Multiphase salt tectonic evolution in NW Germany: seismic interpretation and retro-deformation

Transcription

1 Int J Earth Sci (Geol Rundsch) (2005) 94: DOI /s \ ORIGINAL PAPER M. Mohr Æ P. A. Kukla Æ J. L. Urai Æ G. Bresser Multiphase salt tectonic evolution in NW Germany: seismic interpretation and retro-deformation Received: 20 October 2004 / Accepted: 7 April 2005 / Published online: 11 November 2005 Ó Springer-Verlag 2005 Abstract The Central European Basin is a classic area of salt tectonics, characterized by heterogeneous structural evolution and complex salt movement history. We studied an area on its SW margin, based on prestack depth-migrated 2D and 3D seismic data. We use seismic interpretation and retro-deformation to obtain a better understanding of salt tectonics, structural control, and sedimentary response in this region. The first phase of salt tectonic evolution started with two main events of NW SE extension and rafting in the Triassic before the Upper Bunter and before the Upper Muschelkalk. Rafting was accompanied by first salt diapirism and an increased sedimentary thickness adjacent to the salt structure. After salt supply ceased updip to the salt structure, a mini-basin grew in the intra-raft area. This sedimentary differential loading caused salt movement and growth of a pillow structure basinward. The second phase of salt movement was initiated by the formation of a NNW SSE striking basement graben in the Middle Keuper that triggered reactive diapirism, the breakthrough of the pillow s roof and salt extrusion. The following downbuilding process was characterized by sedimentary wedges with basal unconformities, onlap structures and salt extrusions that ceased in the Jurassic. The third and latest phase of salt tectonic evolution was Unfortunately, the entire article was originally published Online First with errors. The publishers wish to apologize for this mistake. The correct article is shown here. The online version of the original article can be found at dx.doi.org/10.007/s c M. Mohr (&) Æ P. A. Kukla Æ J. L. Urai Geologisches Institut, RWTH Aachen, Wu llnerstr. 2, Aachen, Germany mohr@geol.rwth-aachen.de Tel.: Fax: G. Bresser Gaz de France Produktion Exploration Deutschland GmbH, Waldstr. 39, Lingen (Ems), Germany activated in the Late Cretaceous to Lower Tertiary by compressional tectonics indicated by salt rise and a small horizontal shortening of the diapir. The interpreted salt tectonic processes and the resulting geometries can now be better tied in with the regional heterogeneous framework of the basin. Keywords Salt tectonics Æ NW Germany Æ Retro-deformation Æ Seismic interpretation Æ Structural evolution Introduction Salt tectonics play a major role in many sedimentary basins. One classic area of salt tectonics is the NW German basin. Here, the mobile Permian Zechstein salt formed a large number of salt walls, diapirs and pillows (Trusheim 1960; Jaritz 1973; Lokhorst 1999; Baldschuhn et al. 2001) (Fig. 1) during long periods of salt tectonic activity (Kockel 1998; Jaritz 1973): major changes in sedimentation patterns and structural regimes are common (Kockel 2002, 2003). It is perhaps surprising then, that the most recent studies in salt tectonics which are summarised in a number of review papers (Jackson and Talbot 1994; Jackson 1995; Stewart and Clark 1999) were undertaken outside this area. In Germany, the debate on the reasons for salt movement started at the beginning of the last century with buoyancy (Arrhenius and Lachmann 1912) or tectonics (Stille 1910, 1925) as the driving forces. The buoyancy-driven halokinetic model of Trusheim (1957, 1960) who postulated an autonomous, isostatic rise of salt and piercement of the overburden due to Rayleigh- Taylor instabilities defined the way of thinking of salt tectonics in the North German Basin for the next forty years. Evolutionary stages of the Trusheim model are the pillow stage with the primary peripheral sink, the diapir stage with the secondary peripheral sink and the postdiapiric stage with the tertiary peripheral sink. Because

2 918 Fig. 1 Location of the central part of the Southern Permian Basin and the distribution of salt diapirs and pillows (after Lokhorst 1999). The basin margin is marked by the facies changes of the Zechstein 2 carbonates from slope (grey) to basin (light grey). Our study area in NW Germany is located inside the rectangle a viscous overburden is a necessary requirement in Trusheim s model, and the sedimentary cover s deformation is dominantly frictional, this model is now widely regarded as not relevant in salt tectonics (Vendeville and Jackson 1992a; Weijermars et al. 1993). Supplemented by the theory of prograding gravity instabilities forming successive generations of diapirs and salt dome families (Sannemann 1968) the concept of gravity inversion was later modified by assuming a possible tectonic reason for the initiation of salt movement (Meinhold and Reinhardt 1967; Ru hberg 1976; Jaritz 1973, 1987, 1992; Zirngast 1996). Brink et al. (1992) proposed a strong connection between diapirism and strike-slip faulting, including rim-syncline rotation. Ge et al. (1997) interpreted diapiric families and peripheral sinks in North Germany as the result of prograding sedimentary wedges and lateral migration of salt. Descriptions of raft tectonics during Keuper times (Best 1996; Thieme and Rockenbauch 2001), and recent studies by Kossow et al. (2000) and Scheck et al. (2003) in the eastern part of the North German basin, propose more influence due to decoupling by salt, and structurally-triggered salt movement. Also Baldschuhn et al. (2001) argue for important structural control during several tectonic phases (Kockel 1998), but many details of the process are still unclear. Therefore the aim of our study is the better understanding of the interaction between sedimentation and salt tectonics in the NW German Basin from the Permian to Recent, considering changing tectonic frameworks through time. Our approach is a combination of seismic interpretation including structural and sedimentary analysis and balanced retro-deformation. This iterative process of seismic interpretation and structural reconstruction minimizes uncertainties of interpretation. We focus on a study area located at the SW margin of the Southern Permian Basin and the western flank of the Triassic Ems Low.

