Salt Provinces Research Articles

Innovative solutions to frontier exploration in Eastern Mediterranean salt basins

The Eastern Mediterranean region is expected to hold large undiscovered natural gas resources that could address critical energy supply needs of the surrounding countries. However, exploration is challenged due to regional geologic complexities. The combination of Messinian salt structures and diapiric shales in offshore Cyprus and Egypt often causes strong attenuation of seismic energy and severe seismic imaging illumination shadows, which lead to large uncertainty regarding subsalt reservoir presence and the underlying petroleum system assessment. As part of the efforts to solve these imaging challenges, we present innovative seismic acquisition solutions to address key exploration risks such as reservoir presence. A one-sided wide-azimuth configuration consisting of one streamer vessel and one dedicated source vessel operating under the simultaneous-source mode was used to acquire data in a cost-effective way. An 8000 in3 low-frequency source was used on both vessels to provide higher energy output in the low-frequency bands of less than approximately 4 Hz. Our preliminary results show evidence of seismic image uplift due to enhanced acquisition and processing, leading to greater confidence in interpretation, which is critical for positioning wells in future drilling campaigns. This paper highlights seismic testing results undertaken to design a large exploration-scale seismic survey in the Eastern Mediterranean and early results from the execution of that program. This will help better refine optimal design criteria for future acquisition, signal processing, and imaging in order to unlock additional resources beneath complex salt provinces and identify where additional imaging needs still exist.

Salt Distribution in the South Pyrenean Central Salient: Insights From Gravity Anomalies

AbstractTriassic evaporites represent the regional décollement of the Pyrenees and form two salt provinces north and south of the South Pyrenean Central Salient (SPCS). We present an updated Bouguer and residual Bouguer anomaly map built upon the homogenization of available gravity data of the SPCS together with four new and representative cross‐sections, constrained by geological data acquired in the field, seismic, well, and gravity data (gravity forward modeling). Gravity anomaly maps and cross‐sections are used to characterize the present‐day uneven distribution of Triassic evaporites. Outcropping Triassic evaporites is not necessarily associated with an underlying evaporite accumulation and the absence of it at surface does not involves its non‐existence at depth. Northwest of the salient, a major accumulation of Triassic evaporites floors a thick syn‐orogenic Upper Cretaceous basin. South of it, Triassic rocks core salt‐detached anticlines related to the Pyrenean orogeny. Along the southernmost (and youngest) thrust sheet of the salient, diapirs, and evaporite accumulations are associated with a salt‐inflated area.

Evaluating controls on deformation patterns and styles in the salt-detached Sureste Basin, southern gulf of Mexico: Insights from physical models

The Sureste Basin forms the southern portion of the Isthmian Salt Province in the southern Gulf of Mexico. This basin is unique in this region as it is both part of a passive/divergent margin and also part of an active/convergent margin due to its proximity to Pacific margin plate tectonics. As a result it has experienced both orogenic and gravity-driven deformation resulting in a complex array of deformation styles and patterns. Our goal is to understand some of the structural patterns and impact of various driving forces in this basin using a series of regional-scale physical models. In particular the Sureste Basin illustrates five major complexities that we explore using our models.1) Sureste narrows from south to north — models with a trapezoidal shape illustrate heightened complexities on their margins where constrictional strains associated with convergent basin margins form transpressional systems.2) Entire basin has been shortened — unlike typical salt detached gravity-driven deformation where shortening is focused at the downdip margin of the salt basin, a combination of a gravity driven and tectonically induced shortening in our models produces shortening structures spanning the entire basin.3) Sureste is full of diapirs and allochthonous salt bodies — the presence of a major diapir network coupled with gravity-driven and moving-endwall induced shortening rejuvenates diapirs all the way to the distal pinchout, with diapir linkage forming complex, composite, curvilinear fold and thrust structures encompassing the basin.4) Sureste has been affected by non-parallel driving forces — in models where the shortening direction is 45° oblique to the regional dip direction this results in major transpression against the western margin of the basin, and initial transtension on the eastern margin as the salt flows down this regional dip. Structures display an asymmetric curvature in structural grain from east to west due to these non-parallel driving forces. Introducing thicker model salt and a multi-stage history to our model formed major diapir and canopy systems, a variety of thrust welds, roho extension down the regional dipslope, as well as major structural curvature, partly matching gross structural patterns seen in Sureste, and.5) Variable salt isopach in Sureste — in our final model the salt isopach thinned across the basin towards the East. The presence of this variable salt isopach controlled where major canopies and isolated salt bodies formed in the basin with these locations being a close match to those seen in Sureste.

