Autochthonous Salt Research Articles

Petrophysical Analyses for Supporting the Search for a Claystone-Hosted Nuclear Repository

The Bundesgesellschaft für Endlagerung mbH (BGE; “Federal Company for Radioactive Waste Disposal”) is responsible for identifying a repository site in Germany for the storage of high-level radioactive waste, ensuring the best possible safety for at least 1 million years (StandAG, 2017). The three-phase site selection process is currently in Step 2 of its Phase I. Around 90 potentially suitable sub-areas are evaluated via individual representative preliminary safety assessments. These sub-areas cover all types of potential containment rock: allochthonous and autochthonous rock salt, claystone, and crystalline rock. Such a comprehensive assessment is challenging for a host rock that covers vast areas like claystone or autochthonous salt. One of the steps towards estimating parameters for a site search process requires the analysis of numerous logs from the areas of interest and their vicinity. Unfortunately, computing these parameters is a much more complex problem than it seems. While gamma ray logs have been the petrophysicist’s workhorse for decades, they allow no quantification of clay properties. Nuclear magnetic resonance and neutron activation logs yield critical information but are scarce in old wells. Decades of research have given good algorithms for defining wet clay porosity from resistivity logs, but inverting those for porosity and tortuosity remains challenging. Another log-derived parameter is a formation’s homogeneity, which quantifies the jaggedness of a logging curve. A vertical variogram or variable filter technique can accomplish this.

Dynamo-thermal subsidence and sag–salt section deposition as magma-rich rifted margins move off plume centres along incipient lines of break-up

Magma-rich rifting generally occurs over mantle plumes or rising convection cells; magma-poor rifting does not. Plumes produce up to 2 km dynamic elevation, outcompeting synrift subsidence in active and ancient magma-rich rifts, as evidenced by top-rift unconformities. Magma-rich margins show anomalously fast and large post-tectonic subsidence, evidenced by thick, short-lived little-faulted sag–salt sections. Syndepositional thermal subsidence cannot be responsible alone, and lack of faulting argues against tectonic subsidence. We argue that dissipation of dynamic topography (dynamic subsidence) is the additional component with thermal subsidence. We depict the formation of the magma-rich central South Atlantic and Gulf of Mexico salt-bearing margins relative to former plumes, showing that sag–salt deposition occurs as the margins migrate off plume flanks while supra-plume rifting continues. However, the pre-sag–salt basement must first be extended or eroded during dynamic elevation to fall below its pre-uplift elevation. ‘Dynamo-thermal subsidence’ creates ‘dynamo-thermal accommodation’, the sum and combined result of dynamic plus thermal subsidence, respectively. It is fast enough to negate the need for deep, sub-sea-level, subaerial depressions for accumulation of thick sag–salt sections. Also, differences in dynamic elevation during salt deposition explain differences in autochthonous salt thickness. Supplementary material : Supplementary figures, seismic data, structural restorations, a compilation of igneous activity and an extended figure caption are available at https://doi.org/10.6084/m9.figshare.c.5908896

Salt tectonics of the offshore Tarfaya Basin, Moroccan Atlantic margin

Salt tectonics play a critical role on passive margins evolution controlling aspects like structural style, subsidence patterns and thermal history, amongst others. The salt-bearing Atlantic passive margin of Morocco hosts one of the oldest stratigraphic records documenting the opening history of the Central Atlantic. However, the available seismic data is scarce and some offshore basins are still poorly studied, particularly in southern Morocco. Through the interpretation of an unpublished 2D/3D seismic dataset from the offshore Tarfaya Basin (SW Morocco), this study aims to highlight the key events that conditioned the evolution of this salt-bearing basin. From proximal to distal regions, the structural style of the basin is characterized by expulsion rollovers and salt-cored anticlines delimited by primary welded surfaces, evolving to buried salt sheets surrounded by thick minibasins and finally, diapirs actively deforming the modern seabed. From Late Triassic to Early Jurassic times, salt was deposited with a basinward thickening wedge-shaped geometry on a narrow trough developed over thinned continental crust. During the Jurassic, sedimentation and associated salt withdrawal triggered early salt deformation. Gravity gliding is a common process in salt-bearing passive margins that requires an originally continuous autochthonous salt layer with a minimum slope angle and longitude to thickness ratio of the overburden. However, in the Tarfaya Basin, the narrow geometry of the salt-bearing depocenter hampered this process. Early salt tectonics was probably triggered by slope progradation during the Early Jurassic. During the Early Cretaceous, the progradation of the Tan-Tan Delta promoted a continued basinward expulsion of salt, the development of a local salt-detached gravitational system and the proximal extrusion of salt sheets. Finally, from Late Cretaceous to the Present-day, shortening related to the convergence between Africa and Eurasia resulted in thick-skin inversion and the rejuvenation of precursor salt structures.

