Salt Tectonics Research Articles

Structural setting and evolution of the Mensa and Thunder Horse intraslope basins, northern deep-water Gulf of Mexico: A case study

The Mensa and Thunder Horse intraslope minibasins in southcentralMississippi Canyon, northern deep-water Gulf ofMexico, had a linked structural evolution from the Early Cretaceous through the late Miocene. Analysis of the two minibasins illustrates the complexities of deep-water sedimentation and salt tectonics in intraslope minibasins. This study is based on the integration of a 378-mi2 (979-km2) three-dimensional seismic data set,wire-line logs, and biostratigraphic data. These two minibasins comprise several structural features that affected their geologic evolution: basement faults, autochthonous salt, three allochthonous salt systems (top Barremian, top Cretaceous, and Neogene), a growth fault and raft system, three major turtle structures with associated extensive crestal faults, and strike-slip faults. Remnant allochthonous salt pillows are present above the 125 Ma horizon (approximate top Barremian system) and on the 66 Ma horizon (top Cretaceous system) throughout the Mensa minibasin, whereas the top Cretaceous allochthonous salt system is identified regionally by a salt weld in the Thunder Horse area. These allochthonous salt systems formed weld surfaces beneath the Mensa and Thunder Horse turtle structures. Structural features and associated minibasins evolved during several discrete intervals. From the Early Cretaceous through the latest Oligocene (125 to 24 Ma), an extensive allochthonous salt canopy was present within the Mensa and Thunder Horse minibasins. During this interval, sediments loaded the salt, forming thinwedge-and sheet-formdeposits in theMensa area and a thick, northwest-Trending trough in the ThunderHorse area. A secondary allochthonous salt system extruded at the Top Cretaceous level, as seen by remnant salt bodies. Salt withdrawal from these allochthonous salt systems provided accommodation for bowl-and trough-shaped external stratigraphic forms to develop during the Miocene. High sedimentation rates produced salt evacuation from these allochthonous salt systems and initiated diapirism that formed the Neogene allochthonous salt level. The prominent turtle structures in the two minibasins, critical to the formation of traps to the two major fields, developed at slightly different times: Thunder Horse at 14.35 and Mensa at 11.4 Ma. Copyright © 2017 The American Association of Petroleum Geologists. All rights reserved.

Geometry and kinematics of Neogene allochthonous salt systems in the Mississippi Canyon, Atwater Valley, western Lloyd Ridge, and western DeSoto Canyon protraction areas, northern deep-water Gulf of Mexico

The structural history of the Mississippi Canyon, Atwater Valley, westernDeSoto, and western Lloyd Ridge protraction areas in the northeastern deep-water Gulf of Mexico is of a basin influenced by complex salt tectonics that controlled the formation of intraslope minibasins and sediment distribution. Large volumes of Middle Jurassic autochthonous salt (Louann Salt) were successively mobilized upward into younger sediments, forming three distinctive allochthonous salt layers: At the top Barremian, at the top Cretaceous, and within the Neogene interval. Four types of Neogene allochthonous salt systems are identified based on the geometry of the allochthonous salt bodies and associated faults, folds, and minibasins: (1) basement-controlled, (2) counterregional, (3) roho, and (4) fold-belt-related salt systems. The allochthonous salt systems are defined based on salt-body geometry, salt-stem geometry, associated fault network, and associated stratigraphic geometries. The distribution of the Neogene allochthonous salt systems is controlled by the original autochthonous salt thickness, the basement configuration, and the regional sediment-loading pattern. The basement-controlled Neogene salt systems are present in the eastern and northern part of the study area where little basinward gravitational gliding occurred. The counterregional and roho allochthonous salt systems are associated with the basinward evacuation of salt in response to extensive sediment loading. The fold-belt-related allochthonous salt systems are present in the southern part of the salt province where extensive shortening remobilized salt into and onto contractional structures. The detailed study of those Neogene allochthonous salt systems is used to build conceptual kinematic models for each style of salt system. Copyright © 2017 The American Association of Petroleum Geologists. All rights reserved.

