Abstract A generalized stratigraphic framework for the upper Jurassic is suggested, in which the name Haynesville Formation is utilized, and the name Lower Cotton Valley Lime is suppressed in favor of the Gilmer, where appropriate. Eustatic sea-level fluctuations have resulted in patterns of sedimentation common to upper Jurassic sequences across the entire northern Gulf of Mexico region and hence indicate the wide applicability of this generalized stratigraphic framework. The presently accepted model of the Smackover-Haynesville sedimentation must be modified to take into account sea-level fluctuations, subsidence, and sediment availability. The model developed here is simply one of lower Smackover basin fill during a rapid transgressive phase and upper Smackover regional shoaling during a sea-level stillstand in which sedimentation was in equilibrium with subsidence. The lower and upper Smackover are not necessarily time equivalent, but represent two separate sedimentologic sea-level regimens. The Haynesville Formation is thought to be a separate sedimentologic package that was deposited during the next sea-level rise; the Gilmer Limestone formed a shelf-margin barrier behind which the lagoonal Buckner evaporites were deposited. The evaporites graded landward into quartzose clastics. Predictable regional porosity patterns have developed in the Smackover-Haynesville, in response to early diagenetic overprints, controlled largely by eustatic sea-level subsidence interactions. These patterns include: updip oomoldic porosity in a regional meteoric-water system developed during the upper Smackover sea-level stillstand; downdip porosity preservation under marine conditions along the shelf margin; regional dolomitization associated with reflux of evaporitive waters from the Buckner lagoon behind the Gilmer shelf-margin barrier. Structural hydrocarbon traps associated with salt movement are the most common type of Late Jurassic trap. Buckner evaporites or Haynesville shales usually form the seals in the Late Jurassic reservoirs. Jurassic source rocks are probably lower Smackover limestones and Norphlet shales. Sourcing is generally local with migration into updip areas, particularly where regional dolomitization has occurred. The time of migration, which is a key factor in a viable Smackover exploration strategy, varies across the Gulf in response to the subsidence history of each individual basin. Future Jurassic exploration will center on south Texas, the Gilmer shelf margin, and the updip Smackover along the bounding graben fault systems. Most production will be gas, but some oil should occur along the updip faulted fairways.

Abstract Correct interpretation of the effect of basin infilling on salt mobilization is critical to understanding salt dome growth and stability. The size of salt structures in the East Texas Basin is determined by the original thickness of the underlying Louann Salt (Middle Jurassic): that is, salt structures distinctly increase in size toward the interior of the basin. Initial movement of salt apparently occurred in the marginal areas of the basin during Smackover (Late Jurassic) deposition. This movement seems to have resulted from downward creep that was induced by loading of carbonate units and was enhanced by basinward tilting. During a major shift from carbonate to clastic sedimentation in the Late Jurassic, salt movement became more extensive. This salt migration was caused by uneven sediment loading of fluvial-deltaic systems in the Cotton Valley Group (Upper Jurassic) and the Hosston Formation (Lower Cretaceous). Terrigenous source areas to the west and north persisted throughout Cotton Valley and Hosston time. clastics were delivered to the East Texas Basin by many small streams, rather than by one major stream, because a mature drainage system had not yet formed. The Cotton Valley Group, which is thought to be a fan-delta system, can be subdivided into three types of facies: prodelta deposits, delta-front deposits, and braided fluvial deposits. Fan deltas, supplied by braided streams, prograded from the north, northwest, and west. Dip-oriented sandstone trends dominate in the northwestern part of the basin and change basinward to northeast to southwest strike-oriented trends. During Hosston time, sedimentation in the northwestern part of the basin was dominantly fluvial. The depositional characteristics of sediments in this area are typical of braided streams. In the study area, parallel net-sandstone and sediment thicks are clearly defined in the distal part of the Cotton Valley but are not as well defined in the Hosston. This suggests that most deltaic sedimentation during Hosston time occurred basinward of the study area. A major transgression at the end of Hosston time resulted in deposition of the Pettet Limestone. Apparently, the location of salt domes and salt anticlines was controlled by the position of the Smackover-Gilmer carbonate platform. This platform impeded local subsidence to the extent that fan-delta sediments of the Cotton Valley Group spread laterally across the shelf rather than stacked vertically. Sediment depocenters formed preferentially basinward of the platform, resulting in migration of the underlying salt into ridges that fronted the prograding sediment wedge. As the salt was depleted under these depocenters, subsidence slowed and thereby allowed the fan deltas to override the salt ridges. This resulted in a basinward progradation of deltaic depocenters and produced younger depocenters toward the interior of the basin. Further salt migration and differentiation of salt ridges produced the present complex array of salt domes and anticlines of the East Texas Basin. Seismic and gravity data clearly demonstrate the existence of these salt ridges and intervening sediment thicks.

