Salt Pillows Research Articles

Middle Jurassic – Early Cretaceous rifting of the Danish Central Graben

During the Jurassic – Early Cretaceous, the Danish Central Graben developed as a N–S- to NNW– SSE-trending graben bounded by the Ringkøbing–Fyn High towards the east and the Mid North Sea High towards the west. The graben consists of a system of half-grabens and evolved by faultcontrolled subsidence; three main rift pulses have been recognised. The first pulse ranged from the Callovian to the Early Oxfordian, the second pulse was initiated in the latest Late Kimmeridgian and lasted for most of the Early Volgian, and the third and final pulse occurred within the Ryazanian in the Early Cretaceous. The first pulse was characterised by subsidence along N–S-trending faults. The most pronounced fault-controlled subsidence occurred in the east, especially along N–S-striking segments of the boundary fault to the Ringkøbing–Fyn High. During this period, minor salt movements occurred with the development of salt pillows. The activity along the N–S-trending faults ceased during the Oxfordian. During the second pulse, in Early Volgian times, subsidence was concentrated along new NNW–SSE-trending faults and the main depocentre shifted westward, being most marked within the Tail End Graben, the Arne–Elin Graben, and the Feda Graben. This tectonic event was accompanied by the accumulation of a relatively thick sediment load resulting in the development of salt diapirs, especially within the Salt Dome Province. The third tectonic pulse was essentially a reactivation of the NNW–SSE-trending structures and there is clear evidence of subsidence controlled by faulting and salt movements. Despite the overall extensional tectonic regime, local compressional tectonics resulted in thrusting. For instance, the Gert Ridge is interpreted to have formed by readjustment at the boundary fault between two subsiding blocks. The structural framework during graben evolution controlled, to some degree, the distribution of reservoir sandstones. Reservoir sandstones associated with periods of rotational tilt include Middle Jurassic deposits referred to the Bryne and Lulu Formations, and Upper Jurassic sandstones referred informally to the ‘Fife Sandstone Formation’. Sands deposited during tectonic relaxation are represented by the Heno Formation and Upper Jurassic turbidites interbedded in the Farsund Formation. Sea-level changes were probably most important during periods of tectonic relaxation, particularly with respect to the deposition of lowstand sandstones in basinal areas.

Salt tectonic evolution and hydrocarbon accumulation of Kuqa foreland fold belt, Tarim Basin, NW China

Thick salt beds in the Lower Tertiary were developed in the Kuqa foreland fold belt of the northern Tarim Basin. The salt beds controlled the structural deformation and hydrocarbon accumulation of the belts. Because of the salt beds, the Meso-Cenozoic structures of the Kuqa foreland fold belts can be divided into three major tectono-sequences: suprasalt structures, salt bed structures and subsalt structures. Suprasalt structures include thrusts and fault-related folds. Salt bed structures are dominated by salt pillows, allochthonous salt sheets, salt welds and fish-tail structures. Subsalt structures mainly include imbricated thrust belts, duplex structures, fault-related folds and pop-up structures. Formation of the salt structures in the Kuqa foreland fold belts is controlled by compression, gravitational gliding and gravitational spreading. The basin-mountain coupling and compression resulted in intense shortening of the fold belt. The rising of the South Tianshan Mountains caused the gravity gliding and gravity spreading. Plastic flow deformation of the salt beds resulted in salt thickening from the mountain front to the south of the fold belt. The thick salt bed accumulated in the Klasu and Qiulitag structural belts, and controlled the distribution of the salt-related structural traps and oil–gas pools. The structural traps have a bearing upon salt beds in the Kuqa foreland fold belts. The source rocks are mainly located below the salt beds, which can act as excellent cap rocks and seal the gas pools. The subsalt beds are the favorable places for hydrocarbons to accumulate.