3 919 The data used for the present study consist of prestack depth-migrated 2D and 3D seismic data from a km area, including deep boreholes and a high number of non-depth-migrated seismic sections. Geological framework In NW Germany, the evolution of the Southern Permian Basin (Ziegler 1990) (Fig. 1) began with post-variscan volcanism and latest Carboniferous strike-slip faulting (van Wees et al. 2000). Late Rotliegend to Early Triassic thermal subsidence (Kossow et al. 2000) was accompanied by rifting (Gast 1988; Geluk 1999). On the WNW- ESE striking basin margin in NW Germany, sedimentation began in the Upper Rotliegend with a suite of arid-semiarid sediments arranged around the central Playa lake (Fig. 2). Subsequent deposition of Zechstein evaporites produced an approximate thickness of at least 800 m (Jaritz 1973), but estimates are uncertain due to lateral migration or dissolution loss. The ensuing sedimentation in the Buntsandstein (Bunter) took place under fluvial, aeolian and lacustrine conditions. After a short period of tectonic quiescence during Lower Bunter times, rifting of the basin started again at the beginning of the Middle Bunter. This resulted for example in a syn-rift increase of sedimentary thickness in the Ems Trough (Ro hling 1991). The erosional event in the uppermost Middle Bunter completed this rifting sequence. First movement of salt is described in this interval based on rim-syncline analysis (Jaritz 1973). Extremely thin-skinned extension on the margin of the subsiding troughs caused rafting of the Middle Bunter blocks (Best 1996; Thieme and Rockenbauch 2001; Kockel 2002). During Upper Bunter and Muschelkalk times, rifting continued and accumulated up to 1,000 m sediments in local depocentres, such as the Westdorf Graben on the northwestern margin of the Fig. 2 Time chart of the stratigraphic units (Menning and German Stratigraphic Commission 2002), an outline of the different lithologies of the study area and the interpreted seismic horizons with the used stratigraphic abbreviations. Additionally, the regional tectonic events are compiled from different authors. See the different time scales on both sides of the figure

4 920 Ems Trough (Gaertner and Ro hling 1993). Depositional conditions in Upper Bunter and Muschelkalk times were shallow marine to evaporitic. Intra-formational salt layers are observed in both sequences with locally highly variable thickness distribution (Geluk et al. 2000). Eustatic changes let to a regression which brought about the clastic-evaporitic sedimentation of the Keuper (Ziegler 1990). Thick halite sequences formed in local basin depocentres like the Ems or Glu ckstadt Graben during Middle Keuper times (Beutler 1998). An Early Cimmerian extensional event (starting at 232 Ma) is seen in context of the North Sea rifting (Ziegler 1990); strong dilatation was coeval with the beginning of main salt diapirism, subsequent rim-syncline development and a possible re-sedimentation of Zechstein salt (Frisch and Kockel 1999; Jaritz 1973). Since the Lower Jurassic, the Lower Saxony Basin in the south underwent rifting along a WNW-ESE axes with a peak in the Upper Jurassic (Baldschuhn et al. 2001), whereas the Southern North Sea area was uplifted and eroded during the Late Cimmerian tectonic phase. In this region sedimentation ended at the latest in the Upper Jurassic (Ziegler 1990). Deposition in the Cretaceous reflected several phases of transgressive sequences and tectonic pulses culminating in Late Cretaceous to early Cenozoic inversion phases (Baldschuhn et al. 1991; de Jager 2003). Sedimentation in the Cenozoic of NW Germany was mainly controlled by subsidence and eustatic sea-level changes. Methods The data set (Fig. 3) includes a high-quality km prestack depth-migrated 3D seismic cube. In addition a network of 28 depth-migrated seismic 2D sections with a total length of approximately 430 km was used. Further, we used a high number of non depth-migrated sections for qualitative interpretation. Eleven hydrocarbon exploration wells, all reaching the Rotliegend, serve for stratigraphic calibration of the seismic data. In general, the seismic data images the sub-salt basement very well. Nevertheless, typical difficulties of seismic interpretation such as uncertainties in sub-salt imaging at the flanks of a diapir, are also apparent in the investigated area. In this study, 16 seismic reflectors and a consistent fault network were interpreted in the depthmigrated 2D sections and the 3D seismic cube (Fig. 3). The interpretation was gridded to 3D horizons interpolating for areas without data coverage. We analysed fault pattern and regional dip for several 3D horizons using depth- and non-depth-migrated seismic information. The sub-salt Rotliegend Formation Fig. 3 Simplified map of the study area demonstrating the location of the salt structures (grey), the 3D seismic cube (light grey) and the presented seismic lines and interpretations

