Allochthonous Salt Sheets Research Articles

Impact of Salt Tectonics on Temperatures Distribution Revealed by RSCM Thermometry in the SW Alps (France)

Evaporites have a strong impact on the structural and sedimentary evolution of sedimentary basins and fold-and-thrust belts. They also have a thermal conductivity that can be more important than other sedimentary rocks and are thus able to modify the thermal history of these sedimentary basins and fold-and-thrust belts. Even though this property is known and has been of interest for the oil and gas industry, no field examples have been studied trying to decipher how salt rock impacts temperature distribution in fold-and-thrust belts. In this paper, we use the Raman Spectroscopy on Carbonaceous Material (RSCM) to track the record of the peak thermal event around three salt structures from the southern sub-Alpine fold-and-thrust belt in SE France. These three salt structures are (1) the Astoin allochthonous salt sheet and the associated overturned megaflap, (2) the Rocher de Hongrie and (3) the Daluis diapir. Our results show that the resulting record of peak temperatures around the structures is different depending on the type of salt structure and its kinematic. The Astoin structure shows that salt tectonics during the Jurassic-Cretaceous has impacted the temperature distribution around the allochthonous salt sheet while at Daluis and the Rocher de Hongrie, the temperatures have overprinted an already existing salt-related structure. The impact of the salt structure on temperature distribution is always local but the interpretation of the RSCM temperatures may systematically be difficult without considering early salt tectonics in the structural evolution of the area.

Mud volcanoes guided by thrusting in compressional settings

Mud volcanoes are fed largely by fracture systems, but fracture systems are common in fold-and-thrust belts (FTBs), and not all of them extrude mud volcanoes. One popular hypothesis is that mud volcanoes extrude up normal faults and fractures in the crests of thrust-related anticlines. Using a high-resolution depth-migrated seismic cube in the northwestern Gulf of Mexico, we demonstrate the occurrence of buried mud volcanoes (shale sheets) that appear to have been fed by fluids moving up the thrust faults themselves. Mobile shales in this deepwater FTB detach a mostly welded allochthonous salt sheet. The main phase of shortening was during the Miocene, accompanying the sedimentation. Shortening generated multiple detachment levels, stacked duplexes, thrusts, and folds affecting the Oligocene mobile shales. Above these detachments, syn-sedimentary shortening initially generated detachment folds that progressively evolved into upward-propagating listric thrusts and fault-propagation anticlines. Shale sheets formed near the central thrust domains, associated with higher fault dips and larger fault displacements. We argue that their feeder systems were probably thrust-related shear zones that widened upward and comprised networks of anastomosing fractures. Finally, we propose that the position of mud volcanoes with respect to the underlying thrust systems depends on the path of least resistance to the surface. Although overpressured muds may vertically reach the surface via extensional fractures in the crests of thrust-related anticlines, we have shown to the contrary that when thrusts are emergent in a deepwater FTB, fluids continue up the inclined thrusts and adjacent fractures, forming mud volcanoes at the thrust tips.

Salt geometry in the Central Basin of the Nova Scotia passive margin, offshore Canada based on new seismic data

