Cenozoic evolution of the eastern Danish North Sea

Marine Geology 177 (2001) 243±269

www.elsevier.com/locate/margeo

Cenozoic evolution of the eastern Danish North Sea M. Huuse*, H. Lykke-Andersen, O. Michelsen Department of Earth Sciences, University of Aarhus, Aarhus, Denmark Received 13 June 2000; accepted 26 March 2001

Abstract This paper provides a review of recent high-resolution and conventional seismic investigations in the eastern Danish North Sea and describes their implications for the development of the eastern North Sea Basin. The results comprise detailed timestructure maps of four major unconformities in the eastern Danish North Sea: the Top Chalk surface (mid-Paleocene), near top Oligocene, the mid-Miocene unconformity, and base Quaternary. The maps show that the eastern Danish North Sea has been affected by faulting and salt diapirism throughout the Cenozoic. Carbonate mounds, erosional valleys and pockmark- or karstlike structures were identi®ed at the top of the Upper Cretaceous±Danian Chalk Group. Strike-parallel erosional features and depositional geometries observed at near top Oligocene and at the mid-Miocene unconformity indicate that these major sequence boundaries can be attributed to large-scale lateral changes in sediment supply directions. Increases in sediment ¯ux to the southeastern North Sea at the Eocene/Oligocene transition and in the post-Middle Miocene appear to correlate with similar events world wide and with long term d 18O increases, indicating forcing by global factors, i.e. eustasy and climate. Stratal geometries observed on the seismic data indicate that the so-called `Neogene uplift' of the eastern Danish North Sea may have been hundreds of metres less than previously suggested. It is argued that late Cenozoic uplift of the basin margin and of mountain peaks in southern Norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late Cenozoic denudation and dissection of topography created in the Paleogene. The late Cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late Cenozoic tectonics, the cause of which is conjectural. A series of shallow thrust structures and an associated system of deep, buried valleys were mapped. Thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet, and it was followed by deep incision due to subglacial melt-water erosion, probably during the Elsterian glaciation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Cenozoic; Danish; North Sea Basin; Seismic

1. Introduction The present paper reviews the main results of * Corresponding author. Address: Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Meston Building, Aberdeen AB24 3UE, UK. Tel.: 144-1224273437; fax: 144-1224-272785. Presently at Cardiff University, Wales, UK. E-mail addresses: [email protected] (M. Huuse), [email protected] (M. Huuse).

®ve years (1994±1998) of seismic investigations of the eastern Danish North Sea, and their implications for the understanding of the Cenozoic development of the eastern North Sea Basin. The study involved acquisition, processing, and interpretation of high-resolution multichannel seismic data in the eastern Danish North Sea, and integration of these data with conventional seismic and well data from the area. High-resolution seismic data were acquired every spring/summer

0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00168-2

244

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 1. The Cenozoic North Sea Basin (modi®ed from Ziegler, 1990; Japsen, 1998). The location of our study area and the study area of Michelsen et al. (1995, 1998) is indicated.

throughout the 5-year study period, providing a unique possibility of planning and executing the seismic surveys based on the results of previous years' investigations. The study is a continuation of the extensive multidisciplinary studies of the Cenozoic succession in the eastern North Sea Basin, which were carried out in the early 1990s at the Department of Earth Sciences, University of Aarhus (Fig. 1; Michelsen et al., 1995, 1998). A sequence stratigraphic framework for the post-Danian siliciclastic succession of the eastern North Sea Basin was established, dividing the succession into twenty-one sequences, grouped into seven major sequence stratigraphic units (Fig. 2; Michelsen et al., 1995, 1998). These major sequence stratigraphic units are referred to as `units' in the present paper. The main factors limiting the outcome of these regional studies are: the limited amount of well data available outside the Central Trough, and the poor resolution of the upper 500 ms TWT (c. 0.5 km) of the available conventional seismic data. Thus, the objective of carrying out high-resolution

seismic investigations was to overcome the latter obstacle. The high-resolution seismic data have provided a re®ned picture of the morphology of four of the main sequence boundaries (unconformities) recognised by Michelsen et al. (1995, 1998): Top Chalk (base unit 1), top Eocene (base unit 4), near top Oligocene (base unit 5), and the mid-Miocene unconformity (base unit 7). The detailed mapping provided new insight into the structural, depositional and erosional processes that interacted to shape these boundaries, and thus yield new evidence regarding the Cenozoic development of the eastern North Sea Basin. The high-resolution seismic data also revealed a system of several hundred metres deep, overdeepened and buried Quaternary valleys and largescale glaciotectonic structures, related to the Pleistocene glaciations, in the uppermost 400-m subbottom. Similar structures have been widely recognised both on- and offshore Northwest Europe. The new seismic data provide an unprecedented resolution of the detailed architecture of such structures in the eastern North Sea.

M. Huuse et al. / Marine Geology 177 (2001) 243±269

245

Fig. 2. Chrono-, bio- and sequence stratigraphy of the Cenozoic succession in the eastern North Sea Basin (modi®ed after Michelsen et al., 1998). Lithostratigraphy onshore Denmark is according to Michelsen (1994, 1996). The composite d 18O curve is compiled from Miller et al. (1987, 1998). Absolute ages indicated to the left are according to Berggren et al. (1995).

246

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 3. (a) Late Palaeozoic±Mesozoic structures (according to Vejbñk, 1990; Vejbñk and Britze, 1994) and locations of exploration wells in the eastern Danish North Sea. (b) High-resolution (thick lines) and conventional (thin lines) seismic data and exploration wells used in the present study.

2. Geological setting 2.1. Structural setting of the North Sea Basin During the late Palaeozoic and Mesozoic, the North Sea region was dominated by two E±W trending basins, the Norwegian±Danish Basin and the North German Basin, separated by the Ringkùbing-Fyn High±Mid North Sea High (Ziegler, 1990). The eastern Danish North Sea is located above the late Palaeozoic and Mesozoic Ringkùbing-Fyn High, the eastern Central Graben, the Horn Graben, the North German Basin, and the Norwegian±Danish Basin (Fig. 3a). A thick succession of evaporites accumulated in the basins during the Late Permian. The evaporites were later mobilised as a result of faulting and loading by Mesozoic and Cenozoic sediments, forming numerous salt structures (Ziegler, 1990; KorstgaÊrd et al., 1993; Kockel, 1995; Fisher and Mudge, 1998). During the

late Palaeozoic and Mesozoic, the E±W trending basement highs were cut by the N±S oriented Horn Graben and by the NW±SE oriented Central Graben (Ziegler, 1990). During early Cretaceous time, extension along the basement structures largely ceased and the North Sea region became increasingly dominated by thermal subsidence centred along the axes of the Central Graben and the Danish Basin (Nielsen et al., 1986; Ziegler, 1990; Michelsen et al., 1995, 1998). The Sorgenfrei±Tornquist Zone, bordering the Norwegian±Danish Basin to the northeast, and the Central Graben suffered inversion in Late Cretaceous±Paleocene time (Liboriussen et al., 1987; Vejbñk and Andersen, 1987, 2001). The Sorgenfrei±Tornquist Zone probably constituted a topographic barrier in the early Paleogene (Gemmer et al., 2001), and it could have diverted ¯uvial supply of clastic sediments from Fennoscandia to the northwest and southeast of the Danish area (Clausen and Huuse, 2001).

M. Huuse et al. / Marine Geology 177 (2001) 243±269

247

During the Cenozoic, the North Sea Basin developed as an epeiric sea centred above the Central Graben and bordered to the east and northeast by the Fennoscandian landmass, central Europe to the south and the British Isles to the west (Fig. 1). The largely unfaulted Cenozoic trough, which developed above the Mesozoic Central Graben, is here referred to as the Central Trough. More than 2000 m of Paleogene and Neogene sediments, and approximately 1000 m of Quaternary sediments accumulated in the Central Trough, while the basin margins are characterised by uplift and erosion (Nielsen et al., 1986; DoreÂ, 1992; Jensen and Schmidt, 1992; Jensen and Michelsen, 1992; Jordt et al., 1995; Michelsen et al., 1995, 1998; Riis, 1996; Japsen, 1998; Clausen and Huuse, 1999; Clausen et al., 2000; Huuse, 2001). The age of the base Quaternary subcrop increases towards the basin margin, from Neogene siliciclastic sediments in the main part through Paleogene siliciclastic sediments to Upper Cretaceous±Danian limestone and Lower Cretaceous through to Triassic strata close to the Sorgenfrei±Tornquist Zone to the northeast of the study area (Sorgenfrei and Berthelsen, 1954; Jensen and Michelsen, 1992; Japsen, 1998).

massive deltaic wedges of late Paleocene±Eocene age, which were fed into relatively deep water off the Shetland Platform, are comparatively well preserved at present (cf. Jordt et al., 1995, ®g. 3). In Oligocene to middle Pleistocene time, the central and eastern North Sea Basin was ®lled by pro-deltaic and deltaic sediments supplied progressively from the N, NE, E and SE (Fig. 4; Spjeldnñs, 1975; Bijlsma, 1981; Zagwijn, 1989; Cameron et al., 1993; Jordt et al., 1995; Michelsen et al., 1995, 1998; Sùrensen and Michelsen, 1995; JuÈrgens, 1996; Michelsen, 1996; Sùrensen et al., 1997; Clausen et al., 1999: Fig. 9; Huuse, 2001). Since middle Pleistocene time the North Sea Basin has experienced at least three major glaciations causing the formation of vast ice sheets which repeatedly covered all or parts of the basin (e.g. Ehlers et al., 1984; Ehlers, 1990). The advance and melting of the ice sheets caused the formation of marginal deformation structures and deeply incised valleys throughout the Northwest European lowland and in the North Sea (Ehlers et al., 1984; Zagwijn, 1989; Wing®eld, 1989; Cameron et al., 1993; Huuse and Lykke-Andersen, 2000a,b).

