Over 7000 sq. km of salt and six Plio-Pleistocene biostratigraphic horizons were mapped in the East Breaks and Garden Banks areas using a 12,000 km grid of seismic data and all obtainable well data. Structure mapping of allochthonous Jurassic salt and the six horizons (Globoquadrina altispira, Lenticulina 1, Angulogerina B, Hyalinea B, Trimosina A, and Sangamon Fauna) and isopachs of the intervals between these horizons revealed notable lateral variations in the area underlain by salt, in the degree of salt deformation, and in the size and thickness of associated intraslope basins. East of 94.5° W salt structures occupy 40% of the area and exhibit complex shapes that suggest a high degree of salt deformation. West of 94.5° W salt structures occupy 11% of the area and consist mostly of structurally simple salt stocks. A zone of high-offset north-south trending faults mark the transition between these two areas. Isopach maps of the six Plio-Pleistocene intervals (from 2.9 Ma to the present) reveal major shifts in the rates and locations of sediment accumulation. From 2.9 to 1.0 Ma. sediment-accumulation rates averaged only 0.8-1.3 mm/y with a maximum rate of 2.7 mm/y. From 1.0 to 0.69 Ma. sediment-accumulation rates averaged 5.8 mm/y with a maximum rate of 11.6 mm/y. This interval correlates to sediments deposited between the extinctions of Hyalinea balthica and Trimosina denticulata and recorded a major period of sediment loading/salt withdrawal between 1.0-0.69 Ma. From the end of this time to the present, sediment -accumulation rates averaged 1.7-2.1 mm/y with a maximum rate measured at 6.2 mm/y. Increased sediment influx during 1.0-0.69 Ma coincides with a major third order sea level lowstand and was focused in central Garden Banks. The restriction of such dramatically increased accumulation rates to this area suggests that sediment influx was accompanied by large-scale salt withdrawal. The increase in accommodation space created by salt withdrawal appears to be the most important factor affecting accumulation rates. Salt structural styles found on the upper continental slope are transitional between those found on the lower slope and those on the shelf. The shelf is dominated by isolated, individual salt stocks (km²) surrounded by kilometer thick sedimentary sections. The lower slope is dominated by broad, laterally continuous, allochthonous salt sheets (10³ km²) with moderate to thin sediment cover. The upper slope contains both of these structural styles plus intermediate size (10-10² km²) salt ridges and massifs. Observations made during this study suggest that differential sediment loading is the mechanism causing the changes in structural style. A Loading/Dissection model is presented to explain the formation of the three primary salt structural styles, their genetic relationship, and their observed distribution. Differential loading has dissected large salt sheets into numerous smaller and irregularly shaped ridges and stocks (like those found on the upper slope). Salt found on the upper slope originated in the Jurassic Louann Formation, but is now surrounded by Pleistocene age sediments. To achieve this relationship, it appears that some Jurassic salt has undergone at least two cycles of sediment loading and consequent diapirism. Salt/sediment relationships suggest that virtually all of the mapped salt is allochthonous. Repetitive sediment loading and salt structural development has not been previously documented and represents a step beyond the limits of current salt structural models. / text

The study area is located on the continental slope in the Green Canyon deep-water area of the Gulf of Mexico. This area is being investigated by the Bureau of Economic Geology as part of several active gas hydrate studies across the area. The chirp sonar profiles used in my study were collected with an Autonomous Underwater Vehicle (AUV) utilizing a frequency-modulated seismic (sonar) source that emitted a 2 to 8 kHz sweep (chirp) frequency signal (wavelengths less than 2 meters). The recording time is limited to about 50 milliseconds, with time zero occurring at the altitude of the AUV about 50 meters above the seafloor. The signal images to about 40 meters below the seafloor, and profiles are as long as 5 km. An interpretation of deep-water, near sea-floor geology has been conducted using both chirp sonar profiles and multibeam bathymetry. Seismic reflections from within the shallow sediments are caused primarily by contrasts in density, rather than acoustic velocity. Reflections were successfully simulated using a model with a constant velocity of 1560 m/s and densities of 2.1 g/cc for sand, 1.4 g/cc for mud, and 1.7 g/cc for silty sand. The chirp sonar profiles imaged near-seafloor geology at nearly a meter scale and allowed for detailed interpretation. The interaction of soft sediment deformation, creep movement and neo-tectonic activity related to gas expulsion controlled the actual topography of the sea-floor. The geologic time represented in the 50 milliseconds of chirp data recorded with the AUV, which is about 40 meters of depth, is approximately 0.050 Ma. B.P., consistent with a depositional rate of 0.8 meters per 1000 years. Reflection patterns are interpreted to be related to fluctuations in sea level. High reflectivities (density contrasts) are interpreted to be deposited during the last sea level low stand, and low reflectivities are interpreted as sedimentation during the last high stand. Sediments deposited during the low stand are proximal while others are distal, deposited in deeper water environments. Shallow structures observed in the chirp sonar profiles are mainly related to gas mobilization mechanisms, which shaped the topography of the seafloor in conjunction with soft sediment deformation and creep movement. The geomorphic features are related in some cases to gas expulsion zones such as pockmarks and mud volcanoes. Other structures are related to soft sediment deformation and creep mobilization. Soft sediment deformation is confined to the deepest part of the minibasin, while the other features are not depth dependent. Highly deformed intervals at the bottom of the sequence seem to control subsequent sedimentation. / text

Utilizing 887 kilometers of multi-stacked seismic reflection profiles and data from 27 exploratory wells, five salt domes arranged along two intersecting salt ridges, bounding and separating localized depocenters , were delineated by subsurface mapping of the structure, sediment distribution, and major sand trends of a 1621 square kilometer area on the Louisiana continental shelf. Pervasive normal faulting in the study area developed to accommodate lengthening of sedimentary units resulting from relative vertical displacement caused by uneven deposition and salt dome growth. These faults are domal (crestal and radial), bounding, and transverse in geometry. The genesis of the bounding faults can be visualized by two models; hinge faulting and collapse faulting. The dominant structural features of the study area began developing in early Pliocene. These features developed coincident with, and as the result of, a huge influx of paralic sediments associated with the progradation of the shelf edge across the study area between early Pliocene and early Pleistocene. The two major dip-oriented sand trends within this sediment wedge represent high constructive delta systems. The positions of the two systems were locally controlled by growing salt structures and sediments were therefore concentrated in the evolving depocenters . A hint of the deep structure which predated the influx of paralic sediments in Pliocene and early Pleistocene is revealed by a structural reversal present on seismic profiles at about 4.0 seconds in the northwest part of the study area. Reconstruction of the paleostructure of this area reveals the flank of an ancestral structural and stratigraphic basin. This basin may represent an ancestral depocenter which began developing very early on the abyssal plain. / text

Not available / text