Allochthonous Salt Sheets Research Articles

An investigation of salt tectonic structural styles in the Scotian Basin, offshore Atlantic Canada: 1. Comparison of observations with geometrically simple numerical models

[1] Three primary salt tectonic structural styles of the Scotian Basin are compared with plane strain finite element models in order to investigate their origin. Here, we focus on simplified model salt basins with initial rectangular cross-sectional geometries and follow their evolution in the context of tectonic and parametric thermal subsidence and under various sedimentation regimes. Structural style A, an open-ended roho system with a synkinematic wedge, is reproduced by models including deltaic progradation and seaward spreading/gliding of sediments above a salt detachment. Structural style B, a linked salt tectonic system with landward regional normal faults and allochthonous salt sheets climbing seaward over Late Cretaceous and Paleogene strata, is shown to be a consequence of early aggradation followed by progradation. Structural style C is characterized by salt diapirs and intervening minibasins and is reproduced by models with Rayleigh-Taylor instabilities requiring compaction driven density inversions, weak sediments, and initial perturbations of the overburden-salt interface.

An investigation of salt tectonic structural styles in the Scotian Basin, offshore Atlantic Canada: 2. Comparison of observations with geometrically complex numerical models

[1] Three primary salt tectonic structural styles of the Scotian Basin are compared with plane strain finite element models in order to investigate their origin. Here, we focus on simplified model salt basins with initial rectangular cross-sectional geometries and follow their evolution in the context of tectonic and parametric thermal subsidence and under various sedimentation regimes. Structural style A, an open-ended roho system with a synkinematic wedge, is reproduced by models including deltaic progradation and seaward spreading/gliding of sediments above a salt detachment. Structural style B, a linked salt tectonic system with landward regional normal faults and allochthonous salt sheets climbing seaward over Late Cretaceous and Paleogene strata, is shown to be a consequence of early aggradation followed by progradation. Structural style C is characterized by salt diapirs and intervening minibasins and is reproduced by models with Rayleigh-Taylor instabilities requiring compaction driven density inversions, weak sediments, and initial perturbations of the overburden-salt interface.

Terra infirma: Understanding salt tectonics

Following common usage, we broaden the term “salt” to include all rock bodies composed primarily of halite (NaCl). Salt is mechanically weak and flows like a fluid, even at geologically rapid strain rates. Salt is also relatively incompressible so is less dense than most carbonates and all moderately to fully compacted siliciclastic rocks. Salt's fluid rheology and incompressibility make it inherently unstable under a wide range of geologic conditions. The primary driving force for salt tectonics is differential loading, which may be induced by gravitational forces, by forced displacement of one boundary of a salt body relative to another, or by a thermal gradient. Buoyancy, long considered a key driver for salt tectonics, is of secondary importance in many settings. Two factors resist salt flow: strength of the overburden and boundary drag along the edges of the salt body. Salt will move only if driving forces exceed the resistance to flow. In order for a salt diapir to be emplaced into its overburden, any rock previously occupying that space must be removed or displaced. Emplacement may occur by extension, erosion, or uplift of the overburden or by overthrusting of the salt. Once salt reaches the surface, it can continue to rise by passive diapirism, in which the diapir grows as sediments accumulate around it. A rapidly rising passive diapir may spread over the sediment surface to form an allochthonous salt sheet. A variety of salt-sheet lineages are possible, depending on the geometry of the feeder and the tectonic setting. Because salt is weak, its tectonism is closely tied to regional deformation. In extension or transtension, diapirs rise up graben axes, taking advantage of the space created by thinning and separation of fault blocks. Later, once the salt source layer is exhausted, diapirs may fall as they continue to widen. In addition, salt typically acts as a detachment in both gravity-driven and basement-involved extension. In compression or transpression, preexisting diapirs are rejuvenated as salt is displaced upward by lateral shortening. This rise is enhanced by buckling and disruption of the diapir roof. In the absence of precursor structures, salt's primary role in compression is to act as a detachment. Some salt sheets may be emplaced in the hanging walls of thrust faults.