2.2. Stratigraphic evolution of the eastern North Sea Basin

3. Database

In Late Cretaceous±Danian times, a thick limestone succession was deposited in the North Sea Basin, with a maximum of 2000 m in the Norwegian± Danish Basin (Liboriussen et al., 1987; Ziegler, 1990). In the eastern North Sea, limestone deposition was followed by a comparatively thin succession of hemipelagic clays and marls during the Late Paleocene and Eocene, while thick upper Eocene deposits accumulated in the Central Trough (Fig. 4; Michelsen et al., 1998). While hemipelagic sedimentation prevailed in most of the southern and eastern North Sea Basin in the Late Paleocene±Eocene, minor deltaic wedges accumulated at the NW tip of the Sorgenfrei±Tornquist Zone southwest of Norway (Fig. 4; Michelsen et al., 1995, 1998; Huuse, 2001). Late Cenozoic erosion of the Norwegian mainland and of Paleogene depocentres on the surrounding shelf makes it dif®cult to estimate the amount of sediment supplied from southern Norway in the early Paleogene. In contrast,

3.1. High-resolution multichannel seismic data Most of the high-resolution multichannel seismic pro®les (c. 5000 km) were acquired by the Department of Earth Sciences, University of Aarhus (AU) in 1994±1996, employing R/V Dana (6 fold; DA94, DA95, DA96). In 1997 and 1998 an additional 1400 km were acquired by AU and the Geological Survey of Denmark and Greenland, as part of a ®ve year mapping program, employing S/V Gribben (24 fold; GR97, GR98). Thus, 6400 km of high-resolution seismic data were available for the study (Fig. 3b). The seismic equipment consisted of a 70 cubic inch ( < 1.1 l) sleeve gun cluster ®red at 100 bar air pressure every 12.5 m, and a 24 channel (DA94±DA96) or 96 channel (GR97±GR98) streamer with a hydrophone-group separation of 6.25 m. This con®guration resulted in a common mid point separation of 3.125 m. Source and receivers were towed at 2 and 3 m below surface, respectively. The shallow towing

248

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 4. Cenozoic depocentres and sediment supply directions in the eastern North Sea (compiled from Bidstrup, 1995; Sùrensen et al., 1997; Michelsen et al., 1998; Clausen et al., 1999).

depths resulted in improved resolution but poorer signal/noise ratio when compared to the deeper towing depths of conventional seismic data. Data were recorded digitally and sampled every 1 ms, yielding a Nyquist frequency of 500 Hz. Poststack bandwidth of the migrated seismic data is 40± 180 Hz (DA94) or 40±250 Hz (DA95, DA96, GR97 and GR98). Penetration is of the order of 1±2 s two-

way traveltime (TWT) corresponding to depths of 1±2 km, thus reaching the base of the Cenozoic in the main part of the area (cf. Fig. 1). 3.2. Conventional seismic data Approximately 18 000 km of conventional seismic pro®les were interpreted, including the regional

M. Huuse et al. / Marine Geology 177 (2001) 243±269

249

purposes, TWT (s) may be read as depth (km). This means that seismic time-sections and time-structure maps provide valid representations of subsurface structure of the Cenozoic. 3.5. Seismic resolution

Fig. 5. Plot of depth (m) vs. TWT (ms) to the Top Chalk surface (mid Paleocene) for 11 exploration wells in the eastern Danish North Sea.

seismic surveys: CGT81, DCS81C, RTD81, SP82, and selected pro®les from the NP85N survey. The DK82 survey was used for fault mapping in the southern Horn Graben. Adjacent parts of the German North Sea sector were mapped using the GR86 and SNST83 surveys (Fig. 3b). Post stack bandwidth of the conventional seismic data is of the order of 10±60 Hz. The high energy of the conventional seismic source and the subsequent processing aimed at deep targets have caused the upper 0.3±0.5 s TWT ( < 0.3±0.5 km) to be virtually impossible to interpret due to noise, reverberations, etc. 3.3. Well data Well logs (GR, Sonic, Velocity and Resistivity) and stratigraphic reports, mainly based on cuttings analyses, were available from 13 exploration wells in the mapped area of the Danish and the southeastern Norwegian North Sea (Fig. 3a and b). 3.4. Sediment velocities A plot of depth versus TWT to the Top Chalk surface reveals that the average velocity of the postDanian siliciclastic deposits in the eastern North Sea is very close to 2000 m/s (Fig. 5). Hence, for most

A detailed account of seismic resolution is provided in Appendix A. The vertical resolution of the highresolution seismic data is of the order of 8±10 m whereas the resolution of the conventional seismic data is of the order of 25±35 m. The best possible horizontal resolution of the Cenozoic succession in the eastern North Sea is of the order of 20±25 m for the high-resolution and 70±80 m for the conventional seismic data, deteriorating with depth. In the top 1 s TWT (1 km), the resolution of the high-resolution data is thus about three times better than that of the conventional data, depending on the noise level of individual pro®les. 4. Major Cenozoic sequence boundaries in eastern Danish North Sea 4.1. The Top Chalk surface (base unit 1) The detailed structure of the Top Chalk surface has been mapped in the eastern Danish North Sea and adjacent areas of the Norwegian and German North Sea (Fig. 6; Huuse, 1999). Mapping was based on all available seismic and well data (Fig. 3b). The map provided the missing link between maps of the Central Trough (Britze et al., 1995) and onshore Denmark (Ter-Borch, 1990), and thus facilitated a compilation of Top Chalk maps covering the entire Danish area (Clausen and Huuse, 1999). The Top Chalk surface displays a gentle dip (,0.58) towards the west and southwest, which is disturbed by more than twenty anomalies caused by salt withdrawal and diapirism (Huuse, 1999). A number of faults offset the Top Chalk, with a maximum displacement of more than 500 m across the D-1 fault to the northwest (Fig. 6; Petersen et al., 1992). The northern and southern margins of the Ringkùbing-Fyn High east of the Horn Graben are outlined by trends of faults and ¯exures with offsets of 50±100 m and less (Fig. 6). The distribution of these structures coincide with the pinch-out of mobile Zechstein salt

250

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 6. Top Chalk time-structure map with locations of major erosional valleys indicated. Based on the data shown in Fig. 3b (modi®ed from Huuse, 1999).

(cf. Figs. 3a and 6). Consequently, the structures have been interpreted as collapse structures due to saltwithdrawal, possibly enhanced by salt dissolution (Huuse, 1999). Previous interpretations of the

structures as caused by strike-slip movements along hypothetical, deep-seated fault lineaments (e.g. Cartwright, 1990) have not been con®rmed in the present study (Clausen and Huuse, 1999; Huuse, 1999).

M. Huuse et al. / Marine Geology 177 (2001) 243±269

251

Fig. 7. High-resolution seismic pro®le showing erosional surfaces and carbonate (possibly bryozoan) mounds at the top of the Upper Cretaceous±Danian Chalk Group. See Fig. 6 for location.

A number of mound structures have been observed in the upper part of the Chalk Group on the Ringkùbing-Fyn High. The mounds are 1±2 km across, 50±100 m high, and are characterised by internal downlap and occasionally by truncation at the surface (Fig. 7). The plan geometry of the mounds has not been mapped due the small size of the mounds relative to the density of the seismic grid. Based on their internal structure and on correlations with the Danish land area (Thomsen 1995; Surlyk 1997), the mounds are interpreted as possible bryozoan mounds. Post-depositional erosion appears to have been most pronounced in the palaeo-bathymetric lows between the mounds (Huuse, 1999). Several erosional features have been observed at

the Top Chalk surface, including: erosional valleys (Figs. 6 and 8), minor depressions (Fig. 9), late Cenozoic truncation to the northeast (Fig. 6). Erosional valleys in the Top Chalk surface are widespread in the Norwegian±Danish Basin, on the RingkùbingFyn High and in the southwestern part of the area, whereas erosional features are absent to the southeast (Huuse, 1999). Individual valleys can be up to 2±3 km wide and 50±100 m deep (Fig. 8), and may be traced across the mapped area into the Central Trough (Fig. 6). The valleys have been interpreted as caused by submarine and possibly subaerial erosion associated with a mid-Paleocene sea-level fall, possibly in combination with regional tilting of the surface towards the west and southwest. The location of the

Fig. 8. High-resolution seismic pro®le across the mid-Paleocene Ibenholt Valley. The in®ll may be interpreted as coarse-grained deposits similar to the Lellinge Greensand onshore Denmark. See Fig. 6 for location.

252

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 9. High-resolution seismic pro®le showing small-scale depressions interpreted as mid-Paleocene pockmarks or possible karst structures. See Fig. 6 for location.