The depositional evolution of diapir- and fault-bounded rift basins: examples from the Lusitanian Basin of West Iberia

New data on the evolution of rift basins is presented after analysing the Late Jurassic stratigraphy of the Central Lusitanian Basin (west Iberia). Well, outcrop and regional 2D seismic reflection profiles are used to investigate the differences in stratigraphic signature between diapir- and fault-bounded sub-basins. During the Late Jurassic syn-rift phase, surface rupturing in fault-bounded sub-basins resulted in the formation of tectonic scarps from which footwall-derived gravity flows were sourced. In contrast, the diapir-bounded Bombarral-Alcobaça sub-basin evolved as a distal bowl-shaped depocentre with an axis located up to 10 km away from its basin margins. Low-gradient marginal slopes developed in the Bombarral-Alcobaça sub-basin during the Late Jurassic rifting, while growing salt pillows limited the vertical propagation of basement normal faults. Differences in tectonic evolution, basin physiography and sediment input are the main factors responsible for the distinct sedimentary evolutions recorded in the study area: (1) transverse footwall-derived sediment fans, predominant in fault-bounded regions, give place to axial southwards-prograding fluvial to shallow-marine units in the diapir-bounded sub-basins; (2) growing salt pillows, absent in the fault-bounded sub-basins, formed barriers to and limited the development of transverse drainage systems.

Post‐Jurassic tectono‐sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin)

AbstractSeismic reflection and well stratigraphic data are used to investigate the post‐Jurassic evolution of the Northern Lusitanian Basin, offshore west Iberia. Stratigraphic correlations between 11 exploration wells were attained in order to characterize the variations in depositional facies associated with salt tectonics. Latest Triassic–Hettangian salt, which generated multiple salt pillows during the Jurassic rifting, was reactivated after the early Aptian in two main phases. The first phase stretches from the late Turonian to the Maastrichtian. The second relates to Miocene tectonic inversion. The compression of the post‐salt overburden caused the amplification of Jurassic detachment folds, forming barriers to the westward progradation of sediment into distinct salt‐withdrawal sub‐basins. Particularly during the Miocene, thin‐skinned overburden shortening was accommodated by growing salt structures that suffered thrusting and extrusion. This structural style contrasts with that of salt‐scarce areas where a simple westerly tilted, fault‐bounded monocline was generated.

Gravitational and buckling instabilities of a rheologically layered structure: implications for salt diapirism

SUMMARY We study the Rayleigh‐Taylor instability of a structure consisting of a buoyant layer of viscous fluid overlain by a dense perfectly plastic layer (which is represented by a strongly nonNewtonian fluid with the power-law exponent tending to infinity). The structure is subject to either horizontal extension or shortening and models rocksalt under a brittle overburden. The growth rate and wavelength of the most unstable perturbation to the background pure shear flow are calculated and compared with those of models composed of two viscous layers or of two perfectly plastic layers. The effects of the viscosity and thickness ratios and density contrasts between the two layers are assessed. Considering the viscosity of the buoyant layer to be much less than the effective viscosity of the overlying layer, we obtain the following results. (i) The instability pattern of the plastic‐viscous structure is similar to that of a plastic‐ plastic structure. (ii) The characteristic wavelength, corresponding to the most unstable mode, increases initially with the thickness ratio between the lower and upper layers, but then decreases by a series of abrupt jumps. (iii) The buckling instability induced by rapid horizontal extension or shortening overwhelms the gravitational instability and the growth rate of this instability depends linearly on the effective viscosity ratio. We analyze the energy equation in order to develop an understanding of the mechanisms of instability as the system varies from a viscous fluid through a power-law fluid to a perfectly plastic medium. To test our analytical results we study models of diapirism in the Great Kavir, Iran. We show that a small interdiapir spacing in the salt canopy province and a wide range of spacings in the salt pillow province of the region can be explained by the perfectly plastic sedimentary overburden and horizontal shortening.

Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin

Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin

Subsurface Mesozoic basins in the central Atlas of Tunisia: Tectonics, sequence deposit distribution, and hydrocarbon potential

The Mesozoic and Cenozoic outcrops of the central and meridional Atlas of Tunisia have been studied by structural, biostratigraphic, and sedimentologic approaches. We extend these studies to the subsurface, employing seismic and well data addressing structures, sequence stratigraphy, and hydrocarbon potential. The basin structures are related to strike-slip fault systems associated with Triassic evaporites. Mesozoic sedimentary sections in the areas of the Gafsa, Sidi Ali Ben Oun, Sidi Aich, and Majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls. We identified tectonic corridors where Upper Triassic evaporites intruded younger Mesozoic rocks beginning in the Jurassic. The organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms. Triassic intrusions induced by the strike-slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms. Mesozoic sequence deposits are distributed along the flanks of the basin-platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits. The time-space distribution of the systems tract deposits indicates the possibility of formation of Triassic, Jurassic, and Cretaceous carbonate and sandstone prograding turbidites, reefs, structural and stratigraphic pinch-outs, and unconformities in the flanks of salt domes and basins. The geometry and distribution of reservoir and seal systems tracts permit the consideration of new Triassic, Jurassic, and Cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms. This (Begin page 886) is a new tectonic model of the central Atlas of Tunisia that shows new potential objectives and traps.

Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin

The northern part of the Northeast German Basin contains a large number of Late Permian (Zechstein) salt pillows, whereas diapiric structures are almost completely absent. This lack of diapirs facilitated the study of early stages of salt movement in the basin. Salt pillows and related structures were investigated in terms of distribution, geometry and time of initiation of salt flow within the regional geological context. The primary Zechstein thickness in the study area was reconstructed to gain more insight into the relationship between the geometry of the salt layer and the style of the salt-related structures. In this study, no clear spatial relationship between the salt structures and basement faults has been found and the location of the salt structures in this area appears to be highly independent of the underlying structural grain. The overburden is affected by minor faulting. We propose that buckling of the overburden due to regional compression significantly contributed to the initiation of the Late Jurassic to Early Cretaceous salt structures in the basin. Reverse faulting of the Gardelegen and Haldensleben Faults is related to inversion tectonics and exerted a compression on the basin fill. During the deformation, the Late Permian salt layer acted as an efficient detachment and led to a marked decoupling of the Mesozoic overburden from the underlying pre-Zechstein rocks.

The formation of the Voldum salt pillow in the Danish Zechstein basin, Jutland (Denmark)

On the basis of seismic and well data, supported by Bouguer gravity data, the sequence of events leading to the formation of a Zechstein salt pillow – called Voldum – in east central Jutland is discussed (the study area is delimited in Fig. 1). The initial salt movements, activated by faulting in the Triassic, resulted in the formation of a minor salt pillow on the edge of a graben within the Zechstein (Upper Permian) basin. During a renewed faulting (Voldum fault) of the base Zechstein in Late Jurassic and consequent deepening of the graben, a syncline developed above the salt where thick sediments of Late Jurassic age accumulated. In the process large quantities of salt, due to differential loading, withdrew from the graben and moved laterally up-dip across the older fault scarps. Thus a new and larger salt accumulation (Voldum pillow) formed above the southern flank of the graben. A relict Triassic thin, formed during the growth of the first pillow, remains, but no salt accumulation which could account for this thin is present. The Voldum pillow continued to grow during the Cretaceous and the Tertiary, but the speed of growth decreased considerably during post-Late Cretaceous times, although there are still large quantities of, virtually undisturbed, salt present south of the pillow in the part of the basin corresponding to the Silkeborg Gravity High. The graben area apparently underwent a mild inversion at the close of the Cretaceous. The reasons why the Voldum pillow did not develop into a diapir are considered to be a strong and thick overburden which existed at the beginning of the Voldum pillow formation, the deeply buried salt which probably acted as a deterrent to the rupture of the overburden, and the nature of the Voldum fault which, apparently, had an appreciable strike-slip component in dextral sense.

Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.