This study describes the structural architecture of the central portion of the Scotian Basin and focusses on Mesozoic salt deposition and deformation linked to magma-rich rifting. BP acquired a high-quality 3D seismic survey called the Tangier survey and built a detailed velocity model with RFWI (Reflection Full Waveform Inversion) and tomography. This has allowed for high resolution seismic imaging of target sedimentary reservoirs but also for the first time, imaging of the complex crustal architecture along the central part of the Nova Scotia margin. We evaluated the regional basin architecture and salt tectonic evolution in context of Late Triassic and Early Jurassic rifting followed by passive margin gravity spreading. An exploration well (Aspy D-11) was drilled based on the 3D seismic interpretation and penetrated halite, carnallite and anhydrite salt in an allochthonous salt sheet. Basement is thought to have rifted during deposition of interlayered halite, shale, carnallite and anhydrite. Carnallite may have precipitated from enriched brines that formed at the more distal (southern) end of an elongated seaway of rifted basins which began opening first in the North in the Late Triassic and propagated southward towards Florida (e.g., Jansa, 1980; Jansa and Tucholke, 1986; Roberts and Bally, 2012). Water flowed southwards from Tethys along a chain of complexly interconnected areas of rifted basins. Flow of seawater along Nova Scotia is interpreted to have been southward along a widening and progressively subsiding region of rifts possibly as dynamic elevation associated with the CAMP (Central Atlantic Magmatic Province) dissipated southward much like what occurred in the Central South Atlantic and Gulf of Mexico (GoM) (e.g., Quirk et al., 2013; Morgan et al., 2020; Pindell et al., 2020; Pindell and Heyn, 2022). Brine was enriched along the seaway by evaporation, causing deposition of the most-soluble salt components furthest south along the sequence of basins. The presence of weathered basalt in salt and SDRs on the seismic supports magma-rich Mesozoic rifting and suggests that heating of brine pools may not have been exclusively caused by the sun, but that hydrothermal systems probably contributed to heating and the precipitation of Carnallite.

Mechanical relationship between strike-slip faulting and salt tectonics in the Northern Tunisian Atlas: The Bir-El-Afou salt structure

Wide variety of salt structures –from typical diapirs to large allochthonous salt sheets– are recognized in the Northern Tunisian Atlas. In this study, we present for the first time the presence of a diapiric structure, Bir-El-Afou Salt Structure (BEASS), which has formed in response to strike-slip tectonics. Structural mapping, mesoscale field observations, fault kinematic analysis, and interpretation of gravity data are used to interpret BEASS, which is currently associated with a restraining bend. The present-day NNE– trending restraining stepover is formed as a result of E-W dextral fault system. We assume that the restraining stepover is a product of the inversion of a pre-existing pull-apart basin that formed during the Cretaceous South Tethyan extension. Tectono-sedimentary relationship is used to argue for an early Cretaceous NW-SE regional extension and local transtension. During the opening of the pull-apart basin, salt movement took place in the southwestern corner of the basin which must have thinned due to its extension facilitating diapiric emplacement. Diapir growth during Aptian was followed by brief salt flow from the diapir crest towards the basin forming a salt sheet. A period of Tertiary contraction resulted in the inversion of the pull-apart basin and consequently, squeezing of BEASS.

Anatomy and evolution of the Astoin diapiric complex, sub-Alpine fold-and-thrust belt (France)

The structure of the southwestern branch of the Alpine orogen is affected by the extensive Late Triassic evaporites. These evaporites have been involved in polyphased salt tectonics since the early Liassic, coeval with the Tethyan rifting, and are the décollement level for thrusts in the external parts during Alpine orogeny. The role of salt tectonics in this branch of the Alpine arc is re-evaluated in order to determine the relative importance of early deformation related to salt motion with respect to deformation related to main Alpine compressional events. This paper focuses on one structure identified as diapiric since the 1930’s: the Astoin diapir (Goguel, 1939). Analysis of geological maps together with new field work have allowed to better define diapirism in the Upper Triassic evaporites outcrops around Astoin. Study of the diapir and the surrounding depocenters reveals a major involvement of salt in the structuration of the area, since the Liassic. Several salt ridges are linked to a main diapiric structure, explaining why we call it the “diapiric complex” of Astoin. Salt tectonics was initiated during the Liassic rifting, and a few locations show evidence of reactive diapirism whereas in others evidence of passive diapirism as early as the Liassic is seen. Passive diapirism continued during the post-rift stage of Alpine margin history in the Late Jurassic and Cretaceous when an allochthonous salt sheet was emplaced. Diapirism also occurred during the Oligocene while the Alpine foreland basin was developing in this part of the European margin of the Alps. Serial interpretative cross-sections have been drawn in order to illustrate the lateral variations of diapirism and structural style. Sequential evolutions for each cross-section are proposed to reconstruct the diapiric complex evolution through time. The Astoin diapir shows a complex structural framework with an important along-strike variation of diapiric activity. Most of the geometries are inherited from salt tectonics that occurred during extension, and in some places these early structures are overprinted by Alpine compressional structures.

