Structural development of the Fennoscandian Border Zone, offshore Denmark

Structural development of the Fennoscandian Border Zone, offshore Denmark Olaf Michelsen Geologisk Institut, A a r h u s Universitet, C.F. Mc~llers A l l & DK-8000, Aarhus C, Denmark

and Lars Henrik Nielsen Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark

Received 24 September 1991; revised 13 December 1991; accepted 21 December 1991 The Hans-l, Saeby-1 and Terne-1 wells are located within the Danish part of the Fennoscandian Border Zone and provide significant new data pertaining to the evolution of this important tectonic belt. All three wells encountered Palaeozoic and Mesozoic rocks. The Cambrian to Lower Silurian deposits can be closely correlated to sections known from Bornholm and Scania, and tectonic activity within the Fennoscandian Border Zone cannot be verified during this time interval. The thick Upper Silurian shales were probably deposited in a rapidly subsiding foreland basin, marginal to the speculative Caledonian deformation front which lies south of the Ringkctbing-Fyn High. A major hiatus comprises the Devonian and Early Carboniferous. Upper Carboniferous sediments were deposited before a phase of rifting. Upper Carboniferous intrusive and extrusive volcanic rocks suggest tectonic activities, heralding the rifting phase. Extensive block faulting occurred in the border zone during the Rotliegende and more than 650 m of reworked volcanic rocks were deposited in an alluvial fan environment. Syndepositional erosion created a major unconformity before the deposition of post-rift Zechstein sediments. During the Mesozoic the Fennoscandian Border Zone was divided into two zones: the Sorgenfrei-Tornquist Zone to the west and the Skagerrak-Kattegat Platform to the east. The Sorgenfrei-Tornquist Zone continued to experience tectonic activity in Triassic and Middle Jurassic times. The zone has experienced an uplift of 1700-2000 m during Late Cretaceous to Early Tertiary inversion tectonics and the Late Tertiary uplift of Fennoscandia. Keywords: Fennoscandian Border Zone; tectonic evolution; structural development; Rotliegende rifting; Late Cenozoic uplift; Palaeozoic-Mesozoic stratigraphy

Introduction The purpose of this paper is to evaluate the structural development of the Fennoscandian Border Zone through Phanerozoic time by investigating seismic data and three new wells: Hans-l, Saeby-1 and Terne-1 (Figure 1). The interpretation of basin development in the Kattegat region was previously based on seismic data and data from the adjacent onshore areas. No offshore well data was previously available. The Kattegat is transected by the Fennoscandian Border Zone that separates the Fennoscandian Shield to the north-east from the Danish Basin to the west and south-west. Hans-1 and Terne-1 are located in the Kattegat (Figure 1) and thus yield the first deep well data from this part of the border zone.

Geological setting Sorgenfrei and Buch ( 1 9 6 4 ) described the Fennoscandian Border Zone as a strongly block-faulted zone. The zone has subsequently been regarded as a north-westerly extension of the Tornquist Zone, which forms a continuous fault zone from the Black Sea to the central North Sea (Bergstr6m, 1984; Pegrum, 1984). Pegrum (1984) argued that the zone was a Precambrian zone of weakness that may have subsequently experienced strike-slip movements during the Variscan phase. Triassic and Jurassic extensional tectonics and compression during the Cretaceous and Tertiary were also considered. Bergstr6m (1984) proposed that the

