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Late Triassic structural evolution of the southern margin of the Ringkøbing-Fyn High, Denmark
Marine and Petroleum Geology 16 (1999) 653±665
Late Triassic structural evolution of the southern margin of the Ringkùbing-Fyn High, Denmark Ole Rùnù Clausen*, Per Kent Pedersen Department of Earth Sciences, University of Aarhus, DK-8000, Aarhus, C, Denmark Received 16 December 1998; received in revised form 12 June 1999; accepted 17 June 1999
Abstract A Late Triassic (Carnian) unconformity is observed on seismic sections and in wells along the southern margin of the Ringkùbing-Fyn High. The unconformity is characterised by low angle erosion on the central parts of the eastern RingkùbingFyn High (Mùn High), and by a signi®cant onlap onto the unconformity surface. Structural reconstruction shows that the unconformity is related to dierential subsidence between the Ringkùbing-Fyn High and the North German Basin. The dierential subsidence initiated movements in the mobile Zechstein Salt, causing deformation of the cover sediments. The variability in dierential subsidence observed during the Triassic along the southern margin of the Ringkùbing-Fyn High is interpreted to re¯ect the subdivision of the Ringkùbing-Fyn High into a number of structure blocks. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Triassic; Denmark; Tectonics
1. Introduction
1.1. Structural setting of the study area
The objective of this study is to analyse the Late Triassic structural evolution of the southern margin of the Ringkùbing-Fyn High with special emphasis on the geological signi®cance of a Late Triassic unconformity observed in wells and on seismic sections. The study integrates observations on sedimentology, facies evolution, and hiatuses made in wells in the south-eastern part of the Danish area with structural interpretations of seismic sections. The study uses released convectional 2-D seismic surveys (GY84 T, PRKL80, WGC79, WGC81), high resolution seismic survey DA98 (owned by the Department of Earth Sciences, University of Aarhus), and well logs, cuttings and cores from the wells: Ringe-1, Sùllested-1, Rùdby-1, Kegnñs-1, érslev-1, Tùnder-1, Lùgumkloster-1, Slagelse-1, R-1 and S-1 (Fig. 1).
The study area encompasses the northern margin of the North German Basin and the southern part of the Norwegian±Danish Basin (Fig. 1). In Triassic times northern and central Europe formed a large landlocked epicontinental basin, the Northwest European Basin (Ziegler, 1990), which was composed of numerous sub-basins, such as the Norwegian±Danish Basin and the North German Basin separated by structural highs such as the Ringkùbing-Fyn High (Fig. 1). West of the Central Trough area, the Silverpit Basin is located south of the Mid North Sea High in a similar setting to the North German Basin (Coward & Stewart, 1995). The Ringkùbing-Fyn High forms part of the generally east±west striking system of highs running across the North Sea area (Ziegler, 1990). The individual blocks of the Ringkùbing-Fyn High are referred to according to the nomenclature given by Vejbñk (1997) (Fig. 1). The Ringkùbing-Fyn High is characterised as an area where the crystalline basement is shallow compared to the adjacent basins. The Ringkùbing-Fyn
* Corresponding author. Tel.: +45-89-42-25-18; fax: +45-89-4225-25. E-mail address: [email protected] (O.R. Clausen)
0264-8172/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 8 1 7 2 ( 9 9 ) 0 0 0 2 6 - 4
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Fig. 1. Map showing the study area, dominated by the Norwegian Danish Basin and the North German Basin separated by the Ringkùbing-Fyn High. The Norwegian±Danish Basin is bounded to the north-east by the Sorgenfrei±Tornquist Zone. Wells presented in this paper are shown as ®lled circles. Seismic sections used in this study are indicated by full lines. The names of the blocks of the Ringkùbing-Fyn High is from Vejbñk (1997) as: ENBl: East North Sea Block, GrBl: Grindsted Block, GlBl: Glamsbjerg Block, StBl: Stigsnñs Block, MùH: Mùn High.
High was formed during a pre-Zechstein stretching phase in which the Ringkùbing-Fyn High area suered lesser stretching than the adjacent basins (Vejbñk, 1997). It cannot be excluded that the pre-Zechstein Ringkùbing-Fyn High was fault bounded since the seismic resolution beneath the Top pre Zechstein surface generally is poor. However, the faults at the Top pre-Zechstein surface indicates little or no fault control of the post-Zechstein dierential subsidence (Fig. 1) and the dierential subsidence between the basins and the high thus has had the character of regional in¯exion. The Ringkùbing-Fyn High formed a barrier and separated the Southern and Northern Permian Basins in Late Permian time (Clark & Tallbacka, 1980), and was inundated in early Triassic times (Pedersen, 1998). Thinning of Triassic deposits across the RingkùbingFyn High has previously and mainly been interpreted as due to slower subsidence on the Ringkùbing-Fyn High than in the North German Basin and Norwegian±Danish Basin and partly due to erosion due to Mid-Jurassic relative uplift of the RingkùbingFyn High (Sorgenfrei, 1969; Bertelsen, 1980; Cartwright, 1990; Kockel, 1995). The subsidence of the Norwegian±Danish Basin has been attributed to thermal subsidence following preTriassic rifting (Sùrensen, 1986; Vejbñk, 1990). However, the Horn Graben, which is located in the
eastern North Sea (Fig. 1), shows active rifting and consequent deposition of a very thick Lower and Middle Triassic succession (Bertelsen, 1980; Vejbñk, 1990; Clausen & KorstgaÊrd, 1993). The Sorgenfrei± Tornquist Zone, which is a major strike-slip zone delimiting the sedimentary basin to the north-east (Fig. 1), experienced right-lateral transtensional movements during the Triassic (Mogensen, 1994; Vejbñk, 1990; Michelsen, 1997). Analysis of the character and cause of the thinning of the Triassic succession is the main objective of the present study. 2. Triassic stratigraphy The Triassic succession in the Danish area has been subdivided lithostratigraphically by Bertelsen (1980) and revised by Pedersen (1998) (Fig. 2). The subdivision is based on signi®cant changes in facies, which are re¯ected in changes in lithology (Pedersen, 1998). The lithostratigraphy de®ned south of the study area for the German area by Beutler and SchuÈler (1987) is correlated with the Danish lithostratigraphy by Pedersen (1998) as shown in Fig. 2. Correlation with the UK lithostratigraphy is hampered by poor biostratigraphic control and lateral continuity across the central parts
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Fig. 2. Triassic lithostratigraphic scheme, compiled from Beutler and SchuÈler (1987), Aigner and Bachmann (1992) and Pedersen (1998).
of the North Sea Basin. However, the Schilfsandstein is interpreted to correlate with the Arden Sandstone Member (Mercia Mudstone Group) in the UK southern North Sea (Warrington & Ivemey-Cook, 1992). The Triassic succession in southern Denmark is constituted by Lower Triassic redbeds (Bunter Shale, Bunter Sandstone and érslev Formations), Middle Triassic carbonates (Falster Formation), Upper Triassic redbeds, sandstone and evaporites (Tùnder and Oddesund Formations) and Upper Triassic±Lower Jurassic marine shales and sandstone (Vinding and Gassum Formations) (Fig. 2). The lateral facies variations in the southern Danish area and in the northern part of the North German Basin are so small and gradual that a correlation of the units is possible (Bertelsen, 1980). This mainly non-marine succession is characterised by several essentially synchronous marker beds, such as the basal evaporite bed of the érslev Formation (Figs. 2 and 3). In the Danish and North German basins, the Triassic lithostratigraphic units are generally chronostratigraphically signi®cant as shown in several studies both in the Danish and the German areas (e.g. Bertelsen, 1980; Aigner & Bachmann, 1992; Pedersen, 1998). The lithostratigraphic subdivision in the dierent wells is based on petrophysical log characteristics
supplemented by cuttings and cores when available. Due to the continental character of the sediments, the biostratigraphic information from the boreholes is limited to absent. 3. `Late Triassic unconformity' Beutler and SchuÈler (1978, 1987) interpreted an unconformity, the `Late Triassic unconformity', within the upper Triassic in the Rùdby-1 and érslev-1 wells, which they correlated to the `Altkimmerische Hauptdiskordanz', an angular unconformity observed at outcrops and in wells in the eastern part of the Northwest European Basin (Beutler, 1995). In a revision of the upper Triassic lithostratigraphy by Pedersen (1998), the `Late Triassic unconformity' was shown regionally to occur within the upper part of the Tùnder Formation, e.g. in the Sùllested-1 at the base of thick sandstone interval (Fig. 3). The interpretation of Pedersen (1998) involved all available wells and positioned the unconformity within the Carnian Tùnder Formation (Figs. 2 and 3). This interpretation diers from Beutler and SchuÈler (1978, 1987), who in the Rùdby-1 and érslev-1 positioned the unconformity at the top of the Lower Norian O-3 member of the
Fig. 3. Lithostratigraphic log correlation of the late Lower to Upper Triassic. Note the `Late Triassic unconformity' in the érslev-1, Rùdby-1 and Slagelse-1 wells and thinning of the Tùnder Formation in the Sùllested-1 well. The position of the lithostratigraphic boundaries is de®ned by facies shifts interpreted on the basis of well logs, cores and cuttings.