Salt tectonics in the Sureste Basin, SE Mexico: some implications for hydrocarbon exploration

Abstract The Sureste Basin salt became diapiric soon after deposition in the Bajocian (169 Ma). Salt ridges and diapirs grew throughout the Jurassic–Cretaceous. The Mexican Fold and Thrust Belt propagated into the Sureste Basin at 70–60 Ma, causing squeezing/extrusion of allochthonous salt sheets. The more intense Mid-Miocene ( c. 13–11 Ma) compression (‘Chiapaneco Event’) produced folding and thrusting over a north–south distance of 600 km. The Mid-Miocene–Recent bulk horizontal flow is indicated by anticline and thrust traces mapped from seismic data and bathymetry. In the SE sector of the salt basin, flow is towards N60°E, where thrusting is intense and the salt is thin. The flow swings to a northerly direction in the basin centre, where folding is important. A parabolic fold arc was produced due to drag on the edges of the salt basin. This indicates that both salt and overburden behaved in a ductile manner at a large scale. Mesozoic carbonate slabs were overthrusted up to 20 km during the Mid-Miocene. Intense fracturing and karstification produced potential reservoir rocks with many untested structures that are leaking hydrocarbons. Pre-Mid-Miocene clastic reservoirs may be trapped in folds below the allochthonous salt, which also have not been drilled.

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.

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 Salt Tectonics on the Thermal Evolution and the Petroleum System of Confined Rift Basins: Insights from Basin Modeling of the Nordkapp Basin, Norwegian Barents Sea

Although the thermal effect of large salt tongues and allochthonous salt sheets in passive margins is described in the literature, little is known about the thermal effect of salt structures in confined rift basins where sub-vertical, closely spaced salt diapirs may affect the thermal evolution and petroleum system of the basin. In this study, we combine 2D structural restorations with thermal modeling to investigate the dynamic history of salt movement and its thermal effect in the Nordkapp Basin, a confined salt-bearing basin in the Norwegian Barents Sea. Two sections, one across the central sub-basin and another across the eastern sub-basin, are modeled. The central sub-basin shows deeply rooted, narrow and closely spaced diapirs, while the eastern sub-basin contains a shallower rooted, wide, isolated diapir. Variations through time in stratigraphy (source rocks), structures (salt diapirs and minibasins), and thermal boundary conditions (basal heat flow and sediment-water interface temperatures) are considered in the model. Present-day bottom hole temperatures and vitrinite data provide validation of the model. The modeling results in the eastern sub-basin show a strong but laterally limited thermal anomaly associated with the massive diapir, where temperatures in the diapir are 70 °C cooler than in the adjacent minibasins. In the central sub-basin, the thermal anomalies of closely-spaced diapirs mutually interfere and induce a combined anomaly that reduces the temperature in the minibasins by up to 50 °C with respect to the platform areas. Consequently, source rock maturation in the areas thermally affected by the diapirs is retarded, and the hydrocarbon generation window is expanded. Although subject to uncertainties in the model input parameters, these results demonstrate new exploration concepts (e.g., deep hydrocarbon kitchens) that are important for evaluating the prospectivity of the Nordkapp Basin and similar basins around the world.