Petroleum geology of the Mississippi Canyon, Atwater Valley, western DeSoto Canyon, and western Lloyd Ridge protraction areas, northern deep-water Gulf of Mexico: Traps, reservoirs, and tectono-stratigraphic evolution

The petroleum geology of the Mississippi Canyon, Atwater Valley, western DeSoto Canyon, and western Lloyd Ridge protraction areas, offshore northern Gulf of Mexico, is controlled by the interaction of salt tectonics and high sedimentation rate during the Neogene and resulted in a complex distribution of reservoirs and traps. We evaluate 87 fields and discoveries: 51 with combined structural/stratigraphic traps (three-way closures), 19 with structural traps (four-way closures), and 17 with stratigraphic traps. Three of these discoveries are in Upper Jurassic eolian reservoirs; the remaining discoveries are in Neogene deep-water reservoirs. The tectono-stratigraphic evolution of the area is analyzed at 11 discrete intervals between 24 Ma and the present. Four stratigraphic external forms—troughs, bowls, wedges, and sheets—are integrated with the structural geology to understand the changing shape of subbasins and minibasins, primarily in a slope setting. This analysis shows how the allochthonous salt systems evolved over time and how salt movement affected sedimentation patterns and subbasin evolution. The study area includes some of the largest fields in the northern deep-water Gulf of Mexico, such as the Thunder Horse field, which produces from an anticlinal (turtle) structure, or the Mars–Ursa and associated fields with greater than 1.5 billion BOE estimated ultimate recovery, which developed with a counterregional allochthonous salt system. The remaining fields have considerably smaller reserves, which are controlled by the area within closure and the number of reservoir intervals. Most of fields in the study area are contained within sheet-shaped or wedge-shaped stratigraphic external forms and have four-way or three-way trapping configurations. These findings indicate the profound effect of mobile salt on the petroleum geology of the region.

New stratigraphic and genetic model for the dolomitic Cretaceous Pinda reservoirs in Angola. Part I - The Pinda of Angola, an integrated lithostratigraphic approach

The Pinda Group of the Congo basin in the northern Angolan offshore is an overall transgressive supersequence of late Aptian - ? early Cenomanian age. Due to synsedimentary salt tectonics of the underlying Loeme Salt, the original ramp arrangement is split into discrete structures corresponding to many proven, probable and possible oil and gas plays. Where the biostratigraphic information was not destroyed by pervasive dolomitization, the little that remains is not available to the general public because it is treated as proprietary data by the Sociedade Nacional de Combustiveis de Angola Empresa Publica - Sonangol E.P. Only a limited set of data containing lithostratigraphic information (e.g., some published well logs and photomicrographs of facies) is available. The aim of this paper is to give a little more information about the Pinda stratigraphy. For instance, to be formally defined, this poorly known unit required a type section, a shortcoming that is addressed here. The wireline log signatures of some regional seismic markers are documented because they are used to correlate wells and subdivide the Group into formations. The microfacies corresponding to some key electrofacies as well as the porosity types are also condensed in a microphotograph catalog. Finally, few examples of typical LoC (i.e., Line of Correlation) diagrams are displayed and interpreted.

Salt tectonics and tear faulting in the central part of the Zagros Fold-Thrust Belt, Iran

The central part of the Zagros Fold-Thrust Belt is characterized by a series of right-lateral and left-lateral transverse tear fault systems, some of them being ornamented by salt diapirs of the Late Precambrian–Early Cambrian Hormuz evaporitic series. Many deep-seated extensional faults, mainly along N–S and few along NW–SE and NE–SW, were formed or reactivated during the Late Precambrian–Early Cambrian and generated horsts and grabens. The extensional faults controlled deposition, distribution and thickness of the Hormuz series. Salt walls and diapirs initiated by the Early Paleozoic especially along the extensional faults. Long-term halokinesis gave rise to thin sedimentary cover above the salt diapirs and aggregated considerable volume of salt into the salt stocks. They created weak zones in the sedimentary cover, located approximately above the former and inactive deep-seated extensional faults. The N–S to NNE–SSW direction of tectonic shortening during the Neogene Zagros folding was sub-parallel with the strikes of the salt walls and rows of diapirs. Variations in thickness of the Hormuz series prepared differences in the basal friction on both sides of the Precambrian–Cambrian extensional faults, which facilitated the Zagros deformation front to advance faster wherever the salt layer was thicker. Consequently, a series of tear fault systems developed along the rows of salt diapirs approximately above the Precambrian–Cambrian extensional faults. Therefore, the present surface expressions of the tear fault systems developed within the sedimentary cover during the Zagros orogeny. Although the direction of the Zagros shortening could also potentially reactivate the basement faults as strike-slip structures, subsurface data and majority of the moderate-large earthquakes do not support basement involvement. This suggests that the tear fault systems are detached on top of the Hormuz series from the deep-seated Precambrian–Cambrian extensional faults in the basement.