thick Upper Silurian deposits of the Cotonus Trough, which forms part of the Fennoscandian Border Zone in Scania, probably did not extend further towards the north-east, indicating that the present trough-like structure was a syndepositional feature. The most recent discussions have been presented by Liboriussen et al. (1987) and the EUGENO-S Working Group (1988). Liboriussen et al. (1987) considered the Fennoscandian Border Zone as a continuation of the Tornquist Zone and proposed that it has been tectonically active from Early Palaeozoic to Recent times. Important tectonic phases proposed were Early Palaeozoic tensional tectonics, Late Palaeozoic dextral wrench movements, Triassic to Early Cretaceous transtensional and Late Cretaceous to Early Cenozoic transpressional movements. The EUGENO-S Working Group (1988) considered the structural line through the Kattegat region as a discrete structural element rather than an integrated part of the Tornquist Zone. It was termed the Sorgenfrei-Tornquist Zone, whereas the PolishUkrainian element was referred to as the Teisseyre-Tornquist Zone. The Sorgenfrei-Tornquist Zone cuts through the Precambrian crustal province, whereas the Teisseyre-Tornquist Zone follows the south-western boundary of the province. The two zones converge at the ROnne Graben, offshore Bornholm. The Sorgenfrei-Tornquist Zone is defined as a strongly faulted zone primarily characterized by Late Cretaceous to Early Tertiary inversion tectonics. The

0264-8172/93/020124-11 ©1993 Butterworth-Heinemann Ltd 124

Marine and Petroleum Geology, 1993, Vol 10, April

Structure of the Fennoscandian Border Zone." O. Michelsen and L. H. Nielsen

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M a r i n e and P e t r o l e u m G e o l o g y , 1993, Vol 10, April

125

Structure of the Fennoscandian Border Zone: O. Michelsen and L. H. Nielsen area between this zone and the Fennoscandian Shield Structural development was named the Skagerrak-Kattegat Platform, which is Early Palaeozoic characterized by eastwards thinning and a relatively The shelf and shallow marine sedimentation patterns undisturbed Mesozoic succession. that are known from the Baltic region (e.g. Bornholm) seem to have dominated in the Kattegat part of the Stratigraphy Sorgenfrei-Tornquist Zone during the Cambrian and Ordovician. The greater thicknesses in the study Two of the studied wells, Hans-1 and Terne-l, are area compared with Scania and Bornholm reflect a located within the Sorgenfrei-Tornquist Zone. The more complete stratigraphic succession (Michelsen and third well, Saeby-1, is located on the Nielsen, 1991). Neither the stratigraphic well data nor Skagerrak-Kattegat Platform (Figure 1). The three the seismic data indicate synsedimentary tectonic wells provide significant new stratigraphic data activity during the Cambrian and the Ordovician. pertaining to the evolution of this important tectonic In the Late Silurian a pronounced increase in belt. subsidence rate is recognized to have occurred in All three wells encountered Palaeozoic and Mesozoic the Kattegat region. By combining well and seismic rocks. An integrated study comprising sedimentology, evidence the maximum thickness of the Silurian seismic interpretation, log evaluation, log sequence succession is estimated to be 2600 m (Michelsen and stratigraphy and biostratigraphy was published by Nielsen, 1991), and a likely thickness for the Upper Michelsen and Nielsen (1991). Their stratigraphic Silurian is approximately 2000 m. An 800 m thick subdivision is presented in Figure 2. The wells section of Late Silurian sedimentary rocks is known encountered Cambrian to Silurian, Carboniferous to from the Colonus Trough in Scania, lying within the Zechstein, and Mesozoic successions, which are south-eastern part of the Sorgenfrei-Tornquist Zone correlatable with stratigraphic sections known from (Bergstr6m et al., 1982). Bornholm, Scania and the Danish Basin. The S~eby-1 In view of the indications that the Colonus Trough well was terminated in Rotliegende coarse clastic formed a syndepositional structural element (Bergsedimentary rocks, the Hans-1 well in Upper str6m, 1984) and the presence of large sedimentary Carboniferous sandstones, siltstones and claystones, thicknesses in the Kattegat region, it could be and the Terne-1 well in the Lower Cambrian concluded that the Sorgenfrei-Tornquist Zone was an Hardeberga Sandstone equivalent. active fault zone during the Late Silurian. However, continuation of a truncated Palaeozoic section west of Reservoir and source rocks the Sorgenfrei-Tornquist Zone and the presence of a A review of possible hydrocarbon plays in the Danish thick pre-Rotliegende Palaeozoic succession in the Basin, including the Fennoscandian Border Zone, basin in North Jylland (Figure 3) suggest that high Late identified two important potential source rock Silurian subsidence rates also prevailed in the basin intervals: the Lower Palaeozoic Alum Shale and the west of the fault zone. These data may support the Lower Jurassic claystones (Thomsen et al., 1987). model regarding the zone as a part of the region to the Several reservoir rocks were also recognized, ranging in south-west which acted as a rapidly subsiding foreland age from the Cambrian to Late Cretaceous. basin marginal to the Caledonian deformation front, The Hans-1 well tested the hydrocarbon potential of south of the Ringk0bing-Fyn High (as suggested by the several Triassic and Jurassic sandstone intervals and a EUGENO-S Working Group, 1988). Syndepositional thick Rotliegende interval (Figure 2). Furthermore tectonic activity is, however, suggested by the presence hitherto unknown Upper Carboniferous sandstones of extrusive volcanic rocks, intercalating the Silurian were evaluated. The Sa~by-1 well encountered several Rcmde and Novling Formations in the central basin Triassic and Jurassic sandstone intervals and the west of the fault zone (Figure 4). Rotliegende section. The Terne-1 well tested the Late Palaeozoic Lower Cambrian sandstone, which is oil-producing in Poland. 'The sandstone was heavily cemented and Upper Carboniferous sedimentary rocks were recorded without any reservoir qualities. The well further only in the Hans-1 well; Devonian rocks were not evaluated Triassic and Jurassic sandstones. identified in the three wells. The seismic data indicate a No hydrocarbons were found in the three wells. The general structural conformity between the Silurian presence of multiple sandstone reservoirs was section and the succession referred to as Upper confirmed. The expected source rock intervals were Carboniferous; however, a major hiatus comprising the also present. However, the Lower Palaeozoic Alum entire Devonian and the Lower Carboniferous is Shale, which is very rich in organic material, was interpreted from regional considerations. The present post-mature, probably having lost all hydrocarbon data do not exclude the possibility of Devonian and potential before the formation of the regional Permian Lower Carboniferous deposits being present at other unconformity. The Lower Jurassic and parts of the locations within the Kattegat region. On the seismic Rhaetian successions contained several intervals with section a tentative boundary between the Silurian and organic matter, adequate to provide for a good source the Upper Carboniferous deposits is indicated (Figure rock of both oil and gas. However, the intervals were 5). immature to marginally mature. Carboniferous rocks are known from wells in The major risks in the area remain the timing, the southern Denmark (Sorgenfrei and Buch, 1964; presence of adequate source rocks and potential Michelsen, 1971; Bertelsen, 1972) and from the Oslo leakage caused by the Late Cretaceous to Early Graben (Olaussen, 1981). Carboniferous deposits may Tertiary inversion tectonics and the Late Cenozoic therefore have covered the Kattegat region and the uplift. Danish onshore area.