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Fig. 4. Seismic sections showing the geometry of the Triassic succession from the North German Basin to the Mùn High. The section (a) is a composite section from two seismic surveys. The section (c), which is an enlarged part of (a) ties to the érslev-1 well where the unconformity is observed at the erosive top of the Falster Formation. Note the onlapping re¯ectors of the internal re¯ectors in the succession overlying the unconformity. The map (b) shows the southern margin of the Ringkùbing-Fyn High, the position of the major salt-structures (from Vejbñk & Britze, 1994) and location of the wells and seismic sections used here.
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Fig. 5. Structural reconstruction of the seismic section from Fig. 4. (a)±(g) shows the geometric reconstruction of the section. The dashed line at the basement in (f) and (g) shows the unfaulted Top pre-Zechstein if assumed that the faulting took place during the Triassic. (h) Shows the dip of the basement from the dierent reconstructed sections superimposed unto each other. The lines `f' and `g' dips less that the other lines which indicated that the major change in dip of the basement takes place between section (e) and (f). See text for further discussion.
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Oddesund Formation (see Pedersen (1998) for a detailed discussion). Pedersen (1998) thus argues that locally erosion of the upper part of the Falster Formation took place e.g. in the érslev-1 well (Fig. 3). The erosional character at the top of the Falster Formation is also shown by erosional truncations on seismic sections (Fig. 4). In the Danish area, the `Late Triassic unconformity' is thus interpreted to be located at the top of the Falster Formation in the érslev-1 and Rùdby-1 wells (as indicated in Fig. 2). The `Late Triassic unconformity' has not been recognised on seismic sections in southern Jutland, in spite of the fact that a hiatus interpreted in wells encompasses a considerable period with non-deposition and local erosion in the eastern part of the Ringkùbing-Fyn High (Fig. 2). The regional extent of the `Late Triassic unconformity' and the dierence in the Upper Triassic succession encountered in the two closely spaced Rùdby-1 and Sùllested-1 wells will be discussed below on the basis of seismic interpretation. 3.1. Age of the `Late Triassic unconformity' The hiatus in the Rùdby-1 and érslev-1 wells at the top of the Middle Triassic Falster Formation, described above, encompasses a period from the Ladinian (upper Falster Formation) to the Lower Norian (O-3 member of the Oddesund Formation; Figs. 2 and 3). In the Slagelse-1 well, the calcareous deposits of the Falster Formation are abruptly overlain by arenaceous deposits of the upper part of the Tùnder Formation (Fig. 3). The Tùnder Formation in the Sùllested-1 well is relatively thin compared to elsewhere (Fig. 3). In the Sùllested-1 well, the calcareous deposits of the Falster Formation are gradually overlain by arenaceous redbeds of the Tùnder Formation, similar to the facies transition encountered further to the west (Fig. 3; Pedersen, 1998). Likewise, the upper boundary of the Tùnder Formation is a gradual facies transition from arenaceous deposits to the anhydritic redbeds of the overlying Oddesund Formation (Fig. 3). The base of this upper arenaceous succession is a pronounced log-break with an abrupt increase in gamma ray readings, which is interpreted as a `Late Triassic unconformity' within the middle part of the Tùnder Formation (Fig. 3). The hiatus observed in the Rùdby-1 and érslev-1 wells encompasses in the nearby Sùllested-1 well only the middle part of the Tùnder Formation (Fig. 3). The abrupt facies changes observed regionally by means of well logs and cuttings positioned within the Tùnder Formation is interpreted to correlate with the `Late Triassic unconformity'. The `Late Triassic unconformity' is thus interpreted to be of Carnian age (Fig. 3).
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4. Structural evolution Based on interpretation of seismic sections (Fig. 1), calibrated by the wells investigated, the Mesozoic structural evolution (with special emphasis on the Triassic structural evolution) of the southern ¯ank of the Ringkùbing-Fyn High can be unravelled. We have selected three composite sections which run from the North German Basin onto the Ringkùbing-Fyn High as indicated in Fig. 1 for presentation. The sections cover an area from the east coast of Jutland to the east coast of the island of Falster. The sections are tied to the wells located in the study area. 4.1. The érslev-1 area Fig. 4(a) gives an interpreted seismic cross-section extending from the érslev-1 well in SSW±NNE direction and crosses the southern margin of the Mùn High, a part of the Ringkùbing-Fyn High (Figs. 1 and 4). This composite seismic section shows that the seismic basement (the Top pre-Zechstein) dips to the south±southwest and that the post-Zechstein sediments are cut by a major fault (Fault A, Fig. 4(a)) and a minor fault-zone (Fault-zone B, Fig. 4(a)). Fault A does not cut the Zechstein evaporites and is located at the southern ¯ank of a salt roller composed of Zechstein salt. A fault at the Top pre-Zechstein is located beneath the salt roller but there are no indications of a possible genetic control of the Top preZechstein fault on the initiation of the salt roller. Fault-zone B, is composed of opposed dipping faults which detach at the Top Zechstein. A fault (or ¯exure) cutting the Top pre-Zechstein surface is interpreted below the fault-zone B (Fig. 4(a)). 4.2. Structural reconstruction of the érslev area The interpreted seismic section shown in Fig. 4(a) is restored in time to evaluate subsidence patterns (Fig. 5) using the reconstruction software RESTORE, designed for structural reconstruction in areas with mobile salt (Schultz-Ela & Duncan, 1991). The reconstruction software assumes a ®xed geometry for the basement through time. The dip of the basement can be changed in order to adjust for salt movements and deformation of the cover sediments. This gives control on the amount of salt present in the section (i.e. enables us to account for salt moving in and out of the section). If faulting of the basement takes place contemporaneously with the faulting of the cover sediments, it can be accounted for by manually reconstructing the basement geometry and continuing the reconstruction back in time using the reconstructed basement geometry. During the reconstruction, the thickness of the
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Zechstein salt deposits was assumed to be constant in the érslev-1 well since only 20% of the Zechstein is mobile halites in the érslev-1 well. The halite is concentrated in two layers of 35 m and 30 m thickness out of a total 369 m of Zechstein. The salt thickness to the south of Fault A (Fig. 4(a)) i.e. beneath the hangingwall of Fault A, was assumed too thin or to be constant through time, as salt generally ¯ows away from the area beneath the hangingwall of a fault in the cover sediments. Fault A (Fig. 4(a)) dips to the south and detaches at the top of the Zechstein. The reconstruction in Fig. 5 shows that activity along Fault A was mainly the result of salt accumulating beneath the footwall and that only a very small volume of salt was withdrawn from the hangingwall area. A gentle salt roller thus developed during the Triassic and the salt structure became accentuated during the post-Triassic and was very active during the Cretaceous. The reconstructed geometry shows a thinning of the Oddesund Formation (light shaded in Fig. 5(e)) at the footwall of fault A which indicates that the salt roller below the fault accentuated during deposition of the Oddesund Formation. It is possible that the salt from the area indicated with an asterisk in Fig. 5(e) moved downdip into the salt roller according to the model of Roberts, Price and Olsen (1990), thereby creating a depression in which bi-directional onlap is observed on the seismic sections (Fig. 4(a)). However, it cannot be excluded that salt moved into the salt roller perpendicular to the section. The depression may thus have been generated due to interaction between salt movements and regional tilt of the basement. The dip of the basement through time is summarised in Fig. 5(h), where the position of the érslev-1 well is used as a reference point. Fig. 5(h) shows that the dip of the basement changes signi®cantly between f and e, which corresponds to the period when the `Late Triassic unconformity' is interpreted in this area. The basement geometry was ®xed during this reconstruction. However, it is possible that the faults were active during the Triassic, since early Triassic stretching of the basins is observed (Vejbñk, 1997; Sùrensen, 1986). Assuming that the salt volume was relatively constant, a reconstruction of the eect of the basement faults on the dip of the basement was undertaken. The dashed line at the basement in Fig. 5(g) and (f) thus indicates the geometry of the Top pre-Zechstein with a constant salt volume. It clearly shows that although the Top pre-Zechstein may have undergone faulting during the Triassic, it has not aected the timing of the southward tilting of the basement as indicated in Fig. 5(h).