Ibenholt Valley along the northern ¯ank of the Ringkùbing-Fyn High suggests a possible in¯uence of underlying structures and facies variations of the underlying limestone, although a direct link has not been observed (Huuse, 1999; Clausen and Huuse, 2001). A unit containing prograded (i.e. high-energy) re¯ectors is observed in some of the valleys incised into the upper Cretaceous±Danian limestone. Seismic stratigraphic interpretations tied to the C-1 well (cf. Fig. 6) indicate that the unit is younger than the Danian Limestone and older than the lower Selandian North Sea Marl (Huuse, 1999). It is thus coeval withand possibly lithologically similar to the glauconiteand carbonate-rich, lowermost Selandian Lellinge Greensand described from onshore Denmark (cf. Gry, 1935). The prograded seismic facies is restricted to the valleys, suggesting that the in®ll could be late lowstand or early transgressive deposits (Huuse, 1999; Clausen and Huuse, 2001). Numerous minor depressions have been observed at the Top Chalk surface in the Norwegian±Danish Basin. The depressions are typically 5±10 m deep and 100±300 m wide (Fig. 9) and have similar appearances on seismic sections of all orientations. The similar appearance independent of strike suggests that the depressions are near-circular. This is con®rmed locally by recently acquired 3D seismic data, which reveal the occurrence of numerous circular depressions at Top Chalk in the Norwegian±Danish Basin. Two possible interpretations of the depressions have been considered based on a comparison with morphologically similar structures from the literature: karst structures and pockmarks (Huuse, 1999). Based on the timing of source rock maturity in the area (Neogene,

cf. Jensen and Schmidt, 1993) and on the purity of the carbonates, the latter possibility was considered unlikely, leaving an interpretation of the depressions as possible karst structures (Huuse, 1999). Karst structures at Top Chalk have also been interpreted on 2D seismic data from the southern North Sea by Jenyon (1984) and on 3D seismic data from the northern part of the Norwegian±Danish Basin by Heggland (1995). However, Heggland (1995) did not exclude that the structures could represent palaeo-pockmarks. Interpretation of the depressions as karst structures would have wide-reaching implications for palaeogeographic reconstructions as it would indicate that most of the eastern North Sea was subaerially exposed at the Danian/Selandian transition. Subaerial exposure of Top Chalk is also indicated by the presence of a transgressive lag containing phosphate aggregates and rolled echinoids immediately above the chalk onshore Denmark (cf. Gry, 1935; Thomsen, 1995) and on the East North Sea Block (Clausen and Huuse, 2001). The karst interpretation is, however, ¯awed by the location of the depressions in the Norwegian±Danish Basin, while similar structures are absent on the adjacent Ringkùbing-Fyn High, opposite to the expected distribution of karts features (Huuse, 1999). Moreover, recent drilling of a thick Pleistocene cool-water carbonate succession in the Great Australian Bight yielded large concentrations of biogenic methane and hydrogen sul®de in the shallow subsurface (Swart et al., 2000). The succession drilled in the Great Australian Bight is lithologically similar to the Upper Cretaceous±Danian limestone of the North Sea (cf. Surlyk, 1997) and it is thus possible that signi®cant quantities of biogenic gas was generated within the coccolith ooze of the late Cretaceous and early Paleocene North Sea. The episodic venting of this gas could have led to pockmark formation at the sea ¯oor of the mid-Paleocene North Sea. It is not known, however, whether the quantities of biogenic gas would have been suf®cient to cause intense pockmark formation in the carbonate ooze. Hence, the origin of the circular depressions remains enigmatic. 4.2. Top Eocene (base unit 4) Except for differential in®ll of mid-Paleocene erosional valleys, the upper Paleocene±Eocene succession generally drapes the top of the upper

M. Huuse et al. / Marine Geology 177 (2001) 243±269

253

Fig. 10. (a) High-resolution seismic pro®le showing channels in the top of the middle±upper Eocene Sùvind Marl Formation. (b) Isopach map of the Sùvind Marl Formation (modi®ed from Huuse and Clausen, 2001). See Fig. 6 for location and structural setting.

Cretaceous±Danian limestone throughout the study area (e.g. Bidstrup, 1995; Michelsen et al., 1998). The structure of the top Eocene unconformity thus resembles that of the smoothed-out Top Chalk surface (Huuse and Clausen, 2001). The top Eocene is characterised by downlap of Oligocene clinoforms to the north, whereas further to the south, it occurs within a conformable succession of ®ne-grained distal upper Paleocene to lowermost Miocene deposits (Figs. 10a and 11). Erosion at top Eocene has only been observed in a small area off the westcoast of Jutland (Figs. 6 and 10), where minor SW±NE striking channels cut into a local depocentre of the Middle±Upper

Eocene Sùvind Marl Formation (Fig. 10). The channels are 2±400 m wide, less than 50 m deep (Fig. 10a), up to 10±20 km long and are generally oriented parallel to the contours of the local depocentre (Fig. 10b). The location and orientation of the depocentre and the channels indicate a possible structural control by the SW±NE striking Holmsland Fault, which cuts the crystalline basement (Huuse and Clausen, 2001). Since the channels formed on the basin ¯oor, a considerable distance from the palaeo-coastline, they are most likely due to increased bottom current activity in a structurally controlled bathymetric low at earliest Oligocene time.

254

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 11. High-resolution seismic pro®le showing progradational Oligocene±Middle Miocene clinoforms and estimates of `missing section' (i.e. denudation) based on compaction analyses (Jensen and Schmidt, 1993; Japsen, 1998) and on seismic geometries (this study; modi®ed from Huuse, 2001). See Fig. 6 for location.

Observations onshore Denmark indicate that several periods of non-deposition and erosion occurred in the Late Paleocene±Eocene period (Heilmann-Clausen et al., 1985; Heilmann-Clausen, 1995). Extensive erosion of the basin margins is indicated by the abundant occurrence of reworked microfossils, and clay-mineralogy variations in the middle±upper Eocene succession (Clausen et al., 2000). Variations in the age of the uppermost Eocene deposits onshore Denmark (Dinesen et al., 1977; Clausen et al., 2000) indicate that substantial erosion also occurred on the basin ¯oor in the marginal parts of the basin. It is therefore puzzling that the channels at top Eocene are the ®rst morphological evidence for erosion in the eastern North Sea since the erosion of the top of the upper Cretaceous±Danian limestone (i.e. for more than 25 Myr).

4.3. Near top Oligocene (base unit 5) In the northern one-third of the study area, the near top Oligocene sequence boundary corresponds to the top of a more than 500 m thick deltaic wedge, which prograded towards the south and southwest during the Oligocene (unit 4, Figs. 4 and 11). Extensive postOligocene salt tectonics in the Norwegian±Danish Basin has caused the presence of numerous highs and lows in the area covered by the Oligocene delta (Fig. 12b). At 56830 0 N there is an abrupt increase in gradient of the SW-dipping delta top (Fig. 12b). This break corresponds to the most basinward clinoform breakpoint (of¯ap break) of the Lower to middle Upper Oligocene unit 4. Basinward of the breakpoint, the foreset part of the clinoform de®nes a 150±250 m high palaeo-depositional slope, increasing to more than 500 m towards the southwest (Fig. 12b). These

M. Huuse et al. / Marine Geology 177 (2001) 243±269

variations probably re¯ect lateral variations in palaeowater depth, with depths increasing towards the central North Sea (cf. Huuse, 2001). The deltaic wedge to the north consists of silty micaceous clays intercalated with thick, sandy near-shore deposits containing abundant lignite fragments (Danielsen et al., 1997; Huuse, 2001). Further to the south, basinward of the clinoform breakpoint, sedimentation was characterised by silty micaceous clays deposited in an outer shelf to upper bathyal environment (Laursen, 1995; Michelsen et al., 1998). The near top Oligocene boundary is identi®ed by a marked downward shift in onlaps (Figs. 11 and 13; Michelsen et al., 1995, 1998). Marked downward shifts in onlap are usually associated with major falls in relative sea level (e.g. Vail, 1987). Consequently, the boundary has been interpreted as caused by a major relative sea-level fall (Michelsen et al., 1995, 1998). Such a marked downward shift in onlap positions should be associated with the formation of deeply incised valleys; if the onlaps are coastal onlaps, that is. Detailed analyses of a dense grid of high-resolution and conventional seismic data (Fig. 3b) did not yield any evidence of incised valley erosion along the almost 200 km of the clinoform breakpoint covered by the study area (cf. Huuse and Clausen, 2001). Erosional truncation was observed, but it was parallel to the strike of the clinoform breakpoint (Fig. 12b), i.e. perpendicular to the dip-parallel directions of valley incision normally associated with sea-level falls (cf. Van Wagoner et al., 1990; Fulthorpe et al., 1999). The erosional features form up to 50 m high, concave-upward scarps on seismic dip-sections and resemble slump scars associated with mass-failure (Fig. 13, cf. Hampton and Locat, 1996; Huuse and Clausen, 2001). A prominent onlapping re¯ection was traced from the toe of the slope in the central part of the area towards the east where it onlaps gradually higher on the slope, before ®nally overstepping the clinoform breakpoint in the easternmost part. This pattern indicates that the observed downward shift in onlaps was caused by a lateral shift in sediment input rather than a drop in sea level, and that the onlaps are distal (i.e. not coastal) onlaps (Huuse and Clausen, 2001). The occurrence of inferred slumps scars and the geometry of the onlapping succession indicate that the near top Oligocene clinoform breakpoint

255

experienced sediment starvation and submarine exposure due to a shift in sediment input directions from north to northeast. The erosional and depositional geometries observed at the near top Oligocene sequence boundary thus seems to be a result of large scale delta-lobe switching (Huuse and Clausen, 2001). Similar large-scale lobe-switching is well-documented in the Gulf of Mexico (cf. Galloway, 1989). Moreover, the lateral variations in onlap positions of individual re¯ections indicate that the onlaps should be interpreted as `oblique' distal onlaps, which contain no information about sea-level changes (cf. Huuse, 2001; Huuse and Clausen, 2001). The effect of a latest Oligocene change in sediment input directions (from N to NE) is illustrated by comparing the map of the near top Oligocene unconformity (Fig. 12b) with that of the mid-Miocene unconformity (Fig. 12c). By the latest Oligocene± earliest Miocene, the previously sediment starved eastern Danish North Sea had become the locus of sedimentation, as evidenced by thick lower Miocene progradational wedges observed on seismic data (unit 5; Figs. 4 and 11). The ®rst occurrence of near-shore deposits onshore Denmark (the Vejle fjord Formation; Larsen and Dinesen, 1959; Friis et al., 1998) since the lowermost Selandian Lellinge Greensand thus seems to be a result of changes in the sediment input direction. However, the abrupt decrease in palaeo-water depth represented by the transition from the Branden Clay to the Vejle Fjord Formation also indicates that a sea-level lowering occurred in the latest Oligocene (Michelsen et al., 1998). Such a sea-level fall could be the mechanism triggering the shift in sediment supply. 4.4. The mid-Miocene unconformity (base unit 7) The mid-Miocene unconformity takes on a variety of expressions in the North Sea Basin; it is an onlap surface towards the northeast, a downlap surface in the southern North Sea, and a conformable surface (below a downlap surface) in the Central Trough (cf. Clausen, 1995; Kockel, 1995; Sùrensen et al., 1997; Huuse and Clausen, 2001). In the present study area, the mid-Miocene unconformity constitutes the top of the Oligocene±lower Middle Miocene delta system, which ®lled all available accommodation in the eastern Danish North Sea, except for the

256 M. Huuse et al. / Marine Geology 177 (2001) 243±269 Fig. 12. (a) Top Chalk (mid Paleocene, and similar to top Eocene), (b) near top Oligocene, (c) mid-Miocene unconformity, and (d) base Quaternary time-structure maps. The progressive change in the trend of the contours from (a) N±S to (b) E±W and N±S to (c) NW±SE and ®nally (d) N±S show the progressive ®lling-up of the eastern Danish North Sea during the Oligocene to Pleistocene.

