurassic uplift and erosion of the mid North Sea dome and the Ringkøbing–Fyn High (Michelsen 1978a, b; Gowers & Sæbøe 1985; Ziegler 1990; Underhill & Partington 1993; Andsbjerg & Dybkjær 2003, this volume; Nielsen 2003, this volume). 197 198 10 0 m M ej ru p- 1 H yl le bj er g- 1 Ve ds te d- 1 Bø rg lu m -1 Fl yv bj er g- 1 Fr ed er ik sh av n- 2 G R G R So ni c So ni c SP R es SP R es SP R es SP R es F- lV F- lV F- lll F- ll F- lb F- lb F- lll F- ll F- lb F- la F- la Fl yv b. –H al d. Fj er ri ts le v Fm Bø rg lu m F m Fjerritslev Fm M ar in e m ud st on es Sh al lo w m ar in e sa nd st on es a nd s ilt st on es Pa ra lic a nd n on -m ar in e sa nd st on es , si lts to ne s, m ud st on es a nd c oa ls D an is h Ba si n So rg en fr ei –T or nq ui st Z on e Sk ag er ra k– K at te ga t Pl at fo rm Sk ag er ra k Fm (T ri as si c) Fr ed er ik sh av n Fm Bø rg lu m F m V ed st ed F m G as su m F m (T ri as si c) F- lll F- ll F- lb F- la G as su m F m (Ju ra ss ic ) Fr ed er ik sh av n Fm Fl yv bj er g Fm H al da ge r Sa nd F m V ed st ed F m F- lV F- lll F- ll F- lb Fr ed er ik sh av n- 2 Bø rg lu m -1 Fl yv bj er g- 1 Ve ds te d- 1 H yl le bj er g- 1 M ej ru p- 1 50 k m Fi g. 3 0. W el l- lo g p an el a cr o ss t h e D an is h B as in a n d F en n o sc an d ia n B o rd er Z o n e sh o w in g th e U p p er T ri as si c – Lo w er C re ta ce o u s lit h o st ra tig ra p h ic u n its w ith g en er al is ed d ep o si tio n al e n vi ro n - m en ts i n d ic at ed b y co lo u r. F ly vb ., Fl yv b je rg F o rm at io n ; H al d ., H al d ag er S an d F o rm at io n . 199 10 0 m D an is h Ba si n So rg en fr ei –T or nq ui st Z on e R øn ne G ra be n Bø rg lu m F m F- lV F- lll F- ll F- lb F- la G as su m F m (T ri as si c) Fjerritslev Fm F- lll F- ll F- lb F- la Fj er ri ts le v Fm H yl le bj er g- 1 Te rn e- 1 H an s- 1 Pe rn ill e- 1 St in a- 1 G as su m F m (T ri as si c) Fr ed er ik sh av n Fm Q ua te rn ar y G as su m F m (Ju ra ss ic ) G as su m F m (T ri as si c) U pp er C re ta ce ou s H as le F m Rønne Fm Rønne Fm M un ke ru p M b M un ke ru p M b Q ua te rn ar y So rt ha t Fm H as le F m Tr ia ss ic Tr ia ss ic Fl yv bj er g Fm H al da ge r Sa nd F m So se Bu gt M b G al ge - lø kk e M b G al ge - lø kk e M b So se Bu gt M b Ve ds te d Fm M ar in e m ud st on es Sh al lo w m ar in e sa nd st on es a nd s ilt st on es Pa ra lic a nd n on -m ar in e sa nd st on es , si lts to ne s, m ud st on es a nd c oa ls H yl le bj er g- 1 Te rn e- 1 H an s- 1 Pe rn ill e- 1 St in a- 1 G R So ni c G R So ni c So ni c G R G R So ni c G R So ni c 50 k m Fi g. 3 1. W el l- lo g p an el f ro m t h e D an is h B as in a n d F en n o sc an d ia n B o rd er Z o n e w ith t h e U p p er T ri as si c – Lo w er C re ta ce o u s lit h o st ra tig ra p h ic u n its c o m p ar ed t o t h e P er n ill e- 1 an d S tin a- 1 w el ls fr o m t h e R ø n n e G ra b en , o ff sh o re B o rn h o lm . G en er al is ed d ep o si tio n al e n vi ro n m en ts i n d ic at ed b y co lo u r. The Early Jurassic transgression resulted in the estab- lishment of a well-oxygenated marine environment in the Danish Basin and in the central North Sea Basin (Fig. 33; Michelsen 1975, 1978a; Pedersen 1986). Marine clay- stones interbedded with thin sandstone beds and car- bonate-cemented beds (F-Ia unit) are known from both areas. Along the north-eastern margin of the Danish Basin, fluvial and shallow marine conditions prevailed in the Hettangian – Early Sinemurian, and a stepwise back- stepping of the sandy Gassum Formation took place (Figs 30–32; Nielsen 2003, this volume). In Skåne, south- ern Sweden, the Hälsingborg and Döshult Members rep- resent a transition from deltaic to shallow marine conditions (Sivhed 1984; Erlström et al. 1999; Ahlberg et al. 2003, this volume). On Bornholm, an overall trans- gressive succession of lacustrine, coastal plain and tidal flat deposits is represented by the Rønne Formation, which is thickly preserved in the Rønne Graben (onshore: Galgeløkke-1, -2; offshore: Pernille-1, Stina-1; Figs 5, 31; Gravesen et al. 1982; Nielsen 1995; Surlyk et al. 1995). The depositional environment changed in the Early Sinemurian due to continued sea-level rise. Deposition of fine-grained claystones (F-Ib unit) took place from the Early Sinemurian to the earliest Early Pliensbachian in the basinal areas. To the north, in Jylland, the Gassum Formation was overstepped, and deposition of marine clay (F-Ib unit) extended onto the Skagerrak–Kattegat Platform (Fig. 30). Deposition of fluvial and shallow marine sandy deposits of the Döshult Formation in Skåne was followed by deposition of marine clays of the Pankarp Member, whereas shallow marine sand, tidal flat heteroliths and estuarine mud are found in the cor- relative part of the Rønne Formation on Bornholm (Surlyk et al. 1995; Erlström et al. 1999; Frandsen & Surlyk 