3‐D basin‐scale reconstruction of natural gas hydrate system of the Green Canyon, Gulf of Mexico

AbstractOur study presents a basin‐scale 3‐D modeling solution, quantifying and exploring gas hydrate accumulations in the marine environment around the Green Canyon (GC955) area, Gulf of Mexico. It is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system. Overall, it comprises a comprehensive basin reconstruction, accounting for depositional and transient thermal history of the basin, source rock maturation, petroleum components generation, expulsion and migration, salt tectonics, and associated multistage fault development. The resulting 3‐D gas hydrate distribution in the Green Canyon area is consistent with independent borehole observations. An important mechanism identified in this study and leading to high gas hydrate saturation (>80 vol %) at the base of the gas hydrate stability zone (GHSZ) is the recycling of gas hydrate and free gas enhanced by high Neogene sedimentation rates in the region. Our model predicts the rapid development of secondary intrasalt minibasins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower GHSZ boundary. Consequently, large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upward to recharge the GHSZ. In total, we have predicted the gas hydrate budget for the Green Canyon area that amounts to ∼3256 Mt of gas hydrate, which is equivalent to ∼340 Mt of carbon (∼7 × 1011 m3 of CH4 at STP conditions), and consists mostly of biogenic hydrates.

Upper Triassic lithostratigraphy, depositional systems, and vertebrate paleontology across southern Utah

The Chinle Formation and the lower part of the overlying Wingate Sandstone and Moenave Formation were deposited in fluvial, lacustrine, paludal, and eolian environments during the Norian and Rhaetian stages of the Late Triassic (~230 to 201.3 Ma), during which time the climate shifted from subtropical to increasingly arid. In southern Utah, the Shinarump Member was largely confined to pre-Chinle paleovalleys and usually overprinted by mottled strata. From southeastern to southwestern Utah, the lower members of the Chinle Formation (Cameron Member and correlative Monitor Butte Member) thicken dramatically whereas the upper members of the Chinle Formation (the Moss Back, Petrified Forest, Owl Rock, and Church Rock Members) become erosionally truncated; south of Moab, the Kane Springs beds are laterally correlative with the Owl Rock Member and uppermost Petrified Forest Member. Prior to the erosional truncation of the upper members, the Chinle Formation was probably thickest in a southeast to northwest trend between Petrified Forest National Park and the Zion National Park, and thinned to the northeast due to the lower Chinle Formation lensing out against the flanks of the Ancestral Rocky Mountains, where the thickness of the Chinle is largely controlled by syndepositional salt tectonism. The Gartra and Stanaker Members of the Ankareh Formation are poorly understood Chinle Formation correlatives north of the San Rafael Swell. Osteichthyan fish, metoposaurid temnospondyls, phytosaurids, and crocodylomorphs are known throughout the Chinle Formation, although most remains are fragmentary. In the Cameron and Monitor Butte Members, metoposaurids are abundant and non-pseudopalatine phytosaurs are known, as is excellent material of the paracrocodylomorph Poposaurus; fragmentary specimens of the aetosaurs Calyptosuchus, Desmatosuchus, and indeterminate paratypothoracisins were probably also recovered from these beds. Osteichthyans, pseudopalatine phytosaurs, and the aetosaur Typothorax are especially abundant in the Kane Springs beds and Church Rock Member of Lisbon Valley, and Typothorax is also known from the Petrified Forest Member in Capitol Reef National Park. Procolophonids, doswelliids, and dinosaurs are known but extremely rare in the Chinle Formation of Utah. Body fossils and tracks of osteichthyans, therapsids, crocodylomorphs, and theropods are well known from the lowermost Wingate Sandstone and Moenave Formation, especially from the St. George Dinosaur Discovery Site at Johnson Farm.

