Search

Existing definitions for passive and active salt diapirism are somewhat overlapping and ambiguous. These terms are also equated to downbuilding and upbuilding, respectively, which are problematic concepts as originally conceived and even as subsequently modified. This results in conflicting usage and lack of consensus in the literature, creating confusion and decreasing the utility of the terms. For example, a diapir with a thin roof is defined as active by some but passive by others. In this short contribution, we first review historical definitions and then focus on several inherent problematic aspects including (1) the applicability of the concepts of downbuilding and upbuilding, (2) using the absence or presence and thickness of a diapir roof as criteria to classify diapirs as active or passive, and (3) the related concept of halokinetic sequences. We offer three suggestions in the hope of improving understanding and communication. First, we suggest that the terms downbuilding and upbuilding should be abandoned. Second, we argue that passive diapirism should be used sensu lato in that the presence or absence of a thin roof should not be the key defining factor. Third, we propose that active diapirism should be reserved for two specific scenarios: (1) uplift and folding of a thick roof during diapir rejuvenation and (2) the single brief stage when salt in a precursor structure (e.g., reactive diapir, salt-cored contractional fold, or salt pillow) breaks through its roof to subsequently rise as a passive diapir.

The gas charging influences on dissolution, cementation, and clay transformation are still largely unknown during the migration and accumulation of hydrocarbons. The reservoirs in the upper Huangliu Formation of the XF13 block in the Yinggehai Basin, South China Sea, were charged by CO2 and gaseous hydrocarbons, offering us an ideal opportunity to quantitatively study the diagenetic differences caused by gas charging with different compositions. The techniques and methods used in this study include optical microscopy, scanning electron microscopy, cathodoluminescence, x-ray diffraction, chemical analysis of formation water, stable isotope analyses, and fluid inclusion examination. These analyses demonstrate that the late-stage charging of CO2-rich thermal fluid in the wells near the diapir zone modified porosity, permeability, temperature, fluid pH, and contents of K+, Na+, Mg2+, Fe2+, and other ions. Natural gas accumulations with high maturity and high CO2 contents presented at wells XF13A and XF13B near the diapir zone resulted from episodic mixing and modification. Wells far from the diapir zone (e.g., XF13E and XF13F) mostly show alkane enrichment. Late-stage charging of CO2-rich thermal fluids at wells XF13A and XF13B (in a closed system with episodic semiopen events) has resulted in significant dissolution and caused reservoirs with high porosity but low permeability. In contrast, wells far from the diapir zone were only charged by early-stage hydrocarbons, and the porosity and permeability are relatively high. This study is important in predicting reservoir quality according to gas compositions of reservoirs.

The Romanian Carpathians are of global importance in salt tectonics because they host the type locality for the term “diapir.” Although there are several historical reviews of worldwide salt tectonics, important aspects of how diapirism has been described and defined in the Romanian Carpathians are missing in these works. Many of these publications are difficult to access, and most are written in Romanian, German, French, or Polish. This paper aims to present in detail the original ideas and to describe the historical development of these concepts. We focus particularly on the work of the outstanding Romanian geologist, Ludovic Mrazec, who defined the folds with piercing core and coined the term diapir.

Exposures of salt diapirs and flanking strata in La Popa basin, in the Mexican states of Nuevo Leon and Coahuila, contain world-class examples of salt–sediment interaction that provided the basis for the concept of halokinetic sequences. The basin also contains one of the first secondary salt welds described in outcrop. The two-day field trip described here provides an easily accessible overview of salt–sediment relations within a short distance of Monterrey, the capital city of Nuevo Leon. The first day constitutes an introduction to basin stratigraphy, a traverse through halokinetic sequences at El Papalote diapir, and visits to the Cretaceous–Paleogene boundary bed and a salt-cored detachment fold near the town of Hidalgo, north of Monterrey. The second day is a visit to La Popa salt weld, where stops at several parts of the weld permit comparison of different structural styles developed along the weld. The trip begins and ends at the Marriott Courtyard Hotel, near the Monterrey airport.

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

Megaflaps are steep stratal panels that extend far up the sides of diapirs or their equivalent welds. They have multiple-kilometer fold widths and structural relief and are thus distinct from smaller-scale composite halokinetic sequences. Maximum dips range from near-vertical to completely overturned. Although overturned megaflaps are associated with flaring salt, there is no direct link between megaflap formation and the initiation of salt sheets. Strata within a megaflap are usually convergent, and the lower boundary is typically concordant with the top salt. The upper boundary ranges between a prominent onlap surface and a more diffuse zone of gradual rotation and thinning, and growth strata likewise display both onlap and stacked wedge geometries. We use quantitative cross-section restoration to elucidate the origin and development of megaflaps. Megaflaps typically represent the relatively thin roofs of early salt structures that include single-flap active diapirs, passive diapirs, salt pillows, and salt sheets. They develop during halokinetic drape folding as the minibasin sinks, during contractional squeezing of the diapir and its roof, or during some combination of the two. The kinematics are dominated by either limb rotation or kink-band migration, in which roof strata move through a fold hinge into a lengthening steep megaflap. Both restoration results and direct field evidence suggest that internal strain is minor, with little bed lengthening and thinning. Recognition and understanding of megaflaps are critical to successful petroleum exploration of three-way truncation traps against salt. Megaflaps also have implications for the lateral seal of stratigraphic traps and fluid pressures in minibasins.

We illustrate recently developed techniques of three-dimensional (3-D) geomechanical structural restoration applied to resolve the kinematics of deformation in the sedimentary cover above mobile salt. Our study area is one of the hydrocarbon-bearing domes in eastern Arabia. We used 3-D seismic reflection and well data to build a 3-D structural geomodel for the well-imaged part of the sedimentary cover. The geomodel includes faults and a 3.2-km (2-mi) thick section of Permian to Cenozoic sediments and is restored from the Jurassic to the present day. The development of the structures is characterized by stages of normal faulting in the Jurassic and Cretaceous and a subsequent stage of low-amplitude folding in the Late Cretaceous. We interpret that the development of the structures in the sediment cover is caused by the movement of a deep, nonpiercing salt pillow. The structures grew under the control of gradually changing deforming mechanisms, from dominantly faulting to folding. The transition from normal faulting to domal folding is indicative of a reactive salt diapir. These restoration results improve our understanding about the kinematic history of the structures developed within the Jurassic and Cretaceous sedimentary section, which contains most of the hydrocarbon resources in Arabia. Moreover, they illustrate the potential of geomechanical restoration methods to investigate structures above mobile salt systems.