4.3. Triassic erosion and non-deposition in the érslev area Truncationed re¯ector terminations evident on the seismic sections indicate that erosion took place at the top of the Falster Formation (Figs. 4(c) and 5(a)). During the structural reconstruction, the top Falster Formation re¯ector was extrapolated to the north as indicated by the dashed line in Fig. 5(e). The extrapolation of the Top Falster Formation suggests that in the érslev-1 well approximately 50 to 75 m sediment was eroded from the top of the Falster Formation (Fig. 5(e)). This is in accordance with the observed thickness dierence of the Falster Formation between the Sùllested-1 well (see Fig. 3), where seismic sections show no evidence for erosion at the top of the Falster Formation, and the érslev-1 well. Furthermore, re¯ectors onlapping the `Late Triassic unconformity' observed south of the érslev-1 well, emphasise that the missing upper Tùnder Formation, and lower and middleOddesund Formation in the érslev-1 well as compared to the Sùllested-1 and Slagelse-1 wells (Fig. 3), indicate that the érslev-1 area was an area of non-deposition prior to the deposition of the O-3 member of the Oddesund Formation. Thus, the hiatus observed in the érslev area contains an erosional as well as non-depositional component with the major part interpreted as being due to non-deposition. This suggests that the area around the érslev1 well was located above the erosional baselevel whereas sedimentation was continuous in the area south of the érslev-1 well. The depression, in which relatively thick upper Tùnder Formation and lower and middle Oddesund Formation were deposited, was probably caused by minor salt withdrawal contemporaneously with (and probably triggered by) a change in dip of the basement. The analysis of Vejbñk (1997) and Pedersen (1998) shows that the area north of the RingkùbingFyn High also suered increased subsidence during the deposition of the upper Tùnder Formation and Oddesund Formation compared to the RingkùbingFyn High area. The Ringkùbing-Fyn High was thus hinged along both the northern and the southern margins. The internal re¯ectors of the Oddesund Formation shows bi-directional onlap onto the margins of the depression (Figs. 4(a) and 5(e)). The Oddesund Formation contains two distinct evaporite beds (Bertelsen, 1980; Pedersen, 1998) which indicates that the bedding was originally horizontal (or near horizontal). The bi-directional onlaps thus indicate that the depression was generated prior to the deposition of that part of the Oddesund Formation, which con®rms a pre-Oddesund Formation age of the structure.
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4.4. The Sùllested-1 and Rùdby-1 area The seismic section WGC80-8021 was interpreted using the Sùllested-1 and Rùdby-2 wells (Fig. 6) as calibration points which tie to the section via WGC808022 and WGC79A-7925. The Rùdby-2 and Rùdby-1 wells are both located on the Rùdby salt structure and show very similar evolution whereas the Sùllested-1 well is located adjacent to a salt wall (Fig. 6). The seismic interpretation constrains the log correlation of the earlier described `Late Triassic unconformity' (Fig. 6). The Rùdby-1 well shows deeper erosion at the `Late Triassic unconformity' than is observed in the Sùllested-1 well (Fig. 3) in spite of the more basinward position of the Rùdby-1 well compared to the Sùllested-1 well. This is interpreted to be due to the location of the Rùdby-1 with respect to the underlying saltstructures (Fig. 6). The Rùdby salt structure may either have uplifted the area around the Rùdby-1 well or have generated a relative depression similar to the depression south of the érslev-1 well. That a complex interaction between salt-movements and basin tectonics in¯uenced the unconformity is also suggested by the position of the Sùllested-1 well relative to a saltwall (Fig. 6). The area shows a geological evolution very similar to the érslev section (Fig. 4(a)) where salt-movement and basement subsidence generates the topography in which the sediments were deposited just above the unconformity. There are no erosional truncations observed in the north-western part of the section (Fig. 6), but low-angle erosion truncations cannot be excluded and the seismic interpretation indicates a signi®cant thinning of the Triassic strata located below the unconformity in the north-eastern part. It is also possible that periods of non-deposition took place as inferred in the érslev-1 area. 4.5. The south-western margin of Funen
Fig. 6. Seismic section showing the geometry of the successions from the North German Basin onto the Stigsnñs Block. The unconformity has here the character of an onlap surface. The map shows the position of the section and wells in relation to underlying salt structures (from Vejbñk & Britze, 1994).
The area around and north of the Kegnñs-1 well is illustrated by four seismic sections (Fig. 7). The section GY84 T-8 (Fig. 7(a)) shows that salt-movements caused relative uplift of a salt structure south-west of the Kegnñs-1 well. There are a large number of erosional truncations at the Top Trias. The Kegnñs salt structure was uplifted in Post-Triassic time as indicated by the constant thickness of the Tùnder and older formations. Furthermore, no indications of onlap internally in the Triassic succession onto the Kegnñs salt structure are observed. The section WGC79-7917 (Fig. 7(b)) strikes parallel with the south-western border of the Glamsbjerg Block (Fig. 1) and shows no signi®cant changes in thickness of the érslev, Falster and Tùnder Formations outside the faulted area. The complex fault pattern in the centre of the section is due to sec-
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Fig. 7. Seismic sections from the area around the island Als showing the geometry of the Triassic succession. The position the Kegnñs-1 well is also shown (a). The consistent thickness of the early and middle Triassic succession around the Kegnñs pillow and the erosional truncation near the base Cretaceous (BC), at a horizon which corresponds to the Top Triassic, indicates that the salt movements generating the Kegnñs pillow took place after deposition of the Tùnder, Falster and érslev Formations. No signi®cant change in thickness of these Triassic deposits is observed in the other section running along the island of Als (b and c) and across the Lille Bñlt (d). See text for further discussion. For location of the sections see the inserted map. See Fig. 4 for legend.
tion parallel faults, which strike NW±SW and detach along the Top Zechstein surface. The seismic section WGC79-7918 (Fig. 7(c)) strikes east±west and the section DA98-46 (Fig. 7(d)) continues across the Lille Bñlt to the shore of Funen. All sections emphasise that no signi®cant dierential basement-controlled subsidence took place during deposition of the Tùnder Formation in the area around the island of Als. In the
Ringe-1 well, which is located at the centre of the Glamsbjerg block (Fig. 1), the Tùnder Formation is absent and the Falster Formation is overlain by the Slagelse Member. Previous seismic interpretations carried out by Sorgenfrei (1966) in the area around the Ringe-1 well shows that the Triassic succession is conformable and shows no evidence of lateral thickness changes. Newer seismic sections con®rm this. The
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Fig. 8. Well section showing the reinterpreted correlation of the lithostratigraphy related to the seismic subdivision of Clausen and KorstgaÊrd (1993). The horizon TR1-4 are of Triassic age, and TPZ equals the Top pre-Zechstein. The seismic surfaces are correlated to the same lithostratigraphic unit in both wells, which con®rms the assumption that the lithostratigraphic subdivision used in this study is chronostratigraphic signi®cant. The TR4 is a marked unconformity and correlates to a surface internally in the Tùnder Formation, and is important since it marks the change in subsidence pattern in the Horn Graben area (Clausen & KorstgaÊrd, 1993). See text for further discussion.
absence of the Tùnder Formation in the Ringe-1 well thus suggests that at least the central parts of the Glamsbjerg Block were topographically high during deposition of the Tùnder Formation or that erosion of the Tùnder Formation (if deposited) took place prior to deposition of the Slagelse Member, as suggested for the area around the érslev-1 well.