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.

The tectono-magmatic and subsidence evolution during lithospheric breakup in a salt-rich rifted margin: Insights from a 3D seismic survey from southern Gabon

In this study, we present an interpretation of a 3D depth-migrated dataset imaging the Ocean Continent-Transition (OCT) of the southern Gabon margin. Located in an area where salt is not strongly overflowing the OCT, it allows the geometrical relationships of salt deposition and its relative timing to breakup to be determined. The OCT is formed by extensional propagators floored by exhumed mantle limited by either fault-bounded upper-crustal or pre-salt sediments and/or magmatic additions. Based on our interpretations, we conclude that the pre-salt sequence was deposited over a wide area spanning crustal thinning to the onset of hyperextension and exhumation. The relative timing of salt deposition is constrained by the observations that detachment faults break away in the pre-salt sequence and that some of these faults also strongly offset the base salt and are linked to the creation of accommodation space in the salt. This is direct proof that salt was deposited during mantle exhumation. Salt deposition is interpreted to have occurred at variable depths. Evaporites formed approximately 1 km below sea level in the north, above a 4–6 km thick pre-salt sequence, and as deep as 2 km in the south. In this area, salt is interpreted to directly overlie exhumed mantle, but whether it was deposited on mantle or juxtaposed during exhumation remains more debatable. Space created in the faulted outer trough was filled partly by autochthonous salt and possibly also by downslope flow. Rifting continued after the cessation of salt deposition, followed by magmatic additions in the outer high and ultimately the onset of seafloor spreading and the formation of Penrose oceanic crust.

Salt tectonics in a double salt‐source layer setting (Eastern Persian Gulf, Iran): Insights from interpretation of seismic profiles and sequential cross‐section restoration

AbstractSalt tectonics in the Eastern Persian Gulf (Iran) is linked to a unique salt‐bearing system involving two overlapping ‘autochthonous’ mobile source layers, the Ediacaran–Early Cambrian Hormuz Salt and the Late Oligocene–Early Miocene Fars Salt. Interpretations of reflection seismic profiles and sequential cross‐section restorations are presented to decipher the evolution of salt structures from the two source layers and their kinematic interaction on the style of salt flow. Seismic interpretations illustrate that the Hormuz and Fars salts started flowing in the Early Palaeozoic (likely Cambrian) and Early Miocene, respectively, shortly after their deposition. Differential sedimentary loading (downbuilding) and subsalt basement faults initiated and localized the flow of the Hormuz Salt and the related salt structures. The resultant diapirs grew by passive diapirism until Late Cretaceous, whereas the pillows became inactive during the Mesozoic after a progressive decline of growth in the Late Palaeozoic. The diapirs and pillows were then subjected to a Palaeocene–Eocene contractional deformation event, which squeezed the diapirs. The consequence was significant salt extrusion, leading to the development of allochthonous salt sheets and wings. Subsequent rise of the Hormuz Salt occurred in wider salt stocks and secondary salt walls by coeval passive diapirism and tectonic shortening since Late Oligocene. Evacuation and diapirism of the Fars Salt was driven mainly by differential sedimentary loading in annular and elongate minibasins overlying the salt and locally by downslope gliding around pre‐existing stocks of the Hormuz Salt. At earlier stages, the Fars Salt flowed not only towards the pre‐existing Hormuz stocks but also away from them to initiate ring‐like salt walls and anticlines around some of the stocks. Subsequently, once primary welds developed around these stocks, the Fars Salt flowed outwards to source the peripheral salt walls. Our results reveal that evolving pre‐existing salt structures from an older source layer have triggered the flow of a younger salt layer and controlled the resulting salt structures. This interaction complicates the flow direction of the younger salt layer, the geometry and spatial distribution of its structures, as well as minibasin depocentre migration through time. Even though dealing with a unique case of two ‘autochthonous’ mobile salt layers, this work may also provide constraints on our understanding of the kinematics of salt flow and diapirism in other salt basins having significant ‘allochthonous’ salt that is coevally affected by deformation of the deeper autochthonous salt layer and related structures.