126

Marine and Petroleum Geology, 1993, Vol 10, April

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Figure 4 Correlation of the Carboniferous to Permian successions m the Hans-1 and Saeby-1 wells, and the Silurian sections in the NOvling-1 and ROnde-1 wells M a r i n e a n d P e t r o l e u m G e o l o g y , 1993, V o l 10, A p r i l

127

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Intrusive volcanic rocks of Late Carboniferous age intercalate the Silurian deposits in the Terne-1 well. Extrusive volcanic rocks of possible Late Carboniferous age are present above the Upper Carboniferous sedimentary succession in the Hans-I well. This episode of volcanic activity in the Sorgenfrei-Tornquist Zone is approximately time-equivalent with the earliest volcanic activity in the Oslo Graben (Ro et al., 1990) and with diabase dykes known from Scania (Bergstr6m et al., 1982). A dominantly volcaniclastic succession, conglomerates and poorly sorted sandstones of an assumed Rotliegende age was encountered in the Hans-1 well (662 m thick) and in the S~eby-1 well (approximately 200 m thick). A local source is suggested by the angularity of the pebbles and sand grains. Geosections presented by S0rensen and Martinsen (1987) indicate that the Rotliegende is typically much thinner elsewhere in the basin. The thicknesses of the volcaniclastic succession increase on the downthrown side of the main faults that controlled the half-graben development on the Skagerrak-Kattegat Platform and in the Sorgenfrei-Tornquist Zone (Figures 3 and 5). At the margin of the Oslo G r a b e n - B a m b l e Trough complex a Lower Palaeozoic section about 2 km thick is overlain by almost 2 km of presumed Upper Carboniferous to Lower Permian volcaniclastic rocks