The salt pillow to diapir transition; evidence from unroofing unconformities in the Norwegian-Danish Basin

Salt diapirs in the Norwegian Danish Basin are characterized by the occurrence of marked angular unconformities which dip towards the diapirs. These unconformities developed above salt pillows, showing that the pillows grew close to the surface, or even reached the surface, in the time interval predating diapir formation. By contrast, the unconformities above present-day pillows show that these pillows have never reached close to the surface, strongly suggesting that pillow growth to "near unroofing" was a necessary condition for a transition to diapirism. Evidence from pillows indicates that unroofing in some cases was an asymmetric process involving growth of the pillow on the footwall side of a fault which soles out on the top surface of the salt. A consequence of this mechanism is that a separate stage of salt structure growth, with a combination of both pillow and diapir features, must be included in a general model for the "classical" Zechstein basin salt structures.

Interpretive processing of a Walkaway VSP: Imaging a North Sea pre-Zechstein reservoir

A three-component walkaway vertical seismic profile (VSP) was used to resolve the faulting of a North Sea Rotliegendes reservoir at a depth of 11,000 feet (3400 m), underlying a major Zechstein salt pillow. `Interpretive Processing' was applied to the walkaway VSP, meaning that when applying conventional processing techniques, every accomplished step was interpreted and the interpreted information was used in the design of the next processing step. This data processing philosophy is significantly different from the `routine processing' used in many VSP imaging efforts. A pre-survey reservoir model with lateral velocity variation was iteratively updated during the processing. The traveltimes of large lateral offsets were found to be affected by anisotropy and therefore were not included in the reservoir imaging. Kirchhoff migration of the walkaway VSP, based on the updated velocity model, yielded better fault resolution at the reservoir level than did the surface seismic section. In this instance, the VSP image improved interpretation of the Rotliegendes and pre-Permian structure.

Nøvling: An unusual salt structure on the soutbern margin af the Danish Basin

The N~vling salt structure is located on the southern margin of the Danish Zechstein Basin. The present study of this structure is based on a detailed gravity survey, reflection and refraction seismic data, and well information. Apart from the exploratory well NØvling-l, and a shallower well Herning-l, a number of wells, mainly 20-30 m deep, were drilled in this general area (by the Geological Survey of Denmark). Through this combined study using geological and geophysical methods it is possible to distinguish several stages in the development of the N~vling structure. It is shown that these stages are closely related to faulting at the base Zechstein level. The faulting, which apparently started as normal faulting in the Triassic, triggered the first salt movements from the deeper parts of the basin to this peripheral area. The faulting was reactivated during the late Jurassic, resulting in the formation of a salt pillow. The tectonic regime changed in the post-Paleocene tertiary when transpressional movements along a prominent basement wrench fault became dominant. This ultimately resulted in the formation of a positive flower structure and the uplifting of the Saale Quatemary surface. At shallow depths the salt became dissolved creating a topographic depression above the flower structure. The present depression was formed at the beginning of the last late glacial time. Quatemary landforms in the N~vling area are therefore greatly influenced by recent tectonic events.

Why does salt start to move?

This paper concerns mechanisms of salt (and ductile shale) movement. It investigates salt flow due to differential loading, folding of the overburden during compression and drag by a moving overburden. The approach is to compare the salt flux caused by these processes to that generated by buoyancy. It is demonstrated that overburden folding and drag by the overburden can, under commonly encountered conditions, result in greater amounts of salt movement than that produced by buoyancy or differential loading. These conclusions apply during the early stages of salt anticline, salt pillow and salt roller formation but not during the later stages of salt diapir and salt wall growth when buoyancy dominates. The quantitative significance of these alternatives to buoyancy is determined by considering an elastic plate overlying a viscous fluid. This is the simplest mathematical model that can reproduce the processes considered. The model shows that: (1) Under certain conditions, these mechanisms produce more salt movement than buoyancy. Differential loading dominates when the surface slopes become more than a small fraction of the slope of the salt top. Overburden buckling dominates if the in-plane stress exceeds a critical value. Drag dominates when the salt layer is thinner than a few hundred metres. (2) The strength of the overburden inhibits formation of salt diapirs, even those due to buoyancy, on wavelengths less than about 12 km.