Advance of allochthonous salt sheets in passive margins and orogens

Allochthonous salt sheets advance in four ways: (1) extrusive advance, (2) open-toed advance, (3) thrust advance, and (4) salt-wing intrusion. These mechanisms are determined primarily by the geometry and thickness of the roof that overlies the advancing sheet. An extrusive sheet spreads without a roof or with a roof of negligible mechanical strength. An open-toed sheet is partially covered by a mechanically significant roof but has an extrusive toe. An overthrusting sheet advances along a thrust fault at its leading edge, carrying its roof with it. A salt wing intrudes from the flank of a diapir into a shallower salt layer.Extrusive, open-toed, and overthrusting salt sheets are found in both passive margins and orogens. A sheet typically evolves through two or more of these mechanisms. Three lineages, or evolutionary paths, are common. Plug-fed extrusions emanate from the top of a salt dome or salt wall. Plug-fed thrusts form the base of the hanging wall of thrust faults that are rooted in the tops of salt domes or salt walls. Finally, source-fed thrusts initiate as thrust faults rooted in the autochthonous salt layer. Source-fed thrusts form the largest individual salt sheets, some covering thousands of square kilometers. Salt-wing intrusions form only under special circumstances, so they are not part of the three major lineages. These intrusions are restricted to compressionally inverted basins containing multiple salt layers and are known only in the Zechstein salt basin.

Structural evolution of the Frampton growth fold system, Atwater Valley-Southern Green Canyon area, deep water Gulf of Mexico

The Frampton growth anticline is part of the Atwater Valley-Southern Green Canyon frontal fold belt in the deep-water northeastern Gulf of Mexico. The anticline is located basinward of the allochthonous Sigsbee Salt sheet near the Sigsbee Escarpment. The timing and mechanisms of the formation of the frontal fold have been investigated using palinspatically restored depth sections and by the analysis of the growth stratal architecture preserved on fold limbs. The Frampton anticline is cored by autochthonous Middle Jurassic Louann salt and its western limit is bounded by the Green Knoll diapir. The fold geometry varies along strike, from a symmetric box-fold in the east, to a breached detachment fold in the west. Small-wavelength salt pillows formed during the Late Jurassic-Cretaceous in response to an early contractional deformation. These precursor structures controlled the geometry of Tertiary age folding. The Green Knoll diapir, west of the Frampton anticline, influenced deformation, leading to complex interactions between segments of the fold system and associated thrust faults. A landward-vergent thrust fault accommodated shortening adjacent to the diapir whereas folding was the main mechanism of deformation in the eastern part of Frampton anticline. Analysis of growth strata indicates that the detachment anticline developed according to a progressive limb rotation kinematic model with minor hinge migration. The fold evolution model proposed in this study could be used as an analogue for less well imaged, hydrocarbon-bearing growth folds in the deep-water province of the Gulf of Mexico, particularly those that are partially obscured by overlying allochthonous salt sheets.