257

Fig. 12. (continued)

M. Huuse et al. / Marine Geology 177 (2001) 243±269

258

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 13. (a) High-resolution seismic pro®le showing erosional truncation of the near top Oligocene boundary. See Fig. 12b for location. (b) High-resolution seismic pro®le showing erosional scarp at the mid-Miocene unconformity. See Fig. 12c for location. Modi®ed from Huuse and Clausen (2001).

Fig. 14. Interpreted seismic pro®le across the northern North Sea (after Jordt et al., 1995). See Fig. 1 for location.

southwesternmost part (Figs. 11 and 12c; cf. Huuse, 2001). The deposits below the boundary correlate with those of the marine Arnum Formation and the ¯uvial Ribe and Odderup Formations onshore Denmark (Fig. 2; Rasmussen, 1961; Michelsen, 1994; Laursen, 1995; Huuse, 2001). Disturbance by salt tectonics is marked, although less pronounced than at the near top Oligocene unconformity (cf. Fig. 12b and c). This indicates that signi®cant salt movement took place in the latest Oligocene±early Middle Miocene, but also in post-middle Miocene time. A minor wedge, which onlaps the base of the clinoform constituted by the mid-Miocene unconformity (Fig. 11) contains ®ne-grained sediments coeval with the transgressive marine Hodde Formation onshore Denmark (DGU, 1975; Laursen, 1995). The onlap position of this wedge was mapped along strike of the mid-Miocene clinoform, and it appears to overstep the clinoform towards the east (Huuse and Clausen, 2001). The erosional truncation observed along the mid-Miocene unconformity is markedly similar to the truncation observed at near top Oligocene; it is contour parallel, located at the clinoform breakpoint, and resembles slump scars formed by mass-failure (Fig. 14). In addition, shallow erosional structures, which could have been caused by bottom current erosion, have been observed seaward of the clinoform breakpoint. It appears that the erosional morphology and the onlap geometry observed at the mid-Miocene unconformity is most likely due to a change in sediment input directions, from northeast to east (cf. Huuse, 2001; Huuse and Clausen, 2001). This change may represent large scale autocyclicity of the Miocene delta system or it could be related to midMiocene compressional tectonics and a possible renewed inversion of the Sorgenfrei±Tornquist Zone. The continued ®lling up of the eastern Danish North Sea Basin in post-middle Miocene time is illustrated by comparing the contours of the midMiocene unconformity and base Quaternary (Fig. 12c and d). The southern part of the Danish area was ®lled by the peripheral parts of a huge delta complex, which prograded across the southern (German/Dutch) North Sea in late Middle Miocene± early Pleistocene time (Fig. 4; Gramann and Kockel, 1988; Sùrensen et al., 1997; Huuse, 2001).

M. Huuse et al. / Marine Geology 177 (2001) 243±269 18

5. Major sequence boundaries and the global d O curve: climatic vs. tectonic control on sedimentation Global and regional d 18O records provide detailed records of glacioeustatic and global as well as regional climatic changes for the post-Middle Eocene (Fig. 2; Buchardt 1978; Miller et al., 1987, 1998). Abrupt increases in d 18O values are here termed `d 18O events'. Such events are usually associated with eustatic falls (Miller et al., 1987, 1998; Abreu and Anderson, 1998). Information about the early Paleogene lacks detail and a correlation between the Top Chalk surface (mid Paleocene) and the d 18O record therefore has little meaning (Fig. 2). Uncertainties in the dating of the North Sea sequences (approximately ^2 Myr) makes it dif®cult to unequivocally correlate the North Sea sedimentary record with speci®c d 18O events. There seems to be an overall correspondence between the number and timing of North Sea sequence boundaries and d 18O events for the Oligocene±middle Miocene interval (Fig. 2). This seems to indicate a signi®cant glacioeustatic in¯uence on North Sea sequence development in this period (Clausen et al., 1999; Huuse and Clausen, 2001). However, some of the major boundaries appear not to correlate with signi®cant d 18O events, indicating forcing by factors other than eustatic lowerings. Below, we focus on the correlation between the major sequence boundaries and the global d 18O record in order to extract independent information about the origin of the boundaries and about the factors controlling the evolution of the eastern North Sea Basin. The top Eocene sequence boundary (top unit 3) correlates with an interval of long term d 18O increase (Fig. 2), corresponding to a eustatic lowering and climatic deterioration in the late Eocene±earliest Oligocene. It is accompanied by a marked increase in sediment supply from southern Norway (Michelsen et al., 1998). The near top Oligocene unconformity (top unit 4) and the mid-Miocene unconformity (top unit 6) do not correlate with major d 18O events (Fig. 2). The mid-Miocene unconformity appears to correlate with a local low in d 18O values (Fig. 2) representing a time of high global sea level and relatively warm climate. This could explain the condensed character of the middle Miocene deposits in many parts of the North Sea Basin (Fig. 2: Hodde

259

Formation; Michelsen et al., 1995, 1998; Huuse, 2001). A pronounced long-term d 18O increase begins c. 1 Myr later than the time interval represented by the mid-Miocene unconformity (Fig. 2). The magnitude of the mid-Miocene increase in d 18O values appears to be comparable to the terminal Eocene event. The Eocene/Oligocene transition and the postmiddle Miocene are characterised by abrupt increases in sediment ¯ux in the eastern North Sea (Michelsen et al., 1995, 1998) and globally (cf. Donnelly, 1982; Bartek et al., 1991; Miller et al., 1996, 1998; SeÂranne, 1999; Steckler et al., 1999). Pene-contemporaneous increases in sediment ¯ux along widely separated continental shelves indicates that the increases in sediment supply are associated with global, rather than regional, forcing factors, i.e. that climatic change and eustatic fall, rather than regional tectonics were the dominant forcing factors (cf. Donnelly, 1982; Bartek et al., 1991; Miller et al., 1996, 1998; Steckler et al., 1999; Huuse, 2001; Huuse and Clausen, 2001). The notion of global forcing factors is supported by the close correlation between the global increases in sediment ¯ux and major intervals of long-term d 18O increase at the Eocene/Oligocene boundary and in the post-middle Miocene. It is now well known that these were times of intensi®ed southern hemisphere glaciation and consequent eustatic lowering (e.g. Lear et al., 2000). The North Sea d 18O record (Fig. 2; Buchardt, 1978) also displays major increases at these times, showing that the effects of climatic change were felt as much in the North Sea as elsewhere. These considerations suggest that major increases in the sediment supply to the North Sea, and other areas, at terminal Eocene and in post-middle Miocene time could be related to climatic cooling and eustatic fall. Such an explanation diminishes the need for globally synchronous tectonic uplift and subsidence events, the cause of which are conjectural (cf. Molnar and England, 1990; Huuse, 2001).

6. Cenozoic uplift and denudation 6.1. Terminology The term `Neogene uplift' is often used without speci®cation of what is actually being uplifted and often encompasses uplift and/or denudation, which

260

M. Huuse et al. / Marine Geology 177 (2001) 243±269

occurred during Paleogene and Quaternary time. In fact, `Cenozoic uplift and denudation' would often seem a more appropriate term. Here, we assume that the quantity most often referred to be the uplift of rocks with respect to the earth's surface, i.e. denudation, since this is a quantity that can be estimated from the rock record. Uplift of the earth's surface relative to the geoid or sea level is, on the other hand, notoriously dif®cult to quantify when marine deposits are absent in the uplifted area (Molnar and England, 1990). When discussing uplift, it is important to differ between regional (averaged over areas of 10 3 ± 10 4 km 2) uplift of the earth's surface and the uplift of mountain peaks. Regional uplift of the earth surface requires a tectonic driving force of large magnitude (or a eustatic fall if global sea level is used for reference). In contrast, uplift of mountain peaks and of points within the earth's crust may occur as the isostatic response to denudation and incision of deep valleys (Gilchrest et al., 1994). In theory, the isostatic response to dissection of a high-elevation peneplain could cause mountain peaks to rise to twice their initial elevation, while the mean surface elevation would only be lowered by about 1/5±1/6, depending on the density of the eroded rocks (Molnar and England, 1990). Moreover, it should be borne in mind that due to isostasy, a mean surface lowering of 200 m due to denudation would require about 1 km of the earth's crust to be removed. 6.2. Uplift and denudation of southern Norway It has been proposed that the present-day topography of southern Norway could be due to regional surface uplift of the south Norwegian dome, related to the North Atlantic opening in the early Paleogene, followed by denudation and dissection of the uplifted area (e.g. DoreÂ, 1992; Huuse, 2001). Denudation by dissection of topography in southern Norway would have been accelerated during the middle and late Cenozoic periods of climatic deterioration and eustatic fall, as indicated by d 18O increases (Fig. 2), culminating with full glacial condidtions in the late Pliocene and Pleistocene. This scenario is in agreement with ®ssion track data (Rohrman et al., 1995) and geomorphological investigations (LidmarBergstroÈm et al., 2000) from southern Norway, which indicate that mountain peaks only experienced

