Geological Survey of Denmark and Greenland Bulletin 12, 57-63 57 pyritic part of the Knudshoved Member may correlate with the upper, tuff-poor unit B2 of the Balder Formation. Stronsay Group Knox & Holloway (1992) replaced the Hordaland Group of Deegan & Scull (1977) with two new groups: the Stron- say Group succeeded by the Westray Group (Fig. 3). The two groups together comprise the light grey, green and brown coloured, soft, fissile, marine shales with thin lime- stone streaks that overlie the Rogaland Group and under- lie the Nordland Group. These groups each contain two formations, one representing sandy shelf lithofacies and the other representing basinal mudstone lithofacies. In the central North Sea, and in the Danish sector, the Stronsay Group is represented by its mudstone facies, the Horda Formation (Knox & Holloway 1992). Sandstone units of varying thickness occur at many levels in the Stronsay and Westray Groups along the basin margin in the Norwe- gian and British sectors, and many of these have been defined as formations or members (Deegan & Scull 1977; Hardt et al. 1989; Knox & Holloway 1992). A sandstone unit also occurs in the Horda Formation on the Ringkø- bing–Fyn High in the Danish sector and is described here as a new member (Hefring Member). Horda Formation History. Knox & Holloway (1992) established the Horda Formation for the greenish grey basinal mudstone facies of their Stronsay Group that overlies the grey tuffaceous mudstones of the Balder Formation and underlies the greenish grey to brown mudstones of the Lark Formation (Knox & Holloway 1992). Type well. British sector well 22/1-1A, 2379.5–1992 m MDKB. Danish reference wells. Mona-1, 2930.8–2363.5 m MD- KB (Fig. 46); Siri-1, 2037.9–1916.5 m MDKB (Fig. 47). Distribution and thickness. The Horda Formation extends over the central and northern North Sea and is present in 1916.5 2037.9 Horda Fm Chalk Gp Lark Fm Balder Fm Sele Fm Lista Fm Våle Fm 1900 2000 2100 2200 m Siri-1 GR Sonic Rind Mb Idun Mb Tyr Mb Vile Mb Ve Mb Bue Mb Bue Mb Horda Formation Siri-1 Mona-1 Floki-1 100 200 300 400 500 600 700 800 900 Thickness (m) 25 km Fig. 47. Siri-1, Danish reference well for the Horda Formation. Black bars show cored sections. Fig. 48. Isochore map of the Horda Formation in the study area. The positions of the two Danish reference wells for the Horda Formation, Mona-1 and Siri-1, are indicated on the map. The position of Floki-1, the type well for the Hefring Member, is also indicated. 58 Gulnare-1 Adda-2 Alma-1 NNW SSE 10 km Horda FmHorda FmHorda Fm L4L4L4 L2L2L2 0 500 1000 1500 2000 2500 3000 3500 TWT (msec) TL TF UOU TL2 TH TB TC L3L3 Lark FmLark FmLark Fm 1000 1500 2000 2500 3000 3500 Gert-1 SW NE Mona-1 Elna-1 Sandra-1 Horda FmHorda FmHorda Fm H2H2 H3H3TH2TH2 H2 H3H3TH2 10 km Lark FmLark FmLark Fm TL UOU TH TB TC TWT (msec) TH1TH1TH1 H1H1H1 Fig. 49. NNW–SSE-trending seismic section (RTD81-RE94-17A) in the Central Graben showing south- and eastward thinning of the Horda Formation. The location of the seismic section is shown in Fig. 1. The L2, L3 and L4 subunits of the Lark Formation are indicated, as well as the mudstone-equivalent of the Freja Member. TL, Top Lark; TF, Top Freja; UOU, Upper Oligocene Unconformity; TL2, Top L2; TH, Top Horda; TB, Top Balder; TC, Top Chalk. Fig. 50. SW–NE-trending seismic section (RTD81-RTD94-19A) showing the tripartite subdivision (H1–3) of the Horda Formation in the eastern part of the Danish Central Graben (Gert-1 and Mona-1) and pronounced thinning of the Horda Formation east of the Central Graben (Elna-1 and Sandra-1). The location of the seismic section is shown in Fig. 1. TH1, Top H1 marker; TH2, Top H2 marker; other abbreviations as in Fig. 49. 59 all wells in the Danish sector of the North Sea. However, the lower part of the Horda Formation (Fig. 4; equivalent to Sequence 2 of Michelsen et al. 1998) is lacking in the eastern wells R-1 and S-1 and in the eastern part of the Ringkøbing–Fyn High. The upper part of the Horda For- mation (Fig. 4; equivalent to Sequence 3 of Michelsen et al. 1998) is thin or absent in the same area (Michelsen et al. 1998). The Horda Formation reaches a thickness of 906 m in the Central Graben well Tordenskjold-1, but thins towards the east and south-east to less than 100 m, with minimum recorded thicknesses of 9 m in the Ida-1 well and 4 m in the S-1 well. An isochore map of the Horda Formation is shown in Fig. 48. The overall thin- ning of the Horda Formation towards the south-east, east and north-east is also shown on the seismic sections in Figs 49 and 