EXTENSIONAL EVOLUTION OF THE GULF OF MEXICO BASIN AND THE DEPOSITION OF TERTIARY EVAPORITES

Current interpretations of the evolution of the Gulf of Mexico Basin are guided by the critical assumption that all the salt in this huge basin is Jurassic and time‐equivalent to the Louann salt of the interior evaporite basins of Texas, Louisiana and Mississippi. This assumption forces the interpretation that extensive salt sheets that now reside within Eocene to Plio‐Pleistocene stratigraphic levels are allochthonous, having been emplaced by several episodes of Jurassic salt piercement and horizontal flow as shallow salt sills or sub‐aqueous salt glaciers during Tertiary sedimentation.This interpretation is contested from several geological viewpoints. The uncontaminated Louann salt differs substantially from salt of the coastal, offshore northern Gulf and Mexican Saline Basin, in all of which the salt carries multiple shaley layers and exotic material. The intra‐salt deformation of offshore sheets appears from cores and seismic profiles to be much less severe than would be expected if it had been emplaced via several generations of deep piercement diapirs. The intra‐salt shale laminations in the Belle Isle Salt Mine, Southern Louisiana, carry Oligocene fauna and are so closely conformable with primary salt layering that their contemporaneity with the salt is a valid conclusion. Because the Belle Isle Shale Zone is so similar to shaley zones encountered in offshore sheets, a contemporaneous origin for them is also implied. The mixed Tertiary fauna found within, and below salt sheets is so typical of sedimentary reworking that its origin from evaporite basin margins is a logical conclusion.The Gulf of Mexico Basin is unique as the presumed depository of multiple allochthonous salt sheets. Salt piercements in the remote Kavir Basin of Iran have been proposed as an analogue for the emplacement of salt canopies, but there is no sub‐surface control in the Kavir Basin and the canopy concept is not unanimously accepted. Furthuremore, the popular view that salt sheets were emplaced by submarine salt glaciers during successive episodes of piercement diapirism is doubted because of the complete absence of glaciers in todays halokinetically active basin. The preservation from solution of purported submarine salt extrusions lacks both experimental and credible geological support.Angular sub‐salt contacts of many offshore salt sheets in the northern continental slope have been interpreted as sheer or thrust contacts. However, thrusting seems unlikely in an overall extensional environment and it is here concluded that the angular contacts represent sub‐salt unconformities and that the Cretaceous and older exotic blocks found in some piercement domes have been plucked from a base‐salt unconformity surface.There is much circum‐Gulf evidence for extensional fracturing of the basement during the Tertiary period which, it is thought, supports the expectation of basement block faults, now hidden from view below thick Tertiary sediments; these faults governed sedimentation into depocenters as the basin expanded.It is concluded that salt sheets at Eocene, Oligocene, Mio‐Pliocene and Plio‐Pleistocene levels are autochthonous, a conclusion that has evolved from critical geological observations in the Belle Isle Salt Mine.

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

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

The effect of allochthonous salt evolution and overpressure development on source rock thermal maturation: a two-dimensional transient study in the northern Gulf of Mexico Basin

Sequential two-dimensional forward modelling of source rock maturation along a north–south 600 km megaregional profile across the northern Gulf of Mexico Basin illustrates the coupled influence of allochthonous salt evolution and overpressure development on source rock maturation. Model results indicate that allochthonous salt evolution strongly affected local and regional source rock maturation, while overpressure development had a minor impact. Within the lower slope portion of the profile (south of the Sigsbee escarpment to the Tabular Salt/Minibasin province), localized thermal lows associated with salt stock development locally retarded the maturation of source rocks, while the slow development of overpressures effected little influence on the maturation. Within the centre of the profile (offshore Louisiana shelf), however, the evolution of allochthonous salt sheets and shallow salt structures locally and regionally lowered the oil generation window. The timing of overpressures in this region, related to the progradation of the Cenozoic marine depocentre, may have accelerated the maturation of middle Eocene source rocks, although the deeper Turonian source rocks had reached maturation at the time of greatest overpressure development. In the northern portion of the profile (southern Louisiana), the Terrebonne salt sheet lowered the oil generation window and retarded the maturation of the middle Eocene source rocks.

Structure and Stratigraphy of the Calcasieu Lake Salt-Withdrawal Minibasin, Onshore Louisiana