South Atlantic salt basins – Witnesses of complex passive margin evolution

Cretaceous aged salt and salt-free basins along the continental margins of Brazil and Angola/Namibia exhibit a rather heterogeneous geometry and basin fill which implies a complex tectonic-sedimentary evolution. As a consequence, the origin of the South Atlantic Aptian salt province has been widely discussed with rather contrasting concepts presented for the distribution of the salt depocentres relative to the protracted evolution of both continental margins. The work presented here aims on integrating conjugated basins transects with kinematic reconstruction models to evaluate the evolution and the geodynamic role of salt basins since the early opening of the South Atlantic. Comparing basin pairs along both margins reveals pronounced symmetry changes attributed to a shift in the spreading centre location from the South American to the African side, which at the same time explains changes in salt basin sizes, the continent/ocean crustal locations and seaward dipping reflector distributions. Furthermore, the arrangement of presalt sag sediments on subsiding, rifted continental crust in the proximal basins and newly formed proto-oceanic crust in the distal basins controlled the distribution and thickness of Aptian salt deposits. The subsequent structural and sedimentary record and the response of the salt systems in the South Atlantic indicate a prolonged history of spatial and temporal tectonic impacts that were rather unique for the individual margin segments. In particular phases of postrift margin uplift and tilting with associated salt tectonics and sediment deposition amplified structural differences in conjugated and neighboring salt basins. These also reflect the combined effects of plate tectonic and vertical motions with associated dynamic topography components along both South Atlantic margins.

Combination of improved acquisition system, processing, velocity model and migration for seismic imaging in areas of intense salt tectonics

Various types of hydrocarbon traps associated with intensively developed salt bodies cannot be reliably delineated and then successfully explored by drilling without first obtaining adequate seismic images. This paper shows that such images can only be obtained by a synergic combination of the most advanced options for acquisition system, processing, velocity model and migration as elements of technology as a whole. Currently, such most progressive elements are wide- and full-azimuth acquisition systems, tilted transversely isotropic velocity model, reverse time migration and special processing techniques that enhance the effectiveness of this method. This is substantiated by the actual data examples demonstrated in this paper.

Numerical modeling of the deformation of embedded brittle rock bodies in compressive environment during salt tectonics – A case study from SOSB

Large rock inclusions (called stringers) such as carbonate or anhydrite blocks are embedded in many salt bodies. The observation from seismic data shows different ways of movement and deformation of the inclusions, including displacement, folding and fracturing. There remains much unknown about the deformation process of embedded inclusions in a compressive environment during salt tectonics. In this research, we investigate how the differential displacement of the top salt surfaced caused by downbuilding process leads to the deformation of brittle stringers located in-between two sediment pods in a compressive environment by an interative procedure with adaptive remeshing technique to detect and simulate fracture, overtrusting, folding deformation in a finite element model which is established as the generic model from the observations from South Oman Salt Basin (SOSB). The model shows that the compressive environment induces different deformation ways such as shear failure, folding and thrusting of stringers and the stringers can break soon after the onset of salt tectonics. The orientation of the possible shear failure plane (first break) determines the final complex deformation pattern. The initial depth and position of stringer (parallel or horizontal location) strongly influences the time and frequency of breaks.

Pore pressure and stress regime in a thick extensional basin with active shale diapirism (western Mediterranean)

The Alboran Sea in the Mediterranean is a back-arc basin developed during the Miocene by extensional collapse within an arc-shaped orogen. A major depocenter (>10 km [>6.2 mi]) is located to the west of the basin (West Alboran Basin [WAB]) and contains a diapiric province with overpressured shales and mud volcanoes. Seismic and well data are used to analyze the evolution of the shale structures in the northern margin of the WAB and to estimate the in situ stress tensor. Geomechanical modeling suggests a present-day normal faulting stress regime along the northern WAB, where the maximum horizontal stress is parallel to the coastline. Pore pressure shows a hydrostatic gradient down to 2000 m (6561 ft), where the top of the regional pore pressure ramp is located. Undercompaction is the dominant mechanism generating overpressures in sediments shallower than 5000 m (16,406 ft). At greater depths, thermal mechanisms impose an excess of pore pressure on the sediments feeding the diapirs. This framework is used to discuss the contribution of thermally generated pressures to the triggering of shale diapirism. Increasing thermal pressures in the deepest confined units cause tensile failure of the overburden and subsequently promote mud withdrawal and injection in the overburden. The magnitude of the overpressure conditions the vertical ascent of shale. The more mature structures reuse preexisting normal faults in their ascent toward shallower basin levels. Results provide insights into the current discussion about the triggering factors behind shale diapirism. They also help to explain the differences between shale structures and those shaped by salt tectonics.