5. Discussion 5.1. Variations in structural evolution along the southern margin of the Ringkùbing-Fyn High The `Late Triassic unconformity' observed in the study area is interpreted as related to basement involved relative uplift of the eastern part of the Ringkùbing-Fyn High, causing the initiation of movements of Zechstein salt in basinward areas. The relative uplift of the Ringkùbing-Fyn High is interpreted as due to heterogeneous Triassic stretching of the area (less stretching at the Ringkùbing-Fyn High compared to the basins) and later rapid thermal subsidence of
the basins both north and south of the RingkùbingFyn High (Sùrensen, 1986, Vejbñk, 1997). Late Triassic tectonics are also observed in the German Basin located south of the study area (Wolburg, 1969; Dittrich, 1989, 1991; Beutler, 1995). Clausen and KorstgaÊrd (1993, 1994) show in the Horn Graben area the presence of a major unconformity at the base of the Jurassic succession in accordance with Cartwright (1990). Furthermore, a major intra-Triassic surface was interpreted as marking the change in dominant deposition within the Horn Graben to the area south of the Ringkùbing-Fyn High (Clausen & KorstgaÊrd, 1993). The lithostratigraphic subdivision of the wells R-1 and S-1 in the Horn Graben (Fig. 1), according to the principles de®ned by Pedersen (1998), shows that the intra-Triassic surface is located within the Tùnder Formation, and thus most probably corresponds to the stratigraphic level of the above described Late Triassic unconformity (Fig. 8). The analysis of E±W striking detaching faults within Horn Graben shows that a change in dip of the Top pre-Zechstein surface took place during the late Triassic and is thus comparable with the southward tilt of the North German
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Basin observed in south-eastern Denmark. Assuming that the constant thickness of the Late Triassic succession in the areas Als-Funen and Southern Jutland indicates no dierential subsidence, the areas in southern Jutland and the isles Als and Funen dier from the areas in south eastern Denmark and the Horn Graben. Therefore, the stratigraphic record of the RingkùbingFyn High suggests that the individual blocks, constituting the Ringkùbing-Fyn High, underwent dierent structural evolution, as already suggested by Sorgenfrei and Buch (1964). The subsidence pattern along the Ringkùbing-Fyn High is thus suggested to be a consequence of intra-plate deformation due to the overall E±W extension of the European continent during the Triassic. 5.2. Eect of the salt structures The reconstruction of the section in the south-eastern Danish area shows that mobile salt had an important role during deposition of the Late Triassic succession by modifying the accommodation space. Best, Kockel and SchoÈneich (1983) show that the salt structures located in the southern part of the Horn Graben were similarly active in the Late Triassic to those in the south-eastern Danish area. However, this is in contrast to the observations in southern Jutland and the Als-Funen area where the salt structures are interpreted to be generally younger. The correspondence between change in dip of basement and salt movements as observed in this study corresponds to the observations made along the southern rim of the Ringkùbing-Fyn High in the Danish North Sea (Clausen & KorstgaÊrd, 1996) and along the southern margin of the Mid North Sea High in the UK sector (Coward & Stewart, 1995). A similar trend is observed along the northern margin of the Danish RingkùbingFyn High (Britze, Japsen & Langtofte, 1992; Vejbñk, 1997).
6. Conclusions The integration of well-logs and seismic sections enables the geological analysis of a Late Triassic unconformity and the structural evolution. The analysis has shown that: 1. The dierential subsidence between the RingkùbingFyn High and the North German Basin during the Late Triassic varies along the strike of the Ringkùbing-Fyn High. A signi®cant change in dip of the basement is observed in the south eastern part of the Danish area and in the Horn Graben area. This contrast with southern Jutland where no indications of dierential subsidence or salt move-
ments during the Late Triassic are observed. 2. Late Triassic erosion is shown by an unconformity characterised by local low-angle erosion at the crest of the Ringkùbing-Fyn High and onlap onto the unconformity at the Ringkùbing-Fyn High. 3. Salt-structures in the North German Basin are active during the Late Triassic, but restricted to areas where a signi®cant change in basement dip is observed. It is therefore inferred that the change in basement dip controlled the movements of Zechstein salt. The halokinetic events in¯uenced the deposition of Late Triassic sediments by generating local depressions and highs in the area south of the Ringkùbing-Fyn High. 4. The subsidence pattern of the Top pre-Zechstein surface varies along the southern margin of the Ringkùbing-Fyn High. This indicates that the subdivision of the Ringkùbing-Fyn High into structural blocks played a major role into the evolution of the Triassic succession in the Danish area.
Acknowledgements The referees are thanked for valuable comments, which improved the manuscript. Egon Nùrmark is thanked for the seismic processing of the seismic line DANA98-46, which is part of the Danish Natural Science Research Council 9701761. Per K. Pedersen was ®nanced by the Faculty of Science, University of Aarhus.
References Aigner, T., & Bachmann, G. H. (1992). Sequence-stratigraphic framework of the German Triassic. Sedimentary Geology, 80, 115±135. Bertelsen, F. (1980). Lithostratigraphy and depositional history of the Danish Triassic. Danmarks Geologiske Undersùgelse, Serie B, 4, 1±60. Best, G., Kockel, F., & SchoÈneich, H. (1983). Geological history of the southern Horn Graben. In J. P. H. Kaasschieter, & T. J. A. Reijers, Petroleum geology of the southeastern North Sea and the adjacent onshore areas, vol. 62 (pp. 25±33). Geologie en Mijnbouw. Beutler, G. (1995). Quanti®zierung der altkimmerischen Bewegungen in Nordwest-deutschland, Teil 1 Stratigraphie des Keupers, Hannover, FoÈderungsvorhaben der Hans-Joachim MartiniStiftung. Ð UnveroÈ. Bericht BGR, Archiv-Nr. 113 087, pp. 1± 147. Beutler, V. G., & SchuÈler, F. (1978). UÈber altkimmerische Bewgungen in Norden der DDR und ihre regionale Bedeutung (Fortschrittsbericht). Zeitschrift fuÈr Geologische Wissenschaften, 6, 403±420. Beutler, V. G., & SchuÈler, F. (1987). Probleme und Ergebnisse der lithostratigraphischen Korrelation der Trias am Nordrand der MitteleuropaÈischen Senke. Zeitschrift fuÈr Geologische Wissenschaften, 15, 421±436.
O.R. Clausen, P.K. Pedersen / Marine and Petroleum Geology 16 (1999) 653±665 Britze, P., Japsen, P., & Langtofte, C. (1992). Major structural elements of the Danish Basin. European Association of Petroleum Geoscientists and Engineers, 4th Conference and Technical Exhibition, p. 101. Cartwright, J. (1990). The structural evolution of the RingkùbingFyn High. In D. J. Blundell, & A. D. Gibbs, Tectonic evolution of the north sea rifts (pp. 200±216). Oxford: Clarendon Press. Clark, D. N., & Tallbacka, L. (1980). The Zechstein deposits of southern Denmark. In H. Fuchtbauer, & T. M. Peryt, The Zechstein Basin with emphasis on carbonate sequences, contributions to sedimentology, vol. 9 (pp. 205±231). Clausen, O. R., & KorstgaÊrd, J. A. (1993). Faults and faulting in the Horn Graben area, Danish North Sea. First Break, 11, 127±143. Clausen, O. R., & KorstgaÊrd, J. A. (1994). Displacement geometries along graben bounding faults in the Horn Graben, oshore Denmark. First Break, 12, 305±319. Clausen, O. R., & KorstgaÊrd, J. A. (1996). Planar detaching faults in the southern Horn Graben, Danish North Sea. Marine and Petroleum Geology, 13, 537±548. Coward, M., & Stewart, S. (1995). Salt-in¯uenced structures in the Mesozoic±Tertiary cover of the southern North Sea, UK. In M. P. A. Jackson, D. G. Roberts, & S. Snelson, Salt tectonics: a global perspective, AAPG memoir (pp. 229±250). Dittrich, D. (1989). Der Schilfsandstein als synsedimentaÈr-tectonisch gepraÈgtes Sediment Ð eine Umdeutung bisheriger Befunde. Zeitschrift der Deutschen Geologischen Gesellschaft, 140, 295±310. Dittrich, D. (1991). Vergleiche zwischen ardennischer und vindelizischer Randfacies des Schilfsandsteins (Mittlerer Keuper, Trias). Geol. Bl. NO-Bayern, 41(3-4), 143±168. Kockel, F. (1995). Structural and paleogeographical development of the German North Sea sector, Berlin, Stuttgart, BeitraÈge zur regionalen Geologie der Erde, Bd. 26, GebruÈder Borntraeger, pp. 1±35. Michelson, O. (1997). Mesozoic and Cenozoic stratigraphy and structural development of the Sorgenfrei±Tornquist Zone. Zeitschrift der Deutschen Geologischen Gesellschaft, 148, 33±50. Mogensen, T. E. (1994). Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark. Tectonophysics, 240, 191±214. Pedersen, P. K. (1998). Sequence stratigraphic analysis of the nonmarine to marginal marine Danish Triassic, supplemented by a ®eld study of the alluvial architecture of the Ericson Sandstone (USA), unpublished Ph.D. thesis, University of Aarhus.
