Pre-rift (Altena Group)

The Early Jurassic has been commonly described as a period of relative tectonic quiescence, between rifting events in the Triassic and the Middle to Late Jurassic (Cartwright, Reference Cartwright1991; Coward et al., Reference Coward, Dewey, Hempton and Holroyd2003; Husmo et al., Reference Husmo, Hamar, Høiland, Johannessen, Rømuld, Spencer and Bathurst2003). Wong (Reference Wong, Wong, Batjes and De Jager2007) pictures the Early Jurassic sedimentation as a very uniform blanket and Andsbjerg & Dybkjaer (Reference Andsbjerg and Dybkjær2003) regard the relatively homogenous thickness of the Lower Jurassic successions recorded in Danish wells as an indication for a uniform subsidence history for most of the Early Jurassic within the Danish Central Graben. In a tectonically quiet depositional environment without significant differential subsidence, we would assume an approximately constant original thicknesses (without erosion). During the Middle to Late Jurassic transition, significant erosion took place in the area of the Central Graben, caused by the Mid North Sea Doming event (Underhill & Partington, Reference Underhill and Partington1993), as it is also apparent in the pillar map of the Lower Jurassic pre-rift sediment distribution (Fig. 10a). However, there are considerable thickness differences in areas that were unaffected by the Mid Cimmerian erosion, e.g. the parts of the northern Dutch Central Graben and areas along the MBF (Figs. 8 and 10). Our observation of thickness variations is in accordance with Van Winden et al. (Reference van Winden, de Jager, Jaarsma and Bouroullec2018), who noticed a thickening of the Lower Jurassic Altena Group towards the centre of the Dutch Central Graben. These variations in thickness may result from differential tectonic activity in the Early Jurassic. The Central Graben already experienced a first phase of major tectonic activity in the Late Triassic (Jähne-Klingberg et al., Reference Jähne-Klingberg, Bense and Warsitzka2018). Also, we presume that the Clemens Basin and Clemens Graben constituted a topographic high in the Early Jurassic, indicated by the absence of Lower Jurassic sediments and erosional truncation of the Upper Triassic sediments in the area (Fig. 5 to Fig. 7). This high was then inverted to a topographic low in the Late Jurassic because of fault activity and salt withdrawal (Figs. 13 and 14). The division of the study area by the MCGTZ may also have a significant influence on the stratigraphy and petroleum geology (Fig. 13). Confirmed and prolific deposits of the Posidonia Shale Formation occur in the northern Dutch Central Graben and potentially in the southern German Central Graben (Ineson et al., Reference Ineson, Bojesen-Koefoed, Dybkjær and Nielsen2003; Müller et al., Reference Müller, Arfai, Jaehne-Klingberg, Bense and Weniger2020). The presence of the most important source rock for hydrocarbons in the southern Central Graben, the Posidonia Shale Formation, is limited to areas south of the MCGTZ, in areas with the highest subsidence rates of the Altena Group. However, the subsidence rates are rather low, compared to the subsequent investigated intervals of TMS-1 and TMS-2. (Fig. 8). North of the MCGTZ, there is no known Posidonia Shale Formation. It was assumed to have been eroded during the Mid-Cimmerian uplift (Damtoft et al., Reference Damtoft, Andersen, Thomsen, Brooks and Glennie1987). However, the pillar maps show clearly that large areas around the John Graben and in the Danish part of the study area were unaffected by later erosion (Fig. 10). Also, there are no indications for erosion, or the presence of the Posidonia Shale Formation on the reflection seismic data (see John Graben in Figs. 5 and 6). This may be the result of a hiatus that is not resolvable. Other explanations are that the anoxic conditions were restricted to the areas south of the MCGTZ or that in the areas north of the MCGTZ an increased terrestrial and siliciclastic input attenuated the marine organic matter in the formation.