The influence of base-salt relief, rift topography and regional events on salt tectonics offshore Morocco

Abstract This study integrates borehole-calibrated 2D and 3D seismic interpretation with numerical models to provide a regional analysis of the complex salt tectonics offshore central Morocco. We investigate the mechanisms controlling along-margin structural variations, the effects of thick-skinned shortening and the sequential evolution of allochthonous sheets. Additionally, we analyse how base-salt relief generated complex flow kinematics and alternation of extensional and contractional domains by causing flux variations at both autochthonous and allochthonous salt levels. The area is divided into three structural provinces, with the central, Essaouira segment, having greater downdip translation, structural complexity and volume of allochthonous salt, which suggests the salt was originally thicker and better connected across multiple syn-rift structures. The southern segment, Agadir, is dominated by up-right squeezed diapirs formed by early load-driven rise and late shortening; limited salt and overburden translation and no allochthonous sheets. The northern segment, Safi, is narrower and has a smaller number of salt structures that were affected by abrupt translation due to steep detachment gradient. Late, oblique thick-skinned shortening generated contractional structures in the entire salt basin, which are most prominently developed in Essaouira due to a favourably-oriented NW-SE syn-rift structure, the Talfeney Accommodation Zone. Allochthonous salt sheets formed during four phases from the Albian, Late Cretaceous, Paleocene to Oligo-Miocene by two main mechanisms: contraction and basinward salt expulsion. This work improves understanding of the structural configuration along the Moroccan margin guiding the identification of potential sub-salt plays and contributing to a better comprehension of salt-related deformation along rifted passive margins worldwide.

Diapiric growth within an Early Jurassic rift basin: The Tazoult salt wall (central High Atlas, Morocco)

The Central High Atlas (Morocco) constitutes a diapiric province that hosts a complex array of elongated diapirs and minibasins that formed during the Lower Jurassic rift of the Atlas Basin. This paper aims to study the structure and growth evolution of the Tazoult diapiric wall, located in the Central High Atlas, by means of structural and sedimentological fieldwork integrated with remote sensing mapping. The Tazoult salt wall is a 20 km long x 3 km wide NE-SW trending ridge that exposes Upper Triassic red beds and basalts along its core. The succession flanking the salt wall ranges from Hettangian to Bajocian ages displaying spectacular sedimentary wedges in the SE and NW flanks. The Hettangian-early Sinemurian carbonates mainly crop out as blocks embedded in the core rocks. The ~1-km thick Pliensbachian platform carbonates display large subvertical flap structures along the flanks of the Tazoult salt wall with unconformities bounding tapered composite halokinetic sequences. In contrast, the ~2.5-km thick late Pliensbachian-Aalenian mixed deposits form tabular composite halokinetic sequences displaying small-scale hook halokinetic sequences. Passive diapirism resulted in the lateral extrusion of the evaporite-bearing rocks to form an allochthonous salt sheet towards the adjacent SE Amezrai minibasin. The Bajocian platform carbonates partially fossilized the Tazoult salt wall and thus constitute a key horizon to constrain the timing of diapir growth and discriminate diapirism from Alpine shortening. The Pliensbachian carbonate platform evolved as a long flap structure during the early growth of the Tazoult salt wall, well before the onset of the Alpine shortening.

Allochthonous salt sheet growth: Thermal implications for source rock maturation in the deepwater Burgos Basin and Perdido Fold Belt, Mexico

A better understanding of suballochthonous salt petroleum systems can be achieved by thermal modeling a series of vertical pseudowell profiles that are positioned along ramps and flats identified at the base salt level. This allows the changing 2D shape of the highly conductive salt to be taken into account and assesses the physical parameters that control the thermal evolution. A case study of a typical cross section through the Burgos Basin and Perdido Fold Belt is analyzed in an area of very active oil exploration in northern Mexico. Numerical experiments indicated how the extrusion of salt cools the underlying basin and significantly prolongs the period of Tithonian source rock maturity. The rapid extrusion of thick allochthonous salt sheets, and the associated transient heat flow effects, can also cause peaks in oil generation, in places postdating the trap formation during the Oligo-Miocene folding event. The principal Tithonian source rock is predicted to have remained in the oil window since Paleogene times, despite being buried to present-day depths of approximately 8 km subsea. This upgrades the oil potential of some of the subsalt areas that are currently on offer in the CNH-R01-L04/2015 Mexico deepwater Licensing Round.