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Roberts, A. M., Price, J. D., & Olsen, T. S. (1990). Late Jurassic half-graben control on the siting and structure of hydrocarbon accumulations: UK/Norwegian Central Graben. In R. F. P. Hardman, & J. Brooks, Tectonic events responsible for Britain's oil and gas reserves (pp. 229±257). London: Geological Society Special Publication No. 55. Schultz-Ela, D. D., & Duncan, K. (1991). Restore. User's manual and software (pp. 1±75). Austin: Bureau of Economic Geology, University of Texas. Sorgenfrei, T. (1966). Strukturgeologischer Bau von DaÈnemark. Geologie, 6, 641±660. Sorgenfrei, T. (1969). A review of petroleum development in Scandinavia. In The exploration for petroleum in Europe and North Africa (pp. 191±208). Dorking: The Institute of Petroleum, Adlard and Son Ltd. Sorgenfrei, T., & Buch, A. (1964). Deep tests in Denmark, 1935± 1959. Danmarks Geologiske Undersùgelse, III. Rñkke, 36, 1± 146. Sùrensen, K. (1986). Danish Basin subsidence by Triassic rifting on a lithosphere cooling background. Nature, 319, 660±663. Vejbñk, O.V., (1990). The Horn Graben, and its relationship to the Oslo Graben and the Danish Basin. In E.R. Neumann, Rift zones in the continental crust of Europe Ð geophysical, geological and geochemical evidence: Oslo±Horn graben. Tectonophysics, 178, 29± 49. Vejbñk, O. V. (1997). Dybe strukturer i danske sedimentñre bassiner. Geologisk Tidsskrift, 4, 1±31. Vejbñk, O. V., & Britze, P. (1994). Top pre-Zechstein (two-way traveltime and depth). Geological map of Denmark 1: 750.000. Danmarks Geologiske Undersùgelse Kort Serie, 45, 1±9. Warrington, G., & Ivemey-Cook, H. C. (1992). Triassic. In J. C. W. Cope, J. K. Ingham, & P. F. Rawson, Atlas of palaeogeography and lithofacies, vol. 13 (pp. 97±106). Geological Society Memoir. Wolburg, J. (1969). Die epirogenetischen Phasen der Muschelkalkund Keuper-Entwicklungen Nordwest-Deutschlands, mit einem RuÈckblick auf den Buntsandstein. Geotektonische Forschungen, 32, 1±65. Ziegler, P. A. (1990). Tectonic and palaeogeographic development of the North Sea rift system. In D. J. Blundell, & A. D. Gibbs, Tectonic evolution of the North Sea rifts (pp. 1±36). London: Clarendon Press.
Late Triassic structural evolution of the southern margin of the Ringkùbing-Fyn High, Denmark Ole Rùnù Clausen*, Per Kent Pedersen Department of Earth Sciences, University of Aarhus, DK-8000, Aarhus, C, Denmark Received 16 December 1998; received in revised form 12 June 1999; accepted 17 June 1999
Abstract A Late Triassic (Carnian) unconformity is observed on seismic sections and in wells along the southern margin of the Ringkùbing-Fyn High. The unconformity is characterised by low angle erosion on the central parts of the eastern RingkùbingFyn High (Mùn High), and by a signi®cant onlap onto the unconformity surface. Structural reconstruction shows that the unconformity is related to dierential subsidence between the Ringkùbing-Fyn High and the North German Basin. The dierential subsidence initiated movements in the mobile Zechstein Salt, causing deformation of the cover sediments. The variability in dierential subsidence observed during the Triassic along the southern margin of the Ringkùbing-Fyn High is interpreted to re¯ect the subdivision of the Ringkùbing-Fyn High into a number of structure blocks. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Triassic; Denmark; Tectonics
1. Introduction
1.1. Structural setting of the study area
The objective of this study is to analyse the Late Triassic structural evolution of the southern margin of the Ringkùbing-Fyn High with special emphasis on the geological signi®cance of a Late Triassic unconformity observed in wells and on seismic sections. The study integrates observations on sedimentology, facies evolution, and hiatuses made in wells in the south-eastern part of the Danish area with structural interpretations of seismic sections. The study uses released convectional 2-D seismic surveys (GY84 T, PRKL80, WGC79, WGC81), high resolution seismic survey DA98 (owned by the Department of Earth Sciences, University of Aarhus), and well logs, cuttings and cores from the wells: Ringe-1, Sùllested-1, Rùdby-1, Kegnñs-1, érslev-1, Tùnder-1, Lùgumkloster-1, Slagelse-1, R-1 and S-1 (Fig. 1).
The study area encompasses the northern margin of the North German Basin and the southern part of the Norwegian±Danish Basin (Fig. 1). In Triassic times northern and central Europe formed a large landlocked epicontinental basin, the Northwest European Basin (Ziegler, 1990), which was composed of numerous sub-basins, such as the Norwegian±Danish Basin and the North German Basin separated by structural highs such as the Ringkùbing-Fyn High (Fig. 1). West of the Central Trough area, the Silverpit Basin is located south of the Mid North Sea High in a similar setting to the North German Basin (Coward & Stewart, 1995). The Ringkùbing-Fyn High forms part of the generally east±west striking system of highs running across the North Sea area (Ziegler, 1990). The individual blocks of the Ringkùbing-Fyn High are referred to according to the nomenclature given by Vejbñk (1997) (Fig. 1). The Ringkùbing-Fyn High is characterised as an area where the crystalline basement is shallow compared to the adjacent basins. The Ringkùbing-Fyn
* Corresponding author. Tel.: +45-89-42-25-18; fax: +45-89-4225-25. E-mail address: [email protected] (O.R. Clausen)
0264-8172/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 8 1 7 2 ( 9 9 ) 0 0 0 2 6 - 4
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Fig. 1. Map showing the study area, dominated by the Norwegian Danish Basin and the North German Basin separated by the Ringkùbing-Fyn High. The Norwegian±Danish Basin is bounded to the north-east by the Sorgenfrei±Tornquist Zone. Wells presented in this paper are shown as ®lled circles. Seismic sections used in this study are indicated by full lines. The names of the blocks of the Ringkùbing-Fyn High is from Vejbñk (1997) as: ENBl: East North Sea Block, GrBl: Grindsted Block, GlBl: Glamsbjerg Block, StBl: Stigsnñs Block, MùH: Mùn High.