5 921 and its structural configuration is, as exploration target, the best-analysed geological horizon in the area. The pattern of sub-salt Rotliegend faults were compared with the structures in the overburden and the regional geological framework. We also used data from the regional geo-history for dating of structural events, sedimentary thickness distribution and preference of salt tectonic models. The most likely model of salt movement is presented and tested against alternative models. Several authors (e.g. Rowan 1993; Hossack 1995; Buchanan et al. 1996; Scha fer et al. 1998) have demonstrated that palinspastic restoration in salt regimes is an useful tool to reconstruct the structural and sedimentary evolution through time. Superposed effects of younger strata and structures are removed sequentially to understand the individual structural components and salt tectonic mechanisms. In addition, different possible subsurface geometries from the seismic interpretation can be tested and uncertainties constrained. In salt structure restoration, the overburden evolution is seen as a direct result of salt flow. Salt itself can only be passively restored. In addition, salt area may vary due to dissolution or out-of-section migration and the algorithms developed for retro-deformation of brittle rocks cannot reproduce the kinematics of the salt (Rowan 1993). In a full restoration the effects of sedimentation, compaction, isostasy, thermal subsidence, and the dynamics of faulting and salt movement should be included. Our structural restoration contains sedimentation, the kinematics of faulting and salt movement as the main factors. Variations in isostasy and thermal subsidence are negligible for the balanced section with a length of 30 km. Compaction influences the vertical thickness of sedimentary sequences through time, but is only relevant when lateral variations in thickness or facies are present, and affects mainly the uppermost sequences. Sensitivity analyses have shown that disregarding decompaction did not influence the results for the mechanisms and processes of structural and salt tectonic evolution interpreted from section restoration. Because our primary focus lies on the mechanisms and processes, rather than the rates and masses we did not decompact the sections at this stage of the analysis. An essential assumption in 2D structural balancing is plane strain, and that the section should be orientated parallel to the tectonic transport (Woodward et al. 1989). Our E-W orientated section is sub-perpendicular to the NE-SW structures of Lower Triassic, the NNW- SSE extended structures of Upper Triassic and the N-S direction of the Cenozoic structures. The decoupling by salt necessitates the treatment of the salt layer, the subsalt and supra-salt sequence as three autonomous tectonic systems during the restoration process (Schäfer et al. 1998). In addition the positioning of the regional elevation or target horizon to which a template line is restored is of importance. Regional elevation represents the pre-deformational relief, which is deduced from a point without deformation or a line that defines area balance above and below as a consequence of salt movement and sedimentary response (Hossack 1995). The usual restoration algorithms [as used in the software package 2DMove (Midland Valley 2002)] in salt regimes are oblique shear and flexural slip. Oblique shear preserves area, but line-length is not constant, which causes a significant length loss when restoring steeply-dipping layers. Most workers prefer oblique shear for downbuilding and extension while flexural slip is preferred for structural shortening and active salt diapirism. Fault displacement may be removed by Fault- Parallel Flow, Inclined Shear or using the Move and Rotate tools. Because seismic interpretation has often non-unique results, an iterative process of structural balancing followed by an update of the interpretation should be used. The final model is consistent with all known constraints and is therefore more likely than others, but not necessarily unique (Woodward et al. 1989). Further constraints e.g. by mechanical modelling, can improve this model. 2D and 3D seismic interpretation Interpretation of seismic sections The southernmost W-E running 2D seismic transect (Fig. 4) shows Top Rotliegend rising 780 m from east to west towards the Groningen High. This change in basement topography is concentrated at two major normal faults in the western part of the section. Faulting of upper Rotliegend age occurs along easterly dipping faults, indicating structural activity in this area in the Lower Permian. The section line crosses a sub-salt graben directly beneath the western salt structure. The Lower and Middle Bunter sequences in this section show a sedimentary thickness of 650 m. Normal faults running through these sequences have an easterly dip in the west and a westerly dip in the central part. In the western part of the profile, three gaps of m are interpreted, where this sedimentary sequences are absent (Fig. 4). The seismic reflectors of the Lower and Middle Bunter at the western and eastern edges of this structures run uniform and parallel. Significant syn-depositional interaction to the adjacent salt area, such as converging reflectors or onlap-structures, was not observed. We interpret two disconnected Lower and Middle Bunter blocks in the centre of this structure with reasonable confidence in spite of the low seismic resolution in this area. The Upper Bunter thickness varies between 500 m in the west and 100 m in the centre of the profile. Three disconnected blocks are interpreted, located directly above Zechstein salt. A former connection of these blocks is indicated by good correlation of the seismic reflectors across their broken edges. The disconnected Upper Bunter blocks are covered and surrounded by

6 922 Fig. 4 Depth-migrated W-E seismic section (2D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. The black arrows mark the unconformities mentioned in the text for each section respectively. For used stratigraphic abbreviations, see Fig. 2. The section shows the westward rise of basement, the disconnected Lower/Middle Bunter, three salt structures (Fig. 3) and the large depocentre of the Lower Keuper and lowermost Middle Keuper. The unconformity at the base of the Lower Cretaceous cuts the eastwarddipping layers of the Triassic. This section is used for sequential restoration, see Figs. 12 and 13 displaced Zechstein salt. The Lower and Middle Muschelkalk show a large thickness increase from east to west, from 250 m to 650 m, and are missing in the western part of the section. The lower boundary of the Upper Muschelkalk Formation is formed by an unconformity in the central part of the profile. The