50 and on the log panel in Fig. 51. Lithology. The Horda Formation is characterised by green- ish grey to greyish green fissile mudstone. Subordinate limestone benches and thin layers of black mudstones occur at some levels in the formation. In many wells, par- ticularly in the Central Graben, the lowermost 20–50 m of the Horda Formation consists of red-brown mudstones (Fig. 52). This lithology is apparently lacking in the east- ern wells of the Danish sector. Log characteristics. The Horda Formation is characterised by an overall stable gamma-ray and sonic log motif with a lower gamma-ray response than that displayed by the underlying Balder Formation and the overlying Lark For- mation. In a few wells, the base of the Horda Formation shows relatively high gamma-ray values, which decrease to lower and more stable values over a short interval. The sonic readings decrease slightly upwards from the base to the top of the Horda Formation. Boundaries. The base of the Horda Formation is placed at the change from the laminated, predominantly grey mud- stones with interbedded sandy tuffs of the Balder Forma- tion to the predominantly non-laminated, fissile, green- ish grey or red-brown massive mudstones that form the basal part of the Horda Formation. The Balder–Horda boundary may be conformable or marked by a hiatus. The boundary is often difficult to pick on petrophysical logs. In basinal settings, Knox & Holloway (1992) advocated placing the lower boundary of the Horda Formation at the base of a marked gamma-ray peak believed to repre- sent a glaucony-rich condensed layer in the basal part of the Horda Formation. However, in many sections in the Danish sector there are two or more gamma-ray peaks in the Balder–Horda boundary interval. As the glaucony- rich layer has not been identified with certainty in the few cores taken across the boundary in the Danish sector, it is not possible to identify the key gamma-ray peak unam- biguously. Therefore, it is suggested that the lower bound- ary of the Horda Formation is placed on the basis of the sonic log where a gradual decrease in values in the upper part of the Balder Formation is succeeded by relatively stable, but somewhat lower readings in the Horda Forma- tion (Figs 46, 47). The upper boundary is at the base of the Lark Formation. Subdivision. Knox & Holloway (1992) suggested a three- fold subdivision of the Horda Formation (H1–3), based on lithology and biostratigraphy. A threefold subdivision can also be seen on seismic sections in the Danish sector of the North Sea (Fig. 50). In some Central Graben wells, the subdivision may also be recognised on shifts in log patterns on both gamma-ray and sonic logs (Fig. 51). In these wells, subtle peaks separate the three units on the gamma-ray log and coincide with the top H1 and top H2 seismic markers. Based on analysis of cuttings samples, the subdivision apparently lacks lithological expression in the Danish sector. Knox & Holloway (1992) noticed that the top of unit H1 is close to the HO of the dinoflagellate Eatonicysta ursulae, and that the top of unit H2 is close to the HO of the foraminifer Spiroplectammina spectabilis. This observation is supported by biostratigraphical data from the present study. A sandstone body within the Horda Formation has been encountered in the well Floki-1 on the Ringkøbing–Fyn High) in the Danish sector of the North Sea. This sandstone is defined herein as the new Hefring Member (see below). Macro- and ichnofossils. The Horda Formation is mode- rately to intensely bioturbated. Ichnofossils comprise Chon- drites ispp., Phycosiphon ispp. and Planolites ispp. Microfossils and palynomorphs. In wells where the Horda Formation rests conformably on the Balder Formation, the dinoflagellate events HO Deflandrea oebisfeldensis and HO Dracodinium condylos occur in the lowermost part of the Horda Formation. Characteristic microfossil datums from the lowermost part of the Horda Formation are the HO of the planktonic foraminifer Subbotina ex gr. linap- erta, which occurs abundantly, followed upwards by the almost coeval HOs of the benthic foraminifers Uvigerina batjesi, Turrilina brevispira and Gaudryina hiltermanni. A hiatus between the Horda and Balder Formations is indi- cated in wells in the northern and eastern part of the Dan- ish sector by the absence of D. oebisfeldensis and D. condy- los from the lower part of the Horda Formation. Central 60 1700 1800 1900 1400 GR Sonic GR Sonic Density GR Sonic Density GR Sonic Density 1500 1600 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 