High was formed during a pre-Zechstein stretching phase in which the Ringkùbing-Fyn High area suered lesser stretching than the adjacent basins (Vejbñk, 1997). It cannot be excluded that the pre-Zechstein Ringkùbing-Fyn High was fault bounded since the seismic resolution beneath the Top pre Zechstein surface generally is poor. However, the faults at the Top pre-Zechstein surface indicates little or no fault control of the post-Zechstein dierential subsidence (Fig. 1) and the dierential subsidence between the basins and the high thus has had the character of regional in¯exion. The Ringkùbing-Fyn High formed a barrier and separated the Southern and Northern Permian Basins in Late Permian time (Clark & Tallbacka, 1980), and was inundated in early Triassic times (Pedersen, 1998). Thinning of Triassic deposits across the RingkùbingFyn High has previously and mainly been interpreted as due to slower subsidence on the Ringkùbing-Fyn High than in the North German Basin and Norwegian±Danish Basin and partly due to erosion due to Mid-Jurassic relative uplift of the RingkùbingFyn High (Sorgenfrei, 1969; Bertelsen, 1980; Cartwright, 1990; Kockel, 1995). The subsidence of the Norwegian±Danish Basin has been attributed to thermal subsidence following preTriassic rifting (Sùrensen, 1986; Vejbñk, 1990). However, the Horn Graben, which is located in the
eastern North Sea (Fig. 1), shows active rifting and consequent deposition of a very thick Lower and Middle Triassic succession (Bertelsen, 1980; Vejbñk, 1990; Clausen & KorstgaÊrd, 1993). The Sorgenfrei± Tornquist Zone, which is a major strike-slip zone delimiting the sedimentary basin to the north-east (Fig. 1), experienced right-lateral transtensional movements during the Triassic (Mogensen, 1994; Vejbñk, 1990; Michelsen, 1997). Analysis of the character and cause of the thinning of the Triassic succession is the main objective of the present study. 2. Triassic stratigraphy The Triassic succession in the Danish area has been subdivided lithostratigraphically by Bertelsen (1980) and revised by Pedersen (1998) (Fig. 2). The subdivision is based on signi®cant changes in facies, which are re¯ected in changes in lithology (Pedersen, 1998). The lithostratigraphy de®ned south of the study area for the German area by Beutler and SchuÈler (1987) is correlated with the Danish lithostratigraphy by Pedersen (1998) as shown in Fig. 2. Correlation with the UK lithostratigraphy is hampered by poor biostratigraphic control and lateral continuity across the central parts
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Fig. 2. Triassic lithostratigraphic scheme, compiled from Beutler and SchuÈler (1987), Aigner and Bachmann (1992) and Pedersen (1998).
of the North Sea Basin. However, the Schilfsandstein is interpreted to correlate with the Arden Sandstone Member (Mercia Mudstone Group) in the UK southern North Sea (Warrington & Ivemey-Cook, 1992). The Triassic succession in southern Denmark is constituted by Lower Triassic redbeds (Bunter Shale, Bunter Sandstone and érslev Formations), Middle Triassic carbonates (Falster Formation), Upper Triassic redbeds, sandstone and evaporites (Tùnder and Oddesund Formations) and Upper Triassic±Lower Jurassic marine shales and sandstone (Vinding and Gassum Formations) (Fig. 2). The lateral facies variations in the southern Danish area and in the northern part of the North German Basin are so small and gradual that a correlation of the units is possible (Bertelsen, 1980). This mainly non-marine succession is characterised by several essentially synchronous marker beds, such as the basal evaporite bed of the érslev Formation (Figs. 2 and 3). In the Danish and North German basins, the Triassic lithostratigraphic units are generally chronostratigraphically signi®cant as shown in several studies both in the Danish and the German areas (e.g. Bertelsen, 1980; Aigner & Bachmann, 1992; Pedersen, 1998). The lithostratigraphic subdivision in the dierent wells is based on petrophysical log characteristics
supplemented by cuttings and cores when available. Due to the continental character of the sediments, the biostratigraphic information from the boreholes is limited to absent. 3. `Late Triassic unconformity' Beutler and SchuÈler (1978, 1987) interpreted an unconformity, the `Late Triassic unconformity', within the upper Triassic in the Rùdby-1 and érslev-1 wells, which they correlated to the `Altkimmerische Hauptdiskordanz', an angular unconformity observed at outcrops and in wells in the eastern part of the Northwest European Basin (Beutler, 1995). In a revision of the upper Triassic lithostratigraphy by Pedersen (1998), the `Late Triassic unconformity' was shown regionally to occur within the upper part of the Tùnder Formation, e.g. in the Sùllested-1 at the base of thick sandstone interval (Fig. 3). The interpretation of Pedersen (1998) involved all available wells and positioned the unconformity within the Carnian Tùnder Formation (Figs. 2 and 3). This interpretation diers from Beutler and SchuÈler (1978, 1987), who in the Rùdby-1 and érslev-1 positioned the unconformity at the top of the Lower Norian O-3 member of the
Fig. 3. Lithostratigraphic log correlation of the late Lower to Upper Triassic. Note the `Late Triassic unconformity' in the érslev-1, Rùdby-1 and Slagelse-1 wells and thinning of the Tùnder Formation in the Sùllested-1 well. The position of the lithostratigraphic boundaries is de®ned by facies shifts interpreted on the basis of well logs, cores and cuttings.
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Fig. 4. Seismic sections showing the geometry of the Triassic succession from the North German Basin to the Mùn High. The section (a) is a composite section from two seismic surveys. The section (c), which is an enlarged part of (a) ties to the érslev-1 well where the unconformity is observed at the erosive top of the Falster Formation. Note the onlapping re¯ectors of the internal re¯ectors in the succession overlying the unconformity. The map (b) shows the southern margin of the Ringkùbing-Fyn High, the position of the major salt-structures (from Vejbñk & Britze, 1994) and location of the wells and seismic sections used here.
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Fig. 5. Structural reconstruction of the seismic section from Fig. 4. (a)±(g) shows the geometric reconstruction of the section. The dashed line at the basement in (f) and (g) shows the unfaulted Top pre-Zechstein if assumed that the faulting took place during the Triassic. (h) Shows the dip of the basement from the dierent reconstructed sections superimposed unto each other. The lines `f' and `g' dips less that the other lines which indicated that the major change in dip of the basement takes place between section (e) and (f). See text for further discussion.
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Oddesund Formation (see Pedersen (1998) for a detailed discussion). Pedersen (1998) thus argues that locally erosion of the upper part of the Falster Formation took place e.g. in the érslev-1 well (Fig. 3). The erosional character at the top of the Falster Formation is also shown by erosional truncations on seismic sections (Fig. 4). In the Danish area, the `Late Triassic unconformity' is thus interpreted to be located at the top of the Falster Formation in the érslev-1 and Rùdby-1 wells (as indicated in Fig. 2). The `Late Triassic unconformity' has not been recognised on seismic sections in southern Jutland, in spite of the fact that a hiatus interpreted in wells encompasses a considerable period with non-deposition and local erosion in the eastern part of the Ringkùbing-Fyn High (Fig. 2). The regional extent of the `Late Triassic unconformity' and the dierence in the Upper Triassic succession encountered in the two closely spaced Rùdby-1 and Sùllested-1 wells will be discussed below on the basis of seismic interpretation. 3.1. Age of the `Late Triassic unconformity' The hiatus in the Rùdby-1 and érslev-1 wells at the top of the Middle Triassic Falster Formation, described above, encompasses a period from the Ladinian (upper Falster Formation) to the Lower Norian (O-3 member of the Oddesund Formation; Figs. 2 and 3). In the Slagelse-1 well, the calcareous deposits of the Falster Formation are abruptly overlain by arenaceous deposits of the upper part of the Tùnder Formation (Fig. 3). The Tùnder Formation in the Sùllested-1 well is relatively thin compared to elsewhere (Fig. 3). In the Sùllested-1 well, the calcareous deposits of the Falster Formation are gradually overlain by arenaceous redbeds of the Tùnder Formation, similar to the facies transition encountered further to the west (Fig. 3; Pedersen, 1998). Likewise, the upper boundary of the Tùnder Formation is a gradual facies transition from arenaceous deposits to the anhydritic redbeds of the overlying Oddesund Formation (Fig. 3). The base of this upper arenaceous succession is a pronounced log-break with an abrupt increase in gamma ray readings, which is interpreted as a `Late Triassic unconformity' within the middle part of the Tùnder Formation (Fig. 3). The hiatus observed in the Rùdby-1 and érslev-1 wells encompasses in the nearby Sùllested-1 well only the middle part of the Tùnder Formation (Fig. 3). The abrupt facies changes observed regionally by means of well logs and cuttings positioned within the Tùnder Formation is interpreted to correlate with the `Late Triassic unconformity'. The `Late Triassic unconformity' is thus interpreted to be of Carnian age (Fig. 3).