Abstract Recent discoveries in the Late Jurassic Haynesville and Bossier shales have dramatically increased unconventional gas exploration activity in the mature petroleum provinces of eastern Texas and northern Louisiana. The Haynesville and Lower Bossier shales comprise the uppermost units of a transgressive systems tract of a second-order supersequence, which spans the interval from the top of the Werner Anhydrite/Louann Salt equivalent to the upper Cotton Valley clastics. Depositional variations within the shales are a function of higher-order sequences that resulted from eustatic sea-level fluctuations, paleobasin physiography, and the interplay of local subsidence and sediment input rates. The antecedent topography shaped by underlying carbonates of the Gilmer (Haynesville) Lime (Forgotson and Forgotson, 1976) and subsequent sediment budgets strongly influenced (1) facies development and stacking patterns that vary along the northern rim of the young Gulf of Mexico (GOM) Basin during Haynesville and Bossier time, and (2) the depositional processes, total organic carbon richness, and preservation of the self-sourcing Haynesville and Bossier Shale units. The Haynesville Shale depositional system is an example of a competing carbonate and clastic system that contains contemporaneous retrogradational and progradational facies. In the western part of the system, which is carbonate-dominated and fairly restricted from siliciclastic input, the time-equivalent Gilmer (Haynesville) Lime consists of backstepping carbonate facies. In contrast, to the east, strong progradational stacking patterns, comprised of mainly siliciclastic facies assemblages, dominate in northern Louisiana and western Mississippi because of increased sediment supply from the ancestral Mississippi River, which outpaced subsidence and eustasy. Hence, major bounding stratigraphic events such as higher-order maximum flooding surfaces and condensed sections critical for shale gas exploration appear to change facies laterally whereas the second-order maximum flooding surface, or the turn from retrograding to prograding stacking patterns, appears diachronous along depositional strike. During Bossier time, the youngest carbonates were drowned and siliciclastics became increasingly dominant, expanding westward from northern Louisiana into eastern Texas and ultimately across most of the northern GOM shelf as the Cotton Valley Sandstones and its distal shale equivalents. Depending on the paleophysiography of the depositional shelf setting, some areas of the Haynesville-Bossier system were restricted and relatively sediment starved. These correspond with areas of total organic carbon enrichment and, in turn, lower shale gas exploration risk.

Abstract Sequence and seismic stratigraphic analysis of well logs and 2-D seismic lines from Freestone, Anderson, Leon, Houston, Madison, Robertson and Limestone Counties, Texas, demonstrates that the Bossier Formation of the western East Texas basin can be subdivided into two recognizable sequences separated by a major sequence boundary (SB-2). Similarly, the Bossier Formation is also bracketed by a basal (SB-1) and upper (SB-3) sequence boundary separating it from the Gilmer (Cotton Valley) Lime of the Haynesville Formation below, and the Cotton Valley Sand above, respectively. In seismic sections, the SB-2 boundary in the middle of the Bossier Formation was identified by tracing mounded basal reflectors and sigmoid basal reflectors representing basin floor and slope fans. This boundary was correlated onto the shelf below deltaic sands. In well log sections, basin floor fan log shapes were traced laterally into slope fan and stacked delta log patterns to identify SB-2. These basin floor and slope fans immediately above the SB-2 boundary represent a lowstand systems tract, whereas the lower Bossier (below the SB-2 Sequence Boundary) represents a transgressive systems tract and the upper Bossier (above the SB-2 boundary) represents a prograding complex. Burial history analysis suggests that the lower Bossier accumulated during a time of rapid mechanical subsidence when the East Texas basin was underfilled. A drop in sea level associated with the SB-2 boundary represents a major climate shift from tropical to cooler conditions, favoring rapid influx of sands from the ancestral Mississippi, Ouachita, and Red River systems. These sands developed inner shelf prograding deltaic packages, outer shelf and incised valley fill stacked deltas, and basin submarine fan systems. The stacked deltas and basin fan sand systems all represent prospective gas plays.