7 923 formation entirely covers the western salt structure. Three normal faults cutting this sequence are spatially associated with sedimentation of a local depocentre in the Lower Keuper and lowermost Middle Keuper (Grabfeld-Formation, KM1) with a maximum thickness of 1,000 m. Their seismic reflectors converge eastward to diapir A. The sequences are absent adjacent to the diapir, where the Stuttgart-Formation (Middle Keuper 2, KM2) rests unconformable on Upper Muschelkalk. In the central part of the section, the interpreted Middle Keuper to Lower Jurassic rim-syncline belongs to the salt diapir A described in detail later in this chapter. The base Cretaceous unconformity truncates the Triassic sequences down to Muschelkalk in the west and down to the Jurassic in the east (Fig. 4). Cretaceous to Quaternary sequences dip slightly to the west with an increasing thickness and westward dipping normal faults. The salt structure in the western part of the profile shows large variations in thickness of Zechstein salt below Upper and Lower/Middle Muschelkalk sequences. The sub-vertical diapiric part of this salt structure truncates the sequences at least up to the Middle Keuper. Two W-E and one N-S transects from the 3D cube are presented in Figs. 5, 6, 7. The high quality of sub-salt seismic resolution in Fig. 5 allows us to identify a number of antithetic normal faults in the Rotliegend Formation with a graben in the centre of the section. A thin residue of Zechstein salt is present in the gaps between the tilted sub-salt basement blocks and the Bunter Formation. In this section, which is just south of the diapir A, the Lower Bunter up to the Middle Keuper is truncated by a pair of conjugate normal faults in the centre of the profile. Fault movement started in Upper Bunter times as indicated by a marked increase in sedimentary thickness across the fault. Furthermore, we observe normal faults in the Lower/Middle Bunter in the western part of the profile. Directly above, blocks of Upper Bunter and Lower/Middle Muschelkalk are arranged in domino-like fashion between the layered salt of Upper Bunter and Middle Muschelkalk. Further significant structures are the general decrease of thickness of the Lower and Middle Muschelkalk sequences to the centre of the section, an unconformity at the base of Upper Muschelkalk and irregular reflectors below the base of the Stuttgart-Formation (Middle Keuper 2, KM2), where the Lower Keuper and the Grabfeld- Formation (Middle Keuper 1, KM1) is absent. The maximum depocentre of the Lower Keuper to lowermost Middle Keuper sequences are located in the west of the section directly above an area without Lower and Middle Bunter. Sedimentation of the Stuttgart-Formation (Middle Keuper 2, KM2) started above an unconformity and the Grabfeld-Formation (Middle Keuper 3, KM3) formed characteristic peripheral sinks and turtle back structures. The sedimentary thickness maximum of these sequences moves from the eastern turtleback structure (Stuttgart and lower Grabfeld-Formation, KM2/3) to the central peripheral sink (Arnstadt-Formation, KM4). Another important unconformity is observed at the base of the Arnstadt-Formation in the eastern part of the profile. In this eastern peripheral sink of another salt structure just outside the working area the Jurassic reaches a thickness of about 350 m. Local thickness increase of the Lower Cretaceous above the major unconformity is consistent with a subcrop of inter-layered salt sequences of the Middle Keuper. The northerly section (Fig. 6) through the salt diapir A also shows the absence of Lower/Middle Bunter in the western part and faulting in the central part. In this section the pair of conjugate normal faults, described above, are positioned off the centre of the diapir. The predominantly parallel and flat lying reflectors of the Upper Bunter to lowermost Middle Keuper (KM1) in the east of the profile are truncated by the prominent unconformity at the base of the Stuttgart-Formation (KM2). Thickness variations and geometry of the overlying Middle Keuper sequences has been traditionally interpreted as a secondary rim-syncline. The typical turtleback structure in the eastern part of the section consists of sedimentary wedges with thick salt interlayers of Middle Keuper age. At the salt-sediment interface we interpreted local unconformities, onlapping and salt jags indicating lateral spreading of the salt. Diapir A shows a narrow geometry at the bottom and widens upwards. The peripheral sink represents an asymmetric evolution particularly during sedimentation of the upper Weser- Formation (upper part of KM3) and Arnstadt-Formation (KM4). Prominent seismic reflectors at the roof of the salt diapir are caused by the strong acoustic impedance contrast between the material of the caprock, the overlying sediments and the underlying salt. This reflector is elevated about 500 m in comparison to the regional datum given by the base Cretaceous unconformity. Relating to this, thickness of the Cretaceous and Cenozoic sedimentary strata above the diapir are reduced and faulted. A Cretaceous graben structure at the crest of the turtle back is present directly above subcropping salt of the Middle Keuper. The N-S section in Fig. 7 cuts the structural pattern at low angle and thus represents a longitudinal section. Southward dipping normal faults in the sub-salt basement show a maximum offset of 250 m. The Lower/ Middle Bunter is faulted, truncated and partially eroded in the central part of the profile. In the northern part of the seismic line, these sequences are absent and the Upper Bunter lies directly above Zechstein salt. Blocks of Lower/Middle Muschelkalk and Upper Bunter are domino faulted, tilted and displaced on a unit interpreted as Ro t salt (Upper Bunter). These faulted blocks in the south and the flat lying sequences of Lower/ Middle Muschelkalk in the north are truncated by an unconformity at the base Upper Muschelkalk. Upper Muschelkalk sequences are thickened in the central parts, where they rest upon Zechstein salt. The overlying Lower Keuper (KU) and lowermost Middle Keuper (KM1) Formation has its maximum depocentre in the northern part of the profile, where Lower/Middle Bunter

8 924 Fig. 5 Depth-migrated W-E seismic section (3D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. For used stratigraphic abbreviations see Fig. 2. The section shows antithetic basement faults, a graben in the centre of the overburden and an erosional surface above. Note the secondary rim-syncline of the northerly adjacent diapir A in the central part sequences are absent or incomplete. In addition, these sequences show syn-sedimentary normal faulting and southward converging seismic reflectors. The SW-NE section (Fig. 8) shows salt diapir B positioned above a structural basement high. The strong relief of the Top Rotliegend in the profile reaches about 800 m between the central high and the northeast. The Lower/Middle Bunter Formations have a thickness of about 450 m in the northeast, an increased thickness of about 650 m in the southwest and a triangular block in between. These sequences are absent below the salt diapir B. In the far northeast, the Upper Bunter thickness reaches 400 m. The same formation has a thickness of 150 m southwest adjacent and about 300 m at the southwestern flank of the salt diapir where the sequences are faulted and converge. The Lower/Middle Muschelkalk sequences have two depocentres: one in the northeast and one above the Lower/Middle Bunter gap in the southwest. In the latter depocentre, we interpreted about 300 m inter-layered salt of Middle Muschelkalk age. Above this area, the intra-muschelkalk unconformity separates a northeast-migrating zone of maximum thickness of the Upper Muschelkalk to lowermost Middle Keuper (KM1) sequences. In the northeastern part of the section, these sequences are thinning towards the diapir. Their depocentre at the northeastern end of the profile has a thickness of at least 1,000 m. Above the unconformity, at the base of the Stuttgart-Formation

9 925 Fig. 6 Depth-migrated W-E seismic section (3D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. For used stratigraphic abbreviations see Fig. 2. The section shows the jagged geometry of diapir A and the asymmetric sedimentary record (KM2), the Middle Keuper to Jurassic sequences show a characteristic secondary rim-syncline geometry related to diapir B. The asymmetric configuration of the Late Triassic rim syncline is similar to that of salt diapir A in the south. The thickest sedimentary wedges in the east of the diapir are represented by sequences of the Weser- Formation (KM3). They contain a high amount of interlayered salt. Typical for the salt-sediment interface forming the geometry of the diapir are salt jags, sedimentary onlapping and unconformities at the base of each sedimentary wedge. The diapir has a caprock, and its roof is about 400 m higher than the base Cretaceous as the regional datum. The overlying Cretaceous and Tertiary layers wrap around the top of the structure and show reduced thickness as well as normal faulting in the crestal region. No peripheral sink can be observed in these sequences. A NNW-SSE to NW-SE transect (Fig. 9) runs in its northern part parallel to the strike of the diapirs A and B as well as to the main structural pattern in the sub-salt basement. Therefore the Top Rotliegend shows only minor relief in the north in contrast to the southern part of the section, where the seismic line crosses a small basement graben. In the southern part, the seismic line is very similar to the section in Fig. 5, south of diapir A. The Lower and Middle Bunter is interpreted with a truncated side in the NNW, a steep sided block in the SSE and a triangular block in between. The Upper