ABSTRACT The Calcasieu Lake salt-withdrawal minibasin, central Cameron Parish, Louisiana, forms an E-W elliptical shape of ~15 mi by 10 mi. Complex normal-fault systems occur along the basin's periphery, commonly down-to-the-basin and flanking adjacent salt-cored uplifts, including Hackberry, Big Lake, Sweet Lake and Creole. Calcasieu Lake salt dome, a comparatively small piercement structure, occupies the basin's center. Sub-salt exploration is currently a major play offshore, yet sub-salt production beneath prominent overhangs of the Calcasieu Lake salt dome was established nearly a quarter century ago in October, 1966, producing ~6.0 BCFG and 300 MBO. Calcasieu Lake minibasin formation was preceded by emplacement of an allochthonous salt sheet into Upper Eocene Jackson strata, probably similar to the modern salt canopy offshore. Rapid sedimentary loading in the Oligocene caused salt remobilization, forming or enhancing peripheral salt uplifts, and an eventual salt weld to form. The Middle Miocene marked renewed sedimentation and basin formation, likely related to formation of the now-detached Calcasieu Lake salt dome by remobilization of salt from a central residual mound. In particular, a sandrich depositional sequence, up to 3,000 ft or more in thickness, is confined to the basin itself, without equivalent strata on peripheral highs. Local upturning of Middle Miocene basinal strata by the Calcasieu Lake salt dome allowed for the sub-salt production at Calcasieu Lake. The paucity of wells drilled along the basin's flanks leaves open many exploration opportunities to test these sand-rich strata as they onlap and pinch-out up the flanks. End_of_Record - Last_Page 409-------

ABSTRACT: Emplacement, Inflation and Folding of an Extensive Allochthonous Salt Sheet in the Late Mesozoic (Ultra-Deepwater Gulf of Mexico)

ABSTRACT: Emplacement, Inflation and Folding of an Extensive Allochthonous Salt Sheet in the Late Mesozoic (Ultra-Deepwater Gulf of Mexico)

Salt extrusion at Kuh-e-Jahani, Iran, from June 1994 to November 1997

Abstract Kuh-e-Jahani is one of the largest extrusions of salt currently active in the Zagros mountains. Salt rises from about 4 km below sea level to nearly 1.5 km, above, where, unable to support its own weight, it spreads over the surrounding scenery in a process responsible for present and past allochthonous salt sheets elsewhere. We report vertical movements and apparent horizontal displacements of 43 markers dispersed over this mountain of salt for 4.5 years in three consecutive intervals, the first of 18 months and two others of 12 months. The geometry and inferred flow rate of the salt changed between surveys emphasizing that the gravity spreading is not steady. Our field readings of the dimensions and velocities of the salt at Kuh-e-Jahani are used to tune a simple numerical model and constrain the viscosity of the salt to between 10 16 and 10 17 Pa s −1 , its rate of surface dissolution to 2–3 cm a −1 , and its rate of rise out of its orifice at 2–3 m a −1 for c. 55 ka. These results imply that vigorous extrusion of salt at Kuh-e-Jahani is probably close to evacuating its deep source and that this mountain will soon begin to waste as salt dissolution overtakes extrusion. This progress report is warranted because our results have significant implications for sophisticated engineering in salt.

Abstract: Spatial Variations in Formation Water Salinities, South Pelto and South Timbalier Areas, Eastern Louisiana Continental Shelf 

ABSTRACT Spatial variations in pore fluid pressure, temperature, and salinity were determined from borehole logs along a 100-km long north-south transect from the Louisiana coastline to the shelf edge in the South Pelto and South Timbalier outer continental shelf areas to help identify driving forces and pathways of regional fluid flow and solute transport, information potentially useful also in identifying pathways of hydrocarbon migration. The Pleistocene to Upper Miocene sediments were deposited in fluvial, deltaic, and open marine environments and hence contained waters of fresh to brackish to normal marine salinities, <1 to 35 g/L, at the time of their deposition. Most of these sediments, however, now contain hypersaline fluids having salinities of up to 140 to 170 g/L. Exceptions are shallow Upper Pleistocene fluvial and deltaic sediments that contain fluids less saline than sea water, even though this section is now overlain by seawater. These shallow brackish fluids probably represent the remnant of a regional freshwater lens formed during the last low-stand of sealevel. A regional plume of water having salinities in excess of 140 g/L dips gulfward to the shelf edge at depths of 3 to 5 km below the seafloor. The locus of this plume is within the sandier Pliocene section. Potential sources of dissolved salt within this plume include updip shallow salt structures and the underlying remnants of an allochthonous salt sheet at depths of approximately 6 km. Regional faults in the area may have acted as vertical conduits for upward transport of brines. Down-dip fluid flow was presumably in part gravity-driven. The distal end of this regional salinity plume near the shelf edge is now highly overpressured, and fluid flow should be in the opposite direction, i.e., updip. Hence, this part of the regional salinity structure was probably established prior to over pressuring. The results of this study establish the dynamic nature of formation water flow which has occurred in the outer continental shelf. Further work is now needed to quantify the magnitude of the driving forces and rates of these processes.