Fig. 13. Illustrates the depocenters and most prominent faults within or in the vicinity of the study area for the TMS. During deposition of TMS-1 N-S trending faults were active and sediment depocenters were located at the axis of the graben structures and along the main eastern boundary fault of the Central Graben. During deposition of TMS-2, NNW-SSE trending faults were active and depocenters were oriented along these faults but also shifted towards the rising salt structures. The main eastern boundary fault of the Central Graben became less significant for the subsidence in the study area. In TMS-3 and TMS-4 rifting decreased and salt tectonics dominated subsidence. A roughly W-E trending strike-slip fault with normal fault tendencies (MCGTZ) was probably active during deposition of TMS-1 to TMS-4. The figure is compiled and modified from Cartwright (Reference Cartwright1987), Møller and Rasmussen (Reference Møller and Rasmussen2003), Arfai et al. (Reference Arfai, Jähne, Lutz, Franke, Gaedicke and Kley2014).

Fig. 14. (a) Interpolation of the initial Zechstein salt thickness in and around the study area. Modeled from data from Smith et al., (1993) and ten Veen et al. (Reference ten Veen, van Gessel and den Dulk2012). The presumed initial thickness of the salt reaches a maximum thickness of around 1000m in the area of the German and the northern Dutch Central Graben and decreases towards the Danish graben part. (b) Today’s thickness of the Zechstein salt in and around the study area (based on Thöle et al., Reference Thöle, Jähne-Klingberg, Doornenbal, den Dulk, Britze and Jakobsen2021). Salt thicknesses of up to 6000 m are reached in the diapirs of the region.

TMS-1

At the transition from the Aalenian to the Bajocian, sedimentation recurred to the Central Graben, propagating from the north to the south due to the renewed rifting following the thermal doming in the early Middle Jurassic (Andsbjerg & Dybkjaer, Reference Andsbjerg and Dybkjær2003). At first, a terrestrially dominated environment developed with fluvial deposits of the Bryne Formation in the Danish Central Graben (Michelsen et al., Reference Michelsen, Nielsen, Johannessen, Andsbjerg and Surlyk2003; Mellere et al., Reference Mellere, Mannie, Longhitano, Mazur, Kulausa, Brough and Cotton2017). Its base is the equivalent of the base of TMS-1 offshore Denmark. This period of the initiation of syn-rift subsidence corresponds to phase 3 of basin evolution of Andsbjerg and Dybkjaer (Reference Andsbjerg and Dybkjær2003). Rifting of the Dutch Central Graben started again a few million years later in the Late Callovian and is expressed by the deposition of fluvial and deltaic sediments of the Lower Graben Formation (Figs. 2, 11, and 12). Younger sedimentary equivalents were deposited further south of the Dutch Central Graben, outside our study area, in form of the Friese Front Formation. Estuarine and lacustrine deposits of the Middle Graben Formation follow the fluvial deposits. Subsequently, marine conditions re-established in the southern Central Graben, starting with shoreface and lagoonal deposits of the Danish Lulu Formation in the Callovian and of marginal marine sediments of the Dutch Upper Graben Formation at the earliest Late Oxfordian (Michelsen et al., Reference Michelsen, Nielsen, Johannessen, Andsbjerg and Surlyk2003; Bouroullec et al., Reference Bouroullec, Verreussel, Geel, De Bruin, Zijp, Kőrösi, Munsterman, Janssen and Kerstholt-Boegehold2018; Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018). The results of our mapping (TMS1-1 thickness map) indicate that the transgression propagated from the Danish to the Dutch Central Graben along the axis Rosa Basin – John Basin – Johannes Basin – northern Dutch Central Graben (Fig. 8). At the top of TMS-1, fully marine conditions existed in large areas of the Danish Central Graben at the Callovian-Oxfordian transition (Lola Formation; Andsbjerg & Dybkjaer, Reference Andsbjerg and Dybkjær2003). This transgression is attributed to another rift-pulse (phase 4 of Andsbjerg and Dybkjaer, Reference Andsbjerg and Dybkjær2003; Table 2). Open marine conditions reached the Dutch Central Graben at the Late Oxfordian – Early Kimmeridgian, which is expressed by the deposition of the Kimmeridge Clay Formation (Fig. 11).

In the study area, TMS-1 and its subdivisions reflect this sedimentary development. Terrestrial and marginal marine sediments of the Bryne and Middle Graben Formations in the Danish Salt Dome Province and of the Lower, Middle, and Upper Graben Formations in the German and Dutch Central Graben are overlain by marine mudstones of the Lola and Farsund Formation in the Danish and by marine mudstones of the Kimmeridge Clay Formation in the German and Dutch Central Graben (Fig. 11).