128

(Figure 6). Furthermore in the tilted fault block in North Jylland a thick succession of 1.5 km of volcaniclastic rocks is present (Figure 3). Dipmeter data in Hans-I indicate structural dips that range from 20-24 ° at the base to 4 - 6 ° towards the top of the Rotliegende section, which together with the seismic data indicate a syntectonic sedimentation pattern. Fault block tilting probably led to contemporaneous erosion of the footwall blocks and deposition of volcaniclastic material on alluvial fans located in the hanging walls. This syn- and post-depositional erosion has obscured the original extent of the Rotliegende deposits. The rifting along the Fennoscandian Border Zone is contemporaneous or slightly later than rifting of the Oslo Graben as also suggested by Ro et al. (1990). A detailed correlation of the well data with the seismic data (Figure 7) allows subdivision of the rifting events in the Fennoscandian Border Zone. The extrusive volcanic rocks in Hans-1 are overlain by a 30 m thick interval of coarse-grained volcaniclastic rocks interbedded with thin claystone beds. This interval corresponds to a seismic sequence which thickens dramatically towards the east and shows a diverging reflection pattern (sequence C, Figure 7). The sequence records the initial rifting; judging from the reflection pattern and the well information it

Marine and Petroleum Geology, 1993, Vol 10, April

Structure of the Fennoscandian Border Zone: O. Michelsen and L. H. Nielsen consists of proximal alluvial fan deposits. The overlying whereas a shift back towards the north-east is indicated sequence consists of non-fossiliferous red claystones, by the upper sequence (G). Thus deposition of the presumably a lacustrine deposit, which drape the fan lowermost part (C) of the rift sequence was probably deposits (sequence D in Figure 7). A south-westwards limited to the present half-grabens and it was shift in the depocentre is indicated. The overlying terminated with the fine clastic sediments (D) recorded coarse-grained volcaniclastic succession corresponds to in Hans-1. It was followed by a more coarse clastic infill three seismic sequences characterized by an almost ( E - G ) which may have covered larger parts of the parallel reflection pattern (sequences E, F and G in Fennoscandian Border Zone. The present extent of the Figure 7). The three seismic sequences are identified by Rotliegende deposits is a result of both syn- and onlaps and truncations. The reflection pattern and the post-rift erosion, which created a major regional thickening towards the north-east of the lower unconformity (Figures 3, 5, 7 and 8). , sequence (E) suggest renewed movements along the In the Hans-1 well a thin section of non-marine, main fault. During deposition of the middle sequence siliciclastic Zechstein deposits overlies the Rotliegende (F) the depocentre moved towards the south-west, succession. A corresponding minor Zechstein basin is

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129

Structure of the Fennoscandian Border Zone: O. Michelsen and L. H. Nielsen S•

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Figure 7 Part of seismic line K 83-005 through the Hans-1 well showing details of the Upper Carboniferous pre-rift sequence (A-B), the Rotliegende synrift sequences (C-G) and the Zechstein and Triassic post-rift sequences (I-H). The detailed correlation of well data and seismic data was facilitated by plots of log measurements against two-way travel time. The significance of the chosen seismic reflectors was confirmed by construction of synthetic traces. (A) A well bedded unit of sandstones, siltstones and claystones dipping 24° towards the ENE; (B) extrusive volcanic rocks; (C) corresponds to two 12-13 m thick beds of volcaniclastic rocks interbeded with thin claystone beds in Hans-l. Dipmeter data in the well indicate dip of 22-24 ° towards the ENE. The seismic sequence thickens dramatically in the same direction towards the fault; (D) red claystones; (E, F and G) corresponds in the well to a succession of coarse grained volcaniclastic rocks which show gradually decreasing dips upwards to 4 - 6 ° at the top of G. The seismic data indicate that these sequences may have covered larger parts of the Fennoscandian Border Zone than the present half-graben (see Figure 5); (H) siliciclastic Zechstein sediments - - the deposition was controlled by the Rotliegende structural pattern; and (I) Lower Triassic deposits