Diapir kinematics in a multi-layer salt system from the eastern Persian Gulf

The eastern Persian Gulf hosts several salt structures sourced from two different evaporite layers, the Neoproterozoic-Cambrian Hormuz Salt and the Oligocene-Miocene Fars Salt. Based on seismic interpretation, we discuss the kinematics and dynamics of this autochthonous multi-layer salt system for three case studies: the Tunb and Hengam diapirs, and the Taftan salt anticline. Halokinetic sequences related to Hormuz salt evacuation suggest three main stages of evolution: i) onset of Hormuz Salt diapirism (Paleozoic), ii) diapir squeezing, and emplacement of allochthonous Hormuz Salt (Late Cretaceous to Oligocene-Miocene), and iii) development of secondary and tertiary salt welds (Upper Miocene). Since Fars Salt structures are absent in areas devoid of allochthonous Hormuz Salt, our structural model interprets that the driving mechanism for Fars Salt diapirism is directly related to the emplacement of allochthonous Hormuz Salt during the Zagros-Oman contractional event. The related halokinetic sequences suggest that the Fars Salt diapir kinematic style depends on to the timing of Hormuz Salt extrusion with regards to Fars Salt deposition. Our results provide new insight into the interaction between salt structures sourced from two distinct autochthonous salt layers.

Salt thickness and composition influence rift structural style, northern North Sea, offshore Norway

Abstract“Salt” giants are typically halite‐dominated, although they invariably contain other evaporite (e.g. anhydrite, bittern salts) and non‐evaporite (e.g. carbonate, clastic) rocks. Rheological differences between these rocks mean they impact or respond to rift‐related, upper crustal deformation in different ways. Our understanding of basin‐scale lithology variations in ancient salt giants, what controls this and how this impacts later rift‐related deformation, is poor, principally due to a lack of subsurface datasets of sufficiently regional extent. Here we use 2D seismic reflection and borehole data from offshore Norway to map compositional variations within the Zechstein Supergroup (ZSG) (Lopingian), relating this to the structural styles developed during Middle Jurassic‐to‐Early Cretaceous rifting. Based on the proportion of halite, we identify and map four intrasalt depositional zones (sensu Clark et al., Journal of the Geological Society, 1998, 155, 663) offshore Norway. We show that, at the basin margins, the ZSG is carbonate‐dominated, whereas towards the basin centre, it becomes increasingly halite‐dominated, a trend observed in the UK sector of the North Sea Basin and in other ancient salt giants. However, we also document abrupt, large magnitude compositional and thickness variations adjacent to large, intra‐basin normal faults; for example, thin, carbonate‐dominated successions occur on fault‐bounded footwall highs, whereas thick, halite‐dominated successions occur only a few kilometres away in adjacent depocentres. It is presently unclear if this variability reflects variations in syn‐depositional relief related to flooding of an underfilled presalt (Early Permian) rift or syn‐depositional (Lopingian) rift‐related faulting. Irrespective of the underlying controls, variations in salt composition and thickness influenced the Middle Jurassic‐to‐Early Cretaceous rift structural style, with diapirism characterising hangingwall basins where autochthonous salt was thick and halite‐rich and salt‐detached normal faulting occurring on the basin margins and on intra‐basin structural highs where the salt was too thin and/or halite‐poor to undergo diapirism. This variability is currently not captured by existing tectono‐stratigraphic models largely based on observations from salt‐free rifts and, we argue, mapping of suprasalt structural styles may provide insights into salt composition and thickness in areas where boreholes are lacking or seismic imaging is poor.