ABSTRACT A contour map on a prominent mid-Jurassic surface (MJS) outlines four crustal types in the northeastern Gulf of Mexico basin: 1) continental crust underlying northern Florida, 2) thick transitional crust characterized by large E-W trending basement highs and lows (Wiggins uplift, Apalachicola basin, Middle Ground arch-Southern platform, Tampa embayment, and Sarasota arch), 3) thin transitional crust underlying the West Florida basin, and 4) oceanic crust underlying the deep central Gulf basin. This basin configuration developed during the early breakup of Pangea, as South America-Africa separated from North America. Over much of the area the MJS is a prominent unconformity that truncates Late Triassic-Early Jurassic rift sediments as well as all older Paleozoic and Precambrian sedimentary, igneous and metamorphic rocks. It is overlain by mid-Jurassic pre-marine evaporites (Louann salt and equivalent rocks), which are thick in basinal areas and thin or absent over adjacent highs. The evaporites were deposited toward the end of the period of crustal attenuation and the formation of transitional crust but prior to the emplacement of oceanic crust. This crustal structure and basin configuralion of the MJS influenced the distribution and development of the overlying Upper Jurassic and Lower Cretaceous sequences. During Smackover time a broad shelf to prograding ramp partially filled the basinal areas, while scattered carbonate buildups developed along the surrounding basement highs. In areas of thick salt sediments thicken into small basins formed along listric growth faults caused by salt withdrawal (salt rollers). The shelf margins continued to prograde basinward during Haynesville time, depositing a thick section in the Tampa embayment, while a starved carbonate margin (Gilmer?) developed in the western Apalachicola basin. During the latest Jurassic-earliest Cretaceous (Cotton Valley time) a broad, dominantly clastic, prograding ramp filled the basins and overlapped the highs, while a deep-sea fan system developed in the adjacent West Florida basin. Cotton Valley deposition culminated with the development of the Knowles carbonate margin along a tectonic hinge-zone (THZ) at the thick/thin transitional crust boundary. During the Lower Cretaceous a rimmed carbonate margin controlled by the THZ continued to develop along the present-day Florida escarpment. This margin became steeply-dipping to the south, separating a broad carbonate platform from a relatively starved basin. To the north in the De Soto canyon area, the margin remained gently-dipping, reflecting the influence of a narrow shelf and the influx of terrigenous clastics to the adjacent slope. In the West Florida basin mounded and lobate geometries are interpreted as deep-sea fan systems. Low velocity channel fills on the adjacent platform suggest that siliciclastic sediments may have bypassed the Lower Cretaceous carbonate margin during sea level lowstands and been deposited as potential siliciclastic reservoir rocks within portions of the deep-sea fan systems. As the carbonate margin adjacent to the West Florida basin continued to aggrade and steepen during the Early Cretaceous, southward-flowing deep-sea currents intensified and became focused along the base of the margin, altering depositional patterns and processes and culminating in the deposition of a large southward-prograding contourite mound. Near the end of the Early Cretaceous, extremely intense currents, possibly due to sea level drops, preferentially eroded these lobes and mounds, culminating in the widespread mid-Cretaceous sequence boundary (MCSB). A subsequent rise in relative sea level terminally drowned the Lower Cretaceous platform margin and caused the retreat of the margins to more landward positions during the Upper Cretaceous.

Abstract A diverse and abundant Late Pleistocene pteropod (pelagic gastropod) fauna is described from marine cores near the island of Montserrat, Lesser Antilles. In several of the cores, there are ‘floods’ of pteropods at particular levels, usually associated with glacial periods within the Late Pleistocene. These levels of abundant pteropods appear to be of regional significance, having been reported from other locations in the Caribbean Sea, the Gulf Coast of Florida and other ocean basins. The concentrations appear to reflect the enhanced preservation of aragonite during cooler periods within the Pleistocene.