minor Cenozoic denudation while adjacent valleys were deeply incised (.1 km) during the late Cenozoic. Large-scale depositional geometries observed on seismic pro®les across the northern North Sea show that the bulk of the Paleogene sediments preserved in the northern North Sea were derived from Scotland and the Shetland Platform (Fig. 14; Jordt et al., 1995). This observation seems to contradict a scenario involving early Paleogene uplift of southern Norway. However, these pro®les also show that the Paleogene and early Neogene sedimentary wedges are largely complete on the Shetland Platform whereas they are truncated at a high angle towards southern Norway (Fig. 14). Further north in the North Sea, remnants of thick Paleocene wedges are preserved locally off the Norwegian mainland (Jordt et al., 1995, 2000). The highly truncated wedges indicate that substantial volumes of sediments of Norwegian provenance were deposited offshore southern Norway during the Paleogene, only to be eroded during the late Cenozoic in response to eustatic lowering and uplift in response to denudation of the adjacent landmass. Hence, signi®cant topography must have existed in southern Norway during the Paleocene. It is well documented that denudation may lag uplift by several tens of Myr (Summer®eld and Brown, 1998). Hence, it can be argued that tectonic uplift of southern Norway occurred in the early Paleogene and that the resulting topography may have persisted to the late Cenozoic. The amount of surface uplift is dif®cult to quantify, but mean surface elevation subsequent to uplift may well have been much like the present, i.e. a broad dome of about 1±1.5 km maximum elevation (when averaged over 10 3 ±10 4 km 2). The highly varying present-day topography of southern Norway thus probably re¯ects the combined effects of relict topography, early Paleogene tectonic uplift, and subsequent dissection of the topography in response to climatic deterioration and eustatic fall during the late Cenozoic. Hence, it appears that Neogene tectonic mechanisms need not be invoked in order to explain the regional denudation pattern of the eastern North Sea Basin and the presentday relief of the south Norwegian dome. 6.3. Denudation of the eastern Danish North Sea A variety of proxies such as ®ssion tracks, vitrinite

M. Huuse et al. / Marine Geology 177 (2001) 243±269

re¯ectance and compaction trends have been used to infer denudation in the North Sea area. This has led to widely varying denudation estimates for the eastern North Sea, although all studies show increased denudation towards the northeast (Fig. 11; Jensen and Michelsen, 1992; Jensen and Schmidt, 1992, 1993; Japsen, 1998; Japsen and Bidstrup, 1999). The maximum denudation estimates are derived from sonic compaction trends of Jurassic shales, yielding estimates of up to 1000 m denudation in the northeastern Danish North Sea (Fig. 11; Jensen and Michelsen, 1992; Jensen and Schmidt, 1992, 1993). Estimates based on compaction trends of the Upper Cretaceous±Danian Chalk Group are somewhat lower, yielding estimates of up to 650 m of the northeastern Danish North Sea (Fig. 11; Japsen, 1998). Revised chalk compaction studies integrated with basin modelling and vitrinite re¯ectance data yield even lower estimates of 4±500 m in the northeastern Danish North Sea (Japsen and Bidstrup, 1999). These estimates are all based on borehole measurements and samples, while seismic data have not been incorporated. In Fig. 11, the estimates of denudation from seismic geometries are compared to those of shale and chalk compaction. The denudation estimate from extrapolation of stratal geometries observed on the seismic data amounts to about 200 m in the northern part of the pro®le, which corresponds to the northeastern Danish North Sea. The denudation estimate based on stratal geometries is thus several hundreds of metres less than estimates based on compaction trends. Hence, it appears that previous estimates based on compaction trends may have severely over-estimated the amount of denudation in the eastern Danish North Sea. This begs a re-evaluation of borehole-based denudation studies performed elsewhere in the North Sea by incorporating stratal geometries observed on seismic data.

7. Quaternary 7.1. Base Quaternary The base of the Quaternary was mapped as a smooth continuous surface (Fig. 12d). This was done in order to provide as clear structural information as possible. However, the true morphology of the

261

base Quaternary is heavily modi®ed by deeply incised middle and late Pleistocene valleys, which cut up to 400 m into the Cenozoic of the eastern North Sea (Fig. 15; Salomonsen, 1993, 1995; Huuse and LykkeAndersen, 2000a). The overall structure of the surface is a smooth WSW-dipping trend, which has been affected locally by minor salt-induced faulting. Occasionally, the base Pleistocene has been offset by glaciotectonic thrust structures (Fig. 16). Erosional features other than subglacial valleys are largely absent at the base Quaternary in the western half of the study area. Further to the east and northeast and onshore Denmark the base Quaternary is a regional erosional unconformity truncating increasingly older strata towards the northeast. 7.2. Quaternary valleys An intricate system of completely buried, overdeepened valleys has been mapped in the eastern Danish North Sea, based on the approximately 6400 km of high-resolution seismic pro®les available (Figs. 3b and 17). The overall pattern of buried valleys is a N±S trending (including NE±SW and NW±SE), anastomosing system (Huuse and Lykke-Andersen, 2000a). The trend of this system is at variance with an E±W trending system mapped earlier, using conventional seismic data (Salomonsen, 1993, 1995). The conventional seismic data used in that study are of very poor quality in the top 300± 500 ms TWT, and hence it is believed that the map presented here (Fig. 17) is considerably closer to the actual picture. However, additional complexities are likely to be revealed by future acquisition of highquality, high-resolution seismic data. Individual valleys are normally characterised by irregularity, in cross-section, longitudinal pro®le and traceÂe. The width of the valleys varies between some hundred metres and a few kilometres, and depth of incision amounts to 350±400 ms TWT (.300 m subbottom, Fig. 15), although depths of 250±350 ms TWT are more common (Fig. 17). The valleys generally begin and terminate rather abruptly. Valleys west of Jutland can be traced for several tens of kilometres, in one case more than hundred kilometres. The pattern of valleys in the northern and western parts of the survey area is less well de®ned. This is partly due to insuf®cient seismic coverage and partly due to a more

262

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Fig. 15. High-resolution seismic pro®le across one of several deep buried valleys (modi®ed from Huuse and Lykke-Andersen, 2000a). See Fig. 17 for location.

widespread and generally shallower depth of erosion towards the north. In the southeastern part of the Danish North Sea valleys are almost absent (Fig. 17). Comparisons between the mapped distribution of valleys and structural lineaments indicate that the distribution of valleys is independent of subsurface structures (Huuse and Lykke-Andersen, 2000a: Fig. 11). A comparison with the distribution of coarsegrained vs. ®ne-grained sediments in the Cenozoic

of the eastern North Sea shows that the intensity of valley erosion was greatest in areas of low-permeability substrate (Huuse and Lykke-Andersen, 2000a). The valley ®ll may be chaotic or it may have a seismically well-ordered internal structure. Several erosion/®ll cycles may be discerned although individual surfaces/units are dif®cult to trace between seismic pro®les with the present line spacing (10±20 km). The valleys mapped in the eastern Danish North Sea

Fig. 16. High-resolution seismic pro®le showing inferred glaciotectonic structures detaching above the mid-Miocene unconformity (modi®ed from Huuse and Lykke-Andersen, 2000b). See Fig. 17 for location.

M. Huuse et al. / Marine Geology 177 (2001) 243±269

263

Fig. 17. Map of overdeepened Quaternary valleys and glaciotectonic thrust structures in the eastern Danish North Sea (modi®ed from Huuse and Lykke-Andersen, 2000b).

264

M. Huuse et al. / Marine Geology 177 (2001) 243±269

seem to be analogous to overdeepened Quaternary valleys known from onshore Denmark, northern Germany, Poland, and from the central Danish, UK, German and Dutch North Sea areas (cf. Pasierbski, 1979; Ehlers et al., 1984; Ehlers and Linke, 1989; Wing®eld, 1989; Laban, 1995; Schwarz, 1996; Praeg, 1997; Huuse and Lykke-Andersen, 2000a: Fig. 4). Timing of incision and the lithology of the sediments in the valleys offshore Denmark are poorly constrained and rely on comparisons with morphologically similar valleys in the adjacent North Sea and onshore areas (see Ehlers et al., 1984; Huuse and Lykke-Andersen, 2000a, and references therein). The major valleys of Northwest Europe are largely ®lled with melt-water sands and marine/lacustrine clays and have generally been dated as Elsterian (e.g. Ehlers et al., 1984). An Elsterian age also seems probable for the offshore Danish valleys, although a Saalian age can not be excluded, as both glaciations covered the study area entirely, while the Weichselian glaciation only reached the northeastern part (Ehlers et al., 1984; Huuse and Lykke-Andersen, 2000a). The in®ll of the offshore Danish valleys may be interpreted in terms of chaotic or well-ordered seismic facies, possibly representing melt-water sand and lacustrine/marine clay/silt, respectively. Small, seismically chaotic units bounded by high amplitude re¯ections may represent more coarse-grained (till?) deposits (Fig. 15; Huuse and Lykke-Andersen, 2000a). The formation of major overdeepened valleys has mainly been attributed to peri- or subglacial erosion by melt water, erosion by glacier ice or a combination of these (see Pasierbski, 1979; Ehlers and Linke, 1989; Huuse and Lykke-Andersen, 2000a and references therein). The major valleys in the eastern Danish North Sea are often associated with compressional deformation structures, which are interpreted as glaciotectonic thrust structures. In places, the structures have been truncated by valleys, indicating that deformation occurred prior to valley incision (Huuse and Lykke-Andersen, 2000a,b). As extensive glaciotectonic deformation appears to pre-date subglacial valley erosion within the study area, it appears that subglacial meltwater was the main erosional agent responsible for cutting the valleys in the eastern Danish North Sea.