Integrating kinematic restoration and forward finite element simulations to constrain the evolution of salt diapirism and overburden deformation in evaporite basins

In evaporite basins, salt deformation including inflation, diapirism, and salt canopy emplacement is inherently non-coaxial and ductile and thus it presents challenges for two-dimensional kinematic restorations that rely on line-length and area-balancing assumptions. Also, because salt flow and the resulting deformation of adjacent cover units can be driven by temporally and spatially transient salt pressure gradients, kinematic restorations are generally unable to predict the magnitude and distribution of subseismic deformation that results from a particular structural scenario. Here, we use a case study from the Atwater fold belt, Gulf of Mexico, to test a new workflow that involves comparison of kinematic restoration models with forward numerical (finite-element) models of structural evolution to examine the physical validity of solutions derived from the kinematic restorations and to determine the nature and spatial distribution of the resultant subseismic deformation. In the Atwater fold belt, which represents the downdip portion of a linked updip (landward) extensional-downdip (seaward) contractional system, our kinematic restorations indicated that major anticlines likely result from early short wavelength folding followed by (1) inflation of the autochthonous salt to drive failure of the overburden, (2) collapse of the updip limb of the major salt-cored anticline as the salt evacuates updip, and (3) rapid emplacement of the allochthonous salt canopy. In margin scale finite element models of the same system, progradation of the sedimentary wedge above the weak salt substrate leads to basinward migration of the salt and produces inflation of the major downdip salt-cored folds, as predicted by the kinematic model. However, in relatively strong overburden materials (equivalent friction angle = 32°), such salt flow only sustains inflation of the anticlines and is unable to reproduce the interpreted collapse of the anticlinal backlimb or emplacement of the salt canopy. Alternate model runs that include a significant reduction in material strength (equivalent friction angle = 18°) allow salt in the anticlinal crest to drive both reactive and active diapirism and ultimately lead to rapid emplacement of allochthonous canopies. In all of these models, diapirism drives substantial seismically-resolvable and subseismic deformation of wall rocks. Additionally, these models clearly show that the stress field, and particularly the K value (horizontal-vertical stress ratio) of the sediments adjacent to salt structures used for estimating stress magnitudes for drilling predictions, is fundamentally dependent on what point along the evolutionary path from autochthonous salt, to diapir, to salt sheet, that each structure resides. These results highlight the need to test complex kinematic restorations with physics-based techniques. Additionally, they demonstrate that integrating kinematic restorations with these finite element solutions can substantially increase our ability to predict both subseismic reservoir damage in sediments adjacent to salt structures and the K values used for forecasting drilling conditions, particularly in young basins filled with poorly consolidated sediments.

Estimated location of the seafloor sources of marine natural oil seeps from sea surface outbreaks: A new "source path procedure" applied to the northern Gulf of Mexico

Marine oil reservoirs are generally characterized on the sea surface by the presence of natural oil seeps (Sea Surface outbreaks - hereafter SSO). The latter are easily evidenced with Synthetic Aperture Radar (SAR) images because of the dampening effect that oil has on the capillary and associated small gravity waves (Bragg waves). The sea surface outbreaks of oil seeps are offset from their source on the seabed (seafloor sources - SFS) by hundreds meters or even kilometres. This displacement all along the sea water column is a function of the upward velocity of the oil droplet size, and the presence of lateral marine currents. This paper proposes a Vertical Drift Model (VDM) that combines both SAR images to get the SSO and the hydrodynamic model (HYCOM) function of the oil droplet size to estimate the SFS. After oil seeps detection from SAR images, the VDM proceeds to a regression in time and space based on the upward velocity of the oil, based on Stokes law, and the hydrodynamic conditions (HYCOM) to estimate the location of the seep source on the seafloor. The upward velocity depends strongly on the unknown droplet size. We propose herein a new VDM method named "sources paths" that allows to estimate the oil seeps sources on the seafloor without a priori knowledge of the oil droplet size by finding, for each oil seep, the seafloor sources corresponding to different diameters. We call "sources path" the line that joins the seafloor sources for an oil seep. The seafloor sources ought to be at the intersection of the maximum sources paths. The methodology has been applied to the northern Gulf of Mexico where the locations of many prolific oil seep sites are well known. A first validation of the source path procedure is that the obtained SFSs correspond to the seafloor sources of oil droplets having the same diameter and seeped at different times. Another validation has been performed through the comparison of SFS locations and those of the outcropping shallow salt. This comparison shows a good correlation and suggests that the oil seeps may be situated above the allochtonous toward autochthonous salt connections.

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.

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.