The northern Flinders Ranges and eastern Willouran Ranges, South Australia, expose Neoproterozoic salt diapirs, salt sheets, and associated growth strata that provide a natural laboratory for testing and refining models of allochthonous salt initiation and emplacement. The diapiric Callanna Group (∼850–800 Ma) comprises a lithologically diverse assemblage of brecciated rocks that were originally interbedded with evaporites that are now absent. Using stereonet analysis to derive three-dimensional information from two-dimensional outcrops of stratal geometries flanking salt diapirs and beneath salt sheets, we evaluate 10 examples of the transition from steep diapirs to salt sheets, 3 of ramp-to-flat geometries, and 2 of flat-to-ramp transitions. Stratal geometries adjacent to feeder diapirs range from a minibasin-scale megaflap to halokinetic drape folds to high-angle truncations and appear to have no relationship to subsequent allochthonous salt development. In all cases, the transition from steep diapirs to salt sheets is abrupt and involved piston-like breakthrough of thin roof strata, which permitted salt to flow laterally. We suggest two models to explain the transition from steep diapirs to subhorizontal salt: (1) salt-top breakout, where salt rise occurs inboard of the salt flank, thereby preserving part of the roof strata beneath the sheet; and (2) salt-edge breakout, where rise occurs at the edge of the diapir with no roof preservation. Lateral emplacement of salt sheets is dependent on the interplay between the rate of salt supply to the front of the sheet and the sediment-accumulation rate. When the ratio of salt-supply rate to sediment-accumulation rate is high to moderate, thrust advance produces base-salt flats and truncation ramps, respectively. Halokinetic folds are absent because the thrust emerges at the base of the sea-floor scarp and mass-transport complexes are rare as a result of relatively low scarp relief. If the ratio is low, pinned inflation leads to drape folding of the top salt and cover into a fold ramp, with occasional slumping of the sheet and its roof and further breakout on thrust or reverse faults. In the shallow-water depositional environments of South Australia, lateral emplacement of salt sheets occurred through some combination of thrust advance, extrusive advance, and open-toed advance, with no evidence for subsalt thrust imbricates, shear zones, or continuous rubble zones. In deep-water environments, such as the northern Gulf of Mexico, thrust imbricates and rubble zones, which represent slumped carapace, are more common. The presence of slumped carapace is caused primarily by higher topographic relief related to thicker hemipelagic roofs, a lack of dissolution, and gravity-driven transport of overburden strata to the toes of large canopies.

Understanding the factors controlling the development of accommodation above collapsing salt diapirs and their influence on reservoir distribution is critical in reducing exploration risk in salt-influenced sedimentary basins. In this study, we use an integrated subsurface data set (three-dimensional and two-dimensional seismic reflection, wire-line-log, core, and biostratigraphic data) from the Upper Jurassic of the Cod terrace, Norwegian North Sea, to understand the influence of rifting on accommodation creation and shallow-marine deposition during the initial-stage collapse of salt diapirs. We demonstrate that rifting resulted in the rise and fall of salt diapirs, and the formation of supra-diapir minibasin-style depocenters that became sites for deposition and preservation of up to 500 m (1640 ft) thick net-transgressive shallow-marine sandstone reservoirs. Maximum thickness is recorded in the axis of minibasins with a reduction in thickness of up to 65% noted on their flanks. The stratigraphic architecture of individual minibasins is variable. Proximal-to-distal facies variations from shoreface to offshore shelf and commensurate changes in reservoir quality occur over scales larger than individual minibasins. These deposits contain large sand volumes, and are not confined to areas of localized sandstone subcrop. In combination, these features suggest that the minibasins formed a linked network supplied by regional sediment-routing systems. The results of this study provide a new tectono-stratigraphic model for prediction of reservoir presence, thickness, and continuity in diapir-collapse minibasins along salt walls in the Central North Sea, and in other less mature, data-poor basins where reservoirs have been identified in depocenters above salt walls.

The style of faulting in offshore Louisiana, Gulf of Mexico, is characterized by short, arcuate regional and counterregional growth faults, which commonly form complex transfer zones above shallow, Miocene level salt bodies. South Timbalier Block 54 (ST54) constitutes one such area where a basinward-dipping regional and a landward-dipping counterregional growth fault form a convergent transfer zone. Structural interpretation using three-dimensional (3-D) seismic and well data reveals that the eastern and western flanks of the structure contain salt in the footwalls of the main regional and counterregional faults. The salt rises to a much shallower stratigraphic level in the central part of the transfer zone, forming a symmetric salt diapir. Secondary antithetic and synthetic faults adjacent to the two main faults and extending into the transfer zone accommodate slip between the main faults. Kinematic restoration of a series of north–south-trending cross sections across the structure show that upslope evacuation of salt is the result of sediment loading and growth fault movement, and the location of the transfer zone is possibly controlled by the allochthonous salt. The entire area is characterized by down-to-basin movement, with the major regional and counterregional faults displaying footwall and hanging-wall fixed deformation, respectively. The presence of the crestal graben above the salt high and the timing of maximum salt evacuation from the flanks suggest that active or reactive diapirism occurred during part of the deformation history. A 3-D structural model using depth-converted horizons, balanced cross sections, and well tops accurately portrays the subsurface structure. Understanding the evolution of the structure in ST54 provides insight on similar structures in other areas in offshore Louisiana and the relationship between salt evacuation and transfer zone development.

Thirty-seven mudstone samples were collected from the uppermost Lower Mudstone Member of the Potrerillos Formation in El Gordo minibasin within La Popa Basin, Mexico. The unit is exposed in a circular pattern at the earth's surface and is intersected by El Gordo diapir in the northeast part of the minibasin. Vitrinite reflectance (Ro) results show that samples along the eastern side of the minibasin (i.e., south of the diapir) are mostly thermally immature to low maturity (Ro ranges from 0.53% to 0.64%). Vitrinite values along the southern, western, and northwestern part of the minibasin range between 0.67% and 0.85%. Values of Ro immediately northwest of the diapir are the highest, reaching a maximum of 1.44%. The results are consistent with two different possibilities: (1) that the diapir plunges to the northwest, or (2) that a focused high-temperature heat flow existed along just the northwest margin of the diapir. If the plunging diapir interpretation is correct, then the thermally immature area south of the diapir was in a subsalt position, and the high-maturity area northwest of the diapir was in a suprasalt position prior to Tertiary uplift and erosion. If a presumed salt source at depth to the northwest of El Gordo also fed El Papalote diapir, which is located just to the north of El Gordo diapir, then the tabular halokinetic sequences that are found only along the east side of El Papalote may be subsalt features. However, if the diapir is subvertical and the high-maturity values northwest of the diapir are caused by prolonged, high-temperature fluid flow along just the northwestern margin of the diapir, then both of these scenarios are in disagreement with previously published numerical models. This disagreement arises because the models predict that thermal anomalies will extend outward from a diapir a distance roughly 1.5 times the radius of the diapir, but the results reported here show that the anomalous values on one side of the diapir are about two times the radius, whereas they are as much as five times the radius on the other side of the diapir. The results indicate that strata adjacent to salt margins may experience significantly different heat histories adjacent to different margins of diapirs that result in strikingly different diagenetic histories, even at the same depth.