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4. Structural evolution Based on interpretation of seismic sections (Fig. 1), calibrated by the wells investigated, the Mesozoic structural evolution (with special emphasis on the Triassic structural evolution) of the southern ¯ank of the Ringkùbing-Fyn High can be unravelled. We have selected three composite sections which run from the North German Basin onto the Ringkùbing-Fyn High as indicated in Fig. 1 for presentation. The sections cover an area from the east coast of Jutland to the east coast of the island of Falster. The sections are tied to the wells located in the study area. 4.1. The érslev-1 area Fig. 4(a) gives an interpreted seismic cross-section extending from the érslev-1 well in SSW±NNE direction and crosses the southern margin of the Mùn High, a part of the Ringkùbing-Fyn High (Figs. 1 and 4). This composite seismic section shows that the seismic basement (the Top pre-Zechstein) dips to the south±southwest and that the post-Zechstein sediments are cut by a major fault (Fault A, Fig. 4(a)) and a minor fault-zone (Fault-zone B, Fig. 4(a)). Fault A does not cut the Zechstein evaporites and is located at the southern ¯ank of a salt roller composed of Zechstein salt. A fault at the Top pre-Zechstein is located beneath the salt roller but there are no indications of a possible genetic control of the Top preZechstein fault on the initiation of the salt roller. Fault-zone B, is composed of opposed dipping faults which detach at the Top Zechstein. A fault (or ¯exure) cutting the Top pre-Zechstein surface is interpreted below the fault-zone B (Fig. 4(a)). 4.2. Structural reconstruction of the érslev area The interpreted seismic section shown in Fig. 4(a) is restored in time to evaluate subsidence patterns (Fig. 5) using the reconstruction software RESTORE, designed for structural reconstruction in areas with mobile salt (Schultz-Ela & Duncan, 1991). The reconstruction software assumes a ®xed geometry for the basement through time. The dip of the basement can be changed in order to adjust for salt movements and deformation of the cover sediments. This gives control on the amount of salt present in the section (i.e. enables us to account for salt moving in and out of the section). If faulting of the basement takes place contemporaneously with the faulting of the cover sediments, it can be accounted for by manually reconstructing the basement geometry and continuing the reconstruction back in time using the reconstructed basement geometry. During the reconstruction, the thickness of the
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Zechstein salt deposits was assumed to be constant in the érslev-1 well since only 20% of the Zechstein is mobile halites in the érslev-1 well. The halite is concentrated in two layers of 35 m and 30 m thickness out of a total 369 m of Zechstein. The salt thickness to the south of Fault A (Fig. 4(a)) i.e. beneath the hangingwall of Fault A, was assumed too thin or to be constant through time, as salt generally ¯ows away from the area beneath the hangingwall of a fault in the cover sediments. Fault A (Fig. 4(a)) dips to the south and detaches at the top of the Zechstein. The reconstruction in Fig. 5 shows that activity along Fault A was mainly the result of salt accumulating beneath the footwall and that only a very small volume of salt was withdrawn from the hangingwall area. A gentle salt roller thus developed during the Triassic and the salt structure became accentuated during the post-Triassic and was very active during the Cretaceous. The reconstructed geometry shows a thinning of the Oddesund Formation (light shaded in Fig. 5(e)) at the footwall of fault A which indicates that the salt roller below the fault accentuated during deposition of the Oddesund Formation. It is possible that the salt from the area indicated with an asterisk in Fig. 5(e) moved downdip into the salt roller according to the model of Roberts, Price and Olsen (1990), thereby creating a depression in which bi-directional onlap is observed on the seismic sections (Fig. 4(a)). However, it cannot be excluded that salt moved into the salt roller perpendicular to the section. The depression may thus have been generated due to interaction between salt movements and regional tilt of the basement. The dip of the basement through time is summarised in Fig. 5(h), where the position of the érslev-1 well is used as a reference point. Fig. 5(h) shows that the dip of the basement changes signi®cantly between f and e, which corresponds to the period when the `Late Triassic unconformity' is interpreted in this area. The basement geometry was ®xed during this reconstruction. However, it is possible that the faults were active during the Triassic, since early Triassic stretching of the basins is observed (Vejbñk, 1997; Sùrensen, 1986). Assuming that the salt volume was relatively constant, a reconstruction of the eect of the basement faults on the dip of the basement was undertaken. The dashed line at the basement in Fig. 5(g) and (f) thus indicates the geometry of the Top pre-Zechstein with a constant salt volume. It clearly shows that although the Top pre-Zechstein may have undergone faulting during the Triassic, it has not aected the timing of the southward tilting of the basement as indicated in Fig. 5(h).
4.3. Triassic erosion and non-deposition in the érslev area Truncationed re¯ector terminations evident on the seismic sections indicate that erosion took place at the top of the Falster Formation (Figs. 4(c) and 5(a)). During the structural reconstruction, the top Falster Formation re¯ector was extrapolated to the north as indicated by the dashed line in Fig. 5(e). The extrapolation of the Top Falster Formation suggests that in the érslev-1 well approximately 50 to 75 m sediment was eroded from the top of the Falster Formation (Fig. 5(e)). This is in accordance with the observed thickness dierence of the Falster Formation between the Sùllested-1 well (see Fig. 3), where seismic sections show no evidence for erosion at the top of the Falster Formation, and the érslev-1 well. Furthermore, re¯ectors onlapping the `Late Triassic unconformity' observed south of the érslev-1 well, emphasise that the missing upper Tùnder Formation, and lower and middleOddesund Formation in the érslev-1 well as compared to the Sùllested-1 and Slagelse-1 wells (Fig. 3), indicate that the érslev-1 area was an area of non-deposition prior to the deposition of the O-3 member of the Oddesund Formation. Thus, the hiatus observed in the érslev area contains an erosional as well as non-depositional component with the major part interpreted as being due to non-deposition. This suggests that the area around the érslev1 well was located above the erosional baselevel whereas sedimentation was continuous in the area south of the érslev-1 well. The depression, in which relatively thick upper Tùnder Formation and lower and middle Oddesund Formation were deposited, was probably caused by minor salt withdrawal contemporaneously with (and probably triggered by) a change in dip of the basement. The analysis of Vejbñk (1997) and Pedersen (1998) shows that the area north of the RingkùbingFyn High also suered increased subsidence during the deposition of the upper Tùnder Formation and Oddesund Formation compared to the RingkùbingFyn High area. The Ringkùbing-Fyn High was thus hinged along both the northern and the southern margins. The internal re¯ectors of the Oddesund Formation shows bi-directional onlap onto the margins of the depression (Figs. 4(a) and 5(e)). The Oddesund Formation contains two distinct evaporite beds (Bertelsen, 1980; Pedersen, 1998) which indicates that the bedding was originally horizontal (or near horizontal). The bi-directional onlaps thus indicate that the depression was generated prior to the deposition of that part of the Oddesund Formation, which con®rms a pre-Oddesund Formation age of the structure.
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4.4. The Sùllested-1 and Rùdby-1 area The seismic section WGC80-8021 was interpreted using the Sùllested-1 and Rùdby-2 wells (Fig. 6) as calibration points which tie to the section via WGC808022 and WGC79A-7925. The Rùdby-2 and Rùdby-1 wells are both located on the Rùdby salt structure and show very similar evolution whereas the Sùllested-1 well is located adjacent to a salt wall (Fig. 6). The seismic interpretation constrains the log correlation of the earlier described `Late Triassic unconformity' (Fig. 6). The Rùdby-1 well shows deeper erosion at the `Late Triassic unconformity' than is observed in the Sùllested-1 well (Fig. 3) in spite of the more basinward position of the Rùdby-1 well compared to the Sùllested-1 well. This is interpreted to be due to the location of the Rùdby-1 with respect to the underlying saltstructures (Fig. 6). The Rùdby salt structure may either have uplifted the area around the Rùdby-1 well or have generated a relative depression similar to the depression south of the érslev-1 well. That a complex interaction between salt-movements and basin tectonics in¯uenced the unconformity is also suggested by the position of the Sùllested-1 well relative to a saltwall (Fig. 6). The area shows a geological evolution very similar to the érslev section (Fig. 4(a)) where salt-movement and basement subsidence generates the topography in which the sediments were deposited just above the unconformity. There are no erosional truncations observed in the north-western part of the section (Fig. 6), but low-angle erosion truncations cannot be excluded and the seismic interpretation indicates a signi®cant thinning of the Triassic strata located below the unconformity in the north-eastern part. It is also possible that periods of non-deposition took place as inferred in the érslev-1 area. 4.5. The south-western margin of Funen
Fig. 6. Seismic section showing the geometry of the successions from the North German Basin onto the Stigsnñs Block. The unconformity has here the character of an onlap surface. The map shows the position of the section and wells in relation to underlying salt structures (from Vejbñk & Britze, 1994).