Numerical modelling study of mechanisms of mid‐basin salt canopy evolution and their potential applications to the Northwestern Gulf of Mexico

AbstractSalt canopies are present in many of the worldwide large salt basins and are key players in the basins' structural evolution as well as in the development of associated hydrocarbon systems. This study employs 2D finite‐element models which incorporate the dynamical interaction of viscous salt and frictional‐plastic sediments in a gravity‐spreading system. We investigate the general emplacement of salt canopies that form in the centre of a large, autochthonous salt basin. This is motivated by the potential application to a mid‐basin canopy in the NW Gulf of Mexico (GoM) that developed in the late Eocene. Three different salt expulsion and canopy formation concepts that have been proposed in the salt‐tectonic literature for the GoM are tested. Two of these mechanisms require pre‐existing diapirs as precursory structures. We include their evolution in the models to assure a continuous, smooth evolution of the salt‐sediment system. The most efficient canopy formation takes place under the squeezed diapir mechanism. Here, shortening of a region containing pre‐existing diapirs is absorbed by the salt (the weakest part of the system), which is then expelled onto the seafloor. The expulsion rollover mechanism, which evacuates salt from beneath evolving rollover structures and expels it both laterally and to the surface, was not successfully captured by the numerical models. No rollover structures developed and only minor amounts of allochthonous salt emerged to the seafloor. The breached anticline mechanism requires substantial shortening of salt‐cored, pre‐weakened folds such that the salt breaches the anticlines and is expelled to the seafloor. The amount of shortening may be too large to occur in the central part of a salt basin, but may explain canopy evolution closer to the distal end of the allochthonous salt. When applying the different concepts to the northwestern GoM, none of the models adequately describes the entire system, yet the squeezed diapir mechanism captures most structural features of the Eocene paleocanopy. It is nevertheless possible that different mechanisms have acted in combination or sequentially in the northwestern GoM.

Halokinetic sequences in carbonate systems: An example from the Middle Albian Bakio Breccias Formation (Basque Country, Spain)

In diapir flanks, unconformity-bounded sedimentary packages associated with gravity-driven deposits, controlled by the ratio between the rates of sediment accumulation and diapir growth can be interpreted in the context of halokinetic sequences. The Bakio Breccias Formation (Basque Country, Spain) corresponds to redeposited carbonate deposit that developed in response to the Bakio diapir growth during the Middle Albian. These deposits provide on of the rare documented example of carbonate-dominated halokinetic sequences. The Bakio Breccias Formation consists of an alternation of clast- and matrix-supported breccias, calcirudite, calcarenite and marl, deposited along the flanks of the diapir. The description and the analysis of the Bakio Breccias Formation lead to a new model for carbonate-dominated halokinetic sequences. These sequences differ from their siliciclastic counterpart because sediment accumulation rate is controlled by carbonate platform growth on the topographic relief top of the diapirs, while sediments are preferentially deposited in the mini-basins adjacent of the diapirs, in siliciclastic settings. During transgressive system tract, carbonate platform are able to keep up with the sea level rise and to aggrade on top of the diapirs, forming thick and resistant roof, which is assumed to limit the diapir growth and thus to favour the development of halokinetic sequences with low angle unconformities (wedge halokinetic sequences). During late highstand system tract deposition (and lowstand system tract if present), platform progradation results in high sediment accumulation in the adjacent depocenters, loading the autochthonous salt layer and promote diapir growth and creation of topographic relief. In addition, if the diapir roof reaches emersion, karstification of the carbonate platform top may also favour roof destruction and diapir growth. Depending on the thickness of the roof developed previously and the amplitude of the sea level fall, the halokinetic sequences with the emersion and the karstification of the carbonate platform may display high angle unconformities (hook halokinetic sequences). Furthermore, gravity-driven deposits are assumed to be more common in carbonate-dominated halokinetic sequences, compared to their siliciclastic counterparts, since carbonate platform aggradation creates steep slopes on the diapir margins, leading to the partial collapse of the margin, even when limited diapir growth occurs. The carbonate-dominated halokinetic sequence model proposed here is an important tool for the prediction of potential reservoir distribution, seal and hydrocarbon migration in flanks of salt diapirs where carbonate platform developed.

