Abstract Seismic reflections interpreted to be top Oligocene, top Wilcox (approximately base middle Eocene), top Cretaceous, top Jurassic, and basement were mapped across portions of the Green Canyon, Keathley Canyon, Walker Ridge, Lund, Sigsbee Escarpment, Amery Terrace, and Lund South OCS areas of the central northern Gulf of Mexico ( Fig. 1 ). 3D Pre-stack depth migrated data were used for mapping the areas covered by allochthonous salt. 2D Pre-stack time migrated data were used for mapping the area on the abyssal plain beyond the Sigsbee Escarpment. These data cover approximately 50,000 km 2 (19,500 miles 2 ). Well control was obtained from data available through the Minerals Management Service. Figure 1. Location map. Black line encloses the area of data coverage. Dashed line marks the transition from 3D prestack depth migrated (PSDM) data to the west and north to 2D pre-stack time migrated (PSTM) data to the east. Numbered segments refer to figures with those numbers. Abbreviations for deep-water OCS areas: AC–Alaminos Canyon; AM–Amery Terrace; AT–Atwater Valley; EB–East Breaks; GC–Green Canyon; GB–Garden Banks; KC–Keathley Canyon; L–Lund; LS–Lund South; MC–Mississippi Canyon; SE–Sigsbee Escarpment; WR–Walker Ridge. Structure maps on the top Oligocene, top Wilcox, top Cretaceous, and basement formed the regional surfaces between which isopach/isochron maps were created to analyze depositional patterns. As might be expected, basement structure displayed the greatest relief and complexity. Outboard from the allochthonous salt of the Sigsbee Escarpment, half-graben structures indicative of rift basin topography were clearly imaged ( Fig. 2 ). Elsewhere on the abyssal plain isolated, sharp-peaked, elevated basement features were observed between more numerous gently sloped highs. These basement structures typically had reflection terminations against their margins or flanks and continuous reflections draping them. Figure 2. 2D Pre-stack time migrated line showing rift basin structure in the basement, Wilcox strata down lapping onto the Cretaceous and thinning to the east, and Oligocene strata down lapping onto the Wilcox and thinning to the north. The vertical scale is in seconds of two-way time (TWT). The horizontal scale is in feet (100,000 feet ~ 18.94 miles or 30.55 kilometers). Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). The top Cretaceous and top Wilcox surfaces show broad regional similarities and show less structural complexity than the basement. Outboard of the Sigsbee Escarpment, both surfaces are broadly lobate and have relatively gentle inclinations which rise to the east. The main observable differences between the two are: ( A ) the Cretaceous surface has several isolated high points reflecting underlying basement structures and ( B ) the Wilcox surface has a more lobate/interdigitate contour character. The top Oligocene surface is less lobate in appearance than either the Cretaceous or Wilcox surface and rises to the southeast ( Fig. 3 ). Figure 3. Time structure map on the top Oligocene. The contour interval is 50 milliseconds. Isochron maps between the four structural surfaces reflect the underlying structure and depositional trends of the interval. Thus the basement to Cretaceous isochron shows thick Jurassic infill, Cretaceous drape in the grabens ( Fig. 2 ), and thin to no cover over highs in the rifted basement topography. The Cretaceous to Wilcox isochron has a broad lobate form that thins gently from west to east. A very subtle down-lapping pattern is visible within the Wilcox interval on Figure 2 . Deviations from this pattern occur primarily where basement structures produce isolated thins. The Wilcox to Oligocene interval shows a regional gradient of north to south thickening and only a slight influence from deeper structure. Down-lapping and thinning to the north strongly suggest a southerly source for the Oligocene interval. Beneath the allochthonous salt of the Sigsbee Escarpment, all surfaces deepen northward and show much greater local variability. Basement is only occasionally visible as it generally lies below the fifteen kilometer limit of the available PSDM data. The deepest area mapped is in Green Canyon where the top Oligocene approaches twelve kilometers depth, the top Wilcox approaches thirteen kilometers, and the top Cretaceous almost fourteen and one half kilometers. These surfaces shallow to less than eight kilometers deep on the abyssal plain. Three coincident lows roughly oriented north-south suggest preferred sediment pathways and possibly areas of thicker original autothonous salt. A change on the structure and isopach maps from smooth broadly spaced contours on the abyssal plain to highly variable tightly spaced contours suggests the location for the original limits of salt deposition in this area. This location often lies close to but not exactly in line with the present day Sigsbee Escarpment ( Fig. 1 ). Of