Three-dimensional geometry and evolution of a segmented detachment fold, Mississippi Fan foldbelt, Gulf of Mexico

The frontal fold of the deep-water Mississippi Fan foldbelt is used to investigate the relationships between folding and faulting in detachment folds. Seismic coverage shows the entire three-dimensional geometry, from termination to termination, and the deformation history as recorded by asymmetric growth strata on fold limbs. The fold is a salt-cored detachment fold cut by reverse faults on both limbs. Its three-dimensional geometry is complex, consisting of four separate culminations, each associated with a distinct fault or fault segment. Consequently, profile geometries vary significantly, but unsystematically, along strike. Data analysis and structural restoration suggest a three-stage evolution during Mio-Pliocene shortening: (1) preexisting, en-echelon salt pillows served as buckling instabilities for the initiation of detachment folds that experienced relatively minor lateral propagation during growth and linkage; (2) an increase in shortening rate was accommodated by break-thrust folding; and (3) the faults became inactive upon a decrease in shortening rate, such that further fold amplification occurred by rotation and uplift of the backlimb. There is a direct correlation between fold and fault geometries, and abundant evidence indicates that the geometries of individual fold segments dictated fault geometries.

Kinematics and Dynamics of Salt Tectonics  Driven by Progradation

Scaled physical models illustrate the importance of progradation as a trigger for salt tectonics and formation of allochthonous sheets. Regional extension and contraction were excluded in the models. In our experiments, prograding wedges above a tabular, buoyant salt layer with a flat base expelled the salt basinward, forming the following structures proximally to distally: (1) sigmoidally distorted initially planar wedges, (2) relict salt pillows and salt welds, (3) basinward-dipping expulsion rollover and crestal graben, (4) rollover syncline, (5) landward-facing salt-cored monocline, and (6) distal inflated salt layer. This deformation zone amplified and advanced basinward during progradation; however, no diapiric salt structures formed. Over a buoyant salt layer whose basement had steps facing landward, progradation initially formed a broad anticline where salt flow was restricted across each basement step. Distal aggradation pinned the anticline and enhanced differential loading. The anticline actively pierced its crest, which ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 2, 1995; revised manuscript received May 9, 1996; final acceptance October 16, 1996. 2Bureau of Economic Geology, University of Texas at Austin, and Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78713. 3Bureau of Economic Geology, University of Texas at Austin, Austin, Texas 78713. All modeling was done at the Applied Geodynamics Laboratory of the Bureau of Economic Geology, with financial support by grant number 3658-178 from the Texas Advanced Technology Program and from the following companies: Agip S.p.A, Amoco Production Company, Anadarko Petroleum Corporation, ARCO Exploration and Production Technology, BP Exploration, Chevron Petroleum Technology Company, Conoco and Dupont, Exxon Production Research Company, Louisiana Land and Exploration Company, Marathon Oil Company, Mobil Research and Development Corporation, Petroleo Brasileiro S.A., Phillips Petroleum Company, Societe Nationale Elf Aquitaine Production, Statoil, Texaco, and Total Minatome Corporation. The Department of Geological Sciences and the Geology Foundation at the University of Texas at Austin and Phillips Petroleum Company provided additional financial support for Hongxing Ge. Dan Schultz-Ela helped us depth convert and restore seismic sections. Mark Rowan, Mike Hudec, Lee Fairchild, Sharon Mosher, and Tucker Hentz provided invaluable suggestions for improving the paper. Publication authorized by the Director, Bureau of Economic Geology, University of Texas at Austin.