7.3. Glaciotectonic thrust structures The distribution of inferred glaciotectonic thrust structures in the eastern Danish North Sea has been mapped based on the 6400 km of high-resolution multichannel seismic data (Figs. 3b and 17). The individual thrust segments are generally 400±1000 m long and 150±250 m thick with up to 200 m of lateral displacement (Fig. 16; Huuse and Lykke-Andersen, 2000b). The regional distribution of thrust structures is rather patchy (Fig. 17). This is at least partly due to truncation by the deep subglacial valleys, which postdate the formation of the thrust structures, but it may also re¯ect an originally patchy distribution. The individual patches are generally located along a NW±SE trend from 6820 0 E, 56840 0 N to 8815 0 E, 55815 0 N (Fig. 17; Huuse and Lykke-Andersen, 2000b). In the main part, the thrust direction was towards the southwest with minor components towards the south and west, whereas in the southeasternmost part, thrusting was towards the west (Fig. 17). The thrust directions are thus parallel to inferred directions of ice movement during the Elsterian and Saalian glaciations, with the main southward push being generated by Norwegian glaciers and the westward push in the southeast supplied by an ice advance from the Baltic (cf. Ehlers, 1990). The thrust directions are, however, also parallel to the dip of the underlying surface of detachment. The detachment surface corresponds to the mid-Miocene unconformity in the east and to the base Quaternary towards the west. Detachment is everywhere located at depths of 150±300 ms TWT (120±240 m) below the presentday sea ¯oor (Huuse and Lykke-Andersen, 2000b). The overall picture of N±S trending subglacial valleys and southward directed thrust structures is in agreement with a model of southward directed ice advance from southern Norway into the NW European lowland, followed by deep erosion by subglacial meltwater ¯owing southward towards the ice margin. Locally, westward thrusting was caused by an ice advance from the Baltic depression. Such advances took place both during the Elsterian and the Saalian glaciations, but the Elsterian glaciation left the most pronounced trace in the form of deep valleys onshore NW Europe (Ehlers et al., 1984; Huuse and Lykke-Andersen, 2000a). If the pattern in the North Sea is similar, this would constrain the timing of both

M. Huuse et al. / Marine Geology 177 (2001) 243±269

thrusting and valley erosion to be within the Elsterian. However, as there is no direct connection between the well-dated valleys in northern Germany and the valleys in the eastern Danish North Sea, this relationship has yet to be con®rmed by precise dating of the North Sea valleys or by a ®rm correlation with and dating of subglacial valleys onshore Denmark. 8. Conclusions Detailed time-structure maps of four major sequence boundaries (unconformities) in the eastern North Sea: the Top Chalk surface, near top Oligocene, the mid-Miocene unconformity and base Quaternary show that the eastern Danish North Sea has been affected by faulting and salt diapirism throughout the Cenozoic. Previously unrecognised erosional features were mapped at Top Chalk, top Eocene, near top Oligocene, and at the mid-Miocene unconformity. Possible bryozoan mounds were observed in the Danian above the Ringkùbing-Fyn High, indicating a shallowing of the early Paleocene North Sea across the ancient basement structure. Valleys and possible karst structures at Top Chalk suggest that parts of the area was subaerially exposed at the Danian/Selandian transition. Seismic facies interpretations indicate that deposits in®lling the valleys at Top Chalk may resemble the coeval Lellinge Greensand observed onshore Denmark. Erosional morphologies observed along near top Oligocene and at the mid-Miocene unconformity resemble slump scars caused by mass-failure. The slump scars indicate that erosion occurred during a transgression caused by large-scale switching of the delta complex, which prograded across the Danish area in Oligocene±Miocene time. Large-scale switching of the deltas may have been entirely autocyclic, but it is also possible that avulsion was triggered by inversion tectonics in the hinterland. The combined time-structure maps and palaeogeographic reconstructions show that large sediment wedges (deltas) progressively ®lled up the eastern and central North Sea Basin from the north, NE, E, SE and ®nally from the south. Clinoform heights of the deltas indicate that water depths in the central parts of the basin were larger than 500 m, while the eastern and

265

southern parts of the present-day North Sea were characterised by basinal water depths of the order of 150±250 m. Correlation of major North Sea sequence boundaries with long-term trends of regional and global (inferred climatic/eustatic) d 18O records indicates that the marked increases in sediment supply to the North Sea at terminal Eocene and in post-middle Miocene time be related to global climatic cooling and eustatic fall. Large-scale stratal geometries observed on the seismic data indicate that late Cenozoic uplift and erosion of the eastern Danish North Sea was less marked than suggested by previous studies. It is suggested that the present topography of southern Norway and the denudation pattern on the adjacent shelf may be due to early Paleogene uplift of the south Norwegian dome followed by denudation and dissection of the topography, accelerating in the late Cenozoic. Acceleration of denudation and incision rates was controlled by long-term eustatic lowering and climatic deterioration through the middle and late Cenozoic, culminating with full glacial conditions during the Plio-Pleistocene. Geo-dynamic modelling involving mass-balanced palaeogeographic reconstructions need to be carried out in order to test this hypothesis. The high-resolution seismic data facilitated detailed mapping of an intricate system of more than 300 m deep, 2±3 km wide and several tens of km long, buried and overdeepened Quaternary valleys. The valleys are interpreted to be of Elsterian and possibly Saalian age and they were probably caused mainly by subglacial meltwater erosion with direct glacial erosion being of secondary importance. The valleys pre-date a system of spectacular, inferred glaciotectonic, thrust structures detaching at the midMiocene unconformity to the east and at the base Quaternary to the west. It is concluded that the high-resolution multichannel seismic data have brought signi®cant new insight into the Cenozoic development of the eastern North Sea Basin. However, substantially more high-resolution 2D seismic data or, preferably, 3D seismic data are required to fully unravel the morphology and origin of the structural, depositional and erosional features observed at several levels within the Cenozoic succession of the eastern Danish North Sea.

266

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Acknowledgements Mads Huuse was supported by the Faculty of Science, University of Aarhus and by the Danish Natural Science Research Council, who also sponsored the seismic surveys DA94, DA95, and DA96 (grant nos. 9401161 and 9502760). The ®nal draft of this article was produced as part of the EFP-2000 programme (project no. ENS-1313/00-0001). We thank colleagues at the Department of Earth Sciences, University of Aarhus, and at Lamont-Doherty Earth Observatory of Columbia University for many enlightening discussions. Reviews and comments by Thomas D.J. Cameron, Ole V. Vejbñk and Herve Chamley are greatly appreciated.

Appendix A A1. Seismic resolution Seismic resolution may be de®ned as the ability to image closely spaced (or narrow), re¯ecting features and place them in their correct spatial position (cf. Yilmaz, 1987; Christie-Blick et al., 1990). Alternatively, resolution may be viewed as the ability to tell that more than one feature is contributing to an observed event (Sheriff, 1985). Vertical resolution may be de®ned as the closest vertical spacing of two re¯ecting interfaces, which can be discerned on a seismic section. A measure often used is a quarter of the dominant wavelength of the seismic data (l /4; e.g. Sheriff, 1985; Yilmaz, 1987). This value is equal to the tuning/dimming thickness and is likely to overestimate the effective resolution, especially in the presence of random noise and multiples. Consequently, half the dominant wavelength may be a more realistic measure of vertical seismic resolution. The dominant wavelength, l is calculated as the interval velocity, n [m/s] of the stratigraphic interval in question, divided by the dominant frequency, f [s 21]: l ˆ n=f : The dominant frequency of the high-resolution seismic data is of the order of 100±120 Hz, whereas that of the conventional seismic data is of the order of 30±40 Hz. Since the average interval velocity of the post-Danian Cenozoic succession in the eastern Danish North Sea is close to 2000 m/s (Fig. 5), it

follows that half the dominant wavelength (and thus the resolution) of the high-resolution seismic data is of the order of 8±10 m. Similarly, the resolution (l /2) of the conventional seismic data is of the order of 25±35 m, i.e. a factor of 3 poorer than the highresolution data. The difference in resolution decreases with increasing depth of investigation as the higher frequencies of the high-resolution seismic data are attenuated more rapidly than the lower frequencies of the conventional seismic data. Moreover, the comparatively low energy of the high-resolution seismic source and the shallow towing depths make the high-resolution seismic data more sensitive to random noise than the conventional seismic data. It is estimated that the resolution of the two data types attain comparable values (25±35 m) at a depth of 1±1.5 km, depending on subsurface geology and on the noise level of individual pro®les. Horizontal resolution may be de®ned as the smallest horizontal feature, which can be discerned on a seismic pro®le. On unmigrated seismic sections, this value equals the width of the ®rst Fresnel zone (Sheriff, 1985; Yilmaz, 1987). Three-dimensional migration collapses the Fresnel zone to approximately the dominant wavelength (Yilmaz, 1987). On 2D migrated seismic sections, like those used in this study, the Fresnel zone is collapsed along the seismic line but unaffected perpendicular to the line (Yilmaz, 1987). The exact value is thus dif®cult to quantify, but it is somewhat larger than the dominant wavelength (Yilmaz, 1987). The best horizontal resolution of the Cenozoic succession in the eastern North Sea is thus likely to be of the order of 20±25 m for the highresolution and 70±80 m for the conventional seismic data, deteriorating with depth. References Abreu, V.S., Anderson, J.B., 1998. Glacial eustasy during the Cenozoic: sequence stratigraphic implications. Am. Assoc. Petrol. Geol. Bull. 82, 1385±1400. Bartek, L.R., Vail, P.R., Anderson, J.B., Emmet, P.A., Wu, S., 1991. Effect of Cenozoic ice sheet ¯uctuations in Antarctica on the stratigraphic signature of the Neogene. J. Geophys. Res. 96 (B4), 6753±6778. Berggren, W.A., Kent, D.V., Swisher III, C.C., Aubry, M.-P., 1995. A revised Cenozoic geochronology and chronostratigraphy. In: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology, time scales and global stratigraphic