indicated by the seismic data further towards the south-west (Figure ,5). The seismic data suggest that deposition in these local Zechstein deposits was controlled by the Rotliegende structural pattern. The depocentre at Hans- 1 has moved further south-westwards. Terne-1 recorded a thin veneer of marginal marine, siliciclastic Zechstein deposits overlying the Lower Palaeozoic succession (Figures 2 and 8).

Mesozoic to Early Cenozoic The Triassic deposits belong to the Northern Marginal Facies Province (sensu Bertelsen, 1980). There is a clear difference between the thicknesses of the sedimentary sequences that accumulated on the Skagerrak-Kattegat Platform and in the Sorgenfrei-Tornquist Zone, which indicates greater subsidence rates in the latter zone. It is believed that the Sorgenfrei-Tornquist Zone continued to experience tectonic activity dominated by vertical movements with subordinate strike-slip movements (Pegrum, 1984; Vejba&, 1990). 130

The extreme thicknesses of the Middle Jurassic sandstones are the only anomalous feature of the Jurassic sequence compared with those known from the Danish Basin. A comparable thickness of the Middle Jurassic sandstones is seen in the Haldager-I well, further north-west in the Sorgenfrei-Tornquist Zone. The presence of a Middle Jurassic depocentre within the Sorgenfrei-Tornquist Zone suggests tectonically controlled local subsidence within this zone. Upper Cretaceous deposits were encountered in the S~eby-1 well on the Skagerrak-Kattegat Platform, whereas pre-Late Cretaceous deposits underlie the Quaternary sections in the two wells in the Sorgenfrei-Tornquist Zone. The seismic data indicate that the latter zone has experienced tectonic inversion. These data are in accordance with the Late Cretaceous to Early Tertiary tectonic inversion and subsequent erosion described by Pegrum (1984), Liboriussen et al. (1987) and the EUGENO-S Working Group (1988).

Late Cenozoic A

Marine and Petroleum Geology, 1993, Vol 10, April

considerable

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Structure of the Fennoscandian Border Zone: O. Michelsen and L. H. Nielsen avoid misinterpretations caused by differences in Fennoscandian Shield has been described by various lithology, the examination is limited to the two lower workers on the basis of analyses of North Sea data. members of the Lower Jurassic Fjerritslev Formation. Jensen and Schmidt (in press) have analysed seismic The two members consist of claystones and are data, petrophysical logs and vitrinite reflectance data uniformly developed in the entire area. They are from the southern Norwegian sector and the northern present in a large number of wells. Danish sector. They suggest that north-east Jylland has The sonic trends of the two Lower ,lurassic members experienced an uplift of 1000-1500 m, indicating an in these wells are presented in Figure 9. The O-1 well is increasing amount of uplift from the North Sea towards located in the southern Danish Central Trough and the east. They indicate the amount of uplift further east records the deepest known position of the two in the Kattegat to be more than 1500 m. members. The well has penetrated a nearly complete As a preliminary contribution to the discussion of the Mesozoic and Cenozoic stratigraphic succession. The Late Cenozoic uplift of Fennoscandia we have upper part of the Lower Jurassic is, however, missing, examined logs from the three wells presented here, one indicating a mid Cimmerian uplift and subsequent well located in central Jylland and a number of wells erosion (Michelsen et M., lt~87). It is therefore from the Danish North Sea sector (see Figure 9). The suggested that the sonic velocity values in the well preliminary results derive from the sonic (interval reflect a subsidence of at least 30{)0-3100 m below the transit time) log, which is related to the amount of present sea level. However, the preconditions are that subsidence (Magara, 197(~: Rider, 1986). The sonic the claystones are normally compacted, e.g. that a measurements primarily reflect the character of the significant overpressure is not present. Although these contact surfaces between the grains in the sedimentary preconditions may not be fully met, the measured sonic rock, indicating the degree of compaction and velocity values arc used as the basis for constructing the diagenesis. As these processes are irreversible, at least sonic trend related to depth (Fig,re 9). in claystones, the results of the subsidence experienced is preserved in the sedimentary rock. Among other The sonic trend is constructed on semilog paper as a things the sonic measurements also reflect lithology. To straight line between the interwd transit time values in