The Tarim Basin is one of the most important hydrocabon-bearing evaporite basins in China. Four salt-bearing sequences, the Middle and Lower Cambrian, the Mississippian, the Paleogene, and the Neogene, have various thickness and areal distribution. They are important detachment layers and intensely affect the structural deformation in the basin. The Kuqa depression is a subordinate structural unit with abundant salt structures in the Tarim Basin. Salt overthrusts, salt pillows, salt anticlines, salt diapirs, and salt-withdrawal basins are predominant in the depression. Contraction that resulted from orogeny played a key function on the formation of salt structures. Growth strata reveal that intense salt structural deformation in the Kuqa depression occurred during the Himalayan movement from Oligocene to Holocene, with early structural deformation in the north and late deformation in the south. Growth sequences also record at least two phases of salt tectonism. In the Yingmaili, Tahe, and Tazhong areas, low-amplitude salt pillows are the most common salt structures, and these structures are commonly accompanied by thrust faults. The faulting and uplifting of basement blocks controlled the location of salt structures. The differences in the geometries of salt structures in different regions show that the thickness of the salt sequences has an important influence on the development of salt-cored detachment folds and related thrust faults in the Tarim Basin. Salt sequences and salt structures in the Tarim Basin are closely linked to hydrocarbon accumulations. Oil and gas fields have been discovered in the subsalt, intrasalt, and suprasalt strata. Salt deformation has created numerous potential traps, and salt sequences have provided a good seal for the preservation of hydrocarbon accumulations. Large- and small-scale faults related with salt structures have also given favorable migration pathways for oil and gas. When interpreting seismic profiles, special attention needs to be paid to the clastic and carbonate interbeds within the salt sequences because they may lead to incorrect structural interpretation. In the Tarim Basin, the subsalt anticlinal traps are good targets for hydrocarbon exploration.

Three aspects of basement structure and rift-related salt distribution have especially influenced the evolution of the deep-water northern Gulf of Mexico: (1) creation of a basement high (Toledo Bend flexure), separating a chain of interior basins from the central Louann salt basin, (2) segmentation of the central Louann salt basin by the Brazos transfer fault into eastern and central domains, and (3) salt provinces formed during basin opening. The Toledo Bend flexure was reactivated as a hinge during the Cenozoic uplift of the North American craton. This uplift triggered gravity gliding, forming fold belts in the seaward parts of the continental margin. The geometry of the Toledo Bend flexure influenced the position of these fold belts. The Brazos transfer fault separates the west sector of the study area from the central and east sectors. Most of the salt in the deep-water northern Gulf of Mexico lay in the central sector, which sourced most of the Sigsbee salt canopy. The western sector was narrower and was subdivided by the East Breaks basement high. Splitting the Callovian salt basin in two as the gulf opened created a southward-thinning wedge of salt at the seaward end of the northern Gulf of Mexico. We divide this wedge into a series of provinces on the basis of the geometry of the base of the deep salt. Original salt thickness influenced diapir location, the geometry of the Sigsbee canopy, the geometry and style of later compressional fold belts, and petroleum systems.

The three-dimensional (3-D) geometry of fractures and fault-related dolomite is difficult to access with classical subsurface prospection tools. Therefore, we have investigated an outcrop to improve the subsurface prediction for complex dolomite bodies. This outcrop is located in the Etoile massif (southeastern France) within a fault-bend anticline. The sedimentary units are of Upper Triassic to lower Barremian age. The fold results from the Pyreneo-Provencal shortening during the Late Cretaceous to the Eocene. The anticline hosts three types of dolomite bodies: (1a) massive dolomite of middle to late Oxfordian age, (1b) syndepositional stratabound dolomite of Tithonian age, and (2) isolated dolomite bodies associated with fractures and faults. Large-scale geometries of fault-related dolomite bodies have been modeled in 3-D. The 3-D geometries of these bodies show diapir-, finger- and wall-like structures. These bodies are located close to the main thrusts, in strata of middle Oxfordian to early Barremian age and are linked to the compressive fold-bending phase during the Late Cretaceous. Fault-related dolomitization occurred because of magnesium removal from the hydraulic brecciation and the pressure solution of type 1 dolomite with overpressured fluids. These fluids flushed upward along the main thrust and laterally by following the reservoir property contrasts in the host rocks. Fault-related dolomite bodies are either spread far apart from faults in grainy limestones with good initial reservoir properties or are restricted to fault vicinity in muddy limestones with poor initial reservoir properties. The study of the structural and stratigraphic framework was essential in the understanding of the dolomitization process.

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

The lower continental slope of Morocco's west coast consists of Triassic-age salt manifested in the form of diapirs, tongues, sheets, canopies, and toe thrusts. Active salt diapirism and regional tectonics greatly influence the morphology of the modern sea floor, forming a severely rugose expression with ongoing minibasin development and episodic submarine failure. Detailed mapping of a 1064-km2 (411-mi2) seismic survey acquired in the Safi Haute Mer area revealed that Jurassic to Holocene salt mobilization continually affected distribution of sediment, causing a range of depositional flow styles, from slumps to sheet slides and mass-transport complexes (MTCs). Large sediment waves (20 km [12 mi] long, 1.5-km [0.9-mi] wavelength) were also documented at the end of the Aptian. An east-west–trending structural anticline downdip of the salt activated during initiation of the Atlas uplift in the latest Cretaceous to earliest Tertiary and shaped much of the lower slope into the Tertiary with a persistent canyon system and slope channels. The largest of the debris flows is a Cretaceous-age MTC, a 500-m (1640-ft)-thick flow that spans an area of up to 20,000 km2 (7722 mi2). Composing the MTC are (1) chaotic, mounded seismic facies; (2) internal syndepositional thrusts; and (3) transported megablocks (3.3 km2 [1.3 mi2]) with preserved internal stratigraphy. The MTC originated from an upslope collapse of a narrow shelf during the earliest phases of the Alpine orogeny.

This article documents the application of techniques in quantitative seismic geomorphology in quantifying the morphometrics and architecture of deep-marine leveed-channel systems within an about 10,000-km2 (3861-mi2) study area offshore eastern Trinidad, West Indies. The principal goal of this study is to assess the relationship, if any, between sea-floor morphology and channel and levee architecture and morphology toward the development of predictive models of reservoir distribution and channel-system morphology that might be applicable to the interpretation of these types of deposits in similar settings around the world. Seven channel systems, classed into three types, within a 200-ms interval of data immediately below the modern sea floor provided the data for analysis. Results suggest that local structural features and sea-floor slopes exert more influence on channel morphology and occurrence than do eustatic sea level factors. Sinuosities, channel widths, meander-belt widths (MBWs), and meander-arc height (MAH) all increase as the channel systems age. Slope and sinuosity are directly related to one another, with sinuosity increasing as slope increases. Levee heights and widths increase downslope in areas of lower slope gradients. Channel sinuosity, MAH, and MBW increase immediately downslope from localized diapirs, and channels appear to migrate updip over time because of regional inflation of distal arc prism areas. Diapirs and uplifts cause overbank splays to become more confined and cause levees to shorten and taper rapidly. Regional tilt to the south appears to affect accommodation, creating a sink for sediments near the toe of slope. Regional tilt also affects flow processes in the channels, causing an increased overbanking of flows toward the south, away from the plate margin, resulting in higher, wider levees on the south sides of the channels.