Enigmatic structures within salt walls of the Santos Basin—Part 1: Geometry and kinematics from 3D seismic reflection and well data

Understanding intrasalt structure may elucidate the fundamental kinematics and, ultimately, the mechanics of diapir growth. However, there have been relatively few studies of the internal structure of salt diapirs outside the mining industry because their cores are only partly exposed in the field and poorly imaged on seismic reflection data. This study uses 3D seismic reflection and borehole data from the São Paulo Plateau, Santos Basin, offshore Brazil to document the variability in intrasalt structural style in natural salt diapirs. We document a range of intrasalt structures that record: (i) initial diapir rise; (ii) rise of lower mobile halite through an arched and thinned roof of denser, layered evaporites, and emplacement of an intrasalt sheet or canopy; (iii) formation of synclinal flaps kinematically linked to emplacement of the intrasalt allochthonous bodies; and (iv) diapir squeezing. Most salt walls contain simple internal anticlines. Only a few salt walls contain allochthonous bodies and breakout-related flaps. The latter occur in an area having a density inversion within the autochthonous salt layer, such that upper, anhydrite-rich, layered evaporites are denser than lower, more halite-rich evaporites. We thus interpret that most diapirs rose through simple fold amplification of internal salt stratigraphy but that locally, where a density inversion existed in the autochthonous salt, Rayleigh–Taylor overturn within the growing diapir resulted in the ascent of less dense evaporites into the diapir crest by breaching of the internal anticline. This resulted in the formation of steep salt-ascension zones or feeders and the emplacement of high-level intrasalt allocthonous sheets underlain by breakout-related flaps. Although regional shortening undoubtedly occurred on the São Paulo Plateau during the Late Cretaceous, we suggest this was only partly responsible for the complex intrasalt deformation. We suggest that, although based on the Santos Basin, our kinematic model may be more generally applicable to other salt-bearing sedimentary basins.

Geological and geophysical expression of a primary salt weld: An example from the Santos Basin, Brazil

Primary salt welds form at the base of minibasins in response to complete evacuation of autochthonous salt. Analytical and numerical models suggest it is difficult to completely remove salt from a weld by viscous flow alone, which is especially true in multilayered evaporites, within which flow is likely heterogeneous due to lithologically controlled viscosity variations. Welds are important in the hydrocarbon industry because they may provide a hydrodynamic seal and trap hydrocarbons, or may allow transmission of fluids from source to reservoir rocks. Few papers document the subsurface expression of welds, principally because they have not been penetrated by wells or because the associated data are proprietary. We use 3D seismic and borehole data from the Santos Basin, offshore Brazil to characterize the geological and geophysical expression of a primary weld associated with flow of Aptian salt. The seismic data that we evaluated suggested that, locally, presalt and postsalt rocks are in contact at the base of an Upper Cretaceous minibasin, implying that several apparent welds, separated by low-relief salt pillows, are present. However, borehole data indicated that 22 m of anhydrite, carbonate, and sandstone are present in one of the welds, indicating that this and other welds may be incomplete. We find that seismic data may be unable to discriminate between a complete and incomplete weld, and we suggested that, during the subsurface analysis of welds, the term apparent weld is used until borehole data unequivocally proves the absence of salt. Furthermore, we speculate that preferential expulsion of halite and potash salt from the autochthonous layer during viscous flow and welding resulted in the formation of an incomplete weld, which, when compared with the initial autochthonous layer, is volumetrically enriched in nonevaporite lithologies and relatively viscous evaporite lithologies (anhydrite). The composition and stratigraphy of the autochthonous layer may thus dictate weld thickness and seal potential.

The Messinian Salinity Crisis: New seismic evidence in the West-Sardinian Margin and Eastern Sardo-Provençal basin (West Mediterranean Sea)

Abstract We processed and interpreted a newly acquired geophysical dataset (WS10 project) integrated with vintage seismic profiles to study the evolution of the Sardinian passive margin of the Sardo-Provencal basin. The most prominent features of the area of study are a consequence of the Messinian Salinity Crisis (MSC) which produced a thick salt layer in the deep basin. Complex halokinetic tectonics deformed the overlying Messinian Upper Unit (UU) and the Pliocene–Quaternary (PQ) sediments, sometimes also bending the sea bottom. A thinner autochthonous salt in the UU suggests at least one further low stand during the late MSC. The Messinian Erosional Surface (MES) separates the erosional truncation of the pre-Messinian sedimentary sequence from the overlaying Plio-Pleistocene sequence with a clear angular discordance, which almost completely masks possible earlier erosional events. The MES is present across the middle and upper slopes of the Sardinian margin and can be easily recognized down to the lower slope, where it interfingers with the Messinian evaporites which gradually thicken basinward. We propose a correlation between the main erosional surface of the lower slope and the salt layer recognized in the UU as a consequence of a falling stage during the last part of the MSC. The seismic profiles now available along the West Sardinian margin provide novel and crucial information to correlate the Messinian depositional units in the deep basin with those present in the lower continental slope and with the erosional surface of the middle and upper slopes. Our data and interpretation highlight the main units of the Messinian trilogy and further internal sequences that are probable related to cyclic fluctuations during their deposition.