Minibasins and salt canopy in foreland fold-and-thrust belts: The central Sivas Basin, Turkey

The Sivas Basin in the Central Anatolian Plateau (Turkey), which formed in the context of a foreland fold-and-thrust belt (FTB), exhibits a typical wall and basin (WAB) province characterized by symmetric minibasins separated by continuous steep-flanked walls and diapirs. Extensive fieldwork including regional and detailed local mapping of the contacts and margins of minibasins, and interpretation of a set of 2-D regional seismic lines, provide evidence for the development of a shallow evaporite level separating two generations of minibasins within the WAB province. Here beds of symmetric exposed minibasins along diapir flank are younger than minibasins observed over autochthonous evaporites. Laterally away from the WAB province, increase in wavelength of the tectonic structures suggests a deepening of the decollement level. We interpret that a shallower evaporite level developed in the form of an evaporite canopy, triggered by significant lateral shortening. The Upper Eocene-Lower Oligocene autochthonous Tuzhisar evaporite level was remobilized by the northward migrating sedimentary load and the tilting of the southern basin margin during propagation of the foreland fold-and-thrust belt. Asymmetric and symmetric primary minibasins were overrun by an allochthonous sheet forming a canopy. A second generation of salt withdrawal minibasins subsided into the allochthonous salt sheet. The polygonal pattern of the WAB province influences the growing fold-and-thrust belt system during the late stage of the secondary minibasins development. The Sivas FTB basin is the result of the interaction between fold-and-thrust belt propagation, evaporite remobilization, and interaction between evaporite flow and sedimentation in the minibasins.

Enigmatic structures within salt walls of the Santos Basin—Part 2: Mechanical explanation from physical modelling

Jackson et al. (this volume) used 3D seismic reflection data to describe intrasalt deformation in salt walls in the Santos Basin. They focused on the origin of enigmatic allochthonous salt sheets of older evaporites (A1 unit) emplaced above overlying stratified evaporites (A2–A4 units). Their kinematic model incorporates: (i) initial inward flow and thickening of A1 salt within the rising wall, and arching of A2–A4 overburden; (ii) breaching of the arched overburden, ascent of mobile A1 evaporites along single or multiple feeders, and emplacement of upper-wall sheets or canopies; and (iii) a component of regional shortening within the salt. This companion paper uses physical modelling to explain how and why these structures occur and proposes a mechanical basis for the kinematic model. Our first two models simulated salt having uniform internal density, with walls growing by (i) initially symmetric differential loading and (ii) initially symmetric differential loading plus shortening. These models reproduced anticlines and injection folds seen in the simpler deformed walls in the Santos Basin. However, neither model reproduced the most complex structures within the Santos evaporites, which are: (i) allochthonous intrusions, (ii) steep feeders, and (iii) recumbent synclines. Thus, differential loading and shortening alone are insufficient to generate these complex structures. In our third model, a less-dense lower evaporite (A1) was overlain by denser upper evaporites (A2–A4), similar to the density structure found by Santos Basin wellbores. The wall rose solely by differential loading. In this model, A1 breached the overlying evaporites to form vertical diapirs feeding salt sheets and salt wings in the upper part of the salt wall. Breakthrough of A1 folded A2–A4 evaporites into recumbent synclines. Model sections closely resemble Santos seismic examples, suggesting that the key to forming these complex intrasalt structures is a density inversion within the evaporite sequence that creates a Rayleigh–Taylor instability.