This study characterizes Miocene chronostratigraphy and plays in the Burgos Basin and adjacent south Texas within an area of approximately 39,700 km2 (15,300 mi2), onshore and offshore (to the 500-m [1640-ft] isobath). Using greater than 40,000 linear kilometers (25,000 mi) of two-dimensional seismic lines, 115 onshore wells, 9 offshore wells, and paleontological data, we established a correlation framework of 9 key surfaces (upper Oligocene to lower Pliocene) representing major (probably third-order) sequence boundaries and maximum flooding surfaces throughout the basin. Five of the Burgos Miocene surfaces coincide with regional chronostratigraphic surfaces from the Veracruz and Laguna Madre-Tuxpan basins, thus establishing a consistent correlation framework throughout much of the Mexican Gulf Coast Basin. Twenty Miocene plays are defined by four age divisions (lower Miocene, middle Miocene, upper Miocene_1, and upper Miocene_2) and four paleogeographic settings (unexpanded and expanded shelf, proximal slope, and distal slope). Because of proven high productivity in salt-bounded basins in the northern Gulf of Mexico, the onlap of strata onto diapirs in the eastern Burgos salt province was evaluated as a fifth setting. The paleogeographic provinces and onlap areas exhibit characteristic seismic facies, stratal geometries, and structural relations; a characterization of each one of these being key to the overall play evaluation. This play framework provides the means for continuing exploration of Miocene strata and evaluation of key play elements (reservoir presence and quality, seal, trap, source, and migration and timing) in this structurally complex, underexplored basin. The relative importance of these play elements varies systematically for each play, especially between the onshore shelf plays and the offshore deep-water plays, where fault complexity and stratigraphic variability are greater.

Major Miocene structural elements of the Burgos Basin include a regionwide detachment system that connects extensional fault systems throughout the basin with an active diapir belt downdip, a regionwide pattern of downthrown extensional rollover folds, pervasive secondary faults, and salt and shale diapiric masses in the eastern part of the basin. An interpretation of two-dimensional seismic data suggests that the Burgos Basin Miocene section can be divided into four structural domains: expanded zone, Lamprea trend, Corsair-Wanda trend, and diapir belt. The westernmost unexpanded zone is the footwall of the expanded system part of the basin, which overlies a domain of Oligocene extension. Remaining trends represent an extensional accommodation related to the basinward migration of mobile salt and shale, which has produced a relatively uniform structural style in the Miocene section. The structural style observed in the Burgos Basin appears to define a transitional zone between gravitational collapse in the offshore Laguna Madre-Tuxpan shelf to the south and salt-related raft tectonics of the south Texas Gulf Coast.

The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africa's Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeirogenic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleoslope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward.

In the South Oman salt basin (SOSB), diapirs of infra-Cambrian Ara Salt enclose isolated, commonly overpressured carbonate reservoirs. Hydrocarbon-impregnated black rock salt shows that it has repeatedly lost and then regained its sealing capacity. The black staining is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil. The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In our model, the oil pressure in the carbonate reservoirs increases until it is equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below the minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values.

Fluviolacustrine strata host significant hydrocarbon volumes in basins characterized by syndepositional growth of passive salt diapirs. An understanding of salt-sediment interaction is critical to the prediction of reservoir distribution and architecture in these strata. Large-scale stratal geometries and thickness changes resulting from salt movement are commonly apparent on seismic data, but to date, there are few predictive models for facies architecture at subseismic, reservoir scale. This article uses a high-quality outcrop data set of fluviolacustrine strata in an exhumed salt basin (Upper Triassic Chinle Formation, Paradox Basin, Utah) as an analog for improved understanding of subsurface data sets of similar structural and sedimentological setting. Salt-sediment interaction in the Chinle Formation is expressed by localized lateral variations in stratigraphic thickness, angular stratal relationships, and changes in facies architecture. Based on these criteria, there is evidence for salt-sediment interaction across a series of syndepositional salt structures, including anticlines above buried salt pillows, salt walls exposed at surface, and salt-withdrawal minibasins. Stratigraphy and facies architecture across these structures reflect the following controls: regional subsidence, localized differential accommodation space, and localized paleogeomorphology. Both localized controls were driven by syndepositional salt movement, which exhibited subtle spatial and temporal variations during the deposition of the Chinle Formation. The outcrop data set is used to develop generic predictive models of facies distributions and architectures resulting from different conditions of regional tectonic subsidence and/or fluvial energy. Analysis of stratigraphic expansion across syndepositional passive diapirs suggests that the outcrop-derived models are applicable to many subsurface data sets.

This article presents the results of a finite-thickness, three-dimensional (3-D) kinematic model for the fault-related deformation above an active diapir. Based on field examples, the models are (1) circular in map view; (2) offset by radially distributed normal faults; (3) faulted or smooth at the top of the salt; (4) relatively unstrained on the flanks; and (5) highly deformed in the central graben, if present. Field examples suggest that active diapirs may initiate with either two or three master faults crossing near the dome crest. Subsequent evolution depends mainly on whether the master faults cut the salt or detach on the salt top. If the top of the salt is faulted, no minor faults are required except in the central graben. If the top of the salt is unfaulted, additional faulting occurs in the flank grabens. In models with three master faults, very different structural patterns appear in cross sections cut parallel and perpendicular to any of the master faults. The domes are asymmetrical in the fault-perpendicular cross sections, but are symmetrical in the fault-parallel cross sections. The initial fault pattern may influence the piercement efficiency. An active dome with faults offsetting the top of the salt grows more rapidly than a dome with a smooth top and may develop into a passive diapir more rapidly. The model map patterns are used to reinterpret the 3-D structure of the Clay Creek dome, Texas, and the Reitbrook dome, Germany. The model predictions are confirmed by the cross sections, and small, but potentially significant, refinements are proposed.

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.

Gas seepages along the Ionian coast of the northwestern Peloponnesus (Greece), at Killini, Katakolo, and Kaiafas reflect deep hydrocarbon-generation processes and represent a real hazard for humans and buildings. Methane microseepage, gas concentration in offshore and onshore vents, and gas dissolved in water springs, including the isotopic analysis of methane, have shown that the seeps are caused by thermogenic methane that had accumulated in Mesozoic limestone and had migrated upward through faults, or zones of weakness, induced by salt diapirism. A link between local seismicity and salt tectonics is suggested by the analyses of hypocenter distribution. Methane acts as a carrier gas for hydrogen sulfide produced by thermal sulfate reduction and/or thermal decomposition of sulfur compounds in kerogen or oil. Methane seeps in potentially explosive amounts, and hydrogen sulfide is over the levels necessary to induce toxicological diseases and lethal effects.

Fingerlike bodies of evaporite rocks have been observed in many regions affected by multiple tectonic phases, such as the Atlas, the Pyrenees, or the Zagros Mountains, where they have been interpreted as fault-plane diapiric injections or collapse-related salt welds. In this article, we suggest a new interpretation of these structures as squeezed diapirs based on detailed structural and sedimentological field data of the Bicorb–Quesa diapir (eastern Prebetics) and offer five analog models, which simulate the polyphase deformation of the eastern Prebetics. In this area, diapirs formed from the Oligocene to Langhian during an extensional phase related to the opening of the Valencia Trough. These diapirs were later affected by a Serravallian contractional phase, which inverted the preexisting grabens and created new folds and thrusts. The preexisting diapirs were necked and/or squeezed, forming secondary welds with a fingerlike geometry that isolated the diapir bulbs from their source layer. Extrusion of diapiric material was also accelerated during this phase, but no new diapirs formed. Finally, the area was again affected by an extensional phase during the Tortonian, which reactivated the normal faults and created a new set of diapirs. The new diapirs formed where the overburden was thinner, that is, at the toe of the major reactivated faults. Commonly, these faults coincide with the bounding faults of the major grabens formed during the first extensional phase, and therefore, the new diapirs grow close to the location of the squeezed diapirs. The models also show that the faults created during the initial extension prevailed as the main focus for deformation during the polyphase history. Deformation in the overburden and the viscous layer was mainly accommodated along the major grabens formed during the first extensional stage. During shortening, the initial major grabens deformed as complex anticlines, and during the subsequent extensional phase, most deformation occurred by the collapse of these anticlines along preexisting faults, fault welds, and the flank of the squeezed diapirs. The source layer is compartmentalized, accumulating and withdrawing material in the same locations (the initial grabens and horst). As a result, the source layer is easily depleted beneath the initial horst, forming primary welds.

New regional (two-dimensional) seismic reflection data, published Deep-Sea Drilling Project–Ocean Drilling Program reports, and unpublished shallow-offshore well information characterize the Mesozoic–Cenozoic evolution of the western Iberian continental slope north of 3845N. Two distinct sectors bounded by first-order transfer faults exist between the Galicia Bank and the Nazar fault. The northernmost sector 1 is filled by Triassic–Aptian (prebreakup) sequences, reaching more than 3.5 s two-way traveltime (TWTT) in thickness in distinct half grabens. Salt pillows, salt ridges, minibasins, and salt-detached overburden faults were generated during the Mesozoic and reactivated in the Cenozoic. Sector 2 shows Triassic–Jurassic units more than 2.0 s TWTT thick, underlying east-tilting half grabens of Early Cretaceous age. Salt structures in this sector evolved into mature salt diapirs. Postbreakup units are up to 2.0 s TWTT thick in both sectors. The evolution of the study area replicates evolutionary settings that have previously been proposed for nonvolcanic passive margins. However, some distinct features are noted: (1) widespread Triassic–Berriasian units deposited over rotated tilt blocks represent the early rifting stage; (2) Early Cretaceous subbasins showing rift-climax units, most likely formed during the advanced rifting stage, are spatially constrained to an approximately 100-km (62-mi)-wide region stretched along the continental slope; and (3) listric blocks and their associated low-angle (deep) detachment faults, formed on the distal margin during the advanced rifting and transition to sea-floor spreading stages, show no developed rift-climax units above them. From the early rifting stage onward, Mesozoic faults and halokinetic structures induced local differences in the thickness and character of seismic facies. Cenozoic (Alpine) tectonism promoted the reactivation of older Mesozoic structures.

Using conceptual reasoning and results from physical models, we describe the mechanical and kinematic characteristics of sediment wedges spreading seaward above a viscous evaporitic layer. Spreading can occur if the distal sediment overburden is thin and weak, or if it comprises preexisting salt bodies. Spreading is accommodated by proximal extension and an associated rise of diapiric ridges, by midslope seaward translation, and by distal shortening. Rapid sediment progradation can lead to the reactivation of older distal folds by later extension. Either retrogradation or renewed sedimentation following a long depositional hiatus can reactivate older grabens and diapirs in shortening.

Neogene shelf, slope, canyon, and slope-to-basin-floor transition plays in the southern Laguna Madre–Tuxpan (LM-T) continental shelf reflect a variety of structural and stratigraphic controls, including gravity sliding and extension, compression, salt evacuation, and lowstand canyon and fan systems. The Neogene in the LM-T area was deposited along narrow shelves associated with a tectonically active coast affected by significant uplift and erosion of carbonate and volcanic terrains. This study characterizes 4 structurally defined trends and 32 Neogene plays in a more than 50,000-km2 (19,300-mi2) area linking the Veracruz and Burgos basins. The Caonero trend in the southern part of the LM-T area contains deep-seated basement faults caused by Laramide compression. Many of these faults are directly linked to the interpreted Mesozoic source rocks, providing potential pathways for vertically migrating hydrocarbons. In contrast, the Lankahuasa trend, north of the Caonero trend, contains listric faults, which detach into a shallow horizon. This trend is associated with thick Pliocene shelf depocenters. The dominant plays in the Faja de Oro–Nyade trend in the central part of the LM-T area contain thick lower and middle Miocene successions of steeply dipping slope deposits, reflecting significant uplift and erosion of the carbonate Tuxpan platform. These slope plays consist of narrow channel-fill and levee sandstones encased in siltstones and mudstones. Plays in the north end of the LM-T area, in the southern part of the Burgos basin, contain intensely deformed strata linked to salt and shale diapirism. Outer-shelf, slope, and proximal basin-floor plays in the Lamprea trend are internally complex and contain muddy debris-flow and slump deposits. Risk factors and the relative importance of play elements vary greatly among LM-T plays. Reservoir quality is a critical limiting play element in many plays, especially those in the Caonero trend directly downdip from the trans-Mexican volcanic belt, as well as carbonate-rich slope plays adjacent to the Tuxpan platform. In contrast, trap and source are low-risk play elements in the LM-T area because of the abundance of large three-way and four-way closures and the widespread distribution of organic-rich Upper Jurassic Tithonian-age source rock. The potential for hydrocarbon migration in LM-T plays is a function of the distribution of deep-seated faults inferred to intersect the primary Mesozoic source. Their distribution is problematic for the Lankahuasa trend, where listric faults sole out into the Paleocene. Seal is poorly documented for LM-T plays, although the presence of overpressured zones and thick bathyal shales is favorable for seal development in middle and lower Miocene basin and slope plays.

Unique outcrop exposures of two salt diapirs, a secondary salt weld, and associated syndiapiric strata in northeast Mexico offer an important perspective on salt-influenced petroleum reservoirs by allowing recognition and description of salt-related sandstone depocenters. Spectacular progressive unconformities and halokinetic sequences, coupled with laterally continuous exposures, permit accurate correlation and interpretation of syndiapiric units. Analysis of the syndiapiric Upper Cretaceous, Delgado Sandstone Member (Potrerillos Formation) delineates regional shoreline sediment dispersal locally impacted by diapiric relief and the distribution and internal character of salt diapir-proximal sandstone depocenters. Sequence-stratigraphic correlation defines striking relationships between highstand (HST) and transgressive systems tracts (TST), stratal thinning trends, and salt diapir relief. Transgressive systems tract and highstand systems tract strata show thinning and lithofacies shoaling trends toward diapirs; however, the latter is more pronounced in the HST and occurs at a greater distance from salt diapirs (within 1–2 km [0.6–1.2 mi]). Sandstone depocenters, roughly 0.5–1.0 km (0.3–0.6 mi) wide and 0.5–0.2 km (0.3–0.1 mi) thick, are present in both TST and HST strata and consist of sandier, shallower water facies. However, depocenters are better developed in TST strata as thicker stratigraphic sections on updip diapir margins. We propose that sandstone depocenters formed by preferential sediment reworking and shelf ridge development on landward diapir margins during marine transgression. Elevated diapir relief and higher subsidence rates adjacent to salt diapirs likely enhanced this process. Additionally, depocenters adjacent to El Papalote diapir are smaller and contain deeper water facies than the age-equivalent depocenters adjacent to El Gordo diapir, suggesting that it had higher, broader sea-floor relief.

The Mesozoic history of the Pyrenean region was dominated by the opening of the North Atlantic Ocean and the left-lateral movement of Iberia relative to Europe. Most deformation was tensional or transtensional, but this style was interrupted by several short compressional events, perhaps related to bends in the transform plate boundary or minor changes in plate movement directions. These compressional events were recorded by deformation adjacent to and above salt structures, which were the weakest parts of the system and so served as especially sensitive strain barometers.The alternation of extension and shortening controlled the location and style of salt structures. Evaporites were initially deformed during the latest Jurassic–Early Cretaceous Neocimmerian transpression, which formed salt-cored anticlines above west-northwest–east-southeast–trending basement faults. Transtension in the Aptian–Albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest-southeast– and northeast-southwest–trending basement structures. Transtension was at least locally interrupted by brief periods of transpression at the Aptian–Albian and Albian–Cenomanian boundaries. Transpression shortened the diapirs, causing rotation, uplift, and erosion of beds near the salt. Most diapirs were buried during the Late Cretaceous. In some cases, postburial hydrothermal circulation dissolved much of the halite, causing the diapirs to sag and the basins to form in roof strata. Finally, the entire region was shortened during the Late Cretaceous–Tertiary Pyrenean orogeny, which greatly distorted many of the preexisting geometries.

Seismic data from the Angolan margin and laboratory experiments on brittle-ductile models are used to study thin-skinned deformation above the salt at margin scale, which is characterized by extension upslope and contraction downslope. The initial geometry of the salt basin was wedging out, both landward and seaward, and the salt was entirely covered by sediments at the onset of gravity-driven deformation. Downslope contraction is accommodated by upslope extension. The upslope extensional domain is subdivided into three subdomains with (1) sealed tilted blocks, (2) growth fault and rollover systems, and (3) extensional diapirs. We interpret this particular arrangement of extensional structures as directly resulting from variations in mechanical coupling between brittle (sediments) and ductile (salt) layers. The downslope contractional domain is subdivided into three subdomains with (1) diapirs squeezed at late stage, (2) polyharmonic folds and thrust faults developed at early stage, and (3) folds and thrusts developed at late stage. This structural zoning results from the initiation of contraction at a distance from the salt toe and further migration of contraction immediately downslope and upslope.

Geochemical and isotopic data indicate the presence of CO2 of both organic and inorganic origin in the natural gas reservoirs of the Yinggehai Basin. The natural gases with inorganic CO2 commonly show a high content of CO2, ranging from 15 to 85%; a heavier carbon isotope value of CO2, from 0.56 to 8.16; and a lower 3He/4He ratio, ranging from 0.20 to 6.79 107, indicating a crustal origin. These gases occur locally, commonly related to diapir structures. Natural gases rich in hydrocarbons occur widely and are characterized by a low CO2 content, from 0.1 to 5.0%, and a lighter C1 carbon isotope value from 10.59 to 20.7, indicating an organic origin. Geological background and geochemical data indicate that the Sanya and Meishan formations are the main source of hydrocarbon gases and the organic CO2. Pyrolysis experiments on Tertiary calcareous shales and thermal history modeling both suggest that the calcareous shales occurring in the lower Miocene strata are the main source of the inorganic CO2 gas, whereas thermal contact metamorphism of the Paleozoic carbonates and/or magmatic CO2 may have made only a small contribution. Abnormally high paleogeothermal gradients (4.25–4.56C/100 m; 12.09–12.26F/100 ft) and a rapid heating rate caused the lower Miocene calcareous shales to reach the threshold temperature (about 300C [570F]) of their thermal decomposition at the burial depth of about 6500 m (21,300 ft) and to generate great volumes of inorganic CO2 gas. Diapir faults acted as the main pathways for the upward migration of deep inorganic CO2 gases into reservoirs connected with shale diapirism along the central Yinggehai Basin. The heavier carbon isotope values of associated methanes and a strong thermal anomaly in the CO2-rich gas reservoirs provide evidence that the inorganic CO2 gas migrated into the reservoirs later than their associated hydrocarbon-rich gases. This suggests that the earlier formed traps and sandstone reservoirs distant from shale diapir structures may have greater potential in the exploration for hydrocarbon-rich gases.

Halokinetic and slope-instability processes have sculpted numerous morphological features on the flanks of the intraslope basins in the Bryant Canyon area. High-resolution geophysical data and long sediment cores (as much as 20 m [66 ft] long) were used to define the time and spatial evolution of sediment failures and their relationship to halokinetic processes. Two episodes of increased salt-tectonic activity are defined: (1) The first acted at the beginning of interglacial oxygen isotope stage 5 as salt adjusted to the abandoned environments of the Bryant and Eastern Canyon systems, and (2) the second occurred during the last glacial period and is characterized by the seaward propagation of salt masses. Three types of slopes are recognized in the intraslope basins: (1) highly inclined slopes with low-relief morphologic features resulting from shallow, translational slump complexes, (2) highly inclined slopes with high-relief morphologic features resulting from deep, rotational slump complexes, and (3) highly inclined slopes dissected by high-relief canyonlike landslide troughs resulting from channelized rotational slumps. The first two slope types occur mainly on the northern flanks of the basins, whereas the third type occurs on the southern flanks. We propose that the slump complexes on types 1 and 2 slopes were triggered by the oversteepening of the flanks by the seaward mobilization of underlying salt masses. The channelized rotational slumps on type 3 slopes are interpreted to result from the development of salt diapir bulges that lead to locally increased gradients on the basin flanks. Most of the sediment failures have been transformed into debris flows and led to the most recent phase of infilling of the basin floors.

This article introduces a technique for three-dimensional inverse modeling of geothermal heat conduction through heterogeneous media. The technique is used to determine the basal geometry of a diapiric salt structure found on the continental slope offshore Texas. Salt is two to four times more thermally conductive than other sedimentary rocks. The geothermal field is perturbed by the presence of salt and results in an anomaly in the heat flow through the seafloor. The spatial variation pattern of the anomalous heat flow reflects the geometry of the salt body. The inverse modeling obtains a model for the thermal-conductivity structure that causes the heat-flow anomaly observed on the seafloor. The inversion algorithm systematically searches for an optimal thermal-conductivity model by iteratively minimizing the misfit between the model-predicted and the observed heat flow. To reduce the problem of nonuniqueness, the inversion incorporates a priori information constrained independently, such as the upper surface geometry of the salt and the lateral extent of the salt body, which can be delineated by a limited coverage of two-dimensional seismic data. In addition, it is assumed that the thermal conductivity of the sedimentary strata surrounding the salt body is well constrained. This inversion method is applied to a heat-flow data set obtained over a salt structure on the Texas continental slope. The salt structure was first surveyed with the single-channel seismic reflection, which yielded the a priori information necessary. The base of the salt was not imaged seismically. Then, three dozen heat-flow measurements were obtained on the seafloor over and off the salt feature. The inverse heat-flow modeling performed here shows that this structure is a salt tongue, which has a diapiric root on one side. According to the most optimal thermal-conductivity model obtained, the root seems to extend to 6 km below the seafloor. Refinement in the model geometry and additional constraints on thermal conductivities of the surrounding strata should yield a model that is more detailed and would allow more thorough geological interpretation of the salt structure.