The Evolution of Allochthonous Salt Along a Megaregional Profile Across the Northern Gulf of Mexico Basin

Sequential restorations of a north-south megaregional cross section across the north-central Gulf of Mexico Basin from east-central Louisiana to the abyssal plain define a dynamic, complex history of sedimentation, salt flowage, and salt evacuation. Proprietary composite seismic profiles (590 km), 33 wells, and published depth-to-basement maps were used to constrain the section in depth. Thirteen sequential structural restorations, incorporating both decompaction and isostatic subsidence (thermal and tectonic), were then constructed from the Late Cretaceous to the Holocene. The restorations highlight and constrain a protracted history of deformation that is primarily controlled by gravity and the progradation of Cenozoic sediments over salt. Early stages of the tectonic history of the northern Gulf of Mexico Basin were related to differential thermal subsidence resulting from Early-Middle Jurassic rifting. During the Cenozoic, the evolution of the basin was dominated by the influx of large clastic depocenters, which caused the basinward evacuation of autochthonous Jurassic salt. Salt extrusion from the autochthonous layer was accomplished by inclined salt bodies, which flowed into salt glaciers or sheets near the sea floor. Evacuation of allochthonous salt layers provided significant sediment accommodation, and unusually thick sedimentary sections were deposited, such as the Terrebonne Trough of southern Louisiana (3-7 km of Miocene strata). Salt sheet formation and evacuation occurred progressively basinward through time in response to basinward shifts of major Cenozoic sedimentary depocenters. As a salt sheet neared complete evacuation, the underlying autochthonous salt layer would begin to evacuate, providing additional sediment accommodation that caused autochthonous salt flowage basinward, and the formation of the next allochthonous salt sheet basinward of the depocenter. The area of autochthonous salt progressively decreased through time and currently represents at most 45% of its maximum along this transect. The total area of salt through time was more stable, with variations of only 30% from its maximum. This relationship is a function of lateral salt flow into and out of the plane of section, and possible salt dissolution. The restorations indicate that very little translation or extension (1.46%) occurred at the autochthonous salt level during the evolution of the basin. The majority of translation or extension occurred above allochthonous salt sheets (25%) and was compensated laterally by salt flow. Displacements above allochthonous salt sheets were driven by gravitational instabilities caused by the slope gradient.

Physical Modeling of Structures Formed by Salt Withdrawal: Implications for Deformation Caused by Salt Dissolution

By creating 15 physical models, we investigated deformation above subsiding tabular salt, salt walls, and salt stocks. Dry quartz sand simulated a brittle sedimentary roof above viscous silicone representing salt. The modeled diapiric walls had linear planforms and rectangular, semicircular, triangular, or leaning cross sectional shapes; the stock was cylindrical. In models where the source layer (or allochthonous salt sheet) was initially tabular, a gentle, flat-bottomed syncline bounded by monoclinal flexures formed above a linear zone where the silicone was locally removed. Above all subsiding diapirs, the deformed roof was bounded by an inner zone of steep, convex-upward reverse faults and an outer zone of normal faults. Above subsiding diapiric walls, extensional and contractional zones were balanced. Above the subsiding salt stock, conical, concentric fault zones comprised inner reverse faults and outer normal faults. Sediments were added both before (prekinematic) and during (synkinematic) salt withdrawal. In entirely prekinematic roofs, reverse fault zones and normal fault zones both widened with time. Reverse faults propagated upward from the corners of the withdrawing diapirs. New reverse faults formed in the footwalls of reverse faults, each nearer the center of the deepening roof trough. Conversely, new normal faults formed successively outward from the sagging trough. Synkinematic deposition retarded faulting, but the pattern of inner reverse and outer normal faults was repeated; however, reverse faults formed successively outward, whereas normal faults formed inward. New conceptual models suggest that salt dissolution forms similar structures to those physically modeled for salt withdrawal. The appropriate physical models resemble natural dissolution structures above tabular salt. Extension alone above diapirs is not caused merely by salt withdrawal or dissolution, but by regional extension or active diapirism.