10 926 Fig. 7 Depth-migrated N-S seismic section (3D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. For used stratigraphic abbreviations see Fig. 2. The section shows the absence of the Lower/Middle Bunter in the north. Faulted and tilted blocks of Upper Bunter and Lower/Middle Muschelkalk are truncated by an unconformity at the base of Upper Muschelkalk Bunter shows significant lateral changes in thickness, with discrete changes across faults directly above the Zechstein salt in the central part of the section. The overlying Lower/Middle Muschelkalk is absent in the SSE part. The intra-formational salt in the Middle Muschelkalk reaches about 300 m thickness. In the NNW its overlying sequences are block-faulted and have rotated counter-clockwise. As compared to the Upper Bunter to Middle Muschelkalk sequences, the maximum depocentres of the Upper Muschelkalk to lowermost Middle Keuper (KM1) are located further to the south and lie directly above the triangular block of Lower and

11 927 Fig. 8 Depth-migrated SW-NE seismic section (2D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. For used stratigraphic abbreviations see Fig. 2. The section shows the diapir B placed above a basement high. The intra-raft areas are filled by two different sedimentary depocentres of Upper Bunter to Middle Muschelkalk and Upper Muschelkalk to Middle Keuper. Similar to the south (Fig. 6), the secondary rim-syncline has an asymmetric geometry Middle Bunter. Two syn-depositional normal faults of a graben structure frame the 800 m thick depocentre. Another remarkable feature is an anticlinal flexure in the centre of the profile affecting the Keuper stratigraphy below the Cretaceous unconformity. Interpretation of seismic horizons The 3D model of the study area is based on 2D and 3D seismic data (cf. Fig. 3). For gridding of the horizons, we used a data density of m and interpolated between the seismic sections. The colour maps reveal the 3D relief of the horizons, overlain by the mapped fault structures, thereby reflecting the complex structural and salt tectonic evolution of the area. The top Rotliegend 3D horizon (Fig. 10a), interpreted as the sub-salt basement, shows a N-S trending high in the west and a low in the east respectively. The two local basement highs in the central area are located directly below the salt diapirs A and B. The exact depth position of the northern structure is confirmed by two boreholes. The most prominent set of faults in the subsalt basement strikes NNW-SSE to N-S. In the west, this system consists primarily of easterly dipping normal faults. In the east, the faults are primarily westerly dipping antithetic faults. Consequently, an asymmetric halfgraben is formed. Subordinate fault sets are striking NE- SW and WNW-ESE. An exact chronology and dating of fault activity is not possible due to the decoupling effect of the salt layer and possible later reactivation of the fault sets under changing stress regimes during multiple tectonic events. The base Muschelkalk 3D horizon (Fig. 10b) lies extremely deep in an area 1 to 10 km wide and at least 25 km long, that trends NNE-SSW to NE-SW. In this area, Lower and Middle Bunter sequences are absent or only small separated blocks are underlying the Muschelkalk layers. The Upper Bunter to Lower Keuper sequences have their thickest depocentres in this area. Seismic interpretation in the 3D cube allowed us to detect a fine network of normal faults located at the margin of the depocentre, where small blocks of hundreds of meters length and width moved downward on

12 928 Fig. 9 Depth-migrated NNW- SSE seismic section (2D data located in Fig. 3) and geological interpretation. The top of this section represent the surface line at sea level. For used stratigraphic abbreviations see Fig. 2. The section shows the Lower Triassic extension and the two depocentres above the intra-raft area. To the SSE, a reduced Upper Bunter to Middle Keuper sequence is located below the sediments of the secondary rim-syncline of diapir A the underlying Upper Bunter salt. The relatively high position of the 3D horizon in the northeast shows an oval, NNE-SSW elongated shape that corresponds to an area with thickest present-day salt (approximately 800 m) in the study area. The base Lower Keuper 3D horizon (Fig. 10c) is absent in the southern central part of the investigated area where Muschelkalk is partially present. This area has a NNE orientated long axis, at an angle to the later diapir, which runs NNW-SSE. The depocentre of the Lower Keuper sequences has the same configuration as the underlying formation but shows a shift of about 1 km to the ESE. The depth structure of the base of the uppermost halite layer of the Weser-Formation (KM3) (Fig. 10d) is dominated by the NNW SSE structural orientation not observable in the underlying Lower and Middle Triassic sequences. The two central salt structures A and B and their peripheral sinks are tracing this direction, which is known from the fault pattern of the sub-salt basement. The relatively high position of this horizon in the northeast corresponds to an area with thickest presentday salt of about 800 m in the study area. In the eastern part the horizon subcrops against the base Cretaceous unconformity and is eroded over a large area. The base Cretaceous horizon (Fig. 10e) is a major unconformity and all horizons above have an average regional dip to the west and southwest, in contrast to the older sequences. A small graben structure in the southeast is positioned directly above the crest of the Middle Keuper turtle back, where inter-layered salt of Middle Keuper age subcropped. The salt structures are clearly reflected by a prominent positive relief. Peripheral sedimentary sinks are minor or absent. Diapir-parallel conjugate faults directly above the roof of the central and southern diapir and normal faulting above the northern diapir are observed. Interpretation of salt tectonic evolution We present our preferred model of salt tectonic evolution, resulting from a series of iterations where seismic interpretations and reconstructions were adopted when conflicts with geological data or requirements of balancing were found. Figures 12 and 13 show the seismic