Balancing salt dome uplift and withdrawal basin subsidence in cross-section

Idealized geometric models are presented for cases of cross-sections drawn perpendicular to elongate salt domes, where a two-dimensional model is adequate, and cross-sections drawn through the center of hemispherical salt domes, where a three-dimensional model is needed. Primary attention is devoted to domes during the salt pillow stage of uplift; however, a simple model is provided for the piercement stage as well. Mathematical relationships are derived between the cross-sectional area of the dome and the cross-sectional area of the associated withdrawal basin. Relationships are also derived relating non-dimensional dome uplift to non-dimensional basin subsidence. These relationships must be taken into account when constructing and restoring cross-sections of salt domes. Failure to do so can result in significant errors in estimating the amount of salt lost to dissolution. In addition, the relationships in this paper imply that there is a change in either salt dome height or withdrawal basin depth near the ends of an elongate dome, and predict that restored cross-sections of mini-basins developed on thick layers of salt typically will contain more salt than the present-day section.

Salt tectonism in the Persian Gulf Basin

Abstract The Persian Gulf Basin is an elongate, margin sag-interior sag, sedimentary basin spanning the last 650 Ma along the northeastern subducting margin of the Arabian Plate and is the largest basin with active salt tectonism in the world. This basin is asymmetrical in NE-SW cross section with sediments thickening from 4,500 m near the Arabian Shield to 18,000 m beside the Main Zagros Reverse Fault. Halokinesis in the Persian Gulf Basin originates where major intersecting basement faults cut the buoyant salt beds of the Neoproterozoic Hormuz Series (1.5 to 2.5 km thick) and the equivalent Ara Formation of Oman where salt thickness reaches 4 km. Another source of salt is the Oligo-Miocene Gachsaran Formation, forming Lali salt plug. Major basement fault trends control salt tectonism in the basin, namely the extensional N-S Arabian Trend, the sinistral NE-SW Aulitic Trend, the dextral Erythaean Trend and the extensional E-W Tethyan Trend. The main types of salt structures in the Persian Gulf Basin are salt domes, salt pillows, salt walls, salt piercements, rim anticlines, turtleback structures, disharmonic folds, orogenic fold fillings and dissolution drapes. Diapiric oil fields, account for 60% of the 600 billion barrels of recoverable oil reserves of the Persian Gulf Basin and have grown continuously since Late Jurassic, or since Permian in some large structures, such as Bahrain.

Synthesis of salt tectonics in the southern North Sea, UK

A structural model encompassing the southern North Sea basin west of the Central Graben has been developed. This model consists of a rift system affecting the post-salt section around the basin margin and a large area of detached compressional buckle folds within the basin. This pattern is initially a response to gravity sliding of the post-salt section on the salt within the basin during the late Triassic to late Jurassic. A close relationship between the location and trend of the peripheral graben system and basement structures in the pre-salt is noted. Pre-Jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre-salt (Rotliegend) level and shortening across salt-cored buckle folds in the post-salt section. Salt pillows and swells passively infilled the cores of these gravity-induced buckle folds. Cretaceous and Tertiary inversion involved basin tilt and renewed movement on basin-bounding basement faults; notably, reverse movements did not propagate from basement structures up into the peripheral graben systems. The post-salt sedimentary section experienced gravity spreading in response to inversion-related uplift, resulting in syn-inversion extensional faulting in the Sole Pit High, where the Mesozoic section was thickest. This extension, combined with a loss of fault heave in the pre-salt section, was balanced by amplification of salt-cored buckle folds in the centre of the basin. In the context of the model described here, salt pillows represent passive infill of thin-skinned, compressional buckle folds which later amplified during thick-skinned basement shortening. Crestal collapse of such folds occurs via normal faulting, accompanied by reactive diapirism. Such reactive diapirs establish conduits through which salt may leak, leading to pillow deflation and ultimately conduit preservation as a salt wall (flanked by rim synclines in areas where the buckle folds were emergent). The salt structures described here are related to cover folds and faults, which in turn reflect episodes of basin extension, tilting and inversion. Hence individual salt structures can be said to be only remotely connected with regional, intraplate stresses.

Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences

Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences