Stress-Strain Behavior and Stress State of Gulf of Mexico Clays in Relation to Slope Processes

Abstract

Abstract Four geotechnical stratigraphic units, with different strength characteristics, are identified from four Jumbo Piston Cores taken in stable plateau regions on the Continental Slope in the Northwest Gulf of Mexico where normally consolidated conditions exist. A comparison of data from the plateau regions and the basin slope regions shows clear evidence of past slope failures. From the stress state determined on a slope of a basin, it is estimated that 8 to 12 meters was removed from the site prior to deposition of the existing five meters of overburden. Introduction The results presented in this paper are part of a research project to investigate seabed processes on the continental slope in the northwest Gulf of Mexico. The overall objectives of the project are to understand the linkages between sediment properties, environmental conditions, and seabed processes related to geological studies and engineering design. The objective of this study was to determine the stressstrain/strength behavior and stress-state of plateau regions with low slope angles (generally less than 1°) and compare the overall properties with sediments on basin slopes where slope failures have occurred. The results of the plateau sediments are combined with index properties and the Multi-Sensor Core Logger (MSCL) data to determine the geotechnical stratigraphy of these areas. The geotechnical stratigraphy is then used as a "baseline" to compare with other areas such as the basin slopes within the study corridor. Additional information about the project "Seabed Slope Processes in Continental Margin, Northwest Gulf of Mexico" can be found in (1). A companion paper entitled "Comparison of Geotechnical Properties from Large-Diameter Cores and Borings in Deep Water Gulf of Mexico" (2) contains additional information on three specific locations outside the main study area; the Auger, Jolliet, and Marlin TLP sites. Geological Setting and Study Locations The Texas-Louisiana slope of the Gulf of Mexico encompasses approximately 120,00 km2 and extends from the shelf break to the Sigsbee escarpment in water depths from 200-3400 m (Fig. 1). The processes that have determined most of the morphology and physiography of this region are related to the movement of underlying salt masses (3, 4). The large overburden pressures created by the accumulation of sediments from late Jurassic to the present, has caused the almost continuous salt sheet to move, flow, and sometimes extrude to the surface (salt diapirism). The halokenesis processes are largely responsible for the formation of seafloor surface expressions such as local lows, highs, basins, canyons, and the Sigsbee escarpment, which marks the southern extent of the salt nappe. In some cases the inter-basin and intercanyon slopes exceed 22° and a number of instabilities have been identified in the geophysical records. These include slumps, debris flows, turbidity currents, and creep movements. The thick layers of sediment that were rapidly deposited on the continental slope during the Pleistocene epoch are associated with cyclic glaciation and fluctuations in sea level (5).