13 929 section of Fig. 4 restored to 16 time slices. The geological and salt tectonic evolution are presented in three phases beginning with the oldest. Distinct phases of basin evolution are described in forward direction. First phase of salt movement: Zechstein to Middle Keuper Preferred interpretation and modelling concept Fig. 10 The colour maps present fault pattern and relief of suband supra-salt horizons (red high, blue low). Gridded 2D and 3D seismic interpretations are used as data base Our interpretation is based on the general idea that decoupled basement extension at the basin margin was the trigger for initial salt movement (Jackson and Vendeville 1994; Vendeville and Jackson 1992a). Detached faulting of the cover sequences and continued extension initiated salt diapirism and rafting of the sedimentary blocks. Salt flows towards the extensional structures in the suprasalt, in particular above the basement fault and towards the distal zone of detached extension, (Koyi et al. 1993). Between these structures, differential sedimentary loading created an increased sedimentary thickness until a salt weld is formed and lateral salt flow stops. As a consequence, the diapir falls and a mini-basin grows in the intra-raft area (Vendeville and Jackson 1992a, b). Salt migrates laterally along dip and forms a pillow-like salt structure basinward adjacent. In the investigated area, we interpret a major spatial gap between the blocks of Lower and Middle Bunter, and thus a significantly shorter section than the overlying sediments (Figs. 4, 9). Seismic reflectors at the edges of the Lower/Middle Bunter blocks are truncated by faulting and several faulted blocks lie isolated in the centre of the gap. The starting phase of this rafting event at the end of Middle Bunter is illustrated in Fig. 11, where a listric fault and the collapse of a roll over anticline in the hanging wall is indicated by antithetic normal faults. In Fig. 11 Upper Bunter layers cover undisturbed the fault blocks suggesting a pre-upper Bunter age of the first extensional faulting event. In the sub-salt basement of the working area no corresponding normal faults running parallel to the rafting structure are detected. This argument for thinskinned extension is also supported by the regional subsidence trend to the east (Ro hling 1991). In addition to rafting at the northwestern margin of the Ems Trough described here, other coeval rafting events are known from marginal positions of the basin (Kockel 2002). Outside the working area an early Triassic NE-SW directed basement step at the western basin margin (Baldschuhn et al. 1991) is documented. Basement faulting caused salt diapirism directly above (Baldschuhn et al. 1991), detached supra-salt faulting and increased Middle Bunter to Middle Muschelkalk layer thickness at the hangingwall both visible in our data (Fig. 4). Two extensional phases are documented by the rafted blocks of Lower/Middle Bunter and Upper Bunter. We propose that salt reached the surface in both events, but was covered subsequently by younger sediments.

14 930 Fig. 11 Lower part of a depthmigrated W-E seismic section (2D data located in Fig. 3) and geological interpretation. For used stratigraphic abbreviations see Fig. 2. The section shows the Lower/Middle Bunter sequence characterised by a listric fault in the west and a collapsed roll-over anticline. This is interpreted as a starting configuration for rafting The sedimentary thickness distribution shows a southeastward migration of the Upper Muschelkalk to lowermost Middle Keuper (KM1) depocentres onto Zechstein salt and normal regional thickness above the former depocentres in the west (Fig. 4, 10c). Therefore we suggest that the primary salt-source layer in the northwest was exhausted, resulting in the fall of the diapir and the development of a mini-basin with increased sedimentary thickness in the intra-raft area. In the northern continuation of this zone, the Westdorf Graben, an extreme increase in thickness of Upper Bunter to Lower Keuper sediments is described (Gaertner and Ro hling 1993), that corresponds with a zone of absent Lower/Middle Bunter sequences (Ro hling 1991). West of the intra-raft area we observed erosion, reduced thickness of Upper Bunter to lowermost Middle Keuper (KM1) sequences and a crestal collapse structure. Additionally domino-like faulted and tilted blocks of Upper Bunter to Lower/Middle Muschelkalk were displaced towards the intra-raft indicating a local dip to the northwest. We interpret these as the result of the growth of a pillow-like salt structure caused by differential loading and lateral basinward salt flow. The interpretation of a primary salt pillow at the southeastern flank of the intra-raft is further supported by the results of Best et al. (1993) and Baldschuhn et al. (1991). Distinct tectonic phases from retro-deformation Top Zechstein (Fig. 12a) is modelled with 0.3 eastward dip and Top Rotliegend starts without faults and with minor relief dipping 0.6 to the east. We assume an original thickness of the Zechstein salt between 820 m and 940 m increasing to the east based on our modelling results. The assumed amount of salt thickness was restored step by step using sedimentary thickness induced by salt withdrawal as a function of total subsidence minus regional subsidence. Regional subsidence was seen as unaffected by local salt diapirism. The total amount of salt area in our section is not constant for the different time slices because salt loss is qualitatively observable especially since the beginning of deposition of the Stuttgart-Formation (KM2) (Figs. 12 and 13). We interpret this as the consequence of the dissolution processes during a phase where the central diapir was close to the surface. Out of section movements of salt into the perpendicular growing diapir can be another reason for the changing salt area. The Lower and Middle Bunter sequences (Fig. 12b) are interpreted to initially completely cover the salt with an eastward thickness increase from 540 m to 760 m corresponding to the regional dip. The (missing) light pink area of Lower/Middle Bunter at the eastern end demonstrates the difference of section length to sequence length and thus a regional extension between Top Middle Bunter times and present. Before Upper Bunter times (Fig. 12c) the first extensional event brought 950 m (3%) extension in the supra-salt by faulting and rafting, whereas the sub-salt stayed unfaulted. (Differential extension at section scale caused by structural decoupling must be balanced in the complete intracontinental basin (Letouzey et al. 1995). Basement faulting elsewhere at the basin margin and shortening in the basin centre can compensate this thin-skinned extension.) The early stage of extension and rafting initiated reactive salt diapirism, followed by active piercement of thinned overburden and possible salt extrusion (Fig. 12c). The configuration at the Base Upper Bunter (Fig. 12c) is modelled as either the result of 180 block rotation of the embedded Bunter block or as the effect of erosion at the eastern edge of the salt structure and 45 counter clockwise rotation of the Bunter block. Both were caused by the upward flow of salt. Sediments of the Upper Bunter covered this early salt diapir (Fig. 12d) and show a laterally variable thickness distribution with increased sedimentary thickness to the west and a decreased thickness adjacent to the intra-raft due to differential loading and lateral basinward salt