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Marine and Petroleum Geology, 1993, Vol 10, April

131

Structure of the Fennoscandian Border Zone: O. Michelsen and L. t4. Nielsen SONIC - TRENDS of the lower members of the Lower Jurassic Fjerritslev Formation

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Figure 9 Sonic transit times of the t w o l o w e r m o s t m e m b e r s of the Lower Jurassic Fjerritslev Formation plotted against the present depth of burial in the F-l, Hans-l, Hyllebjerg-1, K-l, J-l, O-1, Saeby-1 and Terne-1 wells. The trend of upwards decreasing sonic values in relation to depth of burial is constructed on the basis of the sonic values in O-1 and a sonic transit time value of 200 ps/ft at the surface (cf. Magara, 1976). The sonic values measured in each of the other wells indicate the depth of subsidence before uplift. The a m o u n t of uplift is estimated from the difference in depth of a certain sonic value in one well compared with the depth of this sonic value on the sonic trend determined by O-1. The estimated a m o u n t of uplift after the Early Cretaceous at each well location in relation to the present depth of burial at the O-1 well location is given

the O-1 well, at approximately 3000 m and 200 ps/ft at the surface (cf. Magara, 1976). The surface refers here to the mean sea level (MSL) as the present sea bottom and the land surface only show minor deviations from this reference level. Magara (1976) located a shale interval with normal compaction features and extrapolated the sonic trend to the surface. The difference between the sonic value found and 200 ps/ft was used for calculating the amount of uplift. However, the sonic trend in each of the wells used by us varies too much to use this method, probably due to the small thickness of two members. In five of the wells the thickness is less than 200 m. Furthermore the Late Cretaceous to Early Tertiary inversion tectonics (J-I and T e r n e - l ) and locally also halokinetic movements (K-l) may have caused fracturing of the sedimentary rocks. The sonic values measured in each of the wells compared with the values of O-1 indicate that a post-Early Jurassic uplift has taken place. The amount

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of uplift is estimated from the difference in depth of a certain sonic value in one well compared with the depth of the same sonic velocity value on the sonic trend determined by O-1. The differences in depth of the sonic values in the wells presented in Figure 9 indicate an increasing amount of post-Early Jurassic uplift, moving from the central North Sea towards the Skagerrak-Kattegat Platform. All wells west of this latter zone have a Mesozoic succession similar to that of O-1, whereas the younger parts of the Tertiary section are reduced or absent centrally in the Danish Basin. The Tertiary is absent in the J-1 and S~eby-1 wells and the Tertiary and Upper Cretaceous are absent at Hans-1 and Terne-1 in the Sorgenfrei-Tornquist Zone. In Hans-I the youngest Mesozoic deposits belong to the Lower Jurassic (Figure 2). The uplift indicated by the sonic values in the Sorgenfrei-Tornquist Z o n e must therefore have taken place after Early Cretaceous time. The uplift is assumed to be 2000 m at Hans-I and 1700 m at Terne-1

Marine and Petroleum Geology, 1993, Vol 10, April

S t r u c t u r e o f the F e n n o s c a n d i a n B o r d e r Z o n e : O. M i c h e l s e n a n d L. H. N i e l s e n

(Figure 9). The seismic section (Figure 8) and the isopach map of the Upper Cretaceous deposits in Denmark (Liboriussen et al., 1987; Figure 5) suggest that approximately 1000 m of Upper Cretaceous chalk have been eroded in the Sorgenfrei-Tornquist Zone due to these inversion tectonics. Furthermore 600-700 m of Upper Jurassic and Lower Cretaceous deposits are eroded at Hans-1 compared with Terne-1 (Figures 2, 5 and 8). Thus these data suggest a Late Cenozoic uplift of 600-700 m within this part of the Sorgenfrei-Tornquist Zone. The corresponding figures for Saeby-I on the Skagerrak-Kattegat Platform is 1100 m. and for Hyllebjerg-I in the Danish Basin 800 m.