The depocenters in this period were in the vicinity of the MBF, as well as in the centre of the sub-basins, e.g. the John Graben, the Clemens Basin and the Clemens Graben (Figs. 8 and 13). These depocenters, as well as the development of the Clemens Graben and Basin, indicate the dominance of rifting on subsidence patterns. However, the influence of salt tectonics increased steadily from TMS-1.1 to TMS-1.3, which can be recognised by increasing subsidence around salt domes, e.g. around the Johannes Basin and within the southern Salt Dome Province (Fig. 8). The decreasing sediment thickness from TMS-1.2 to TMS-1.3 along the MBF of the eastern Central Graben margin as well as the decreasing thickness trend of TMS-1.3 along the MBF indicate that the shift from rift-dominated subsidence towards salt-dominated subsidence, in combination with local uplift and erosion, took place within this time (Fig. 8). The subdivision of the German Central Graben by the MCGTZ into a southern part, rather belonging to the Dutch Central Graben, and a northern part, rather belonging to the Danish Central Graben is expressed by differences in subsidence rates and sediment thicknesses. In the Dutch Central Graben, the deposition of TMS-1 is rather limited to the graben axis (Bouroullec et al., Reference Bouroullec, Verreussel, Geel, De Bruin, Zijp, Kőrösi, Munsterman, Janssen and Kerstholt-Boegehold2018; Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018; Fig. 13). This depositional pattern is likely related to salt withdrawal beneath the basin axis, where elongated salt structures developed (Van Winden et al., Reference van Winden, de Jager, Jaarsma and Bouroullec2018; Fig. 14). While the northern Dutch Central Graben conforms to this pattern, the depocenters along the MBF correspond to the generally asymmetric half-graben style of the Central Graben (Fig. 13). Single sub-basins as the Clemens Graben south and the Clemens Basin north of the MCGTZ are developed rather symmetrically (Figs. 8 and 13). The depocenters in the Danish Central Graben are strongly associated with faults. Either along the Coffee Soil Fault, emphasising the asymmetric half-graben nature of the Danish Central Graben, or along N-S striking faults like the bounding fault of the Rosa basin, which extension into the German Central Graben is represented by the John Basin (separated by a transversing structure) (Fig. 13). These N-S striking faults were reactivated during the first tectonic pulse during the Callovian to Early Oxfordian (Møller and Rasmussen, Reference Møller and Rasmussen2003). N-S trending faults are probably also related to depocenters of TMS-1 in the German as well as in the Dutch part of the study area (Fig. 13). It is also likely that the N-S trending Clemens Graben and Clemens Basin developed due to this tectonic pulse. The Clemens Graben and Clemens Basin area were probably a topographic high during the Late Triassic and Early Jurassic, which changed to a topographic low due to salt mobilisation and rifting (see thickness map of TMS-1.2 in Fig. 8). Faults in the Mesozoic to Cenozoic are mostly soft-linked to thin-skinned coupled to the basement faults, the faults mostly do not cross the Zechstein salt but clearly stimulated the salt flow (Fig. 4 to Fig. 7; ten Veen et al., Reference ten Veen, van Gessel and den Dulk2012).The subdivision of the German Central Graben by the MCGTZ may have had consequences for the development of the local (litho-)stratigraphy. Higher subsidence rates during TMS-1.1 in the northern Dutch Central Graben resulted in the deposition of thick sediments of the Central Graben Subgroup. Intercalated coal seams may have locally produced gaseous hydrocarbons (Herngreen et al., Reference Herngreen, Kouwe and Wong2003; de Jager and Geluk, Reference de Jager, Geluk, Wong, Batjes and de Jager2007). But the Central Graben Subgroup is economically more important as reservoir rocks for oil, condensate and gas that are predominantly sourced from the Lower Jurassic Posidonia Shale Formation in the northern Dutch Central Graben (de Jager and Geluk, Reference de Jager, Geluk, Wong, Batjes and de Jager2007). In the northern Danish Central Graben, equivalent coal-bearing deposits from the Bryne Formation have sourced gas and condensate fields (Petersen and Hertle, Reference Petersen and Hertle2018; Petersen et al., Reference Petersen, Andsbjerg, Bojesen-Koefoed and Nytoft2000). Coaly liquids are even found within the salt dome province in the Alma field and the Tove-1 well within or near the Danish part of our study area (see Figs. 1b and 3 for location). In the northern part of the study area, north of the MCGTZ, gaseous or liquid hydrocarbons from coals within TMS-1.1 may have been generated in minor quantities, as this interval lies at the present within a hydrocarbon generating temperature and depth window (Petersen and Hertle, Reference Petersen and Hertle2018; Müller et al., Reference Müller, Arfai, Jaehne-Klingberg, Bense and Weniger2020). However, hydrocarbons in significant accumulations, e.g. in chalk fields, probably migrated into the area from the northern part of the Danish Central Graben due to the significantly lower thickness of TMS-1.1 in the study area compared to the northern Dutch (Central Graben Subgroup) or Danish Central Graben (Bryne Formation). The marine mudstones of TMS-1.2 and TMS-1.3 consist of the lower parts of the Kimmeridge Formation, the Lola Formation, and the Farsund Formation, respectively. In the Danish Central Graben, the kerogen quality of these formations decreases towards their lower parts, which may be the result of more oxic depositional conditions and a larger input of terrestrial organic matter (Petersen et al., Reference Petersen, Hertle and Sulsbrück2017). The marine influence increased towards the top of the formation. The ratio of siliciclastics to clay minerals is possibly higher in areas with higher subsidence rates, e.g. the John Graben area.