m 1000 1100 1200 700 800 900 1300 1400 1500 1600 1700 1800 1900 2100 2200 m 1700 1800 1900 1400 1500 1600 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 m 1800 2000 1500 2100 2200 2300 2400 2500 2600 2700 2800 3000 m Kim-1 Kim-1 Mona-1 West Lulu-3 Siri-1 Siri-1 1600 1700 1900 2900 2000 Horda H1 H3 H2 Lark L4 TL UOU TL2 TL1 TH TH2 TH1 TB L3 L2 L1 Balder Sele Lista Våle Chalk Grp SW NE 61 above the top of the lower, oxic part of the formation. The occasional influxes of radiolaria recorded through- out the upper part of the formation suggest that deeper marine conditions prevailed periodically. The palynofacies of the Horda Formation is character- ised by a rich and dominant dinoflagellate assemblage with dispersed terrestrial matter (phytoclasts, spores and pol- Facing page: Fig. 51. SW–NE-trending log panel showing eastward thinning of the Horda Formation. The figure also shows the variation in architec- ture and distribution of the Horda units H1–H3 and Lark units L1– 4 between the Central Graben (Kim-1, Mona-1 and West Lulu-3) and the Ringkøbing–Fyn High (Siri-1). Seismic ties: TL1, Top L1; TH2, Top H2; TH1, Top H1; other abbreviations as in Fig. 49. Fig. 52. Core photographs showing red to reddish grey mudstones of the lowermost part of the Horda Formation in the Sofie-1 well. Depths are core depths. cm 1887 m 1888 m Sofie-1 1901 m 1902 m 0 10 20 30 40 50 60 70 80 90 100 Graben wells contain the downhole succession of the dino- flagellate cyst events HO Areosphaeridium diktyoplokum, HO Areosphaeridium michoudii, HO Heteraulacacysta po- rosa and HO Cerebrocysta bartonensis from the top of the Horda Formation, indicating an age as young as earliest Rupelian (Fig. 5b). In wells to the north and east, the top of the Horda Formation seems to be slightly older since A. michoudii is recorded from the top of the formation, indicating a mid-Priabonian age (Fig. 5b). Significant dino- flagellate events from the middle to upper part of the Horda Formation are the succession of the HOs of Eatoni- cysta ursulae, Diphyes ficosoides and Phthanoperidinium clit- hridium in the middle part of the formation, and the HOs of Diphyes colligerum and C. bartonensis in the upper part of the formation. In Central Graben wells, significant microfossil events in the lower part of the Horda Forma- tion include the HO of abundant radiolaria of the genus Cenosphaera and the HO of the planktonic foraminifer Cyclammina amplectens. Key events in the middle and upper parts of the formation are the HOs of Pseudohasti- gerina spp. (planktonic foraminifers), Lenticulina gutticos- tata, Spiroplectammina amplectens and Planulina costata (benthic foraminifers). The top of the Horda Formation contains the HOs of Cibicidoides truncanus and Vaginuli- nopsis decorata. Depositional environment. The lower part of the Horda Formation contains a microfauna that is significantly diffe- rent from that of the underlying Balder Formation. The basal 5–40 m of the Horda Formation are characterised by a diverse fauna of both benthic and planktonic calca- reous foraminifers together with agglutinated foramini- fers. This indicates that the depositional setting was open marine, bathyal and with oxic bottom conditions. The upper part of the Horda Formation is character- ised by an abundant and diverse agglutinated foraminifer fauna. Calcareous foraminifers are very sparse or absent in this interval. The assemblage of Rhabdammina discre- ta, Haplophragmoides spp., Recurvoides spp. and Usbekista- nia charoides indicate that the upper part of the Horda Formation was deposited at upper bathyal depths with dysoxic bottom conditions. Radiolaria occur commonly in several narrow intervals, the lowest of which is slightly 62 Horda Fm Lark Fm Hefring Mb Våle Fm Balder Fm Sele Fm Lista Fm Chalk Gp 1500 1600 1700 1800 m Floki-1 1731.3 1793.4 GR Sonic Neutron/Density Fig. 53. Floki-1, type well for the Hefring Member. len) as a minor component, indicating an open marine environment with only limited influx from surrounding terrestrial areas. Age. In the Central Graben, where the Horda Formation is most complete, the formation spans from the middle Ypresian (Early Eocene) at its base to earliest Rupelian (earliest Oligocene) at its top. In wells to the east and north, the top is as old as middle Priabonian (see also biostrati- graphic section above). This indicates that the top of the Horda Formation is diachronous, younging in a south- westerly direction. This is possibly due to increased ero- sion or longer intervals of non-deposition towards the north-east in the basin, or both. Correlation. The Horda Formation can be correlated with the onshore Danish succession of the Røsnæs Clay For- mation, the Lillebælt Clay Formation and the Søvind Marl Formation (Heilmann-Clausen et al. 1985), and the Vi- borg Formation (Christensen & Ulleberg 1973). The red-brown mudstones near the base of the Horda Formation in the central North Sea can be correlated litho- logically with the Røsnæs Clay Formation and the lower part of the Lillebælt Clay Formation. 