The amount of Late Cenozoic uplift estimated here is 500-900 m smaller than suggested by Jensen and Schmidt (in press), but their figures are not reduced by the amount of uplift caused by the Late Cretaceous to Early Tertiary inversion, Preliminary sequence stratigraphic studies of the North Sea Cenozoic succession show truncation of the Tertiary deposits towards the east, indicating a post-Early Miocene uplift of the Scandinavian region. Contemporaneous increases in subsidence rates in the central North Sea are indicated by the seismic and well data. The importance of the Late Cenozoic uplift for the evaluation of the hydrocarbon source rocks and reservoirs of the eastern part of the region is yet to be concluded. Further investigations of seismic data and vitrinite reflectance data must be carried out to verify and calibrate the data derived from the sonic logs.

During the Mesozoic, the Fennoscandian Border Zone was divided into two zones: the Sorgenfrei-Tornquist Zone to the west and the Skagerrak-Kattegat Platform to the east. Tectonic activity during the Mesozoic can only be testified for the Sorgenfrei-Tornquist Zone. The presence of Triassic and Middle Jurassic depocentres within the Sorgenfrei-Tornquist Zone suggests tectonically controlled local subsidences within this zone. The Late Cretaceous to Early Tertiary period of tectonic inversion is supported by the new data. Examination of sonic log trends indicates that the Kattegat region experienced a post-Early Cretaceous uplift amounting to 1700-2000 m. Approximately 1000 m of uplift is assumed to be caused by Late Cretaceous to Early Tertiary inversion tectonics, and another 600-700 m of uplift is assumed to have taken place during the Late Cenozoic. This latter uplift corresponds with regional uplift of the Fennoscandian Shield and it is contemporaneous with the significant increase in subsidence rates known from the central North Sea basin.

Acknowledgements This paper was written while Lars H. Nielsen was in receipt of financial support from the Danish Research Academy. Kim Gunn Maver at the Geological Survey of Denmark is thanked for the construction of synthetic seismic traces and the plots of log measurements against two-way travel time. Eva Melskens was responsible for the artwork.

Conclusions Tectonic activity during the Cambrian and Early Silurian along the Fennoscandian Border Zone cannot be demonstrated. The Upper Silurian section may represent deposition in a rapidly subsiding foreland basin marginal to the speculative Caledonian deformation front, south of the Ringk,abing-Fyn High. A major hiatus comprising the Devonian and the Early Carboniferous ,s interpreted from regional considerations. The hiatus does not appear to be associated with an angular unconformity that might suggest tectonic movements. Late Carboniferous volcanic activity probably heralded the onset of rifting in the Fennoscandian Border Zone. The underlying Upper Carboniferous elastic sequence was deposited before rifting. Coarse-grained volcaniclastic rocks and claystones covering the extrusive volcanic rocks were deposited during rifting. Tilting continued during the deposition of more than 650 m of rcworked volcanics and older sedimentary rocks, probably on alluvial fans flanking fault scarps. The rift sequence lies between Upper Carboniferous extrusive w)lcanic rocks and siliciclastic Zechstein deposits, suggesting that the rifting took place during the Rotliegende. The rifting along the Fennoscandian Border Zone is contemporaneous or slightly later than the rifting of the Oslo Graben. The unconformity at the base of the siliciclastic Zechstein deposits is most pronounced where the Rotliegende section is truncated or absent, supporting the interpretation of both syn- and post-rift erosion. The lithology and stratigraphy of the Mesozoic successions are similar to those of the Danish Basin.

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