The effects of subsalt relief on gravity-driven salt tectonics: Results from analogue modelling

Gravity-driven salt-related deformation on passive margins is commonly interpreted as kinematically-linked domains of updip extension and downdip contraction with an intermediate, undeformed zone of translation. Our study uses analogue models to show how salt flow is affected by the subsalt geometry resulting in complex deformation and the alternation of extensional and contractional domains in space and time in translational salt provinces. In our work, the analogue models incorporate subsalt relief associated with half-graben and graben that are realistic geometries along passive margins and combine the effects of gravity spreading in which deformation is driven by differential sedimentary loading, and an early-stage of gravity gliding induced by basin tilt. This approach allows for analysis of how salt flow and overburden deformation are affected by: (1) smoothly dipping subsalt relief; (2) subsalt relief characterized by half-graben with a gently landward dipping extensional fault; (3) subsalt half-graben connected laterally with a distal symmetric graben. For each of these models, we evaluate: 1) the interplay between slipping deformation corresponding to gravity gliding, gravity spreading mechanisms, and subsalt geometry; 2) the spatial and temporal distribution of salt-related structural domains; and 3) the contrasting styles of salt tectonics for different subsalt geometries. In addition, some of the experiments have been repeated considering thicker salt and pre-kinematic overburden and different sedimentation rates. We show that the structures of the subsalt relief variably resist salt flow producing flow mismatch. This variable resistance controls the orientation of the extension and contraction supra-salt structures. Salt surplus and contraction occur over landward dipping base-salt ramps, and a zone of salt depletion and subsidence limited by extensional and contractional hinges occurs over the basinward dipping base-salt ramp. Variations in width and the dip of these base-salt ramps result in stronger flow mismatches and deformation complexity. We also discuss variations in salt flow profile in response to the gravitational forces that drive deformation, allowing a good first-order approximation of viscous salt flow at a regional scale that can be directly compared to examples from various salt-bearing continental margins, particularly the relatively less understood but extremely economically important distal domains of Brazil's Campos and Santos basins.

Reconstructing the Iberian Salt‐Bearing Rifted Margin of the Southern Pyrenees: Insights From the Organyà Basin

AbstractReinterpretation of the Organyà Basin, based on new detailed field observations and subsurface data, emphasizes the key contribution of Upper Triassic evaporites in the tectono‐sedimentary evolution of the South‐Central Pyrenees. Results are integrated in a 65‐km long restored cross‐section through the Serres Marginals, Montsec and eastern Organyà salt‐related depocenters. The reconstructed part of the Jurassic–Cretaceous northern Iberian salt‐rich rifted margin shows a template characterized by inherited Permo‐Triassic basement normal faults and an initial salt thickness of 0.7 km to the south and 1.5 km to the north. The Organyà Basin is part of the South Pyrenean Diapiric Province, a large system of salt related depocenters and minibasins, that is limited to the north by the more than 120‐km long Senterada salt wall complex separating the supra‐salt and sub‐salt domains in the Southern Pyrenees. Three main stages of diapiric activity are recognized along the northern Iberian margin from Asturias to the Eastern Pyrenees: a Jurassic early salt mobilization; a latest Jurassic–middle Albian main diapiric evolution associated with rifting; and a Campanian–Miocene diapiric reactivation during basin inversion that produced salt welds and thrust welds and translated the salt province some 60 km to the south.