2003, this volume; Nielsen 2003, this volume). Continued sea-level rise in the Early Pliensbachian led to open marine conditions and deposition of silty clay (F-II member) in the Danish Basin and more sandy deposits on the Skagerrak–Kattegat Platform (Fig. 30). Farther to the east and south-east, in the Fennoscandian Border Zone, marine sand and clay (Katlösa Member in Skåne) and marine sand and silt (Hasle Formation on Bornholm) were deposited (Troedsson 1951; Surlyk & Noe-Nygaard 1986; Norling et al. 1993; Frandsen & Surlyk 2003, this volume; Nielsen 2003, this volume). After a regressive event with deposition of minor sands and erosion on the Skagerrak–Kattegat Platform, deposition of clay again dominated in the Danish Basin (F-III member) in Late Pliensbachian – Early Toarcian times (Fig. 30). At the end of the Pliensbachian, the ben- thic faunas disappeared due to increasing anoxia, prob- ably related to a sea-level rise, corroborated by sedi- mentology, organic geochemical analyses and paly- nology (Pedersen 1986; Michelsen 1989b; Dybkjær 1991; Nielsen 2003, this volume). The sea-level rise culminated in the Early Toarcian. Towards the end of the Toarcian, a regression began with deposition of interbedded clay and sand (F-IV Member) in a more shallow and restricted marine environment (Figs 30, 31). A regressive tendency also occurred further south in the Fennoscandian Border Zone during the Late Pliensbachian as reflected by dark grey, grey-green and red-brown marine sandstones and siltstones (Rydebäck Member) in Skåne. The benthic foraminiferal fauna decreased markedly in density and diversity from the Pliensbachian to the Toarcian, but ammonites occur throughout the formation (Norling 1972). On Bornholm, sandy and clayey deposits interbedded with thin coal beds (the Sorthat Formation) were deposited in brack- ish coastal plain environments (Koppelhus & Nielsen 1994; Petersen & Nielsen 1995; Petersen et al. 2003b, this volume). The Lower–Middle Jurassic boundary is difficult to identify in the Danish Basin due to relatively poor bio- stratigraphic data and is conventionally placed between the Fjerritslev and Haldager Sand Formations (Michelsen 1978a, 1989a). Recent studies indicate that the Lower– Middle Jurassic transition is situated within the upper- most part of the Fjerritslev Formation in the Sorgen- frei–Tornquist Zone; elsewhere, it coincides with the marked erosion surface between the Fjerritslev and Haldager Sand Formations (Nielsen 2003, this volume). The maximum thicknesses of the uppermost member (F-IV) of the Lower Jurassic Fjerritslev Formation and the Middle Jurassic Haldager Sand Formation are found in the Sorgenfrei–Tornquist Zone, indicating continu- ous subsidence contemporaneously with the Middle Jurassic uplift of the Ringkøbing–Fyn High (Michelsen & Nielsen 1991; Nielsen 1992, 1994, 2003, this volume). The gradual eastwards transgression during the Early Jurassic known from the Fennoscandian Border Zone is also recorded in north-eastern Germany (Meinhold et al. 1960; Bertelsen 1978; Michelsen 1978a). To the south-west, in the Dutch Central Graben, a continuous succession of Lower–Middle Jurassic deposits is included in the Altena Group, comprising the Aalburg, Posidonia Shale and Werkendam Formations (Fig. 34; NAM & RGD 1980; van Adrichem Boogaert & Kouwe 1994; Herngreen et al. 2003, this volume). The overall lithol- ogy of the Altena Group is similar to that of the Fjerritslev Formation. The Aalburg Formation is of Hettangian– Pliensbachian age, and the sediments seem to have 200 201 C le o- 1 L- 1 In ez -1 F- 1 K -1 Bø rg lu m -1 G r U . J ur . Tr ia ss ic G as su m F m H al da ge r Sa nd Fm Bø rg lu m F m G as su m F m T ri as si c F lV V ed st ed F m Fr ed er ik sh av n Fm Bø rg lu m F m Fl yv bj er g Fm H al da ge r Sa nd F m F- lV F- lll F- ll F- lb F- lb F- la Gassum Fm (Jurassic)Fjerritslev Fm Fe lic ia -1 J- 1 10 0 m C en tr al G ra be n R in gk øb in g– Fy n H ig h N or w eg ia n– D an is h Ba si n So rg en fr ei –T or nq ui st Z on e So ni c So ni c G R G R So ni c So ni c G R SP R es G R So ni c So ni c G R G R So ni c M ar in e m ud st on es Tu rb id ite s an ds to ne s an d si lts to ne s Pa ra lic a nd n on -m ar in e sa nd st on es , si lts to ne s, m ud st on es a nd c oa ls Sh al lo w m ar in e sa nd st on es a nd s ilt st on es Bø rg lu m -1 Fe lic ia -1 J-1 K -1 F- 1 In ez -1 C le o- 1 L- 1 Fr ed er ik sh av n Fm Lo w er C re t. Fl yv bj er g Fm C ha lk G r Br yn e Fm Lu lu Fm T ri as si c Lo la Fm Fa rs un d Fm 50 k m Fi g. 3 2. W el l- lo g p an el r u n n in g fr o m t h e So rg en fr ei – To rn q u is t Z o n e to t h e n o rt h -e as te rn D an is h C en tr al G ra b en s h o w in g th e U p p er T ri as si c – Lo w er C re ta ce o u s lit h o st ra tig ra p h ic u n its w ith g en er al is ed d ep o si tio n al en vi ro n m