TMS-2

As outlined before, the MCGTZ plays an important role for the local tectonostratigraphy and subdivides the study area in a northern part with relatively low subsidence rates and a southern part with significantly higher subsidence rates. The main depocenter during TMS-2 was in the northern Dutch Central Graben (Figs. 8 and 13). Comparable or even greater sediment thicknesses only occur in the Tail End Graben north of the Salt Dome Province in the Danish Central Graben (Møller & Rasmussen, Reference Møller and Rasmussen2003). Sediment thicknesses in the remaining study area are significantly lower, indicating that the bounding faults in the area of the Dutch Central Graben were longer active than those in the rest of the study area. A distinctive feature of TMS-2 is the initiation of subsidence in adjacent basins outside of the axis of the Central Graben. Our study area, though, does not record this development, because it mainly comprises the Central Graben. Adjacent basins and plateaus like the Terschelling Basin offshore The Netherlands, the Outer Rough Basin or the Heno and Gertrud Plateau offshore Denmark were affected by the initiation of active faulting along NW-SE trending faults due to the change in the direction of the extensional regime (Zanella et al., Reference Zanella, Coward, Evans, Graham, Armour and Bathurst2003; Coward et al., Reference Coward, Dewey, Hempton and Holroyd2003; Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018). Verreussel et al. (Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018) and Bouroullec et al. (Reference Bouroullec, Verreussel, Geel, De Bruin, Zijp, Kőrösi, Munsterman, Janssen and Kerstholt-Boegehold2018) therefore further subdivided TMS-2 into four tectonostratigraphic sequences related to the opening of different adjacent regions. Subsidence along the MBF mainly ceased (Figs. 8 and 13). Fault activity influenced sediment thickness in the study area mainly in the Clemens Graben and potentially in the northern Dutch Central Graben (Fig. 8). In the Central Graben as a whole, the tectonic regime changed, and older roughly NW-SE trending faults were reactivated or newly created, along which depocenters in the Danish Central Graben are oriented (Møller & Rasmussen, Reference Møller and Rasmussen2003; Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018; Fig. 13). A roughly NW-SE trending fault also influenced the development of the depocenters of the German John Graben, at least by ultimately separating it from the Danish Rosa Graben and the northern Dutch Central Graben (Fig. 13). However, the influence of salt tectonics on subsidence can be noticed clearly during deposition of TMS-2 in the study area, which is located within the area of maximum presumed initial Zechstein salt thicknesses, with up to 1000 m (Fig. 14a). In the northern Dutch Central Graben, however, TMS-2 reaches thicknesses of up to 1600 m. This implies that salt withdrawal, even if dominating in the German part, is not the single driving force for subsidence. At least in the Dutch part of the Central Graben, there must have been also a strong fault-driven or thermal component. The geometry of deposits