Comparison and Results of a Borehole Gravity Survey with Core and Log Data through an Allochthonous Salt Sheet in Ship Shoal South Additions, Offshore Louisiana : ABSTRACTS

Comparison and Results of a Borehole Gravity Survey with Core and Log Data through an Allochthonous Salt Sheet in Ship Shoal South Additions, Offshore Louisiana : ABSTRACTS

Salt flow by aggrading and prograding overburdens

Abstract The mechanisms responsible for segmentation of salt sheets and their emplacement into higher stratigraphic levels are not separable and act simultaneously. Three sets of centrifuge models with strongly planar anisotropic (anisotropy, δ, the ratio between the effective viscosities in pure and simple shear, ranging between 4.8 and 16.4) microlaminate overburdens are used to study the effect of aggradation and progradation on segmentation and emplacement of allochthonous salt sheets. In the first set of models, a tabular buoyant source layer was overlain by tabular anisotropic overburden simulating aggradation. During centrifuging, the underlying ductile source layer was segmented into individual wall-like diapirs by the subsiding blocks formed due to extension and faulting of the overburden. The extensional zone in these models started at the spreading edge (free face) of the model and migrated backwards. In the second set of models, a tabular buoyant source layer was overlain successively by wedges of anisotropic overburdens simulating progradation. During centrifuging, the buoyant layer was displaced from the back of the model, where loading was higher, towards the free face in the front of the model, where overburden units were thinner. Overburden units extended at the back and the middle of the model while contractional structures dominated at the front, where asymmetric diapirs formed overhangs that spread ‘basinward’ to form ‘salt’ sheets. In the third set of models, a wedge-shaped buoyant source layer was overlain successively by wedges of anisotropic overburden simulating progradation. The overburden wedge created a lateral pressure gradient ranging from 144 Pa at the back of the model to 80 Pa at the front when deformed in the centrifuge. In these models, as in the second set of models, the underlying buoyant mass was displaced ‘basinward’ by the subsiding thicker overburden units at the back of the model. Contractional structures dominated the deformation at the leading edge of the wedge. Comparison of model results suggests that progradation (as in the second and third sets of models) loads underlying ‘salt’ differentially, displaces it downdip and segments it. As it segments at the back, the ‘salt’ flows laterally to areas of lower loading by intruding through the thinner overburden units and forming secondary ‘salt’ sheets at the front. On the other hand, aggradation of uniform overburden segments a buoyant sheet into two-dimensional salt walls or stocks, as in the first set of models.

Relation of Subsalt Structures to Suprasalt Structures During Extension

Recent discoveries of hydrocarbons beneath allochthonous salt sheets have heightened interest in the location and geometry of subsalt structures, including faults offsetting the base of salt sheets. Salt extrudes to form flat-topped sheets with discordant or stepped lower contacts that then deform in response to sedimentation and regional or local stretching. We simulate the interacting growth of subsalt and suprasalt structures during regional extension using two-dimensional finite-element models. Fault arrays above and below salt comprise combinations of three characteristic patterns: simple grabens, reactive grabens, and drape grabens. Thickness variations in the roof of a salt sheet strongly influence the initial location of suprasalt faults. Fast extension or thin sa t enhances coupling between subsalt and suprasalt structures, summarized by these predictive guidelines: (1) subsalt faults are laterally offset from suprasalt drape grabens, and the offset and graben symmetry increase as coupling decreases, (2) symmetric reactive grabens tend to overlie zones of no subsalt faulting, and (3) suprasalt sags with little faulting overlie subsalt grabens. Flow in a deeper salt layer increases structural relief beneath a shallower sheet but decreases surface relief. The modeling exposes several possible interpretation pitfalls concerning true and apparent regional elevations, apparent feeder stems, subsalt ramps and flats, and effects of salt withdrawal vs. extension.