Release faults are rift cross faults, which develop to accommodate the variable displacements of the hanging-wall block along the strike of normal faults. Release faults are nearly perpendicular or obliquely oriented to the strike of the normal fault they are related to. They have maximum throws adjacent to the parent normal fault and die out in the hanging wall away from it. They form to release the bending stresses in the hanging wall and do not reflect the orientation of the regional stress field in a basin. Commonly, they show normal-oblique displacements and are preferentially located along the strike ramps. Release faults may also act at the scale of an entire basin, reaching displacements of thousands of meters. Joints, shale, and salt diapirs may develop in association with release faults. Because all these structures represent domains of stress release, they may work as conduits for oil migration and oil traps in extensional basins. This is the case of the Reconcavo basin in northeastern Brazil, a Cretaceous failed rift, connected to the eastern Brazilian continental margin basins. In the Reconcavo basin, two large-scale release faults, with displacements in the order of 3 km, developed in the hanging wall of the rift border faults and control the location of the main oil fields.

Drag folds bordering salt diapirs are commonly attributed to shear from rising salt. Finite-element models demonstrate that shear deformation is only significant for extremely weak overburdens, such as those having high overpressure and ductile interlayers. Protrusions of overburden into and onto the diapir are most susceptible to folding. So-called drag folds are much more likely to initiate as drape folds of strata onlapping a downbuilding diapir, here termed flap folds for clarity. Potential for flap folding is greatest where the salt/overburden contact dips moderately and for episodic or variable timing and thickness of deposition. Moderate dip allows strata to onlap far across the diapir crest and be carried upward into a high-amplitude fold with the rising salt. Depositional hiatuses allow time for such rise without additional layers that would strengthen the onlapping wedge. Previously folded strata may block onlap of later layers. Stretching and rotation of the layers adjacent to steep parts of the diapir may cause disaggregation, slumping, and debris deposits that can be overridden as the diapir crest spreads laterally onto the overburden surface. Large changes in the relative rates of salt rise and sediment deposition can create cycles of onlap, flap folding, and spreading.

Strata adjacent to exposed diapirs in La Popa basin, northeastern Mexico, comprise stacked halokinetic sequences consisting of unconformity-bounded packages of thinned and rotated strata cut by radial faults. Deformation results from shallow drape folding over the flanks of the rising diapirs and not from deep drag folding in diapir-peripheral shear zones. Subsurface analogs from the Gulf of Mexico have diapir-flanking geometries ranging from similar, wide zones of upturned and thinned strata to undeformed, constant-thickness strata. Subhorizontal salt tongues display little subsalt deformation and thinning.We propose a general model for passive diapirism and flank deformation that includes (1) gradually varying salt-flow rates, (2) superposed episodic sedimentation that results in changing bathymetric relief, (3) rotation of near-surface strata as salt inflates relative to the adjacent basin, (4) failure and erosion of strata in the steepening bathymetric halo, and (5) bedding-parallel slip surfaces that converge on unconformities and onlap surfaces. A primary factor influencing flank geometries is the width of the bathymetric high extending beyond the diapir edge. This is largely dependent on the thickness of the halokinetic sequence onlapping the diapir, which in turn is controlled mostly by the interplay between salt inflation/deflation rates and sedimentation rates. Other factors include the amount of concurrent shortening, which produces a wider but less intense zone of deformation, and the position on the scarp of salt breakout and extrusion.Our model is important for exploration and production in diapir-flank and subsalt settings because of its implications for trap size and geometry, reservoir distribution, trap compartmentalization and pressure seals, and hydrocarbon charge. It can help in explaining complex and enigmatic well data and in better assessing risk in areas of poor seismic imaging.

The stratigraphy adjacent to the El Papalote diapir in the La Popa basin, northeastern Mexico, displays depositional thinning, abrupt lateral facies changes, and intense local deformation near the diapir. The strata comprise a series of halokinetic sequences that provide a means of local correlation of stratal packages in an otherwise complex patchwork of seemingly disparate facies. Halokinetic sequences are relatively conformable successions of growth strata genetically influenced by near-surface or extrusive salt movement and are locally bounded at the top and base by angular unconformities that become disconformable to conformable with increasing distance from the diapir. Halokinetic sequences differ from traditional depositional sequences in scale and mechanism of formation. Halokinetic sequences at El Papalote diapir could not be traced farther than 1 km from the diapir, whereas depositional sequences are typically basin wide. Halokinetic sequences form as the rate of net vertical diapiric rise varies relative to the local sediment-accumulation rate, whereas depositional sequences form as the accommodation rate varies relative to the regional sediment-accumulation rate. Angular unconformities form when the net diapiric-rise rate exceeds the local sediment-accumulation rate, allowing diapiric inflation at the surface to generate steep, unstable slopes along which subjacent growth strata are either truncated by attendant slope failure or by current or shoreface erosion. In the case of slope failure, the sequence-bounding unconformity is typically overlain by mass-transport deposits derived from gravitational failure of the domed salt body. Increasing the local sediment-accumulation rate relative to the net diapiric-rise rate results in diapir onlap and overlap, which suppress diapiric surface topography and erosion. Halokinetic sequences are previously unrecognized but probably common features around near-surface or extrusive salt bodies in salt basins found elsewhere in the world. Their understanding may be used to predict the geometry, distribution, and quality of reservoir facies directly adjacent to salt bodies and provide critical data to determine the complex evolution of migrating passive salt bodies.

The Davis Hill dome is a relatively large salt stock located in the northeastern part of the Houston salt dome province. The minimum depth is about 800 ft to the cap rock and about 1,200 ft to salt. The source layer (Jurassic Louann Salt) lies at a depth of at least 30,000 ft. Seismic and well log interpretation suggests that diapir growth took place from Late Cretaceous to Plio-Pleistocene. Passive growth (downbuilding) of the dome ceased during the Early Oligocene (Vicksburg) and was followed by active growth (upbuilding) of the dome. Present-day topographic relief at Davis Hill is 175 ft above the surrounding coastal plain. A radial pattern of oxbow lakes above the dome suggests that the dome still is rising, but the topographic maximum does not correspond to the apex of the dome. Studies of cores indicate that the cap rock is about 440 ft thick and comprises, from bottom to top, anhydrite, gypsum and calcite zones. The anhydrite zone represents the less soluble components of the salt that accumulated as the halite was dissolved by pore fluids. The calcite zone formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone in these cores contains significant amounts of sulfide minerals and oil. The sulfiderich calcite zone is approximately 40 ft thick and is dominated by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool ground waters.

We used three- and two-dimensional seismic data, well and biostratigraphic constraints, and structural restorations to evaluate the evolution of the Mahogany salt body and surrounding strata on the central Louisiana outer shelf of the northern Gulf of Mexico. The history of emplacement and subsequent modification of the Mahogany salt body is divided into six stages: (1) growth of a basinward-leaning salt ridge sourced from a deep allochthonous salt sheet prior to 7.5 Ma; (2) loading-induced evacuation of a large volume of salt from the sheet and ridge through a narrow feeder, resulting in rapid salt flow and consequent radial growth of a bulb-shaped salt stock between 7.5 and 4.3 Ma; (3) gravitational collapse of the inflated salt stock after depletion of the deep source layer, leading to basinward extrusion of a subhorizontal salt tongue and its rafted overburden between 4.3 and 3.65 Ma; (4) burial of the composite salt body during a period of slow sedimentation between 3.65 and 1.95 Ma; (5) basinward translation of the overburden resulting from gravity gliding/spreading on the upper slope between 1.95 and 0.5 Ma, causing extension, reactive diapirism, and matching contraction; and (6) cessation of lateral translation once the shelf margin prograded past the salt body, and subsequent loading of the salt driving active diapirism in the footwalls of normal faults. The Mahogany salt body provides an excellent case study that illustrates both the value and limitations of published simple models for allochthonous salt, and the reconstructed evolution yields insights into the complex interactions between salt deformation and sedimentation. The results also suggest that the Mahogany salt body did not influence the trap style of the subsalt Mahogany field or hydrocarbon migration into the pay sands but that it did affect sediment transport pathways and, to a lesser degree, reservoir facies distribution.