Exposed evaporite diapirs and minibasins above a canopy in central Sverdrup Basin, Axel Heiberg Island, Arctic Canada

AbstractAxel Heiberg Island (Arctic Archipelago, northern Nunavut, Canada) contains the thickest Mesozoic section in Sverdrup Basin (11 km). The ca. 370‐km‐long island is second only to Iran in its concentration of exposed evaporite diapirs. Forty‐six diapirs of Carboniferous evaporites and associated minibasins are excellently exposed on the island. Regional anticlines, which formed during Paleogene Eurekan orogeny, trend roughly north on a regular ca. 20‐km wavelength and probably detach on autochthonous Carboniferous Otto Fiord Formation evaporites comprising halite overlain by thick anhydrite. In contrast, a 60‐km‐wide area, known as the wall‐and‐basin structure (WABS) province, has bimodal fold trends and irregular (<10 km) wavelengths. Here, crooked, narrow diapirs of superficially gypsified anhydrite crop out in tight anticline cores, which are separated by wider synclinal minibasins. We interpret the WABS province to detach on a shallow, partly exposed canopy of coalesced allochthonous evaporite sheets. Surrounding strata record a salt‐tectonic history spanning the Late Triassic (Norian) to the Paleogene. Stratigraphic thinning against diapirs and spectacular angular unconformities indicate mild regional shortening in which diapiric roof strata were bulged up and flanking strata steepened. This bulging culminated in the Hauterivian, when diapiric evaporites broke out and coalesced to form a canopy. As the inferred canopy was buried, it yielded second‐generation diapirs, which rose between minibasins subsiding into the canopy. Consistent high level emplacement suggests that all exposed diapirs inside the WABS area rose from the canopy. In contrast, diapirs along the WABS margins were sourced in autochthonous salt as first‐generation diapirs. Apart from the large diapir‐flanking unconformities, Jurassic‐Cretaceous depositional evidence of salt tectonics also includes submarine debris flows and boulder conglomerates shed from at least three emergent diapirs. Extreme local relief, tectonic slide blocks, steep talus fans and subaerial debris flows suggest that many WABS diapirs continue to rise today. The Axel Heiberg canopy is one of only three known exposed evaporite canopies, each inferred or known at a different structural level: above the canopy (Axel Heiberg), through the canopy (Great Kavir) and beneath a possible canopy (Sivas).

Factors controlling early stage salt tectonics at rifted continental margins and their thermal consequences

AbstractWe use 2‐D thermomechanical models to investigate the early evolution of rifted margin salt tectonics in terms of the competition among margin tilt, salt flow, and sediment aggradation. Model experiments include initial geometry of the rifted margin and autochthonous salt basin, subsequent synrift and thermal subsidence, sediment and water loading, and sediment compaction. We also calculate the thermal evolution of the system to investigate the impact of the high thermal conductivity of the salt (halite). Model Set 1 demonstrates a two‐phase response to salt deposition: short‐term thermal equilibration between the salt and crust and longer‐term relaxation in which the salt basin thermal image penetrates to a depth on the order of its width. Set 2 addresses the competition among margin tilt, salt flow, and sediment aggradation. Set 3 examines other factors, the salt basin width and depth, and the rifted margin width, which potentially affect the system evolution. Set 4 shows that sawtooth subsalt topography, representing faulted basement grabens, does not strongly impede salt flow. The model results are discussed in terms of a ternary diagram with apices representing tilt, salt flow, and sedimentation rates. Characteristic styles include the following: (1) tilt and rapid salt flow draining salt to the distal basin before the sediment aggrades, (2) an equivalent system with faster aggradation that captures draining salt as diapirs between minibasins, and (3) rapid sediment aggradation in which diapir‐minibasins systems develop before the salt drains. Thermal consequences of these styles are discussed. A preliminary comparison shows that salt structures resembling these styles occur in the southwest Nova Scotian margin.