Submarine allochthonous salt sheets: Gravity-driven deformation of North African Cretaceous passive margin in Tunisia – Bled Dogra case study and nearby salt structures

We used structural, stratigraphic and sedimentologic data, together with a comparison of nearby structures and a Bouguer gravity map, to evaluate the evolution of the Bled Dogra salt structure (northern Tunisia) during the Cretaceous. Triassic salt sheets are recognized in the northwestern region of the Tunisian Atlas. These salt sheets are the result of Cretaceous thick and/or thin-skinned extension along the south Tethyan margin. The Bled Dogra salt structure is one of these submarine allochthonous salt sheets, which was emplaced during the Early Cretaceous. The geologic framework, during this period, produces conditions for a predominantly gravity-driven deformation: extension has produced space for the salt to rise; vigorous differential sedimentation created differential loading that resulted in the emplacement and extrusion of a large volume of Triassic salt and formation of large submarine salt sheets. Geologic field data suggest an interlayered Triassic salt sheet within Albian sequences. Salt was extruded at the sea floor during the Early–Middle Albian and was initially buried by Middle–Late Albian strata. The Coniacian corresponds to a second transgressive cover onto the salt sheet after the gliding of the first salt cover (Late Albian–Turonian). In addition, this northwest Tunisian area exposes evidences for salt flow and abundant slump features at the base of a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian. Two gravity deformation processes are recognized: gravity gliding and gravity spreading. Acting concurrently, these two processes appear indistinguishable in this geologic context. Like the present-day salt-involved passive margins – such as the northern Gulf of Mexico, the Atlantic margin of Morocco, the Brazilian Santos basin, the Angola margin, Cadiz in western Iberia, and the Red Sea – the North African Cretaceous passive margin in Tunisia provides evidences that deformation in a passive-margin salt basin is predominantly gravity-driven deformation.

Breakout of squeezed stocks: dispersal of roof fragments, source of extrusive salt and interaction with regional thrust faults

AbstractWe used seven scaled physical models to explore the near‐surface structural evolution of shallowly buried, actively rising salt stocks. The models consisted of dry sand, ceramic microspheres and silicone. Previously dormant stocks rose because of lateral squeezing or pumping of salt from below. The pressure of rising salt created a dynamic bulge in the crest of the diapir, which arched the overlying roof sediments. Eventually this dynamic bulge collapsed and its overlying roof broke into rafts along subradial grabens. The rafts were dispersed outwards by shear traction of spreading salt, surmounting an upturned collar of country rock and eventually grounding at the front of the extrusive flow. Flow of salt around these stranded fragments created a lobate extrusion front, common in submarine salt sheets in the Gulf of Mexico and subaerial salt glaciers in Iran. Stock geometry, regional dip and roof density affected extrusion rates and spreading directions. Stocks leaning seaward extruded salt faster and farther than did upright stocks. Dense roofs foundered and plugged the vent, limiting surface extrusion. In tilted models, broad salt sheets spread asymmetrically downslope. Stock contents were inverted within the extruded salt sheet: successively deeper parts of the stock's core rose to the surface and overran salt extruded from the shallower parts of the diapir. As shortening continued, salt from the source layer reached the surface after being driven out by thrusting. A central thrust block, or primary indenter, moved ahead of surrounding thrust blocks, impinging against and squeezing the stock into an elliptical planform. After high shortening, secondary indenters converged obliquely into the salt stock, expelling salt from the periphery of the diapir. The models shed light on (1) the origin and fate of large rafts or carapace blocks atop allochthonous salt, (2) cuspate margins of salt sheets and (3) interaction of thrusting, diapir pinch‐off and emplacement of allochthonous salt sheets.

Characteristics and deformation mechanism of salt-related structures in the western Kuqa depression, Tarim basin: Insights from scaled sandbox modeling

Abstract Three scaled analog (sandbox) models are constructed to simulate the evolution of thick-skinned shortening on broad and narrow salt basins. These models simulate the evolution of the Cenozoic salt (salt-related) structures in the westernmost Kuqa salt-bearing depression (Quele area), Tarim basin. The formation mechanism and the controlling factors of structural deformation in this area are investigated and discussed systematically. Model results show that the variable depositional width of the salt basin has a major controlling influence on the differential structural deformation in different longitudinal segmentations from east (broad) to west (narrow) in the western Kuqa depression (Quele area). Model results also demonstrate that the early salt structure (salt diapir) has an important controlling role on the subsequent compressional structural deformation. The pre-existing thin overburden Quele diapir, which acts as a stress concentration zone during the subsequent shortening, is deformed to the Quele salt nappe. On the basis of the model results, it is suggested that the formation of the long-distance Quele allochthonous salt sheet is related with the pre-existing Quele diapir, which displaces up paralleling the hanging wall and flows to the surface during the subsequent shortening.