Structural framework of the Lufilian Fold Belt in the Domes region of North Western Province, Zambia from interpretation of geophysical data

The area around the Kabompo and Mwombezhi Domes in the North Western Province of Zambia is characterized by poor outcrop, and as is common with other areas in Zambia, correlation of units between isolated outcrops is complicated by the similarity of lithology in many sections of the stratigraphy. Aeromagnetic data, airborne radiometric and electromagnetic data were used to provide control on correlation between outcrops, and therefore allowed interpretation of a robust structural framework. The two domes are Paleoproterozoic basement-cored anticlines unconformably overlain by Neoproterozoic Roan Group rocks of the lower Katangan Supergroup. The Katangan Supergroup was deposited in an extensional basin formed as part of the breakup of the supercontinent Rodinia.From the geophysical interpretation several normal faults are identified that probably formed during this stage of basin formation, and there is good evidence for at least four regional transfer zones having formed at this time. The fault architecture initiated during the Katangan extensional tectonic event is shown in this interpretation to have strongly influenced the geometry of the Lufilian Fold Belt. Evidence in the domes area suggests that salt tectonics may explain the localization of the regional décollement at the top of the Lower Roan, and provide a structural mechanism for the formation of high-pressure metamorphic rocks in the area.Sinistral reverse faulting and associated folding dominates the Lufilian structural fabric in the east of area, and these faults appear to sole out on the regional décollement. Sigmoidal fold profiles have developed between adjacent fault zones, with east-west oriented fold axes in more distal areas rotating into parallelism adjacent to the faults, suggesting synchronous folding and faulting during north-directed compression. In the north-western part of the study area, fold hinge orientations and basement-cover interactions, indicate north north-westerly directed compression.The study demonstrates that interpretation of combined geological and geophysical mapping at a scale suitable for resolving unit correlations, the deformation sequence and local compression directions, is effective at resolving the extensional and compressional structures that now control the Lufilian Fold Belt structural framework.

The origin of salt-encased sediment packages: Observations from the SE Precaspian Basin (Kazakhstan)

Intrasalt sediment packages containing siliciclastic sediments, carbonate sediments, or non-halite evaporites such as gypsum or anhydrite are common within most salt sequences. Intrasalt sediment packages may have been deposited before, during, or after salt deposition and be incorporated into the salt by various processes. Understanding the origin and evolution of intrasalt sediment packages may yield important insights into the tectonic and geodynamic history of the basin, and also into the understanding of salt tectonics. Despite the importance of intrasalt sediment packages, currently there is no systematic description of their possible origins and their distinguishing criteria.This work is divided into three parts. The first part outlines the possible origins of intrasalt sediment packages, as well as criteria to determine if they originated as subsalt, suprasalt or intrasalt sequences. The second part examines how sediment packages that originated on top of salt, such as minibasins, can be encased within salt. Four key encasement processes are proposed: a) salt expulsion from beneath a minibasin experiencing density-driven subsidence; b) salt expulsion from beneath adjacent subsiding minibasins; c) salt expulsion associated with lateral shortening; d) override of minibasins by a salt sheet sourced from elsewhere. The third part of the paper presents a case study from the SE Precaspian Basin, Kazakhstan, where, using a borehole-constrained 3D seismic reflection dataset, the proposed criteria are applied to an area with abundant, newly discovered sediment packages within salt.

Diversion and morphology of submarine channels in response to regional slopes and localized salt tectonics, Levant Basin