key interest to hydrocarbon explorationists are any factors that would effect Wilcox deposition. We have observed three factors that influence the deposition and thickness of Wilcox age strata in this area: Pre-existing basement highs have caused the Wilcox to be thin or absent around those structures. Although basement topography is mostly smoothed over by the end of the Cretaceous, a few large structures still influenced deposition in the Wilcox on the abyssal plain beyond the Sigsbee Escarpment. Salt nappes and salt pillows have caused thinning of Wilcox strata over those structures. Our interpretation indicates multiple kilometer thick salt nappes extruded beyond the limits of the original salt basin during the Cretaceous ( Figs. 4 and 5 ). Inflated salt pillows associated with the nappes lay along the boundary of the salt basin. Though now deflated, the presence of these salt pillows and other salt pillows updip are recorded by the depositional thinning of Wilcox strata above them. These allochthonous bodies provided the core structure over which Wilcox and Miocene reservoirs are folded or draped at Chinook, Atlantis, Das Bump, and other important deepwater discoveries. The location of allochthonous salt at the onset of Wilcox deposition is apparently coincident with the pronounced increase in northerly dips of the Mesozoic and Paleogene strata. This relationship is consistent with originally thick autochthonous salt above the deepest mapped basement. Sites of continued salt withdrawal from the autochthonous level into growing salt structures directly affected Wilcox sediment thickness. Such sites would have been primary candidates for the location of Wilcox sediment fairways. Identification and elimination of salt feeders would help in refining/defining these pathways. Figure 4. 3D Pre-stack depth migrated line showing a Cretaceous age salt nappe and its associated deformation front. Both features lie just basinward of the modern Sigsbee Escarpment. Thinned Wilcox and Oligocene strata show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Figure 5. 3D Pre-stack depth migrated line showing a second Cretaceous age salt nappe. This one lies about thirty kilometers shoreward of the Sigsbee Escarpment. Thinned Wilcox strata and an Oligocene turtle structure show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Deposition of the Wilcox strata can be broadly divided into two paleogeographic domains: ( A ) a relatively complex north-westerly region characterized by pre-existing, elevated sea-floor, salt-cored structures and sites of contemporaneous salt evacuation, and ( B ) a relatively simple south-easterly region characterized by a near flat and smooth sea-floor rarely punctuated by unburied basement structures. The transition between these two regions should mark changes in Wilcox depositional styles. In the more complex topographic region, Wilcox depositional events were forced to interact with relatively rapid changing sea-floor dips. Whereas in the more simple region to the southeast, a much more unconfined sea-floor presented limited impediment to widespread expansion of depositional events exiting the more complex region to the north-west. Drilling of Wilcox strata to-date has been mainly in the simpler south-easterly region and in the transition zone to the more complex Wilcox geometries towards the north-west. Figure 4 shows an example of one salt nappe and its contractional deformation front that lies in close proximity but basinward of the Sigsbee Escarpment. Thrust relationships suggest that the nappe continued to move/inflate until the end of the Cretaceous. An inflated salt pillow associated with the nappe is present through the Oligocene but then deflates during the Miocene. This interpretation is supported by the thin but depressed Wilcox and Oligocene section behind the nappe today. We predict that the edge of the salt basin lies behind the nappe, below where the Wilcox and Oligocene intervals begin dipping to the north. Figure 5 shows another example of a salt nappe that lies in about thirty kilometers inside of the Sigsbee Escarpment. This nappe does not have a deformational front associated with it. But an inflated salt pillow is associated with this nappe as in Figure 4 . Similar to Figure 4 , the interpretation is supported by a thin but depressed Wilcox section behind the nappe. In contrast, evacuation of the pillow begins in the Oligocene, as evidenced by the Oligocene age turtle structure. Evacuation continues into the Miocene until the pillow is completely deflated. The nappe remnant is all that remains of this salt body. Unique to these two examples, but possibly typical of most salt pillows around the edge of the salt basin, loading has forced salt backwards (updip) into the salt basin. In Figure 4 , the reversal of salt movement is about ten kilometers. In Figure 5 , the reversal of salt movement may be twenty to twenty-five kilometers.

Abstract With the lifting of economic sanctions, western companies have come back to explore for hydrocarbons in Libya, onshore and offshore. However, virtually no modern marine seismic data has been acquired over the past twenty-five years to assist in this renewed exploration effort. During the past year, new 2D pre-stack time migrated seismic data has been acquired and used to examine the large-scale structural and depositional features of the Libyan shelf and slope. The data cover approximately 38,000 line kilometers in water depths ranging between 15 to 2200 meters. The present day Libyan shelf margin has a demonstrably progradational character. Thick, laterally extensive deltaic deposits dominate the shallow shelf and upper slope. These deposits display classical clinoform geometries that suggest multiple phases of progradation during the past 3-5 Ma. Seismic resolution within the clinoform packages is high, as growth faulting, distributary channels, slump scars, and rotated blocks within the delta front are readily visible. Clinoform geometries visible below, but truncated by, the Messinian unconformity indicate that the early to middle Miocene margin of Libya was also progradational at certain times. Recent deltaic deposits sit upon and within a deeply eroded and scarred paleotopography, suggesting large-scale retrogradation of the shelf margin. The erosional surface extends for nearly 500 km along strike in the Sirt Embayment. A 65 km long portion of this erosional surface displays high relief truncated strata, healed fault scarps, and related deep-seated faults. In this area it is likely that a very large volume of shelf margin strata is missing. The Libyan margin is tectonically active today and has been through most of the Cenozoic. Many faults penetrate from deeply underlying Mesozoic strata to the ocean bottom. The close association of active faults scarps, truncated strata, a potentially large missing section, and a laterally extensive erosional unconformity combine to suggest the possibility of catastrophic margin failure. The exact timing of margin retrogradation is uncertain at present but erosional relationships hint that margin failure occurred either coincident with or following the Messinian salinity crisis.

Abstract Examination of 2D seismic data in South Texas has identified what is now interpreted to be a large, rafted block of Eocene, Paleocene, and Cretaceous strata, analogous to rafts identified in the Kwanza Basin of Angola. Preliminarily named the “Wilcox raft” because of its association with the Wilcox depotrough, it has been identified in the subsurface extending from Starr County on the Texas–Mexican border, northward over 200 kilometers into Live Oak County, Texas. The actual extent of rafted material may extend farther to the north and/or south. The raft’s detachment surface is interpreted to be at the base of the Jurassic Louann salt. The Wilcox raft contains one primary block more than 150 kilometers long and 15 to greater than 30 kilometers wide. The primary raft block may be segmented, and the entire rafted unit may include a number of smaller branching arms, ramps, and offset fault blocks. Various portions of the raft have downdip displacements from 5 to greater than 30 kilometers. The raft is bound on the west by expanded upper Wilcox (early Eocene) strata and on the east by expanded Queen City (middle Eocene) strata. Other incompletely detached blocks lie to the west of the raft across the Wilcox depotrough. Raft geometries suggest that at least one additional rafted block lies farther basinward of the Wilcox raft, possibly beneath expanded Vicksburg (early Oligocene) strata. A proposal for rafting in this area of South Texas is not entirely new. Earlier modeling and restorations across the Wilcox depotrough have incorporated rafts. However, these models are predicated on large-scale salt withdrawal and incorporate more than three kilometers (>10,000 feet) of autochthonous salt occupying the area of the Wilcox depotrough. We believe that a much thinner autochthonous salt layer existed beneath South Texas. In other areas of the northern Gulf of Mexico, where thick autochthonous salt existed, salt stocks are abundant. Although a few salt structures do exist in South Texas, there are very few compared with other interior salt basins. Forward modeling suggests that large sedimentary structures in the Wilcox depotrough, which can be misinterpreted as turtle structures, are related strictly to deposition during raft extension and not salt withdrawal. The geometries can be produced purely by extension on multiple detachments (Louann and lower Paleocene Midway shales) linked by ramps that dip both basinward and landward.