Closure of open wellbores in creeping salt sheets

Safe exploration and production of pre-salt (or subsalt) hydrocarbons require that drilling operations be optimized. We introduce analytical models of wellbore closure, accounting for variations in both the wellbore net pressure and far-field flow rate of an autochthonous or allochthonous salt sheet penetrated by the wellbore. We demonstrate how closure rates of such a wellbore evolve for increasingly stronger Rankine flow. For high viscosity salt (?10^18 Pa s) the wellbore closes by a pure sink flow without any Rankine shift from its vertical trajectory path. For low viscosity salt (?10^16 Pa s) Rankine flow dominates.Wellbore closure in salt sheets may vary within the same wellbore due to differential tectonic creep rates at different depths. Mitigation of wellbore closure by, for example, reaming, jarring, brine solution and thermal control, is most effective if spatial variation in closure rates is understood and quantified. Evaluation of wellbore closure rates due to salt creep should be customarily included in wellbore stability analyses before drilling and during well monitoring.

Depositional and halokinetic-sequence stratigraphy of the Neoproterozoic Wonoka Formation adjacent to Patawarta allochthonous salt sheet, Central Flinders Ranges, South Australia

Abstract Parts of two third-order Neoproterozoic (Marinoan) depositional sequences are documented in the Wilpena Group (Wonoka Formation and Bonney Sandstone) at Patawarta diapir, located in the central Flinders Ranges, South Australia. These sequences represent an overall regressive succession transitioning upwards from outer to middle wave-dominated shelf deposits to a tidally dominated barrier bar to coastal plain. The lower, middle, upper limestone and green mudstone informal members of the Wonoka Formation comprise the Highstand Systems Tract of the lower sequence. The Sequence Boundary is at the top of the Wonoka green mudstone member and is overlain by the Lowstand Systems Tract of the upper sequence, which includes the lower dolomite, sandstone and upper dolomite beds of the Patsy Hill Member of the Bonney Sandstone. The upper sequence Transgressive Systems Tract comprises the Bonney Sandstone. These units comprise one complete tapered composite halokinetic sequence (CHS). The lower halokinetic-sequence boundary is associated with the Maximum Flooding Surface of the lower depositional sequence and the upper halokinetic-sequence boundary is interpreted as the Transgressive Surface of the overlying depositional sequence where an angular truncation of up to 90° is documented.

Some consequences of mechanical stratification in basin-scale numerical models of passive-margin salt tectonics

Abstract Two-dimensional plane-strain numerical experiments illustrate the effects of variable evaporite viscosity and embedded frictional-plastic sediment layers on the style of salt flow and associated deformation of the sedimentary overburden. Evaporite viscosity exerts a first-order control on the salt flow rate and the style of overburden deformation. Nearly complete evacuation of low-viscosity salt occurs beneath expulsion basins, whereas significant salt is trapped when viscosity is high. Embedded frictional-plastic sediment layers with yield strength partition salt flow and develop transient contractional structures (folds, thrust faults and folded faults) in a seaward salt-squeeze flow regime. Multiple internal sediment layers reduce the seaward salt flow during sediment aggradation, leaving more salt behind to be remobilized during subsequent progradation. This produces more seaward extensive allochthonous salt sheets. If there is a density difference between the embedded layers and the surrounding salt, then the embedded layers fractionate during deformation and either float to the surface or sink to the bottom, creating a thick zone of pure halite. Such a process of ‘buoyancy fractionation’ may partially explain the apparent paradox of layered salt in autochthonous salt basins and pure halite in allochthonous salt sheets. Supplementary material: Animated gif files of the model results are available at http://www.geolsoc.org.uk/SUP18500 .