Neogene Structural Characteristics of Louisiana Shelf with Emphasis on Growth-fault Interplay with Salt Tectonics: ABSTRACT

Growth faulting and salt tectonics are the most pronounced structural features in the offshore Louisiana. Regional examination of seismic data(4 mile x 4 mile grid) of OCS area Suggests that polyphase halokinesis happened along the whole shelf area during the Neogene sedimentation but with different type of salt movement and faulting in time and space. The sublinear and regularly basinward-dipping lower and middle Miocene faults predominate beneath the present inner shelf region with scarcity of shallow salt features. These fault trends detached on bottom Miocene decollement related ductile shale and salt welds. The shorter and more arcuate-shaped upper Miocene and Pliocene faults complicated by abundant near-surface salt bodies characterize the farther offshore beneath the outer-shelf region. Most of these fault root into or related to salt diapirs, but at the West Cameron and at the South Timberlier, families of growth-faults were developed on the upper Miocene allochthonous salt sheet detachment and played an important role in segmenting salt sheet in this salt raft regime. The more irregularly oriented Pleistocene faults with combination of shallow basinward-dipping and counter-basinward-dipping detachment on top of large and thick upper Pliocene allochthonous salt mass occurred in the present shelf edge and slope region. This studymore » suggest at least three episodes of salt movement occurred at early Miocene, late Miocene-early Pliocene and early Pleistocene and the existence and possible position of large right-slip transfer faults.« less

An Emplacement Model for Allochthonous Salt Sheets with Implications Toward Subsalt Exploration: ABSTRACT

A model for allochthonous salt sheet emplacement is preserved to explain overthrusting of thick sediment columns above these sheets. This model proposes that initial emplacement, either as extrusive sheets or horizontal intrusions near the sea floor, sometimes yields to shearing along thrust fault zones immediately beneath the salt sheets. Anomalously high pore fluid pressures drive shearing beneath the sheets by reducing the effective normal stress, thereby facilitating lateral movement of the sheets and their overlying sediments. Several observations support this model: (1) stratigraphic sections above salt repeated below salt, (2) thick sediment escarpments near the toes of some salt sheets, (3) compressional features in front of some salt sheets, (4) low velocity zones near the base of some salt sheets, (5) lithologically distinct zones beneath many salt sheets, and (6) overpressured sediments below many salt sheets. The mode of salt sheet emplacement has significant implications toward subsalt hydrocarbon exploitation. Our model is consistent with the following implications: (1) subsalt strata may be younger and originate from a more landward depositional environment than otherwise expected, (2) the overpressured shear zone may be a path for hydrocarbon migration, a seal, or a reservoir, (3) one may be able to estimate the hydrocarbonmore » column height of a nearby reservoir based on the detection of hydrocarbons in the shear zone, and (4) hydrocarbon reservoirs trapped against shear zones may have significantly different up dip limits and depletion patterns than expected.« less

Free thermohaline convection beneath allochthonous salt sheets: An agent for salt dissolution and fluid flow in Gulf Coast sediments

Basinward migration of Jurassic salt in the U.S. Gulf of Mexico has resulted in the emplacement of large allochthonous salt sheets into shallow Miocene to Holocene sediments. Although comparatively little direct information is available on the environment below these salt bodies, it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below. We derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures. From these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport, even if the underlying sediments have permeabilities as low as 10−17 m2 (0.01 mD). The calculated maximum Darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability. When the vertical permeability of the underlying sediment is 10−17 m2 (0.01 mD), salt is dissolved from the base of the sheet at an average rate of 3–5 m m.y.−1 The corresponding fluid velocities are such that over a 10 m.y. period the integrated fluid flux in the underlying sediments would be ∼104 m3 m−2. By comparison, integrated fluid flux for compactive expulsion is &lt;103 m3 m−2. Thus, for the offshore sediments of the Gulf of Mexico, thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport, diagenesis, and salt tectonics.