Structure, age, and tectonic evolution of the Gulf of Mexico

The Gulf of Mexico is an isolated oceanic basin whose nature, structure and age are not fully elucidated, mostly because seafloor spreading isochrons have not been identified in this basin so far. We compiled and processed all publicly available marine magnetic data to produce a new magnetic anomaly map of the Gulf of Mexico. This map reveals a fan-like set of intermediate-wavelength (>100 km) magnetic anomalies related to seafloor spreading. Our magnetic anomaly-based plate reconstructions (1) support a counterclockwise rotation of the Yucatán Block around a pole located NW of Cuba, (2) accommodate the fracture zone trends depicted by the gravity data, and (3) suggest that the Continent-Ocean Boundary lies immediately south of the Houston magnetic anomaly, close to the shoreline, implying that oceanic crust underlies a significant part of the Sigsbee salt province. Our attempt to identify the intermediate wavelength anomalies by comparison with filtered Geomagnetic Polarity Time Scales dates the onset of seafloor spreading before the Tithonian (>150 Ma) and its cessation at the Berriasian (∼140 Ma).

Cenozoic structural deformation between the southern Lamprea fold-belt and Salina del Bravo salt province by interacting salt and shale detachments, western Gulf of Mexico

The Lamprea fold-belt is a 300 km-long, 20-60 km-wide, deep-water, passive margin fold-belt located southeast of the Salina del Bravo salt province and trending parallel to the eastern continental margin of Mexico. The Lamprea fold-belt detaches along Eocene shale layers to form a compressional toe of the gravitationally-driven system that forms a transitional domain between the salt province to the north and the shale-detached Mexican Ridges fold-belt to the south. Area-depth strain measurements performed for ten characteristic folds across two regional seismic profiles through the Lamprea fold-belt provide insight into its timing of deformation, depth to detachment, and controls on the pre-growth and syn-growth sedimentary deposits. Miocene-age deformation across the Lamprea fold-belt coincides with a phase of renewed salt canopy emplacement driven by ongoing up-dip sediment loading and extensional deformation west of the Salina del Bravo salt province. These results are consistent with early Miocene fold growth onset controlled primarily by the advancing salt canopy and only minor influence from underlying autochthonous salt. The timing of trap formation, tectonic interaction of salt bodies with shale structures, and the mechanisms for fold-belt formation all control hydrocarbon migration from deep Mesozoic source intervals into overlying Oligocene fold-belt reservoirs.

Salt tectonics and controls on halokinetic-sequence development of an exposed deepwater diapir: The Bakio Diapir, Basque-Cantabrian Basin, Pyrenees

This work evaluates growth strata adjacent to the Bakio Diapir in the Basque Pyrenees, aiming to discuss the application of halokinetic-sequence concepts, mainly developed in shallow-water to subaerial environments, to deepwater depositional settings. This is one of the few exposed passive diapirs developed in deepwater environments, with both synkinematic carbonate and siliciclastic strata. Thus, considering that large hydrocarbon producing areas are located in salt provinces developed in deepwater environments, this study is of interest not only to researchers in salt tectonics, but also to industry geoscientists focusing on hydrocarbon exploration and production. We present a 3D analysis of this outstanding salt structure by integrating detailed geological cartography, high-resolution bathymetry, seismic, and well data. The resulting reconstruction enables us to trace the extent of the diapir offshore and decipher its evolution from its formation as a salt wall developed above the overlap of two basement-involved faults until its squeezing during the Pyrenean compression. But more significantly, it allows us to discern the roles played by bathymetry, subsidence, and sedimentation type in the configuration of halokinetic sequences developed in deepwater environments. Thus, our study shows that: 1) the geometry of halokinetic sequences is defined by the thickness of the roof edges, the dip of the salt-sediment interface in the limbs of the active drape fold, and the onlap angle of the synkinematic sediments over the salt; 2) the roof thickness is controlled not only by the ratio between salt-rise and regional sediment-accumulation rate, but also by the water depth of the diapir roof and the depositional environment that can promote vertical aggradation of a supra-diapir carbonate buildup; and 3) high surface slopes and consequent debrites are not exclusive to, and characteristic of, hook halokinetic sequences and tabular composite halokinetic sequences.