15 931 Fig. 12 Sequential retro-deformation of the interpreted W-E seismic section (2D data) of Fig. 4 from the base of the uppermost halite layer of the Weser-Formation (Middle Keuper) (h) to the Top Zechstein (a). See Fig. 13 for the second part of the restoration flow. A slight thickness increase to the eastern part of the section is interpreted as the result of regional dip to the east. The Lower/Middle Muschelkalk (Fig. 12e) shows a slight westward migration of its depocentre in comparison to the underlying sequence. The primary salt source at this stage is nearly exhausted in the western part of the section. We interpreted another extensional event before the beginning of Upper Muschelkalk times (Fig. 12f) that also triggered rafting and diapirism. Extension calcu-

16 932 Fig. 13 Sequential retro-deformation of the interpreted W-E seismic section (2D data) of Fig. 4 from the base of the uppermost halite layer of the Weser-Formation (Middle Keuper) (h) to the present day geometry (o). See Fig. 12 for the first part of the restoration lated from the balancing for this phase was 1650 m (5.4%). Salt rise was accompanied by drag folding and erosion at the flanks of the Middle Muschelkalk diapir. An unconformity truncated this sequence before the beginning of Upper Muschelkalk sedimentation that covered the former diapir.

17 933 Reconstruction to the Top Grabfeld-Formation (KM1) (Fig. 12g) shows major mini-basin growth of 1,100 m above the intra-raft area and eastward adjacent 300 m erosion down to the Top Middle Bunter sequences. Salt flow from the west then stopped initiating the fall of the diapir due to the eastward regional dip. Differential loading, sedimentary prograding and lateral salt flow resulted in the formation of the Lower Keuper depocentre, the pillow-like salt structure and the primary rim-syncline in the eastern end of the section. Erosion and faulting at the crestal zone of the salt anticline weakened its roof and provided the possibility of the collapse of the structure. Alternative models An alternative explanation for the Zechstein to Middle Keuper phase of salt movement and the absence of Lower/Middle Bunter in this area is structurally controlled erosion. The development of horst and graben structures at the end of Middle Bunter produced primarily a high position under erosion which evolved to a graben structure before Upper Bunter times (Baldschuhn et al. 1991). Both should be caused by vertically coupled basement faulting. However in the sub-salt basement with the Top Rotliegend horizon as one of the best interpreted seismic layers no fault system was detected which runs parallel to the supra-salt structure (Fig. 10a). The thick salt layer of about 800 m in comparison to about 450 m of sedimentary overburden should mechanically result in a strong decoupling of sub- and supra-salt instead of vertical coupling of the faulting, as shown by the analogue modelling of Withjack and Callaway (2000). It seems furthermore improbable that during one short tectonic phase of presumably less than one million year at least 450 m of Lower/Middle Bunter should have eroded from the top of a structural high that evolved subsequently into a graben. The early growth of a pillow-like salt structure as another concept could explain erosion and collapse of Lower/Middle Bunter sediments without extension (Koyi et al. 1993; Coward and Stewart 1995). The isolated blocks in between the Lower/Middle Bunter gap could therefore be the remains of the former coversediments (c.f. Hudec and Jackson 2002). But peripheral sinks with changing sedimentary thickness at the flanks of such a hypothetical salt pillow have not been observed, and faulted edges of the Bunter blocks showing less erosion do not support this concept. Static downbuilding, where sediments sink into the salt layer from the beginning of Bunter times, could be another possible model. But characteristic features of syn-depositional interaction to the adjacent salt area like converging reflectors, onlap-structures, salt jags and unconformities at the base of distinctive sedimentary units are missing. In contrast, parallel seismic reflectors are sharply truncated at the edges of the blocks and extensional faults of pre-upper Bunter age are interpreted (Fig. 11). In addition, it is difficult to explain in this model why Bunter blocks occur inside the downbuilding diapir. Second phase of salt movement: Middle Keuper to Lower Cretaceous Preferred interpretation and modelling concept The second phase of salt movement is interpreted to have started as the consequence of sub-salt extension that triggered normal faulting in the overburden as described in the concept of reactive diapirism (Vendeville and Jackson 1992a; Jackson and Vendeville 1994; Stewart and Clark 1999). Previous erosion and faulting on top of a pillow-like salt structure can weaken a particular location to localize supra-salt extension (Coward and Stewart 1995). Reactive diapirism followed by a short phase of active diapirism passes into passive diapirism (Vendeville and Jackson 1992a), also known as downbuilding (Barton 1933; Jackson and Talbot 1991; Vendeville and Jackson 1991, 1992a; Buchanan et al. 1996). This special type of differential loading involves deposition while salt remains close to the surface. Sediments sink into the salt layer, while salt migrates into the growing diapir. Passive diapirism ceases only when salt migration cannot keep up with sedimentation because of increasing sedimentation rate or when the evacuation of salt forms a salt weld (Rowan et al. 2003). Typical features are near-diapir onlapping, salt re-sedimentation, increased thickness, unconformities and salt jags at the edge of the down-built sedimentary wedges (Giles and Lawton 2002); Rowan et al. 2003). These concepts are the base for our retro-deformation of salt movement from Middle Keuper to Lower Cretaceous times in the working area. Above the major unconformity at the base of the Stuttgart-Formation (KM2), the depocentres migrated to the flanks of the present diapirs. Since then, the sedimentary thickness distribution is directed along a NNW-SSE axes parallel to diapir A and B (Fig. 10). The underlying older suprasalt sequences in contrast show erosional surfaces at the flanks of these diapirs and a NNE-SSW to NE-SW directed thickness distribution and faulting oblique to latter structures (Fig. 10). A striking change in structural patterns is therefore inferred for the end of the Grabfeld- Formation (KM1). Major faulting in the sub-salt basement, seen as the reason for this evolution, formed a graben structure that corresponds to these NNW-SSE structural direction of the supra-salt. This Early Cimmerian extensional phase was caused by intensified North Sea rifting that brought about major extension and faulting in the onshore Ems region or in the Glu ckstadt Graben (Ziegler 1990; Frisch and Kockel 1999). Strong dilation was associated with the beginning of major salt diapirism, subsequent rim-syncline development and extrusion of Zechstein salt in this and many