offshore Denmark is still asymmetric, with the depocenter close to the Coffee Soil Fault, indicating continuous activity of the NW-SE trending section of the MBF in the northern Danish Central Graben (Møller & Rasmussen, Reference Møller and Rasmussen2003; Fig. 6). The area of the Danish Central Graben and the southwestern German Central Graben were affected by salt withdrawal and evolution of salt structure related depocenters (Fig. 4 to Fig. 7). From the northern Dutch Central Graben to the Danish Salt Dome Province, sedimentation of marine mudstones of the Kimmeridge Clay Formation or the Lola and Farsund Formation, respectively, continued unchanged from TMS-1 to TMS-2 (Fig. 11). These predominantly deep marine sediments covered most areas of the southern Central Graben. Basin floor sands, dolomites and terrestrial sediments of the Main Friese Front Member were deposited in the southern Dutch Central Graben and the Terschelling Basin (Bouroullec et al., Reference Bouroullec, Verreussel, Geel, De Bruin, Zijp, Kőrösi, Munsterman, Janssen and Kerstholt-Boegehold2018). Despite the apparent uniformity of the marine clays of TMS-2, we believe that tectonics influenced the local lithostratigraphy. The subsidence and thickness map of TMS-2 show that most of the study area, especially north of the MCGTZ, constituted a topographic high in contrast to the deeper subsided northern Dutch Central Graben (Fig. 8). This structural high extended further north into the Danish Central Graben (Thöle et al., Reference Thöle, Jähne-Klingberg, Doornenbal, den Dulk, Britze and Jakobsen2021). In general, the upper parts of the Farsund Formation contain more organic matter with predominantly oil-prone Type II kerogen of marine origin (Ponsaing et al., Reference Ponsaing, Bojesen-Koefoed, Thomsen and Stemmerik2018). However, the kerogen quality decreases significantly in the Danish Central Graben towards the south in the Salt Dome Province and changes from predominantly type II to type III kerogen (Damtoft et al., Reference Damtoft, Andersen, Thomsen, Brooks and Glennie1987). The decrease in kerogen quality possibly relates to a location with higher terrestrial input which possibly resulted in increased rations of siliciclastics and inert organic matter. Also, lower subsidence rates may have led to a rather shallow marine depositional setting with potentially higher carbonate contents of the shales in contrast to a presumably rather deep marine setting within the northern Dutch Central Graben. A higher portion of marine organic material is known from the deeper, northern parts of the Danish Central Graben and we presume a similar composition of the upper Kimmeridge Formation also for the deeper subsided parts of the northern Dutch Central Graben (Damtoft et al., Reference Damtoft, Andersen, Thomsen, Brooks and Glennie1987).

TMS-3 and Lower Cretaceous

During TMS-3 sediments of the Scruff Greensand Formation and the Hot Shales were deposited in the study area. They are embedded within the mudstones of the Lutine Formation in The Netherlands (Clay Deep Member) and within the mudstones of the Mandal Formation in Denmark (Bo Member) (Figs. 2 and 12).