en ts i n d ic at ed b y co lo u r. 202 4° E 8° E 12 °E 16 °E 58 °N 5 6° N La te A al en ia n – Ba th on ia n 4° E 8° E 12 °E 16 °E 58 °N 56 °N H et ta ng ia n 4° E 8° E 12 °E 16 °E 58 °N 56 °N Si ne m ur ia n – Ea rl y Pl ie ns ba ch ia n 4° E 8° E 12 °E 16 °E 58 °N 56 °N To ar ci an – Ea rl y A al en ia n Sw ed en N or w ay G er m an y T he N et he rl an ds Sw ed en N or w ay G er m an y T he N et he rl an ds Sw ed en N or w ay G er m an y T he N et he rl an ds Sw ed en N or w ay G er m an y D en m ar k D en m ar k T he N et he rl an ds 10 0 km 10 0 km 10 0 km 10 0 km D en m ar k D en m ar k 203 D en m ar k O ffs ho re m ar in e (m ud -d om in at ed ) D ee p m ar in e (s an ds , g ra ve ls ) N on -d ep os iti on /e ro si on Pa ra lic a nd n on -m ar in e sa nd st on es , si lts to ne s, m ud st on es a nd c oa ls Sh al lo w m ar in e sa nd st on es a nd s ilt st on es 4° E 8° E 12 °E 16 °E 58 °N 56 °N C al lo vi an 4° E 8° E 12 °E 16 °E 58 °N 56 °N K im m er id gi an 4° E 8° E 12 °E 16 °E 58 °N 56 °N Vo lg ia n – Ea rl y R ya za ni an 10 0 km Sw ed en N or w ay G er m an y T he N et he rl an ds Sw ed en N or w ay G er m an y T he N et he rl an ds Sw ed en N or w ay G er m an y T he N et he rl an ds 10 0 km 10 0 km D en m ar k D en m ar k Fi g. 3 3. P al ae o ge o gr ap h ic m ap s sh o w in g th e in fe rr ed d is tr ib u tio n o f gr o ss d ep o si tio n al e n vi - ro n m en ts d u ri n g th e Ju ra ss ic . been deposited in a lower shoreface to deep offshore environment similar to that of the lower Fjerritslev Formation. The Posidonia Shale Formation is of Early Toarcian age and seems to have been deposited under dysoxic conditions as were the upper part of the F-III and the lower part of the F-IV members of the Fjerritslev Formation. The Werkendam Formation was deposited in a shallow, open marine environment during Late Toarcian – earliest Bathonian times. Middle Jurassic uplift During Late Aalenian – Early Bajocian times, the cen- tral North Sea area including the Ringkøbing–Fyn High became uplifted and formed a broad arch (Michelsen 1978a; Koch 1983; Ziegler 1990; Andsbjerg et al. 2001; Nielsen 2003, this volume) rather than a dome as sug- gested by Underhill & Partington (1993, 1994). The uplift influenced a large area from England to Denmark and widespread erosion occurred. The Sorgenfrei– Tornquist Zone seems to have been mainly unaffected by uplift and erosion, but farther south and south-west- wards in the Danish Basin, erosion cut deeply into the Fjerritslev Formation and totally removed these strata from the Ringkøbing–Fyn High (Michelsen 1978a; Koch 1983; Nielsen 2003, this volume). On the eastern part of the Ringkøbing–Fyn High, erosion also removed the upper part of the Triassic succession, and Triassic sed- iments are absent from the western part of the high. The main rifting of the Central Graben was initiated during the Middle Jurassic subsequent to the uplift of the central North Sea (Fig. 33). Down-faulting took place along the eastern bounding Coffee Soil Fault leading to preservation of Lower Jurassic deposits in the south-east- ern part of the Danish Central Graben. Erosion cut deeply into the pre-Jurassic deposits in the north-western part of the Central Graben due to simultaneous half-graben tilting, and Permian and Carboniferous rocks subcrop post- Lower Jurassic deposits in this area (Fig. 35). Middle Jurassic block-faulting and subsidence char- acterised the Sorgenfrei–Tornquist Zone, where the entire Lower Jurassic – lowermost Middle Jurassic suc- cession is preserved (Michelsen & Nielsen 1991; Nielsen 2003, this volume). The regional uplift also affected Skåne where renewed faulting and local uplift accompanied by volcanism clearly influenced sedimentation in Middle Jurassic time by reduc- ing the depositional area to a narrow basin (Klingspor 1976; Rolle et al. 1979; Norling & Bergström 1987; Norling et al. 1993; Ahlberg et al. 2003, this volume). Middle Jurassic fluvial and paralic deposition The early Middle Jurassic regional uplift of the central North Sea Basin resulted in radical palaeogeographic changes, including closure of the seaway linking the Arctic Sea with the Tethys Ocean. In the Danish area, subsequent rifting changed the basin configuration from the extensive Early Jurassic sea into two fault-controlled zones of deposition, the Central Graben and the Danish Basin where a depocentre was situated in the Sorgenfrei– Tornquist Zone (Fig. 33; Andsbjerg et al. 2001). Regional and local erosion cut deep into the Lower Jurassic and older deposits. The erosional surface was onlapped by fluvial, paralic and marine sediments during Middle–Late Jurassic times owing to renewed regional subsidence (Andsbjerg & Dybkjær 2003, this volume; Johannessen 2003, this volume; Nielsen 2003, this volume). Middle Jurassic deposits occur in the central and