18 934 other parts of the North German Basin (Frisch and Kockel 1999; Baldschuhn et al. 2001; Jaritz 1973). Seismic indications for the downbuilding phase are Middle to Upper Keuper sedimentary wedges with basal unconformities and onlap structures as well as salt jags of the laterally extruded diapirs. The proposed wedges correspond to well-known sedimentary cycles and sequences of the Keuper (Wolburg 1969; Aigner and Bachmann 1992). Jurassic sequences onlapping diapir A and B and covering diapir C indicate slower salt rise probably caused by exhaustion of the salt source layer. The base Cretaceous unconformity truncated the Jurassic at the flanks of the daipirs and the Muschelkalk sequences in the west. This is in good agreement with the results for uplift, tilting and erosion of this area (Kettel et al. 1984) in context of the Upper Jurassic doming event in the Southern North Sea (Ziegler 1990). Distinct tectonic phases from retro-deformation At the end of the Grabfeld-Formation (Fig. 12g, h) the basement underwent extension by normal faulting and formation of an asymmetric graben structure with the major faults in the west. The amount of basement extension inferred from balancing was 260 m (0.9%). Basement extension triggered normal faulting and extension in the overburden with an elongation of 1120 m (3.7%). Supra-salt extension is focussed in two zones. One position in the west was a preferential site because it was adjacent to a major basement fault. It shows low salt thickness and was located above the previous rafting area. The other zone of focussed extension was already weakened by previous faulting and erosion above the central salt structure. Salt pierced the overlying sequences as reaction to extension and extruded to the surface, starting a downbuilding process with peripheral sedimentary sinks as secondary rimsynclines. The following three reconstructions to the base of the uppermost halite of the Weser-Formation, to the Top Weser-Formation and to the Top Keuper times (Fig. 13h, i, j) show the central diapir with ongoing downbuilding and salt at the surface. The migration of the local Middle Keuper subsidence-maxima towards the diapir indicates the successive collapse of the former salt pillow. An almost constant sedimentary thickness of the sequences west of the diapir suggests less salt movement on this side. We suggest that outside the graben structure only a thin section of Middle and Upper Keuper (120 m) is preserved in the west and none in the east, in contrast to the area in between (up to 1,600 m). The western salt structure could not evolve as a downbuilding diapir because of the exhausted salt source and is therefore modelled to be already covered by the Arnstadt-Formation (Fig. 13j). At the eastern end of the profile a pillow-like salt structure evolved above a westward dipping basement fault since the deposition of the Weser-Formation (Figs. 13h, i). Normal faulting after sedimentation of the Weser-Formation resulted in reactive diapirism and subsequent downbuilding at the easternmost salt structure (Fig. 13j). Ongoing basement extension through the Middle Keuper ceased before the beginning of the Upper Keuper. For restoration to the Top Jurassic time slice (Fig. 13k) the eroded Muschelkalk to Keuper sequences and a thickness of m of Jurassic deposits has been assumed (Kettel et al. 1984). The sub-salt basement is modelled with 1.1 regional dip. The termination of downbuilding is proposed for the Middle to Upper Jurassic, when salt was completely removed from beneath the sedimentary pile, salt rise ceased and the diapir was covered by sediment. Before the beginning of sedimentation in the Lower Cretaceous, more precisely the base Upper Hauterivian (128 Ma), the last tectonic pulse of the Late Cimmerian phase took place. We modelled this by regional uplift and tilting (Fig. 13l). Tectonic uplift of 1,170 m in the west and 650 m in the east produces an 2.1 eastward-dipping sub-salt basement, which is in good agreement with the results of Kettel et al. (1984). This enabled erosion of most Jurassic sequences, parts of the Keuper and Muschelkalk and the uppermost 160 m of the salt diapirs roof. We interpreted two disconnected Bunter blocks in the Central salt structure (generated by supra-salt extension and embedded by Zechstein salt, Fig. 13h k). Kinematic and geometrical modelling provide no constraints to reconstruct whether these blocks sink into the salt or even ascend with the rising salt. Using simple mechanical considerations based on Stoke s law, one can estimate the velocity of a sinking Bunter block in salt. For example, we calculate the velocity for the westerly block using 250 m for the radius and 2,500 kg/m 3 for the density of the Bunter. We used 2,160 kg/m 3 as an average value for the density of salt. The critical parameter in the calculation is the salt viscocity which may vary between Pa s for fine grained salt at relatively high temperatures and Pa s for coarser grained salt at relatively low temperatures (Van Keken et al. 1993). This results in a steady-state velocity of 15 m/ma to m/ma. The salt s upward velocity in the opposite direction is estimated between m/ Ma and m/ma for the different time steps. These simple calculations indicate that the Bunter blocks may or may not sink. For a more accurate constraint, more precise data on halite rheology are required. Alternative models Erosion of the overlying sediments of a former salt structure (Coward and Stewart 1995) without extension is an alternative model for this phase of salt movement. Upper Bunter to lowermost Middle Keuper sequences show erosional surfaces at the flanks of the later salt diapir indicating a reduced thickness and weakening of the sedimentary roof. If the overlying sediments were