The occurrence of the shallow marine sandstones of the Scruff Greensand Formation is mostly restricted to the area of the northern Dutch Central Graben (Fig. 12). Only thin deposits extend into the southernmost German Central Graben (see well B18-5 in Fig. 11). The Hot Shales were mostly deposited in a hemipelagic environment with anoxic bottom water conditions (Ineson et al., Reference Ineson, Bojesen-Koefoed, Dybkjær and Nielsen2003). They are present in the Central Graben over the entire area from the Danish Central Graben into the northern Dutch Central Graben (Müller et al., Reference Müller, Arfai, Jaehne-Klingberg, Bense and Weniger2020). The Hot Shales form part of TS-3.2 of Bouroullec et al. (Reference Bouroullec, Verreussel, Geel, De Bruin, Zijp, Kőrösi, Munsterman, Janssen and Kerstholt-Boegehold2018) and Verreussel et al. (Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018) and correspond to phase seven of basin development defined by Andsbjerg and Dybkjaer (Reference Andsbjerg and Dybkjær2003). They replace or overly the sandstones of the Scruff Greensand Formation and were deposited during the Late Cimmerian II pulse (Wong, Reference Wong, Wong, Batjes and De Jager2007). The typical high gamma-ray (GR) reading, which closely correlates with the content of organic material, thickness and maturity of the Hot Shales varies greatly within its area of distribution (Fig. 11). The distribution of TMS-3.2 and the Hot Shale demonstrate again the impact of tectonics (especially the MCGTZ) on stratigraphy. The Hot Shale reaches the highest thickness in the northern Dutch Central Graben and in the northern Danish Tail End Graben with up to 100 (Ineson et al., Reference Ineson, Bojesen-Koefoed, Dybkjær and Nielsen2003). While the GR-readings and probably the organic content is well developed in the northern Dutch Central Graben, they decrease further north across the German Central Graben. The GR-readings increase again in the Danish Salt Dome Province (Fig. 11). While they form a prolific source rock in the Danish Tail End Graben, the Hot Shales are mostly thermally immature to early mature in the German part of the Central Graben. (Ponsaing et al., Reference Ponsaing, Bojesen-Koefoed, Thomsen and Stemmerik2018; Müller et al., Reference Müller, Arfai, Jaehne-Klingberg, Bense and Weniger2020). The subsidence pattern of TMS-3 is rather complex and local tectonics may have caused significantly more local hiatuses and variations in siliciclastic content (Fig. 9).

Tectonically, the southern Central Graben reached an early post-rift stage (Andsbjerg & Dybkjaer, Reference Andsbjerg and Dybkjær2003) and TMS-3 corresponds to a phase of tectonic quiescence between the tectonic pulses 2 and 3 of Møller and Rasmussen (Reference Møller and Rasmussen2003). During this phase the organic rich Hot Shales were deposited. Subsidence rates strongly decreased in the second half of the Volgian, which is recorded by the reconstructed subsidence rates and subsidence pattern of the northern Dutch Central Graben from TMS-2 to TMS-3 (Figs. 8 and 9). But even if subsidence in total began to fade, subsidence affected not only the graben axis and adjacent basin but also the entire region (Andsbjerg & Dybkjaer, Reference Andsbjerg and Dybkjær2003; Verreussel et al., Reference Verreussel, Bouroullec, Munsterman, Dybkjær, Geel, Houben, Johannessen and Kerstholt-Boegehold2018). Activity of the MBF ceased, while the influence of salt tectonics on sedimentation climaxed, expressed by the formation of depocenters close to salt structures (Figs. 9 and 13).

During the Early Cretaceous, rifting as well as salt tectonics ceased while regional thermal subsidence became important, resulting in a regional transgression and the deposition of the Vlieland Claystone Formation (TMS-4) and the Holland Formation (Herngreen & Wong, Reference Herngreen, Wong, Wong, Batjes and De Jager2007). This deposition followed the third tectonic pulse according to Møller and Rasmussen (Reference Møller and Rasmussen2003). The transgressive sediments were originally deposited in large areas of the southern North Sea but were partly eroded during the Late Cretaceous inversion of the southern Central Graben (Herngreen & Wong, Reference Herngreen, Wong, Wong, Batjes and De Jager2007). Our thickness maps show that the influence of salt tectonics was still significant during the deposition of the Vlieland Claystone Formation (TMS-4), while a tectonic influence at the MBF was minor (Figs. 9 and 13). In comparison to the Vlieland Claystone Formation, the distribution of the Holland Formation is more uniform, reflecting the decrease in differential subsidence and salt tectonics. The low subsidence rates in the marine setting may have produced a relative homogenous distribution of the lithofacies. Increased thicknesses at the basin axis of the northern Dutch Central Graben may indicate continuous activity of sub-salt faults in this area.