M. Huuse et al. / Marine Geology 177 (2001) 243±269 correlation. Soc. Sedim. Geol. (SEPM) Spec. Publ. 54, 129± 212. Bidstrup, T., 1995. Seismic sequence stratigraphy of the Tertiary in the Danish North Sea; incl. 7 structural maps of the main sequences, and 35 isochore maps of main sequences and smaller units. EFP-92 project: Basin development of the Tertiary of the Central Trough with emphasis on possible hydrocarbon reservoirs 5. Danish Energy Agency, 25 pp. Bijlsma, S., 1981. Fluvial sedimentation from the Fennoscandian area into the north-west European Basin during the Late Cenozoic. Geologie Mijnbouw 60, 337±345. Britze, P., Japsen, P., Andersen, C., 1995. The Danish Central Graben. Top Chalk and the Post Chalk Group (two-way traveltime, depth and interval velocity). Geological map of Denmark, 1:200 000. Geological Survey of Denmark, Map Series 47. Buchardt, B., 1978. Oxygen isotope palaeotemperatures from the Tertiary period in the North Sea area. Nature 275, 121±123. Cameron, T.D.J., Bulat, J., Mesdag, C.S., 1993. High resolution seismic pro®le through a Late Cenozoic delta complex in the southern North Sea. Mar. Petrol. Geol. 10, 591±599. Cartwright, J., 1990. The structural evolution of the RingkùbingFyn High. In: Blundell, D.J., Gibbs, A.D. (Eds.), Tectonic Evolution of the North Sea Rifts. Clarendon Press, Oxford, pp. 200±216. Christie-Blick, N., Mountain, G.S., Miller, K.G., 1990. Seismic stratigraphic record of sea-level change. In: Revelle, R. (Ed.), Sea-Level Change. National Research Council Studies in Geophysics, pp. 116±140. Clausen, O.R., 1995. Tertiary seismic stratigraphy in the northern Danish Central Trough. Aarhus Geoscience, vol. 2. Department of Earth Sciences, Aarhus University, 94 pp. Clausen, O.R., Huuse, M., 1999. Topography of the Top Chalk surface, on- and offshore Denmark. Mar. Petrol. Geol. 16, 677±691. Clausen, O.R., Huuse, M., 2001. Mid-Paleocene palaeogeography of the Danish area. Bull. Geol. Soc. Denmark 48 (2) in press. Clausen, O.R., Gregersen, U., Michelsen, O., Sùrensen, J.C., 1999. Factors controlling the Cenozoic sequence development in the eastern parts of the North Sea. J. Geol. Soc. London 156, 809±816. Clausen, O.R., Nielsen, O.B., Huuse, M., Michelsen, O., 2000. Geological indications for Palaeogene uplift in the eastern North Sea Basin. Global Planet. Change 24, 175±187. Danielsen, M., Michelsen, O., Clausen, O.R., 1997. Oligocene sequence stratigraphy and basin development in the Danish North Sea sector based on log interpretations. Mar. Petrol. Geol. 14, 931±950. DGU, 1975. Dansk Nordsù S-1x, Lithologisk og biostratigra®sk rapport. Geol. Surv. Denmark, 4 pp., 8 enclosures. Dinesen, A., Michelsen, O., Lieberkind, K., 1977. A survey of the Paleocene and Eocene deposits of Jylland and Fyn. Geol. Surv. Denmark, Ser. B 1, 15. Donnelly, T., 1982. Worldwide continental denudation and climatic deterioration during the late Tertiary: Evidence from deep-sea sediments. Geology 10, 451±454.

267

DoreÂ, A.G., 1992. The Base Tertiary surface of southern Norway and the northern North Sea. Norsk Geologisk Tidsskrift 72, 259±265. Ehlers, J., 1990. Reconstructing the dynamics of the north-west European Pleistocene ice sheets. Quat. Sci. Rev. 9, 71±83. Ehlers, J., Linke, G., 1989. The origin of deep buried channels of Elsterian age in north-west Germany. J. Quat. Sci. 4 (3), 255±265. Ehlers, J., Meyer, K.-D., Stephan, H.-J., 1984. Pre-Weichselian glaciations of north-west Europe. Quat. Sci. Rev. 3, 1±40. Fisher, M.J., Mudge, D.C., 1998. Triassic. In: Glennie, K.W. (Ed.), Petroleum Geology of the North Sea. 4th ed. Blackwell, Oxford, pp. 212±244. Friis, H., Mikkelsen, J., Sandersen, P., 1998. Depositional environment of the Vejle Fjord Formation of the Upper OligoceneLower Miocene of Denmark: a barrier island/barrier-protected depositional complex. Sedim. Geol. 117, 221±244. Fulthorpe, C., Austin, J.A., Mountain, G.S., 1999. Buried ¯uvial channels off New Jersey: Did sea-level lowstands expose the entire shelf during the Miocene? Geology 27, 203±206. Galloway, W.E., 1989. Genetic stratigraphic sequences in basin analysis I: architecture and genesis of ¯ooding-surface bounded units. Am. Assoc. Petrol. Geol. Bull. 73, 125±142. Gemmer, L., Nielsen, S.B., Lykke-Andersen, H., 2001. Differential vertical movements of a heterogeneous lithosphere in a compressional regime exempli®ed by the eastern North Sea. Bull. Geol. Soc. Denmark 48 (2) in press. Gilchrest, A.R., Summer®eld, M.A., Cockburn, H.A.P., 1994. Landscape dissection, isostatic uplift, and the morphologic development of orogens. Geology 22, 963±966. Gramann, F., Kockel, F., 1988. Palaeogeographical, lithological, palaeoecological and palaeoclimatic development of the Northwest European Tertiary Basin. In: Vinken, R. (Ed.), The Northwest European Tertiary Basin, Geologisches Jahrbuch A 100, 428±441. Gry, H., 1935. Petrology of the Paleocene Sedimentary Rocks of Denmark. Geol. Surv. Denmark, Ser. II 61, 171. Hampton, M.A., Locat, J., 1996. Submarine landslides. Rev. Geophys. 34, 33±59. Heggland, R., 1995. Detection of Ancient Morphology and Potential Hydrocarbon Traps Using 3D Seismic Data and Attribute Analysis. Expanded Abstracts of the 1995 Society of Exploration Geophysicists Exposition and Annual Meeting, Houston, October 1995, 8 (13), 316±318. Heilmann-Clausen, C., 1995. Danmarks geologi fra Kridt til i dag. Nielsen, O.B. (Ed.), Aarhus Geokompendier 1, 69±114. Heilmann-Clausen, C., Nielsen, O.B., Gersner, F., 1985. Lithostratigraphy and depositional environments in the Upper Paleocene and Eocene of Denmark. Bull. Geol. Soc. Denmark 33, 287±323. Huuse, M., 1999. Detailed morphology of the Top Chalk surface in the eastern Danish North Sea. Petrol. Geosci. 5, 303±314. Huuse, M., 2001. Late Cenozoic palaeogeography of the eastern North Sea Basin: climatic vs. tectonic forcing of basin margin uplift and deltaic progradation? Bull. Geol. Soc. Denmark 48 (2) in press. Huuse, M., Clausen, O.R., 2001. Morphology and origin of major

268

M. Huuse et al. / Marine Geology 177 (2001) 243±269

Cenozoic sequence boundaries: eastern Danish North Sea. Basin Res. 13 (1), 17±41. Huuse, M., Lykke-Andersen, H., 2000a. Overdeepened Quaternary valleys in the eastern Danish North Sea: morphology and origin. Quat. Sci. Rev. 19, 1233±1253. Huuse, M., Lykke-Andersen, H., 2000b. Large-scale glaciotectonic thrust structures in the eastern Danish North Sea. Deformation of Glacial Materials, Maltman, A., Hambrey, M., Hubbard, B. (Eds.). Geol. Soc., Lond., Spec. Publ. 176, 293±305. Japsen, P., 1998. Regional Velocity-Depth Anomalies, North Sea Chalk: a record of overpressure and Neogene Uplift and Erosion. Am. Assoc. Petrol. Geol. Bull. 82, 2031±2074. Japsen, P., Bidstrup, T., 1999. Quanti®cation of late Cenozoic erosion in Denmark based on sonic data and basin modelling. Bull. Geol. Soc. Denmark 46, 79±99. Jensen, L.N., Michelsen, O., 1992. Tertiñr hñvning og erosion i Skagerrak, Nordjylland og Kattegat. Dansk Geologisk Forening, Ê rsskrift for 1990±1991, pp. 159±168. A Jensen, L.N., Schmidt, B.J., 1992. Late Tertiary uplift and erosion in the Skagerrak area: magnitude and consequences. Norsk Geologisk Tidsskrift 72, 275±279. Jensen, L.N., Schmidt, B.J., 1993. Neogene uplift and erosion offshore south Norway: magnitude and consequences for hydrocarbon exploration in the Farsund Basin. In: Spencer, A.M. (Ed.), Generation, Accumulation and Production of Europe's hydrocarbons III. Eu. Assoc. Petrol. Geoscient. Spec. Publ. 3, 79±88. Jenyon, M.K., 1984. Seismic response to collapse structures in the Southern North Sea. Mar. Petrol. Geol. 1, 27±36. Jordt, H., Faleide, J.I., Bjùrlykke, K., Ibrahim, M.T., 1995. Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas. Mar. Petrol. Geol. 12, 845±879. Jordt, H., Thyberg, B.I., Nùttvedt, A., 2000. Cenozoic evolution of the central and northern North Sea with focus on differential movements of the basin ¯oor and surrounding clastic source areas. Dynamics of the Norwegian Margin, Nùttvedt (Ed.). Geol. Soc. Lond., Spec. Publ. 167, 219±243. JuÈrgens, U., 1996. MittelmiozaÈne bis pliozaÈne Randmeer-Sequenzen aus dem Sektor der Nordsee. Geologisches Jahrbuch A 146, 217±232. Kockel, F., 1995. Structural and palaeogeographical development of the German North Sea sector. BeitraÈge zur regionalen Geologie der Erde, vol. 26. GebruÈder Bontraeger, Berlin 96 pp. KorstgaÊrd, J.A., Lerche, I.A., Mogensen, T.E., Thomsen, R.O., 1993. Salt and fault interactions in the northeastern Danish Central Graben: observations and inferences. Bull. Geol. Soc. Denmark 40, 197±255. Laban, C., 1995. The Pleistocene glaciations in the Dutch sector of the North Sea: a synthesis of sedimentary and seismic data. PhD thesis, University of Amsterdam, 194 pp. Larsen, G., Dinesen, A., 1959. Vejle Fjord Formationen ved Brejning. Geol. Surv. Denmark II 82, 114. Laursen, G.V., 1995. Foraminiferal biostratigraphy of Cenozoic sections in ®ve wells from the Danish area. EFP-92 project: Basin development of the Tertiary of the Central Trough with