Regional thermal maturity modelling of hydrocarbons along the deep-water Yucatan margin, southern Gulf of Mexico

Abstract The stratigraphy, structure and hydrocarbon potential of the deep-water Yucatan margin remains less understood than other margins of the Gulf of Mexico (GoM), as Yucatan has not been systematically mapped and has few well penetrations. Despite the presence of widespread oil slicks along the continent–ocean boundary, the maturity and distribution of source rock kitchen areas have not been well defined. This study uses a grid of 2D post-stack depth migration (PSDM) seismic profiles covering an area of approximately 120 000 km 2 to map the structure and stratigraphy, and to model thermal maturities of three potential Mesozoic source horizons. Thermal 1D modelling of six pseudo-wells, positioned along a dip profile, was performed using estimates of lithospheric thickness and thermal properties. Integrated 3D model results indicate that the principal Tithonian-age source rock reached the oil window within the diapiric salt province during the Oligocene–Miocene. Inherent model uncertainty was addressed using a range of thermal scenarios. The result of modelling was that deeply-buried, salt-related minibasins along the outer marginal trough are low risk and the uppermost slope is high risk for maturity. Large, salt-related structural traps are directly adjacent to oil kitchens within deeply-buried minibasins. Normal faults bounding minibasins provide effective vertical migration pathways as supported by the clustering of overlying oil slicks.

Interconnection salt diapir–allochthonous salt sheet in northern Tunisia: The Lansarine–Baoula case study

Surface and subsurface data are used to illustrate the halokinetic style, structural evolution and kinematics of the Lansarine–Baouala salt structure in the Alpine domain of Tunisia. It corresponds to a salt structure showing a large salt canopy overhanging buried diapir. The salt structure is located along the main fault systems, inherited from the southern Tethyan passive margin periods. The Northern domain of Tunisia is undergoing diapirism, which has been recognized as active during the Upper Aptian and Lower Albian periods. Above the diapir apex, salt flows outward and spreads by gravity downslope. A minibasin has formed during this period of salt outward evacuation. The minibasin has a thickness practically correlated to the budget of salt evacuation toward the diapiric structure. During Cenozoic times, compressional deformation stages do not reflect active diapirism. The Lansarine–Baouala salt structure, consisting of connected salt sheets, was passively transported on the Southeast-verging major thrusts that are deeply rooted in the salt beneath the Mateur basin. The Northern Atlas of Tunisia reveals, as many others salt provinces, an interconnection of salt diapirs and significant laterally spreading allochthonous salt sheets (canopies?). In this domain, the inherited allochthonous Triassic salt sheets interlayered within recent series and connected to buried diapir have a significant role in the shallow structural complexity observed in the sedimentary cover during the subsequent contractional events.

The Impact of Pre‐Salt Rift Topography on Salt Tectonics: A Discrete‐Element Modeling Approach