Influence of supra-regional tectonics

Initial salt mobilisation in the Central Graben is probably connected to first rifting pulses during the break-up of Pangaea (Stollhofen et al., Reference Stollhofen, Bachmann, Barnasch, Bayer, Beutler, Franz, Kästner, Legler, Mutterlose, Radies, Littke, Bayer, Gajewski and Nelskamp2008; Maystrenko et al., Reference Maystrenko, Bayer and Scheck-Wenderoth2013). In the process, the study area was not uniformly deformed, resulting in a dichotomy in the structural pattern. Salt structures south of and within the MCGTZ, including the Dutch Central Graben, started to form in the Middle to Late Triassic (van Winden et al., Reference van Winden, de Jager, Jaarsma and Bouroullec2018; Warsitzka et al., Reference Warsitzka, Jähne-Klingberg, Kley and Kukowski2018). During this period, initial salt mobilisation concentrated on major fault zones and graben structures as the Dutch Central Graben, the Horn Graben and the Glückstadt Graben (Grassmann et al., Reference Grassmann, Cramer, Delisle, Messner and Winsemann2005; Warsitzka et al., Reference Warsitzka, Jähne-Klingberg, Kley and Kukowski2018). The rifting pulses led to differential gravitational unloading of the Zechstein salt due to the extension of its overburden, in combination with differential loading due to accelerated subsidence in the evolving grabens (Scheck-Wenderoth et al., Reference Scheck-Wenderoth, Maystrenko, Hübscher, Hansen, Mazur, Littke, Bayer, Gajewski and Nelskamp2008; Warsitzka et al., Reference Warsitzka, Jähne-Klingberg, Kley and Kukowski2018). Salt movement north of the MCGTZ, in the German Central Graben and the Danish Salt Dome Province, initiated in the Late Triassic (Warsitzka et al., Reference Warsitzka, Jähne-Klingberg, Kley and Kukowski2018). It was the Triassic main phase of salt mobilisation in the Southern Permian Basin and affected also structurally higher areas (Warsitzka et al., Reference Warsitzka, Jähne-Klingberg, Kley and Kukowski2018). The SPB is dominated by two structural trends, a roughly N-S and a roughly NW-SE orientation of basement faults and depocenters (Kley et al., Reference Kley, Franzke, Jähne, Krawczyk, Lohr, Reicherter, Scheck-Wenderoth, Sippel, Tanner, Van Gent, Littke, Bayer, Gajewski and Nelskamp2008). These two main structural patterns are also reflected in the pattern of the salt structures (Kley et al., Reference Kley, Franzke, Jähne, Krawczyk, Lohr, Reicherter, Scheck-Wenderoth, Sippel, Tanner, Van Gent, Littke, Bayer, Gajewski and Nelskamp2008; Scheck-Wenderoth et al., Reference Scheck-Wenderoth, Maystrenko, Hübscher, Hansen, Mazur, Littke, Bayer, Gajewski and Nelskamp2008). Many elongated salt structures follow the roughly N-S striking structural trend that is dominant for the Dutch Central Graben, e.g. B17-SOUTH1 (Fig. 1b). Directly after the collapse of the Mid North Sea Dome, roughly N-S trending faults were active during the deposition of TMS-1, potentially resulting from reactivation of older structures (Fig. 13; Pharaoh et al., Reference Pharaoh, Dusar, Geluk, Kockel, Krawczyk, Krzywiec, Scheck- Wenderoth, Thybo, Vejbæk, Van Wees, Doornenbal and Stevenson2010). The trend of these faults conforms to the general E-W extension of the area, as well as the Viking Graben and the Dutch Central Graben, but cross-cuts the roughly NW-SE trend of the Danish Central Graben. NW-SE elongated salt structures developed over pre-Zechstein basement faults mainly in the eastern SPB. These structures were reactivated during the Late Jurassic to the Early Cretaceous (Scheck-Wenderoth et al., Reference Scheck-Wenderoth, Maystrenko, Hübscher, Hansen, Mazur, Littke, Bayer, Gajewski and Nelskamp2008), which may be reflected in the change from roughly N-S to NW-SE oriented faults from TMS-1 to TMS-2 in the German and Danish part of the Central Graben (Fig. 13). This structural trend is common in the wider area, e.g. the Broad Fourteens Basin offshore The Netherlands, the Polish Trough and the Lower Saxony Basin onshore Germany (Kley et