north-eastern parts of the Danish Basin. There is a marked unconformity between the marine claystones of the Lower Jurassic Fjerritslev Formation and the flu- vial to shallow marine sandstones of the Middle Jurassic Haldager Sand Formation. In the fault-bounded Sor- genfrei–Tornquist Zone, the boundary between the for- mations occurs in the Lower Aalenian and is developed as a marine regressive erosion surface (Poulsen 1996; Nielsen 2003, this volume). The main change in palaeo- geography involving uplift, regression and influx of sand thus seems to have occurred in the Early Aalenian. Subsequent to the uplift event, a Middle Jurassic suc- cession, more than 150 m thick, accumulated in the Sorgenfrei–Tornquist Zone; this succession comprises fine- to coarse-grained, shallow marine sandstones and fluvial sandstones and siltstones interbedded with marine and lagoonal claystones and thin coal beds (Haldager Sand Formation, Figs 30–32). This succession thins to 204 Facing page: Fig. 34. Comparison of the Upper Triassic – Lower Cretaceous lithostratigraphic schemes from the North Sea Central Graben, the Norwegian–Danish Basin and the Fennoscandian Border Zone. Sst., Sandstone; M.W.Mb, Middle Werkendam Member; M.G.Mb, Middle Graben Sandstone Member; U.G., Upper Graben Formation. Modified from Bertelsen (1978, 1980); Michelsen (1978, 1989a); NAM & RGD (1980); Gravesen et al. (1982); Sivhed (1984); Vollset & Doré (1984); Herngreen & Wong (1989); Norling et al. (1993); Richards et al. (1993); van Adrichem Boogaert & Kouwe (1994); Koppelhus & Nielsen (1994); Ahlberg et al. (2003, this volume); Herngreen et al. (2003, this volume). 205 TriassicJurassicCretaceous Upper UpperLower LowerMiddleSeries System St ag e Upper UpperLower LowerMiddleSeries System TriassicJurassicCretaceous N or th S ea C en tr al G ra be n N S W Br iti sh se ct or N or w eg ia n se ct or D an is h se ct or D ut ch se ct or Fi sk eb an k su bb as in D an is h Ba si n Sk ån e Bo rn ho lm N or w eg ia n– D an is h Ba si n Fe nn os ca nd ia n Bo rd er Z on e N E S N N N S S S St ag e V al an gi ni an V al an gi ni an R ya za ni an R ya za ni an V ol gi an V ol gi an K im m er id gi an K im m er id gi an O xf or di an O xf or di an C al lo vi an C al lo vi an Ba th on ia n Ba th on ia n Ba jo ci an Ba jo ci an A al en ia n A al en ia n T oa rc ia n T oa rc ia n Pl ie ns ba ch ia n Pl ie ns ba ch ia n Si ne m ur ia n Si ne m ur ia n H et ta ng ia n H et ta ng ia n R ha et ia n R ha et ia n N or ia n N or ia n V lie la nd C la ys to ne Fo rm at io n C la y D ee p M em be r K åg er öd F or m at io n R yd eb äc k M em be r K at sl ös a M em be r Pa nk ar p M b D ös hu lt M b H el si ng bo rg M em be r Bj uv M b Va llå kr a M b Höganäs FmRya Fm G la ss S an d M b Fu gl un da M b Fo rt un a M ar l Fy le da le n C la y Annero Formation V ita bä ck C la y N yt or p Sa nd Jy de gå rd Fo rm at io n R ob be da le Fo rm at io n R ab ek ke Fo rm at io n Ba gå Fo rm at io n So rt ha t Fo rm at io n H as le F or m at io n Rønne Formation G al ge - lø kk e M b So se B ug t M b M un ke ru p M b R is eb æ k M b Kågeröd Fm Fj er ri ts le v Fo rm at io n W in te rt on Fo rm at io n Va lh al l Fo rm at io n Ve ds te d Fo rm at io n Fr ed er ik sh av n Fo rm at io n Bø rg lu m Fo rm at io n Fl yv bj er g Fo rm at io n H al da ge r Sa nd Fo rm at io n Fj er ri ts le v Fo rm at io n G as su m Fo rm at io n V in di ng Fm Sk ag er ra k Fm O dd es un d Fm Va lh al l F m Å sg ar d Fm Fa rs un d Fm U la Fm M an da l Fo rm at io n K im m er id ge C la y Fo rm at io n R ib bl e Ss t.M b H ea th er Fm Fr es hn ey Ss t.M b Fulmar Fm Fulmar Fm ? ? Rattray Volcanic Member Ron Volcanic Member Pentland Fm Fjerritslev Formation Fjerritslev Formation ?? ? ? ? ? ? ? ? ? ? Sk ag er ra k Fo rm at io n Sm ith B an k Fo rm at io n G as su m Fo rm at io n Br yn e Fo rm at io n H au ge su nd Fo rm at io n El df is k Fm Å sg ar d Fm Le ek M b V yl Fm Bo M b Fa rs un d Fo rm at io n Po ul Fm Lu lu F m M id dl e G ra be n Fm H en o Fm Lo la Fo rm at io n Br yn e Fo rm at io n Sc ru ff G re en - sa nd F m K im m er id ge C la y Fo rm at io n Pu zz le H ol e Fm U .G M id dl e G ra be n Fm M .G . M b Lo w er G ra be n Fm W er ke nd am Fm W er ke nd am FmM . W . M b Po si do ni a Sh al e Fo rm at io n A al bu rg Fo rm at io n Sl ee n Fo rm at io n U pp er K eu pe r C la y M b D ol om iti c K eu pe r M em be r R ed K eu pe r C la y M b Sm ith B an k M em be r Sk ag er ra k Fo rm at io n G as su m Fo rm at io n Fj er ri ts le v Fo rm at io n Br yn e Fo rm at io n Sa nd ne s Fo rm at io n Eg er su nd Fo rm at io n Ta u Fo rm at io n Sa ud a Fo rm at io n Fl ek ke nf jo rd Fo rm at io n ? ? Mariedal fm 206 10 0 m D ee p G or m -1 El ly -2 R av n- 1 G w en -2 Je pp e- 1 G er t- 4 1 C ro m er K no ll G p Fa rs un d Fm Fa rs un d Fm H en o Fm H en o Fm Lo la F m Tr ia ss ic Lo