Abstract A combination of 3D pre stack and post-stack time-migrated seismic data was used to examine salt structures, stratigraphy, and hydrocarbon potential in the BES 2, 100, 200 and BMES 1, 2, and 9 blocks of the Espirito Santo Basin, Brazil. Salt structures display a proximal to distal basinward transition from salt rollers, to vertical diapirs, to diapirs with overhangs and alloch-thonous tongues, and finally to salt canopies. The original autochthonous salt thickness increases following a similar proximal to distal basinward gradient. Deformation, driven by a combination of gravity gliding and gravity spreading, has been a relatively continuous process in the Espirito Santo Basin. Contraction began early in the Albian and continued unabated up to the present-day. However, individual structures ceased movement at different times depending on geometry, salt supply, and overburden thickness. A major thermal pulse affected the basin in the early to middle Eocene, associated with emplacement of the volcanic Abrolhos Plateau. Both intrusive magmas and extrusive flows are interpreted to exist. Intrusive dikes and sills display characteristic saucer shapes in cross section, elliptical to circular shapes on time slices, and cone shapes in 3D. Magmas appear to have used existing salt structures and associated fault planes as preferred pathways to reach shallower levels. Extrusive flows were identified only where seismic character, clear stratal relationships, and direct ties to intrusive geometries allowed. All the elements for excellent hydrocarbon potential exist in the Espirito Santo Basin. The main Syn-rift II source bed found in the Campos Basin exists across the basin. Several other less documented source intervals also exist. Numerous contractional folds, turtle structures, and diapir-flank traps are present. Reservoir intervals exist in Albian carbonates, Upper Cretaceous transgressive sands, and Cenozoic regressive sands. At least one deepwater hydrocarbon system is operating, as evidenced by numerous shallow bright spots, gas chimneys, and a recent major deep-water discovery. The presence of intrusive magmas may adversely affect deeper source intervals in some places, but could locally bring immature source rocks into the oil window.

ABSTRACT The path of Bryant Canyon was identified during the structural and sequence stratigraphic mapping of reprocessed, migrated, 72-fold seismic data in the Garden Banks OCS area. The canyon pathway winds around and through salt structures in Garden Banks and continues to the south where it eventually overrides the allochthonous salt sheet that extends to the Sigsbee Escarpment. Stratigraphic evidence suggests that Bryant Canyon may be 0.2-0.3 Ma in age. It is now almost entirely infilled with shelf-derived (probably deltaic) sediments near the shelf/slope break and with turbidite and/or slump deposits farther down slope. Erosional features and reentrants in paleoshelf margins suggest that earlier canyons may also have occupied this pathway. The path of Bryant Canyon may be a primary and long standing conduit for shelf-derived coarse clastic sediments being transported into deep water. Sediments and sedimentation associated with Bryant Canyon have affected salt body geometries observed within the Garden Banks area in three ways. First, updip and lateral differential loading by sediments infilling Bryant Canyon helped drive salt migration. Variations in size and location of these differential loads influence the direction and rate of salt flow. Second, sediment transported down Bryant Canyon has physically eroded and incised certain salt structures. Although rare, deep water erosion and dissection of salt structures directly affects their observed geometries. Third, erosion has removed the sediment covering some shallowly buried salt structures thus exposing them to sea water. Geometries of the exposed structures are affected by both the disruption of isostatic equilibrium and by the effects of salt water dissolution. Nearest the shelf margin the effect of differential sediment loading is most advanced. There, sedimentation has dissected the allochthonous salt into closely spaced salt stocks. Farther down slope the dissection process is less complete. Bryant Canyon intersects and parallels a dip-oriented salt ridge. In southernmost Garden Banks, the effects of differential sediment loading are least advanced. Sediment transported down Bryant Canyon has created a series of dip-oriented salt-free fensters within the continuous allochthonous salt sheet.