al., Reference Kley, Franzke, Jähne, Krawczyk, Lohr, Reicherter, Scheck-Wenderoth, Sippel, Tanner, Van Gent, Littke, Bayer, Gajewski and Nelskamp2008). The Central Graben is located at the collision triangle of three terranes, Laurentia in the northwest, Baltica in the northeast, Avalonia in the south (Maystrenko et al., Reference Maystrenko, Bayer, Brink, Littke, Littke, Bayer, Gajewski and Nelskamp2008; Smit et al., Reference Smit, van Wees and Cloetingh2016; Fig. 15). Despite the general E-W direction of extension the trend of the Danish Central Graben is roughly NW-SE oriented. We suggest that the graben follows roughly a deep-seated zone of crustal weakness, the Thor suture that separates the two terranes Avalonia and Baltica (Smit et al., Reference Smit, van Wees and Cloetingh2016; Fig. 15). A similar relation of a Mesozoic sub-basin to deep-seated crustal structure is presumed for the Polish Trough in the east of the SPB, whose roughly NW-SE striking structural trend follows the Teisseyre-Tornquist-Zone (TTZ), a major geologic structure and zone of deformation (Narkiewicz et al., Reference Narkiewicz, Maksym, Malinowski, Grad, Guterch, Petecki and Jankowski2015; Grad & Polkowski, Reference Grad and Polkowski2016). Especially the NW-SE trending part of the Central Graben in German and Danish offshore which cut the Mid North Sea High and Ringköbing-Fyn High show several ENE-WSW striking transverse zones (Cartwright, Reference Cartwright1987; Wride, Reference Wride1995). The MCGTZ is the furthest south of this sequence of transverse zones (Wride, Reference Wride1995) and shows clear evidence for influencing the Jurassic tectonostratigraphy in the study area. It is possible that the transverse zones follow precursors in the crust. However, they may also be a consequence of the influence of a E-W directed supra-regional extension on an already existing NW-SE striking graben segment. The main activity of salt tectonics in the German and Dutch Central Graben was in the Late Jurassic to Early Cretaceous (e.g. ten Veen et al., Reference ten Veen, van Gessel and den Dulk2012; Arfai et al., Reference Arfai, Jähne, Lutz, Franke, Gaedicke and Kley2014), in contrast to other grabens of the SPB, e.g. the Gückstadt Graben, where the main activity was in the Late Triassic (Grassmann et al., Reference Grassmann, Cramer, Delisle, Messner and Winsemann2005; Warsitzka et al., Reference Warsitzka, Kley, Jähne-Klingberg and Kukowski2016). This corresponds with our results, which imply that during the deposition of TMS-2 to TMS-3 the influence of salt tectonics dominated the depositional pattern (Figs. 8 and 9). Salt tectonics ceased during the Lower Cretaceous and some of the salt structures have in parts already formed welds. Due to Late Cretaceous to Paleocene compressional tectonics, salt mobilisation was renewed in the study area as well as in other parts of the SPB (e.g. Mazur et al., Reference Mazur, Scheck-Wenderoth and Krzywiec2005; Ahlrichs et al., Reference Ahlrichs, Noack, Hübscher, Seidel, Warwel and Kley2022). The accompanying structural deformation partly complicates the analysis of the Jurassic to Early Cretaceous tectonostratigraphy in the study area.

Fig. 15. Simplified structural overview map of the North Sea area during the Middle Jurassic. The area is affected by the ongoing break-up of the supercontinent Pangea which led to roughly NW-SE extension in the Arctic-North Atlantic mega-rift-system and to roughly E-W extension in the Central Graben. The Central Graben is located at the triple collision zone of the terranes Laurentia in the northwest, Baltica in the northeast, and Avalonia in the south. It cuts the transition of Laurentia to Avalonia and follows roughly the margin of Avalonia. The figure is compiled and modified from Lyngsie & Thybo., (2007), Pharaoh et al. (Reference Pharaoh, Dusar, Geluk, Kockel, Krawczyk, Krzywiec, Scheck- Wenderoth, Thybo, Vejbæk, Van Wees, Doornenbal and Stevenson2010), and Smit et al. (Reference Smit, van Wees and Cloetingh2016).