emphasis on possible hydrocarbon reservoirs 20, 33 pp. Danish Energy Agency. Lear, C.H., Elder®eld, H., Wilson, P.A., 2000. Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite. Science 287, 269±272. Liboriussen, J., Ashton, P., Tygesen, T., 1987. The tectonic evolution of the Fennoscandian Border Zone in Denmark. Tectonophysics 137, 21±29. Lidmar-BergstroÈm, K., Ollier, C.D., Sulebak, J.R., 2000. Landforms and uplift history of southern Norway. Global Planet. Change 24, 211±231. Michelsen, O., 1994. Stratigraphic correlation of the Danish onshore and offshore Tertiary successions based on sequence stratigraphy. Bull. Geol. Soc. Denmark 41, 145±161. Michelsen, O., 1996. Late Cenozoic basin development of the eastern North Sea Basin. Bull. Geol. Soc. Denmark 43, 9±21. Michelsen, O., Danielsen, M., Heilmann-Clausen, C., Jordt, H., Laursen, G.V., Thomsen, E., 1995. Occurrence of major sequence stratigraphic boundaries in relation to basin development in Cenozoic deposits of the southeastern North Sea. Sequence Stratigraphy of the Northwest European Margin, Steel, R.J., Felt, V.L., Johannessen, E.P., Mathieu, C. (Eds.). Norwegian Petrol. Soc. Spec. Publ. 5, 415±427. Michelsen, O., Thomsen, E., Danielsen, M., Heilmann-Clausen, C., Jordt, H., Laursen, G.V., 1998. Cenozoic sequence stratigraphy in the eastern North Sea. Mesozoic±Cenozoic Sequence Stratigraphy of Western European Basins, de Graciansky, P.Ch., Jacquin, T., Vail, P.R. (Eds.). Soc. Sedim. Geol. (SEPM) Spec. Publ. 60, 91±118. Miller, K.G., Fairbanks, R.G., Mountain, G.S., 1987. Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleoceanography 2, 1±19. Miller, K.G., Liu, C., Feigenson, M.D., 1996. Oligocene to middle Miocene Sr-isotopic stratigraphy of the New Jersey continental slope. Mountain, G.S., Miller, K.G., Blum, P., Poag, C.W., Twichell, D.C. (Eds.). Proc. Ocean Drilling Progm., Sci. Results 150, 97±114. Miller, K.G., Mountain, G.S., Browning, J.V., Kominz, M., Sugarman, P.J., Christie-Blick, N., Katz, M.E., Wright, J.D., 1998. Cenozoic global sea level, sequences, and the New Jersey Transect: results from coastal plain and continental slope drilling. Rev. Geophys. 36, 569±601. Molnar, P., England, P., 1990. Late Cenozoic uplift of mountain ranges: chicken or egg? Nature 346, 29±34. Nielsen, O.B., Sùrensen, S., Thiede, J., Skarbù, O., 1986. Cenozoic differential subsidence of North Sea. Am. Assoc. Petrol. Geol. Bull. 70, 276±298. Pasierbski, M., 1979. Remarks on the Genesis of Subglacial Channels in Northern Poland. Eiszeitalter Gegenwart 29, 189±200. Petersen, K., Clausen, O.R., KorstgaÊrd, J.A., 1992. Evolution of a salt-related listric growth fault near the D-1 well, block 5605, Danish North Sea: displacement history and salt kinematics. J. Struct. Geol. 14, 565±577. Praeg, D., 1997. Buried tunnel-valleys: 3D-seismic morphostratigraphy. In: Davies, T.A., Bell, T., Cooper, A.K., Josenhans, H., Polyak, L., Solheim, A., Stoker, M.S., Stravers, J.A. (Eds.),

M. Huuse et al. / Marine Geology 177 (2001) 243±269 Glaciated continental margins: an atlas of acoustic images. Chapman & Hall, London, pp. 68±69. Rasmussen, L.B., 1961. De miocñne Formationer i Danmark. Geol. Surv. Denmark IV 4 (5), 45. Riis, F., 1996. Quanti®cation of Cenozoic vertical movements of Scandinavia by correlation of morphological surfaces with offshore data. Global Planet. Change 12, 331±357. Rohrman, M., van der Beek, P., Andriessen, P., Cloetingh, S., 1995. Meso-Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite ®ssion track thermochronology. Tectonics 14, 704±718. Salomonsen, I., 1993. Quaternary buried valley systems in the eastern North Sea. PhD Thesis. University of Copenhagen. Salomonsen, I., 1995. Origin of a deep buried valley system in Pleistocene deposits of the eastern central North Sea. Proceedings of the 2nd Symposium on: Marine Geology Ð Geology of the North Sea and Skagerrak, Michelsen, O. (Ed.). Geol. Surv. Denmark, Ser. C 12, 7±19. Schwarz, C., 1996. Neue Befunde zur Verbreitung und Dimension pleistozaÈner Rinnensysteme auf dem deutschen Nordseeschelf. Geologisches Jahrbuch, Reihe A 146, 233±244. SeÂranne, M., 1999. Early Oligocene stratigraphic turnover on the west Africa continental margin: a signature of the Tertiary greenhouse-to-icehouse transition? Terra Nova 11, 135±140. Sheriff, R.E., 1985. Aspects of seismic resolution. Seismic Stratigraphy II, Berg, O.R., Woolverton, D.G. (Eds.), Am. Assoc. Petrol. Geol. Mem. 39, 1±10. Sorgenfrei, T., Berthelsen, O., 1954. Geologi og vandboring. Geol. Surv. Denmark III 31, 106. Spjeldnñs, N., 1975. Palaeogeography and facies distribution in the Tertiary of Denmark and surrounding areas. Geol. Surv. Norway 316, 289±311. Steckler, M.S., Mountain, G.S., Miller, K.G., Christie-Blick, N., 1999. Reconstruction of Tertiary progradation and clinoform development on the New Jersey passive margin by 2-D backstripping. Mar. Geol. 154, 399±420. Summer®eld, M.A., Brown, R.W., 1998. Geomorphic factorsin the interpretation of ®ssion-track data. In: Van den Haute, P., De Corte, F. (Eds.), Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Netherlands, pp. 269±284. Surlyk, F., 1997. A cool-water carbonate ramp with bryozoan mounds: Late Cretaceous-Danian of the Danish Basin. CoolWater Carbonates, James, N.P., Clarke, J. (Eds.). SEPM Spec. Publ. 56, 293±307.

269

Swart, P.K., Wortmann, U.G., Mitterer, R.M., Malone, M.J., Smart, P.L., Feary, D.A., Hine, A.C., 2000. Hydrogen sul®de-rich hydrates and saline ¯uids in the continental margin of South Australia. Geology 28, 1039±1042. Sùrensen, J.C., Michelsen, O., 1995. Upper Cenozoic sequences in the southeastern North Sea Basin. Bull. Geol. Soc. Denmark 42, 74±95. Sùrensen, J.C., Gregersen, U., Breiner, M., Michelsen, O., 1997. High-frequency sequence stratigraphy of Upper Cenozoic deposits in the central and southeastern North Sea areas. Mar. Petrol. Geol. 14, 99±123. Ter-Borch, N., 1990. Geological map of Denmark. Structural map of Top Chalk Group (1:500 000). Geol. Surv. Denmark, Map Series 7. Thomsen, E., 1995. Kalk og kridt i den danske undergrund. Danmarks geologi fra Kridt til i dag, Nielsen, O.B. (Ed.). Aarhus Geokompendier 1, 31±69. Vail, P.R., 1987. Seismic stratigraphy interpretation procedure. Atlas of Seismic Stratigraphy, Bally, A.W. (Ed.). Am. Assoc. Petrol. Geol. Stud. Geol. 27, 1±10. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahmanian, V.D., 1990. Siliciclastic Sequence Stratigraphy in Well Logs, Cores and Outcrops. AAPG Meth. Explor. 7, 55. Vejbñk, O.V., 1990. The Horn Graben, and its relationship to the Oslo Graben and the Danish Basin. Tectonophysics 178, 29±49. Vejbñk, O.V., Andersen, C., 1987. Cretaceous±Early Tertiary inversion tectonism in the Danish Central Trough. Tectonophysics 137, 221±238. Vejbñk, O.V., Andersen, C., 2001. Post mid-Cretaceous inversion tectonics in the Danish Central Graben. Bull. Geol. Soc. Denmark 48 (2), in press. Vejbñk, O.V., Britze, P. (Eds.), 1994. Top pre-Zechstein (two-way traveltime and depth). Geological map of Denmark 1:750 000. Geological Survey of Denmark, Map Series 45. Wing®eld, R., 1989. Glacial incisions indicating Middle and Upper Pleistocene ice limits off Britain. Terra Nova 1, 538±548. È ., 1987. Seismic data processing. Soc. Explor. Geophys. Yilmaz, O Invest. Geophys. 2, 526. Zagwijn, W.H., 1989. The Netherlands during the Tertiary and Quaternary: A case history of Coastal Lowland evolution. Geologie en Mijnbouw 68, 107±120. Ziegler, P.A., 1990. Geological Atlas of Western and Central Europe, Shell Internationale Petroleum Maatschappij BV, The Hague, 239 pp.