63 The lower part of the overlying main body of greenish and greyish mudstones in the offshore succession can be correlated with the coeval and lithologically similar upper part of the Lillebælt Clay Formation. The upper part of the Horda Formation can be correlated with the Søvind Marl Formation, which consists of grey marls. The high- est part of the Horda Formation, only observed in Cen- tral Graben wells, may be correlated with the Viborg For- mation on biostratigraphic evidence. Hefring Member new member History. The Hefring Member consists of sandstone depo- sits within the Horda Formation. These sandstones have not previously been recognised as a separate unit in the Danish sector. Derivation of name. After the goddess Hefring. Type well. Danish sector well Floki-1, 1793.4–1731.3 m MDRT (Fig. 53). Distribution and thickness. The Hefring Member is only known from the Floki-1 well located in the northern part of the Danish sector. As the unit currently cannot be iden- tified on seismic sections, its further distribution is un- known. In the Floki-1 well, the member is 62 m thick. Lithology. The Hefring Member consists of greenish grey, fine-grained, immature sandstones with glaucony grains. Log characteristics. The Hefring Member is characterised by a conspicuous blocky signature on the gamma-ray, sonic and density logs (Fig. 53). Gamma-ray responses are lower than those of the enveloping Horda Formation mudstones. The Hefring Member can also be recognised from a combination of the density and neutron logs as the presence of pure sand- stones results in a ‘cross-over’ of the two log curves (Fig. 53). Boundaries. The boundaries with the mudstones of the Horda Formation are sharp and characterised by promi- nent shifts on the gamma-ray and sonic logs (Fig. 53). Depositional environment. No cores have been taken in the Hefring Member, but the sandstones were probably depo- sited from concentrated gravity flows, based on log simi- larity with the other fine-grained sandstone bodies in the nearby Siri Canyon. Age. Lutetian (Middle Eocene) based on the age of the associated Horda Formation mudstones. Correlation. Based on biostratigraphic data, the Hefring Member may be contemporaneous in part with the Lille- bælt Clay Formation onshore Denmark, with the lower part of the Grid Sandstone Member (Knox & Holloway 1992) in the Viking Graben and with the upper part of the Tay Sandstone Member (Knox & Holloway 1992) in the northern part of the Central Graben. Westray Group The Westray Group is the upper of the two groups estab- lished by Knox & Holloway (1992) to replace the Horda- land Group of Deegan & Scull (1977; Fig. 3). In the cen- tral North Sea and in the Danish sector of the North Sea, the Westray Group is represented by the Lark Formation. Lark Formation History. The Lark Formation was established by Knox & Holloway (1992) for the brownish grey mudstone-domi- nated lithofacies of the Westray Group that overlies the more variable association of red and green-grey mudstones, silty mudstones and sandstones of the Horda Formation and underlies the grey, sandy and shelly mudstones, silt- stones and sandstones of the Nordland Group of Deegan & Scull (1977; Fig. 3). The Lark Formation is also recog- nised in the Danish sector although its lithology is more variable than that given in the original description. Type well. British sector well 21/10-4, 1867–1217 m MDKB. Danish reference wells. Mona-1, 2363.5–1598.3 m MD- KB (Fig. 46); Siri-1, 1916.5–819.3 m MDKB (Fig. 54). Distribution and thickness. The Lark Formation extends over the central and northern North Sea and is probably present in the entire Danish sector of the North Sea. Its depocentre is in the central and northern part of the Dan- ish sector, along the eastern boundary of the Danish Cen- tral Graben, where it reaches a thickness of 1194 m in the Siri-3 well. The Lark Formation thins west to a thickness of 389 m in the Tordenskjold-1 well in the Central Graben, and east to a thickness of 240 m in the S-1 well on the Ringkøbing–Fyn High (Fig. 55).