AbstractGravity‐driven salt tectonics along passive margins is commonly depicted as comprising domains of updip extension and downdip contraction linked by an intermediate, broadly undeformed zone of translation. This study expands on recently published physical models using discrete‐element modeling to demonstrate how salt‐related translation over pre‐salt rift structures produce complex deformation and distribution of structural styles in translational salt provinces. Rift geometries defined by horsts and tilted fault‐blocks generate base‐salt relief affecting salt flow, diapirism and overburden deformation. Models show how flow across pairs of tilted fault‐blocks and variably‐dipping base‐salt ramps associated with pre‐salt faults and footwalls produce abrupt flux variations that result in alternation of contractional and extensional domains. Translation over tilted fault‐blocks defined by basinward‐dipping normal faults results in wide, low amplitude inflation zones above footwalls and abrupt subsidence over steep fault‐scarps, with reactive diapirs that are squeezed and extrude salt as they move over the fault. Translation over tilted‐blocks defined by landward‐dipping faults produces narrow inflation zones over steep fault‐scarps and overall greater contraction and less diapirism. As salt and cover move downdip, structures translate over different structural domains, being inverted and/or growing asymmetrically. Our models allow, for the first time, a detailed evolution of these systems in cross‐section and demonstrate the effects of variable pre‐salt relief, salt sub‐basin connectivity, width and slope of base‐salt ramps. Results are applicable to syn‐ and post‐rift salt basins; ultimately improving understanding of the effects of base‐salt relief on salt tectonics and working as a guide for interpretation of complex salt deformation.

Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin. Part II: A regional validation

Abstract The Lower Congo Basin is known to discharge a substantial volume of oil towards the sea surface, from more than one hundred seafloor seep sites distributed throughout the deep province of the Lower Congo Basin. A large geochemical coring survey confirmed the presence of oil on the seabed. The combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed. Multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets. This study aims to test the characteristics previously identified at regional-scale. The active discharge zone is limited to the distal province of the basin. It is characterised by strong compression/shortening due to the sliding of the post-salt super-sequence, resulting in numerous salt diapirs that control the location of seafloor oil seep sites. The paper describes the bathymetric, reflectivity, amplitude and sub-bottom profiler characteristics of a group of thermogenic seep sites. They correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds. Active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high-amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone. The sub-bottom profiler data shows that the hummocky mounds are connected by a network of faults to high-amplitude bodies buried under a consistent sediment thickness. Based on the definition of specific geophysical features (seafloor mounds, complex shape and irregular pockmarks, positive anomalies of seafloor amplitude, high-amplitude vertical pipes), we identified a series of potential oil seep sites at basin scale. The mounds are particularly recognizable using the seismic curvature attribute; we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35–240 per km2. They are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile. Unusual double BSRs occur over the study area; these are also a specific feature of thermogenic seep sites. Barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface. The proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite-based monitoring or gas-dominated seep sites.

Statistical analysis of forward stratigraphic models in complex salt provinces: The central Scotian Basin case study

Predicting vertical and lateral facies variations in various depositional environments remains a major challenge in the oil and gas industry because it impacts petroleum system assessments and the associated exploration-risking phase. The use of multidisciplinary constraints (geomorphology, geology, geophysics) in forward stratigraphic models sheds light on the complex interaction of local, regional, and global driving mechanisms that influence sediment transport and deposition along continuously evolving landscapes. In this paper, we develop an integrated statistical approach to examine the sensitivity of forward stratigraphic models in complex salt provinces to several parameters, including water discharge, sedimentary load, grain size and associated diffusion coefficients, and slope. This statistical analysis was applied to the Barremian–Albian sequence of the central Scotian Basin (Canada) and highlights the influence of complex salt kinematics on sediment pathway diversion and accumulation around salt domes and canopies. Forward stratigraphic modeling results point to regions of higher probability of Lower Cretaceous sandy reservoirs. Automating simulation runs significantly reduced the time required to achieve a statistically valid number of simulations and allowed the sensitivity of the model to be evaluated.

Geophysical characterisation of active thermogenic oil seeps in the salt province of the lower Congo basin part I: Detailed study of one oil-seeping site

Abstract We report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data. This paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites. The oil seep is a complex-shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre-scale seafloor mounds. A strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden. The associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone (BGHSZ) that is used as a fluid migration route towards the crest of the diapir. The combination of local depressions and seafloor amplitude anomalies linked with vertical high-amplitude pipes rooted on the BGHSZ suggests a focused fluid flow towards the seafloor. In peripheral areas, the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub-bottom profiler sections. The combination of restricted-size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas. The proximity of actively oil-supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally. The thermogenic seep site is affected by two consistent and sub-parallel reflections with negative polarity. The first is interpreted as the methane-related BGHSZ, the second could correspond to the base of a thermogenic BGHSZ produced by a mixture of heavier gas. The seafloor roughness and double BSR appear to be specific features of oil seep sites. The geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation.