In the Levant Basin, submarine channels are abundant around the Nile deep-sea fan (NDSF), an area which is also affected by salt tectonics related to the Messinian salt giant. Here we focus on the relationship between submarine channels and obstacles formed by salt tectonics. Initially, we use methods developed for terrestrial morphological analysis and quantify channel sinuosity, width and slope in search for consistent relationships between morphometric parameters and channel response to obstacles. However, this traditional analysis did not yield robust conclusions. Then, we apply two new morphometric parameters suggested here to express the distortion of channels by obstacles: incident angle (α), defined as the acute angle between the regionally influenced channel direction and the strike of the tectonic obstacle and diversion angle (Ω), defined as the angle between the direction of the regional bathymetric slope and the average direction of the channel. These parameters illustrate the influence of the regional-scale basin geometry and the superimposed tectonic-influenced seabed patterns, on channel development. We found hyperbolic relationships between incident angle (α) and diversion angle (Ω) in which channels flowing approximately parallel (α ≈ 0°) to tectonic folds are (obviously) not diverted; channels nearly orthogonal (α ≈ 90°) to obstacles, crosscut them right through and, again, not diverted much. In contrast, channels with a general direction diagonal to the obstacles (α ≈ 40°), are diverted by ten degrees (Ω ≈ 10°). This diversion accumulates along large distances and significantly influences the regional development of channels around the NDSF. Noteworthy, this phenomenon of channel diversion, indirectly deteriorate normal slope-sinuosity relationships known from terrestrial studies. In light of these findings, we suggest that these new parameters can be applied to other basins, where submarine channels interact with seabed obstacles.

Active low-angle normal faults in the deep water Santos Basin, offshore Brazil: a geomechanical analogy between salt tectonics and crustal deformation

Abstract We provide a structural analysis and propose a geomechanical model for an extensional fault system observed in seismic data from the deep water Santos Basin, offshore Brazil. The system includes low-angle normal faults (LANFs) associated with underlying listric faults and salt rollers. The LANFs cut through shallow sedimentary successions up to the seafloor, reaching displacements of 1 km during the last 5 myr, at least one order of magnitude larger than the displacement of the coeval Andersonian high-angle normal faults. We suggest that the LANFs were produced by clockwise rotation of the stress field caused by the presence of salt intruded along the footwall of the main listric normal faults of the extensional system. Numerical models indicate that the proposed geomechanical model can be attained with strain weakening and widespread plastic yielding along the main normal faults. We suggest that the studied LANFs are related to salt flow in the same way that the LANFs described in regional extensional provinces are related to flow of either the lower crust or the mantle. The rheological analogy is remarkable and therefore we suggest that this work will contribute to the solution of the LANFs paradox.

Unreciprocated sedimentation along a mud-dominated continental margin, Gulf of Mexico, U.S.A.: Implications for sequence stratigraphy in muddy settings devoid of depositional sequences

According to widely accepted sequence stratigraphic and fill-and-spill models, sedimentary cyclicity along continental margins is modulated by relative sea-level change, whereas smaller-scale intraslope accommodation is controlled by the filling of pre-existing bathymetric depressions. Although these concepts are presumed to apply to shelf-to-slope settings regardless of grain size, we have tested both hypotheses in the mud-prone lower Pliocene to Holocene of offshore Louisiana, Gulf of Mexico, and reach different conclusions. We determine that over the last ∼3.7 Myr, differential accumulation and accompanying salt tectonism dislocated the fine-grained shelf and slope, prevented the development of sedimentary reciprocity at 10–100 kyr time scales, and inhibited fill-and-spill accumulation. We show that only 3% of “lowstand” mass transport deposits can be correlated to low stands in relative sea level, whereas approximately 30% of the deposits are related to transgressions and high stands; the remaining 67% are poorly constrained. Mass transport deposits also show no clear evidence of up-section increases in bypass. Based on our results, we conclude that the dominant control on stratigraphic architecture in offshore Louisiana was not relative sea-level change or patterns of accommodation, but rather differential deposition and concomitant salt-related subsidence, which controlled the distribution of facies, timing and location of mass transport deposits, and rates of sediment accumulation. Our conclusions highlight the importance of sediment supply and local tectonism, and caution against a priori use of conventional sequence stratigraphic and fill-and-spill models to decipher the stratigraphic evolution of actively-deforming mud-dominated continental margins. We therefore recommend treating stratigraphic models as testable hypotheses, rather than as methods of interpretation, particularly in fine-grained areas devoid of well-developed depositional sequences and in settings lacking intraslope ponded-to-perched accumulations.