la F m Fa rs un d Fm Br yn e Fo rm at io n H en o Fm Lo la F m Fj er ri ts le v Fm M id dl e G ra be n Fm Tr ia ss ic C ro m er K no ll G p Z ec hs te in G p M ar in e m ud st on es M ar in e or ga ni c- ri ch m ud st on es Tu rb id ite s an ds to ne s an d si lts to ne s Sh al lo w m ar in e sa nd st on es a nd s ilt st on es Pa ra lic a nd n on -m ar in e sa nd st on es , s ilt st on es , m ud st on es a nd c oa ls 50 k m D ee p G or m -1 El ly -2 R av n- 1 G w en -1 Je pp e- 1 G er t- 4 Fe da G ra be n H en o Pl at ea u Sa lt D om e Pr ov in ce G R So ni c R es G R So ni c R es G R So ni c R es G R So ni c R es G R So ni c R es G R So ni c R es less than 10 m in central and south-western parts of the Danish Basin. On the Skagerrak–Kattegat Platform, north-east of the Sorgenfrei–Tornquist Zone, the thick- ness of the Haldager Sand Formation varies between 40 m and 90 m. A braided river system running down- slope from the uplifted Ringkøbing–Fyn High has been suggested as the main method of transport for the sed- iments (Koch 1983). The Middle Jurassic deposits are generally poor in fossils, but palynological assemblages suggest an Aalenian–Callovian age (reviewed by Nielsen 2003, this volume). Dinoflagellate cysts from the top part of the Haldager Sand Formation in the Vedsted-1 well section indicate a Late Callovian or younger age (Poulsen 1992, 1996), suggesting that the youngest sediments of the Haldager Sand Formation in the Sorgenfrei–Tornquist Zone were deposited during a Callovian sea-level rise. In the north-western part of the Norwegian–Danish Basin and in the Central Graben, sediments equivalent to the Haldager Sand Formation are referred to the flu- vio-deltaic Bryne Formation. To the south-east, along the Fennoscandian Border Zone in Skåne, southern Sweden, the development corresponds to that of the Sorgenfrei–Tornquist Zone in the Danish area. A change from the marine Rydebäck Formation to fluvial, lacus- trine, lagoonal and shallow marine deposits of the Fuglunda Beds (Mariedal formation) and the Vilhelmsfält Formation (Ahlberg et al. 2003, this volume) took place in the late Aalenian, and deltaic deposition seems to have continued throughout the Bathonian (Figs 2, 33, 34). The Aalenian part of the Rydebäck Formation con- sists of black and variegated siltstones with a varying clay content showing a general regressive tendency, and the foraminiferal fauna is sparse compared to that of the underlying Lower Jurassic succession (Norling 1972). Several conglomeratic horizons suggest breaks in marine deposition (Norling et al. 1993). The Fuglunda Beds consist of alternating sand and clay with thin coal beds. Deposition of clean sand, probably of shallow marine origin (the Glass Sand Member of the Mariedal formation), took place during most of the Bathonian. The clay-dominated, marine Fortuna Marl was deposited in the southern part of Skåne from latest Bathonian time, throughout the Callovian and into the Early Oxfordian (Norling 1972). Marine influence in the Middle Jurassic thus seems to have increased south-eastwards along the Fennoscandian Border Zone (Fig. 2). On Bornholm, an erosional unconformity between the Sorthat and Bagå Formations separates Lower Jurassic paralic deposits from Middle Jurassic fluvial and lacus- trine deposits (Koppelhus & Nielsen 1994). The Middle Jurassic deposits consist of fluvial sand and gravel and lacustrine clay with coal beds deposited during Late Aalenian(?) and Bajocian–Bathonian times. Locally, the succession contains debris flow deposits indicating movements on the eastern bounding fault of the Rønne Graben in Bathonian time (Gry 1969). The top of the Bagå Formation is an erosional unconformity that forms the present-day land surface, and Callovian and Upper Jurassic strata are absent on Bornholm, although the low- ermost deposits of the Rabekke Formation in the Nyker Fault Block may extend down into the Volgian (Gravesen et al. 1982). In the Danish Central Graben, Middle Jurassic depo- sition took place in the down-faulted area west of the Coffee Soil Fault zone. Sandstones with subordinate siltstones and claystones, and laterally extensive coal beds in the upper part of the succession, characterise the Middle Jurassic Bryne and Lulu Formations in the northern Danish Central Graben (Fig. 36). An upwards increase in marine influence is reflected by a change from fluvial or coastal plain deposits in the lower part to marginal and shallow marine deposits in the upper part of the succession (Johannessen & Andsbjerg 1993; Andsbjerg 2003, this volume). In the southern Danish Central Graben, the lower part of the Middle Jurassic succession is characterised by thick sandy deposits interbedded with silt- and claystones and occasional thin coal beds (Bryne Formation). The upper part contains claystones, siltstones and common coal beds (Middle Graben Formation; Figs 35, 36). The depositional environment evolved from a coastal plain environment with strong fluvial influence to a low- energy paralic environment dominated by lagoons or interdistributary bays. The environmental evolution through Middle Jurassic time shows an increasing marine influence culminating in the major Late Jurassic marine transgression, which 207 Facing page: Fig. 35. NW–SE well-log panel through the Danish Central Graben showing the Upper Triassic – Lower Cretaceous litho- stratigraphic units with generalised depositional environments indicated by colour. Note the organic-rich mudstones, locally associated with turbidite sandstones, in the uppermost Farsund Formation of these wells (excluding Gert-4); these are referred to the Bo Member (see above and Ineson et al. 2003, this volume). Lower Cretaceous strata of the Cromer Knoll Group cap the Farsund Formation in this transect with the exception of the Elly-2 well in which the Farsund Formation is uncon- formably overlain by the Upper Cretaceous Chalk Group. led to deposition of deep offshore clay-dominated deposits. The facies and age distribution of the Middle Jurassic deposits recognised in Central Graben well sec- tions from the Norwegian, Danish and Dutch sectors indicate that the transgression came from the north (Michelsen et al. 1987; Johannessen & Andsbjerg 1993; Underhill & Partington 1994). In the Norwegian Fiske- bank Subbasin and the Norwegian sector of the Central Graben, the non-marine Bryne Formation is overlain by shallow marine and coastal deposits of the Callovian Sandnes Formation (Fig. 34; Vollset & Doré 1984). Similarly, the first fully marine Middle Jurassic deposits in the Danish Central Graben belong to the Callovian Lulu Formation, whereas the first comparable fully marine deposits in the Dutch Central Graben (referred to the Upper Graben Formation) are of Middle–Late Oxfordian age (see Herngreen et al. 2003, this volume). Thus the relatively sparse biostratigraphic data indicate a southwards younging of the transgression. The basal part of the Norwegian Haugesund and the Danish Lola Formations are of latest Callovian – Early Oxfordian age (Vollset & Doré 1984; Jensen et al. 1986; Andsbjerg & Dybkjær 2003, this volume). The onset of marine middle to outer shelf conditions thus started in latest Middle Jurassic time in the Central North Sea Basin, but only reached the southern part of the basin in the Oxfordian (Fig. 34; Herngreen & Wong 1989). Late Jurassic marine deepening The structural reorganisation established during early Middle Jurassic uplift and subsequent erosion created the framework for the basin configuration that pre- vailed from the Middle and Late Jurassic into Early Cretaceous times. The Ringkøbing–Fyn High including adjacent areas to the south and north remained an uplifted landmass, forming the southern border of the 208 50 m West Lulu-3 Middle Gr. Fm Fjerritslev Fm Bryne Fm Lola Fm SonicGR Amalie-1 Elly-3 U-1 Sonic SonicGR GR SonicGR Lola Fm Lulu Fm Bryne Fm Triassic Søgne Basin Tail End Graben Heno Plateau Salt Dome Province Marine mudstones Shallow marine sandstones and siltstones Paralic sandstones, siltstones, mudstones and coals Fluvial and estuarine channel sandstones and conglomerates Floodplain and lacustrine mudstones, siltstones and coals 50 km U-1 Elly-3 Amalie-1 West Lulu-3 Fig. 36. North to south well-log panel through the Danish Central Graben showing the Middle – lower Upper Jurassic lithostratigraphic units with generalised depositional environments indicated by colour. Danish Basin. The landmass was submerged late in the Early Cretaceous. The Fennoscandian Border Zone and the easternmost part of the basin acted as a broad tran- sition zone characterised by shallow marine to paralic or non-marine deposition during repeated transgres- sive–regressive cycles. These environmental conditions lasted into the earliest Early Cretaceous, when they were succeeded by deeper marine conditions. The initial shallow marine conditions represented by the uppermost, probably Callovian part of the Haldager Sand Formation were succeeded by transgressive, par- alic to marine deposition (Flyvbjerg Formation) in the Oxfordian. Dinoflagellate cysts from the lowermost part of the Flyvbjerg Formation in wells located in the Sorgenfrei–Tornquist Zone and in a well in the central part of the Danish Basin suggest an Early–Middle Oxfordian age and a marine origin for the basal part of the formation (Poulsen 1992, 1996). The transition from the shallow marine, regressive deposits of the upper- most part of the Flyvbjerg Formation to the transgres- sive, deeper marine deposits of the Børglum Formation seems to be of earliest Kimmeridgian age in the Danish Basin and of Late Kimmeridgian age in the Fenno- scandian Border Zone, thus demonstrating a north-east- wards