Formation of dome-in-dome structures: Results from experimental studies and comparison with natural examples

Dome-in-dome structures are frequent in salt provinces and in high-grade rocks of orogens. Their origin is only poorly understood. In the present study we have modelled dome-in-dome structures, which result from constrictional (radial) in-plane shortening of a competent layer embedded in a rising less competent matrix. Both the layer and the matrix consist of non-linear viscous plasticine, which display a viscosity contrast of 5. The deforming competent layer is characterized by (1) a striking downward drag along the margins, (2) a sample-scale first-order dome, and (3) numerous second-order domes and basins straddling the first-order dome. With increasing layer thickness and strain, the amplitude, A, arc-length, L, and wavelength, λ, of the second-order domes and basins increase. However, their growth rate is significantly smaller than that of non-rising domes and basins, which result from a bidirectional flow. This restricted growth of rising domes and basins is attributed to the simultaneous growth of the first-order dome, and explains, why hair-pin type folds – typical for non-rising domes and basins - are lacking, even at higher finite strain.Similar dome-in-dome structures like those produced in the present study have been described from deeper levels of salt and gneiss domes or from foliation triple points of interfering diapirs.

Structural evolution of salt-influenced fold-and-thrust belts: A synthesis and new insights from basins containing isolated salt diapirs

Lateral shortening is expressed in unique ways in salt basins, especially if pre-shortening diapirs are present. We present an overview of previously-published and new physical models and present new 3-D conceptual models that capture the evolution of shortening structures in salt provinces dominated by precursor isolated diapirs (termed isolated-diapir provinces). In such provinces, isolated diapirs form only a minor volumetric component of a sedimentary basin, however, due to the relative weakness of rock salt and their ability to localize strain, during shortening they have a disproportionately large influence on structural development.We find three key mechanical principles govern the processes and structural styles developed during shortening of isolated-diapir provinces. First, salt diapirs shorten before surrounding sedimentary rocks due to their relative weakness, and so form salients in the thrust front during early shortening. Second, diapirs tend to nucleate folds and faults, which radiate out from the diapirs. Third, as diapir walls converge, the roof must shorten. Extrusive salt sheets are expelled through thin roofs, but thicker roofs resist piercement and so tend to undergo complex folding and faulting.As a result of these principles, the first-order controls on the structural styles expressed across a shortened isolated-diapir province are the configuration the diapir array prior to shortening, the connectivity of these diapirs prior to shortening, total strain magnitude, and diapir roof thickness. Second-order controls include the initial cross-sectional and map-view geometry of diapirs, diapir size, and diapir orientation with respect to the shortening direction.

Active Seafloor Seepage Along Hydraulic Fractures Connected to Lateral Stress From Salt‐Related Rafting: Regab Pockmark, Congo Fan

AbstractSeafloor seepage is a widespread phenomenon within salt‐influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure‐induced fractures. Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front, and its geology and biology have been studied extensively in recent years. We present high‐resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 m below seafloor to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW‐ENE, and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east.