The area around and north of the Kegnñs-1 well is illustrated by four seismic sections (Fig. 7). The section GY84 T-8 (Fig. 7(a)) shows that salt-movements caused relative uplift of a salt structure south-west of the Kegnñs-1 well. There are a large number of erosional truncations at the Top Trias. The Kegnñs salt structure was uplifted in Post-Triassic time as indicated by the constant thickness of the Tùnder and older formations. Furthermore, no indications of onlap internally in the Triassic succession onto the Kegnñs salt structure are observed. The section WGC79-7917 (Fig. 7(b)) strikes parallel with the south-western border of the Glamsbjerg Block (Fig. 1) and shows no signi®cant changes in thickness of the érslev, Falster and Tùnder Formations outside the faulted area. The complex fault pattern in the centre of the section is due to sec-
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Fig. 7. Seismic sections from the area around the island Als showing the geometry of the Triassic succession. The position the Kegnñs-1 well is also shown (a). The consistent thickness of the early and middle Triassic succession around the Kegnñs pillow and the erosional truncation near the base Cretaceous (BC), at a horizon which corresponds to the Top Triassic, indicates that the salt movements generating the Kegnñs pillow took place after deposition of the Tùnder, Falster and érslev Formations. No signi®cant change in thickness of these Triassic deposits is observed in the other section running along the island of Als (b and c) and across the Lille Bñlt (d). See text for further discussion. For location of the sections see the inserted map. See Fig. 4 for legend.
tion parallel faults, which strike NW±SW and detach along the Top Zechstein surface. The seismic section WGC79-7918 (Fig. 7(c)) strikes east±west and the section DA98-46 (Fig. 7(d)) continues across the Lille Bñlt to the shore of Funen. All sections emphasise that no signi®cant dierential basement-controlled subsidence took place during deposition of the Tùnder Formation in the area around the island of Als. In the
Ringe-1 well, which is located at the centre of the Glamsbjerg block (Fig. 1), the Tùnder Formation is absent and the Falster Formation is overlain by the Slagelse Member. Previous seismic interpretations carried out by Sorgenfrei (1966) in the area around the Ringe-1 well shows that the Triassic succession is conformable and shows no evidence of lateral thickness changes. Newer seismic sections con®rm this. The
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Fig. 8. Well section showing the reinterpreted correlation of the lithostratigraphy related to the seismic subdivision of Clausen and KorstgaÊrd (1993). The horizon TR1-4 are of Triassic age, and TPZ equals the Top pre-Zechstein. The seismic surfaces are correlated to the same lithostratigraphic unit in both wells, which con®rms the assumption that the lithostratigraphic subdivision used in this study is chronostratigraphic signi®cant. The TR4 is a marked unconformity and correlates to a surface internally in the Tùnder Formation, and is important since it marks the change in subsidence pattern in the Horn Graben area (Clausen & KorstgaÊrd, 1993). See text for further discussion.
absence of the Tùnder Formation in the Ringe-1 well thus suggests that at least the central parts of the Glamsbjerg Block were topographically high during deposition of the Tùnder Formation or that erosion of the Tùnder Formation (if deposited) took place prior to deposition of the Slagelse Member, as suggested for the area around the érslev-1 well.
5. Discussion 5.1. Variations in structural evolution along the southern margin of the Ringkùbing-Fyn High The `Late Triassic unconformity' observed in the study area is interpreted as related to basement involved relative uplift of the eastern part of the Ringkùbing-Fyn High, causing the initiation of movements of Zechstein salt in basinward areas. The relative uplift of the Ringkùbing-Fyn High is interpreted as due to heterogeneous Triassic stretching of the area (less stretching at the Ringkùbing-Fyn High compared to the basins) and later rapid thermal subsidence of
the basins both north and south of the RingkùbingFyn High (Sùrensen, 1986, Vejbñk, 1997). Late Triassic tectonics are also observed in the German Basin located south of the study area (Wolburg, 1969; Dittrich, 1989, 1991; Beutler, 1995). Clausen and KorstgaÊrd (1993, 1994) show in the Horn Graben area the presence of a major unconformity at the base of the Jurassic succession in accordance with Cartwright (1990). Furthermore, a major intra-Triassic surface was interpreted as marking the change in dominant deposition within the Horn Graben to the area south of the Ringkùbing-Fyn High (Clausen & KorstgaÊrd, 1993). The lithostratigraphic subdivision of the wells R-1 and S-1 in the Horn Graben (Fig. 1), according to the principles de®ned by Pedersen (1998), shows that the intra-Triassic surface is located within the Tùnder Formation, and thus most probably corresponds to the stratigraphic level of the above described Late Triassic unconformity (Fig. 8). The analysis of E±W striking detaching faults within Horn Graben shows that a change in dip of the Top pre-Zechstein surface took place during the late Triassic and is thus comparable with the southward tilt of the North German
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Basin observed in south-eastern Denmark. Assuming that the constant thickness of the Late Triassic succession in the areas Als-Funen and Southern Jutland indicates no dierential subsidence, the areas in southern Jutland and the isles Als and Funen dier from the areas in south eastern Denmark and the Horn Graben. Therefore, the stratigraphic record of the RingkùbingFyn High suggests that the individual blocks, constituting the Ringkùbing-Fyn High, underwent dierent structural evolution, as already suggested by Sorgenfrei and Buch (1964). The subsidence pattern along the Ringkùbing-Fyn High is thus suggested to be a consequence of intra-plate deformation due to the overall E±W extension of the European continent during the Triassic. 5.2. Eect of the salt structures The reconstruction of the section in the south-eastern Danish area shows that mobile salt had an important role during deposition of the Late Triassic succession by modifying the accommodation space. Best, Kockel and SchoÈneich (1983) show that the salt structures located in the southern part of the Horn Graben were similarly active in the Late Triassic to those in the south-eastern Danish area. However, this is in contrast to the observations in southern Jutland and the Als-Funen area where the salt structures are interpreted to be generally younger. The correspondence between change in dip of basement and salt movements as observed in this study corresponds to the observations made along the southern rim of the Ringkùbing-Fyn High in the Danish North Sea (Clausen & KorstgaÊrd, 1996) and along the southern margin of the Mid North Sea High in the UK sector (Coward & Stewart, 1995). A similar trend is observed along the northern margin of the Danish RingkùbingFyn High (Britze, Japsen & Langtofte, 1992; Vejbñk, 1997).
6. Conclusions The integration of well-logs and seismic sections enables the geological analysis of a Late Triassic unconformity and the structural evolution. The analysis has shown that: 1. The dierential subsidence between the RingkùbingFyn High and the North German Basin during the Late Triassic varies along the strike of the Ringkùbing-Fyn High. A signi®cant change in dip of the basement is observed in the south eastern part of the Danish area and in the Horn Graben area. This contrast with southern Jutland where no indications of dierential subsidence or salt move-
ments during the Late Triassic are observed. 2. Late Triassic erosion is shown by an unconformity characterised by local low-angle erosion at the crest of the Ringkùbing-Fyn High and onlap onto the unconformity at the Ringkùbing-Fyn High. 3. Salt-structures in the North German Basin are active during the Late Triassic, but restricted to areas where a signi®cant change in basement dip is observed. It is therefore inferred that the change in basement dip controlled the movements of Zechstein salt. The halokinetic events in¯uenced the deposition of Late Triassic sediments by generating local depressions and highs in the area south of the Ringkùbing-Fyn High. 4. The subsidence pattern of the Top pre-Zechstein surface varies along the southern margin of the Ringkùbing-Fyn High. This indicates that the subdivision of the Ringkùbing-Fyn High into structural blocks played a major role into the evolution of the Triassic succession in the Danish area.
Acknowledgements The referees are thanked for valuable comments, which improved the manuscript. Egon Nùrmark is thanked for the seismic processing of the seismic line DANA98-46, which is part of the Danish Natural Science Research Council 9701761. Per K. Pedersen was ®nanced by the Faculty of Science, University of Aarhus.
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