younging (Poulsen 1992, 1996) and recording the gradual transgression of the basin margin. The deep- ening represented by the Børglum Formation lasted from the earliest Kimmeridgian to the Middle Volgian in the Danish Basin. A regressive development followed with deposition of coarser sediments in most of the basin. In the Fennoscandian Border Zone, two–three coarsening-upwards successions with shallow marine silt and sand beds alternating with transgressive clays were deposited during the Volgian–Ryazanian (Frederiks- havn Formation; Figs 30–32). Farther to the north-west, in the Norwegian– Danish Basin, a nearly time-equiv- alent succession of sedimentary rocks is referred, in stratigraphic order, to the Egersund, Tau and Sauda Formations (Vollset & Doré 1984). To the south-east, in Skåne, the lacustrine and la- goonal variegated claystones and siltstones of the Fyle- dal Clay were deposited during the Oxfordian and early Kimmeridgian, succeeded by prograding coastal plain deposits of the Nytorp Sand (Norling et al. 1993; Ahlberg et al. 2003, this volume). The Jurassic–Cretaceous tran- sitional beds of the Vitabäck Clays are again dominated by brackish to freshwater deposits showing some marine influence. The connection between the Central European basins and the Danish Basin along the Fennoscandian Border Zone was characterised by increased marine influence (Fortuna Marl) during the Callovian (Fig. 34). During Late Jurassic time, marine communication between the Danish Basin and the North German Basin was mainly through the Central Graben, but a connection was peri- odically established through the Sorgenfrei–Tornquist Zone (Christensen & Kilenyi 1970). The environment changed to mainly brackish water, while open marine conditions dominated in the Danish Basin. However, repeated regressive–transgressive events are recorded in Skåne and in northernmost Jylland. Precise strati- graphic correlation between these two areas is not yet possible due to scarce biostratigraphic data. Callovian– Kimmeridgian deposits are absent on Bornholm and in the Rønne Graben. The ?Volgian– Valanginian Rabekke, Robbedale and Jydegård Formations (Fig. 2), preserved east of the Rønne Graben, show a broadly transgres- sive development, probably reflecting the overall Early Cretaceous transgression in Northwest Europe. Late Jurassic graben development and sedimentation Onset of the main rifting following the Middle Jurassic uplift of the central North Sea Basin led to rapid sub- sidence of the Central Graben. Fault activity along the Coffee Soil Fault continued from Middle Jurassic through Late Jurassic times. The Central Graben was developed as a major half-graben, which became compartmen- talised into a number of minor grabens and highs dur- ing the Late Jurassic (Møller 1986; Japsen et al. 2003, this volume; Møller & Rasmussen 2003, this volume). Up to 4000 m of clay-dominated sediments were depo- sited in the Tail End Graben to the east during the Late Jurassic (Møller 1986; Michelsen et al. 1992). The marine middle–outer shelf conditions which were established in latest Callovian time in the Danish Central Graben continued during the Late Jurassic over- all sea-level rise. The Central Graben region was prob- ably connected with the Danish Basin during most of the Late Jurassic through the north-western part of the Norwegian–Danish Basin, north of the Ringkøbing–Fyn High. The sedimentary facies of the Central Graben dif- fer markedly from those of the Norwegian–Danish Basin, and none of the formations known from the Danish Basin can be traced to the Central Graben. These dif- ferences probably resulted from varying distances to the source areas and from differences in structural evolu- tion. Deposition of offshore muds of the Lola Formation in the eastern part of the Central Graben continued 209 210 Fa rs un d Fm Lo la FmH en o Fm Tr ia ss ic 10 0 m Fa rs un d FmPo ul Fm C al ed on ia n ba se m en t C ro m er K no ll G p C ro m er K no ll G p Tr ia ss ic Lo la Fo rm at io n H en o Pl at ea u Ta il En d G ra be n El in -1 V- 1 R av n- 1 U gl e- 1 G R So ni c R es G R So ni c R es G R So ni c R es G R So ni c R es M ar in e m ud st on es Tu rb id ite s an ds to ne s, si lts to ne s an d co ng lo m er at es Sh al lo w m ar in e sa nd st on es a nd s ilt st on es 50 k m El in -1 V- 1 U gl e- 1 R av n- 1 from the latest Callovian, and onlapped westwards onto the Heno Plateau during the Oxfordian–Kimmeridgian (Fig. 35; Michelsen et al. 1992; Johannessen & Andsbjerg 1993; Andsbjerg & Dybkjær 2003, this volume). Thin shal- low marine sands were deposited at the western mar- gin of the basin during deposition of the Lola Formation. This indicates that the sea transgressed westwards and up-dip from the eastern part of the Danish Central Graben. Deposition of offshore muds of the Lola Formation on the