A thick section of sediments was deposited along the lower continental slope in the vicinity of the Atlantis and Mad Dog Developments in the northwest Gulf of Mexico during the time period spanning the last glaciation to the present. Sediments were collected using large diameter piston cores and borings, and studied to understand how variations in sedimentation rate and sediment properties relate to changes in climate and sea level. Sediment bulk density and compressional-wave velocity were measured using a multisensor core-logging device; afterward, the cores were split and the sediment lithologies were described. Based on these data and the interpretation of high-resolution seismic data collected at the core sites, samples were taken from the cores to determine the ages of the sediments using the following methods: radiocarbon dating, downcore changes in the oxygen isotopic composition of foraminifera shells, and nannofossil occurrence.A detailed chronology for the sediments deposited during the period of time spanning the last glaciation to the present was determined and compared to other sediment characteristics. The establishment of this chronology was an essential step in the assessment of potential geohazards because it provides a fundamental framework for understanding how environmental processes affect the geologic and geotechnical characteristics of the seafloor. For example, the ages of regionally persistent high-resolution seismic reflectors were established and related to changes in sediment properties, providing an important temporal framework for the interpretation of the seismic, lithologic and geotechnical data. A significant decrease in the rate of sediment accumulation occurred as sea level rose from the last glacial maximum to the late Holocene. The data supports the suggestion that the sediment accumulation rate is also influenced by changes in the flux of glacial meltwater into the Gulf of Mexico.
The overall objective of the research project is to understand and predict seabed processes on the continental slope in the orthwest Gulf of Mexico, related to the regional geology andengineering properties for the design of seafloor installations. This paper is a report of work in progress with a focus on a one-month research cruise to the main study corridor in deep water (800-2700 m W.D.), during which over 1,000 meters of sediment samples were obtained with a large-diameter long piston coring system and a large-diameter gravity corer. Profiles obtained from logging of the cores exhibit the wide range of variability associated with laminated soft sediments; debris flows, turbidity currents, and overconsolidated sediments. There is very good agreement between long core shear strength results and conventional boring data at two floating platform sites, Auger and Jolliet. Introduction A new research program on seabed processes in deep water Gulf of Mexico is showing the value of undertaking integrated studies to determine geotechnical and geological properties for geohazard analysis, siting of facilities, and foundation design. A major milestone of the project was the August/September, 1998 research cruise to obtain large-diameter long piston cores of the sediment and perform in situ shear strength and pore pressure measurements. The 5-year joint program by the University of Rhode Island (URI) and Texas A&M University (TAMU) is funded principally by the National Science Foundation with supplemental sponsorship by a consortium of five oil companies (Chevron, Texaco, Mobil, Amoco, and Conoco) and participation of two consulting firms (MARSCO and GEOTEK). The northwestern continental margin of the Gulf of Mexico (Fig. 1) has a complicated history of construction and evolution resulting in a very complex bathymetry and morphology, especially on the upper and lower slopes. The primary study area (Fig. 2) is a 180 km × 9 km corridor south of Louisiana in water depths ranging from 800 m to 2700 m, which encompasses a wide range of seabed conditions including: stable plateau regions, slopes with various types of instabilities, basins, faulting, uplift areas, sediment flows, diapirism, areas with fluid expulsion and gassy zones. A more detailed map of Beaumont Basin (Fig. 3) shows a relief of 800 m with the basin floor at 2600 m and slope inclinations of over 25° in some areas. Two examples of subbottom and sidescan records within the primary study corridor are shown in Figs. 4 and 5. The main objective of the project is to understand and predict a wide range of seabed processes related to geological studies and engineering design for seafloor installations such as platforms, pipelines, cables, and moorings. Three other areas in slightly shallower waters where borings have been taken for production platform design were added to the project so that the results of cores and borings could be compared. The program is an integrated, interdisciplinary effort that includes substantial field work, extensive laboratory analyses and modeling of seabed processes. The majority of the field work, including two deep-tow cruises and the long coring cruise, has been completed and the third deep-tow cruise will occur in early 1999. The laboratory program focuses on determining geotechnical and geological information and parameters necessary for computer modeling of seabed processes, especially slope processes, and for geological interpretations.
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