Jurassic rifting to post‐rift subsidence analysis in the Central High Atlas and its relation to salt diapirism

AbstractThe subsidence evolution of the Tethyan Moroccan Atlas Basin, presently inverted as the Central High Atlas, is characterized by an Early Jurassic rifting episode, synchronous with salt diapirism of the Triassic evaporite‐bearing rocks. Two contrasting regions of the rift basin – with and without salt diapirism – are examined to assess the effect of salt tectonics in the evolution of subsidence patterns and stratigraphy. The Djebel Bou Dahar platform to basin system, located in the southern margin of the Atlas Basin, shows a Lower Jurassic record of normal faulting and lacks any evidence of salt diapirism. In contrast, the Tazoult ridge and adjacent Amezraï basin, located in the centre of the Atlas Basin, reveals spectacular Early Jurassic diapirism. In addition, we analyse alternative Central High Atlas post‐Middle Jurassic geohistories based on new thermal and burial models (GENEX® 4.0.3 software), constrained by new vitrinite reflectance data from the Amezraï basin. The comparison of the new subsidence curves from the studied areas with published subsidence curves from the Moroccan Atlas, the Saharan Atlas (Algeria) and Tunisian Atlas show that fast subsidence peaks were diachronous along the strike, being younger towards the east from Early–Middle Jurassic to Late Cretaceous. This analysis also evidences a close relationship between these high subsidence rate episodes and salt diapirism.

Gravity modelling to assess salt tectonics in the western end of the South Pyrenean Central Unit

A new gravity survey (almost 900 new sites and more than 4000 sites from previous studies) reveals the existence of Pyrenean-oriented long-wavelength residual gravity anomalies ranging from −18 to 28 mGal, associated with frontal structures, and short-wavelength gravity lows (up to −16 mGal) distributed along an oblique zone in the western sector of the South Pyrenean Central Unit of the Spanish Pyrenees. These clearly correlate with the occurrence of several bodies and diapirs of Triassic evaporites and mudstones. The subsurface geometry and regional location of these evaporitic bodies is derived from four 2.5D gravity models that tie data from field geology, rock density analysis, seismic reflection profiles, boreholes and our gravity survey. Gravity-constrained cross-sections allow us to distinguish between vertically well-developed and poorly developed diapirs that belong to the Sierras Marginales Unit, and to characterize the relationships between thrusting and diapirism during Eocene–Oligocene time.

Interpretation of gravity and magnetic data with geological constraints for 3D structure of the Thuringian Basin, Germany

We apply a novel method for the separation of potential field sources and their 3D inversion at the regional study area of Thuringian Basin in central Germany. The gravity and magnetic data are separated into long, medium and short wavelengths and then inverted separately. The main goal is to study uniqueness of the solution and its stability in all numerical steps of the interpretation process and to demonstrate, how geological constraints can diminish the degree of non-uniqueness by the interpretation of the gravity and magnetic anomalies. Our numerical experiments with medium wavelengths reveal that if we explain negative anomalies with the topography of near-surface layers, the obtained solution is not supported by borehole data. These negative anomalies are thus explained by restricted bodies (granitic intrusions) at the depths from 4 down to 10km. These bodies are located above a density interface with topography at the depth of approximately 10km. The 3D inversion of magnetic data (at short wavelengths) allows investigating a detailed structure of the upper boundary of the crystalline basement: two uplifts in the depths between 2.0 and 0.7km are found. By using the residual negative anomalies we further study the salt tectonics, showing that the geometry of a salt pillow with a thickness of approximately 200m closely agrees with borehole data.

The use of seismic attributes to enhance imaging of salt structures in the Barents Sea

Seismic imaging and interpretation of salt bodies and surrounding minibasins have been always a challenge in areas with salt tectonics. Complex steeply dipping flanks along salt structures and adjacent stratigraphy remain problematic areas owing to the complex ray paths of the seismic waves travelling through the salt. This fact avoids a clear image of the salt-sediment interface, increasing the uncertainty of the interpretation of salt structures and surrounding minibasins. In this study, a 3D seismic cube located in the Nordkapp Basin (western Barents Sea), has been used to develop suitable interpretation attribute workflows which will allow the interpreter to: (1) improve interpretation and mapping techniques of salt bodies, salt-related structural elements, and adjacent minibasin stratigraphy; and (2) obtain a better understanding of salt kinematics, trapping geometry, and reservoir distribution adjacent to salt structures. The mapping and characterization of salt structures have been carried out using the attribute dip-illumination and variance. In contrast, chaos and anttrack highlight the presence of radial faults and have been used to better understand diapir kinematics. The surrounding minibasins have been interpreted using frequency filters and cosine of instantaneous phase, which have been essential to identify the different types of composite halokinetic sequences and the presence of sedimentary wedges which might act as potential reservoirs.