Heno Plateau was succeeded by shoreface sands of the Heno Formation during the lat- est Early – Late Kimmeridgian (Figs 35, 37; Andsbjerg & Dybkjær 2003, this volume; Johannessen 2003, this volume). Shallow marine and back-barrier sediments of the Heno Formation were also deposited on the Gertrud Plateau and in the southern Feda Graben. Rapid sub- sidence of the individual basins of the Central Graben took place during latest Kimmeridgian time with an increased rate in the Early Volgian. Deposition of off- shore muds dominated the entire graben area during latest Kimmeridgian – Volgian times, periodically with a significant increase in the content of organic matter as seen in the lower part of the Farsund Formation in the Feda Graben and Heno Plateau (Fig. 35; Damtoft et al. 1992). Locally, coarse-grained sediments deposited from sediment gravity flows (Poul Formation) interfin- ger with the claystones of the Farsund Formation adja- cent to active fault zones (Fig. 37). The subsidence rates seem to have decreased in the latter part of the Volgian, and during the Late Volgian – Early Ryazanian, the high- ly radioactive Bo Member developed as a part of the Farsund Formation. Deposition of the organic-rich muds was governed by stratification of the water column, poor circulation, sea-level changes and reduced sediment influx (Ineson et al. 2003, this volume). The Norwegian sector of the Central Graben devel- oped in a similar way to that described for the Danish part. The Callovian – Early Volgian clay deposits are in- cluded in the Haugesund Formation (Fig. 34), which differs from the partly time-equivalent Lola Formation in containing a number of thin sandstone beds and by forming an overall coarsening-upwards cycle (Vollset & Doré 1984; see discussion in Forsberg et al. 1993). The overlying Farsund Formation is widely distributed north of the Danish sector. In the western part of the graben, a sand-dominated turbidite succession of Kimmeridgian age (Eldfisk Formation) forms a wedge between the Haugesund and Farsund Formations. In the Norwegian sector, the Farsund Formation is also capped by an organic-rich mudstone, named the Mandal Formation (Fig. 34). To the south, in the Dutch sector, the Callovian – Upper Oxfordian Lower, Middle and Upper Graben Formations (terrestrial–paralic) of the Dutch North Sea area are succeeded, with some overlap, by the Upper Oxfordian – Kimmeridgian Puzzle Hole Formation (lower delta plain deposits), indicating a northwards shift of the coastline during the Middle Oxfordian (Fig. 34; NAM & RGD 1980; Herngreen & Wong 1989; van Adrichem Boogaert & Kouwe 1994; Herngreen et al. 2003, this volume). Transgression was again initiated in the Late Oxfordian, as indicated by the southwards onlap by the deeper marine Kimmeridge Clay Formation (equivalent to the Lola and Farsund Formations) onto the shallow marine deposits. To the south, a barrier sand complex (the Volgian–Ryazanian Scruff Greensand Formation) seems to have been partly reworked into transgressive sheet sands overstepping the graben mar- gins (Herngreen & Wong 1989). At the Jurassic– Cretaceous transition, anoxic conditions (Clay Deep Member) prevailed in the northern part of the Dutch North Sea area. Acknowledgements The referees G.F. Waldemar Herngreen and Gunver K. Pedersen and the editor Jon R. Ineson are thanked for many valuable suggestions. F.S. acknowledges the sup- port of the Danish Natural Science Research Foundation; L.H.N. and F.S. acknowledge the use of Gravgærde, the field station of the University of Copenhagen on the island of Bornholm. 211 Facing page: Fig. 37. West–east well-log panel from the Danish Central Graben showing Middle/Upper Jurassic – Lower Cretaceous lithostratigraphic units with generalised depositional environ- ments indicated by colour. Note the thin development of organic-rich mudstones (Bo Member) in the uppermost Farsund Formation of the Ravn-1 well; this member is not recognised in the V-1 and Ugle-1 wells close to the eastern flank of the Central Graben and is thick but only weakly developed in the Elin-1 well (see Ineson et al. 2003, this volume). 212 References Ahlberg, A., Sivhed, U. & Erlström, M. 2003: The Jurassic of Skåne, southern Sweden. In: Ineson, J.R. & Surlyk, F. (eds): The Jurassic of Denmark and Greenland. Geological Survey of Denmark and Greenland Bulletin 1, 527–541 (this volume). Andsbjerg, J. 1997: Sedimentology and sequence stratigraphy of Middle Jurassic deposits. Danish and Norwegian Central Graben, 172 pp. Unpublished Ph.D. thesis, University of Copenhagen, Denmark. Andsbjerg, J. 2003: Sedimentology and sequence stratigraphy of the Bryne and Lulu Formations, Middle Jurassic, northern Danish Central Graben. In: Ineson, J.R. & Surlyk, F. (eds): The Jurassic of Denmark and Greenland. 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