Considerable exploration attention has been devoted to salt-withdrawal minibasins and sub-salt traps in the offshore Gulf of Mexico (GOM) province. Investigation is inherently by geophysical and petrophysical means, utilizing sonar, seismic and gravity data, and scattered well logs, with obvious resolution limitations. Outcrop studies in Texas, Utah and Mexico provide an additional dimension, leading to an improved understanding of GOM examples. Outcrops along I-10 between Kerrville and Sonora, Texas, provide a remarkable suite of evaporite-withdrawal (dissolution) structures, including varying exposure levels of withdrawal synclines, related secondary structures, and the primary weld surface. Paradox Basin exposures in Utah include diapirs and evaporite anticlines, revealing intense internal deformation, and various stratigraphic-structural interactions. The La Popa basin in Mexico hosts evaporite-cored anticlines, diapirs, withdrawal synclines, and a previously-recognized secondary weld. These outcrop analogues provide excellent insight into GOM structures. For example, GOM welds form potentially important hydrocarbon pathways, but the coarse scale of seismic resolution, though revealing significant character changes, leaves many questions unanswered. The Edwards Plateau and La Popa welds exhibit remarkable diversity along their traces, even over relatively short distances. Some segments exhibit enhanced permeability through brecciation, fracturing, or sand-sand juxtaposition, whereas shale smearing or remnant salt limits fluid transmission along other portions.

Sour Lake is a large piercement salt dome located in southern Hardin County, Texas. Recent deformation of the cellar and wellhead of one of the salt cavern storage wells initiated a geologic investigation into the cause of the deformation and its future impact. Unlike many of the other Gulf Coast salt diapirs that have a flat or inclined upper salt surface, the upper surface of the salt at Sour Lake is strongly domed with over 1000ft of relief. The geologic investigation showed that the diapir has maintained a near-surface position since at least Yegua time and appears to be characterized by a passive or down-building stage. However, the central area of the diapir has been an area of active uplift since late Frio time resulting in a complex graben system forming over the central area of the dome. The cause of the 1929 sinkhole occurrence at Sour Lake was re-evaluated and provided the basis for mapping the changes in ground level between 1923 and 1965. These changes in elevation show areas of uplift and subsidence that correspond to the underlying graben system. In addition to being located in the area of active uplift, the storage well experiencing the deformation is also located on a smaller and more active area of salt uplift (a salt spine). Since the late 1960's, this spine has begun moving and has resulted in the over-pressuring of the caprock and adjacent sands. Active salt movement at Sour Lake has resulted in uplift and extension of the overlying strata and has produced surface subsidence.

Gulf Coast salt diapirs have complex internal structure, which is shown by light and dark layering. The layering reflects variation in anhydrite content and is interpreted to represent highly stretched and sheared depositional bedding. Structure is dominantly vertical, consisting of repeatedly refolded isoclinal folds having vertical plunge. Geologic mapping is conducted primarily as an aid to mine planning and development; separate maps are produced to record basic structure and display the distribution of clastic stringers, sylvite stringers and lenses, crystal size, wet areas, and gas. The latter characteristics, including coarse salt crystals, are anomalous and tend to be associated (in anomalous areas and trends). Gas is a mining hazard, causing Weeks Island and Cote Blanche Island to be classified by MSHA as gassy mines having many associated regulations. Five miners died in a gas explosion at Belle Isle mine in 1979, and blasting is now normally done while the mines are unoccupied (before the morning shift goes in, for example). Water influx, particularly if derived from outside the mine, is a cause of major concern for obvious reasons; at Avery Island a crew is constantly drilling and grouting to maintain control of leaks, and Jefferson Island mine has been lost as a result of flooding when penetrated by an oil rig. Sand reduces the salability of the commercial product and also increases mining costs. The geologic maps provide structural trend information to help the mine engineers avoid undesirable or potentially dangerous areas including those in proximity to the edge of the diapir. They also cast light on the nature of salt diapirism; linear anomalous zones have routinely been interpreted (sometimes with minimal justification) as boundaries between separate salt spines. Map examples from Weeks Island are included.

Some of the largest deep-water sedimentary bed-forms ever found in the world oceans were recently discovered along the base of the Sigsbee Escarpment in the northwest Gulf of Mexico. Deep-Tow high-resolution geophysical surveys on the continental rise south of the Sigsbee Escarpment have revealed the existence of mega-furrows, ~10 m deep and ~30 m wide, spaced ~100 m apart that occupy an area on the Continental Rise 20 to 40 km wide. A single furrow within a field of furrows in the Green Knoll Diapir area can be traced over a distance of 150 km. The furrows are located at the base of the Sigsbee Escarpment from 90° to 93° West Longitude, from north of Green Knoll to the area just west of Bryant Canyon. The presence of furrows, longitudinal bed-forms, suggests that unidirectional bottom currents up to 100 cm/sec may exist at the base of the Escarpment. The furrows are inferred to be formed by helical secondary circulation. Allen (1968) produced sedimentary furrows in the lab and found a hierarchy of bed forms ranging from uniform furrows at low flow conditions to meandering furrows and flute casts at the highest flow conditions. All the conditions described by Allen are present in the area due south of Bryant Canyon on the continental rise. In terms of hazards relating to deepwater hydrocarbon exploration and production, the formation of mega-furrows demonstrates the existence of previously undocumented high-velocity currents at the base of the escarpment. End_of_Record - Last_Page 410

An improved understanding of the tectonic behavior of salt can be obtained when microstructural data from the salt are used in conjunction with other geologic information including isopach/isochore maps of the sediments surrounding the salt. Weeks Island is an active Gulf Coast salt diapir where sedimentation was often influenced by contemporaneous faulting and episodes of near surface salt movement. Isopach/isochore mapping can be used to analyze the interaction of the salt mass with the surrounding sediments, giving important clues about the growth history of the salt structure. A series of isopach/isochore maps constructed for the shallow flank sediments down to the middle-Miocene "Tex W" indicates that the Weeks Island salt diapir has grown episodically during approximately the last 11.5 million years as two major salt spine complexes. Subgrain formation dominates microstructural development during steady-state flow of salt and it has been shown by Carter, et al. (1993) that subgrain diameter is inversely proportional to stress alone where: D(µm) = 214 -1.15 (MPa) Estimated differential stress levels recorded by subgrain size in the salt at Weeks Island vary from 0.87 to 2.33 MPa. Comparison of the microstructure data with other geologic information indicates that geologic significance can be attached to this variation in stress. At Weeks Island, the more actively moving salt is interpreted to be indicated by higher stress levels and smaller subgrains, which are generally associated with higher surface topographic relief and fluid-rich, coarse-grained, recrystallized salt.