Extensional vs. compressional deformation in the Central High Atlas salt province: A paleomagnetic approach

In this paper we address the problem of the distinction between diapiric, salt-driven and compressional structures, using the outstanding example of the Central High Atlas (Morocco). A remagnetized component carried by magnetite has been isolated in 32 new paleomagnetic sites. It is characterized by: maximum unblocking temperatures around 450 °C, syn-folding behavior and normal polarity.These 33 mean paleomagnetic directions were analyzed together with other 68 from published works around the study area to construct a robust paleomagnetic dataset along a cross-section perpendicular to the main structures. The remagnetization direction (n: 100, Dec: 332.2°, Inc: 34.5°, η: 6.2°, ξ: 2.0°, A/n: 6.427°) and the paleo-dip of beds (the attitude of the beds at the remagnetization occurrence) were calculated through small circle methods. The remagnetization can be dated as ca. 100 Ma. Because of its occurrence between the extensional and compressional periods, this remagnetization offers the possibility of restore the basin to its pre-inversion geometry.Comparison between present-day and pre-inversion structure allows discriminating three different evolutionary patterns: (i) thrusted and welded salt-walls mainly structured during the extensional stage (Ikkou ridge) with steep limbs close to the salt-wall core. (ii) Jurassic salt-walls with weaker deformation, restricted to the areas adjacent to the structure (Tadaghmamt and Timedouine); in this case, Cenozoic compression is limited to welding of the salt-walls and buttressing of the sedimentary sequences against faults. (iii) salt-rollers gently initiated during the Jurassic (Toumliline diapir), thrusted during the Cenozoic compression. Results show the importance of salt tectonics both during extension and compression, as well as the control of the compressional features by the inherited extensional structures. The performed restorations prove that paleomagnetism is a useful, independent tool to obtain palinspastic restorations and to separate, and quantify, the imprint generated during the basinal stage from the inversional features.

Regional structural setting and evolution of the Mississippi Canyon, Atwater Valley, western Lloyd Ridge, and western DeSoto Canyon protraction areas, northern deep-water Gulf of Mexico

The structural framework and evolution from the Middle Jurassic to the present of the Mississippi Canyon, Atwater Valley, western DeSoto Canyon, and western Lloyd Ridge protraction areas consist of a complex history influenced by basement fabric, multiple stages of salt movement, and gravitational gliding. A detailed tectono-stratigraphic interpretation of the study area indicates that three main stages of salt movement controlled sediment dispersal patterns and the formation and evolution of intraslope minibasins. These three stages of salt movement occurred during the Cretaceous, the Paleogene, and the Neogene. Basement structures were the primary control on initial salt kinematics, affecting gravity-driven slope deformation and resulting in a wide variety of structural styles. Basement (acoustic basement) structures (horsts, grabens, and half grabens) formed prior to the deposition of the Middle Jurassic autochthonous Louann Salt. These features are interpreted to have controlled the original thickness of the autochthonous salt layer and subsequent salt-withdrawal patterns. Mesozoic structures, such as extensional-compressional gliding systems and expulsion rollovers, formed above the autochthonous salt. Three levels of allochthonous salt systems are identified: (1) approximate top Barremian, (2) top Cretaceous, and (3) intra-Neogene (between 10 and 4 Ma). Early emplacement of two allochthonous salt layers is present in the northeastern part of the study area, whereas the Neogene allochthonous salt system extends throughout the Mississippi Canyon, western DeSoto Canyon, and northern Atwater Valley protraction areas. Salt from the autochthonous and two deep allochthonous salt layers was expelled vertically and basinward during theNeogene, feeding the younger allochthonous salt systems. The autochthonous and deep allochthonous salt layers were detachments for many of the large Neogene extensional (growth faults and turtles) and contractional (anticlines and thrust faults) structures, whereas the Neogene allochthonous salt system accommodated suprasalt minibasins associated with counterregional and roho salt systems. These three allochthonous salt layers were successively loaded by gravity-flow sediments, resulting in deep (above autochthonous or deep allochthonous salt layers) and shallow (supra-Neogene allochthonous salt) minibasin formations and local development of extensive salt welds. Northwest-southeast-oriented strike-slip structures, active during the Neogene, are present in the salt province within the study area. They are related to basinwide heterogeneities in the salt distribution and are controlled by differential basinward movement of adjacent suprasalt minibasins. 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.