HR: 17:30h
AN: OS24A-06    [Abstracts]
TI: Triggering Mechanisms of Slope Instability Processes on the Gulf of Mexico Continental Slope: Preliminary Results From IODP Expedition 308
AU: * Urgeles, R
EM: urgeles@ub.edu
AF: Dept. of Stratigraphy, Paleontology and Marine Geosciences, University of Barcelona, Facultat de Geologia, Mart¡ i FranquŠs s.n., Barcelona, 08301 Spain
AU: Locat, J
EM: locat@ggl.ulaval.ca
AF: Dept. of Geology and Geological Engineering, Laval University, Facult‚ des sciences et de g‚nie, Ste. Foy, QUE G1K 7P4 Canada
AU: Flemings, P
EM: flemings@geosc.psu.edu
AF: Dept. of Geosciences, Penn State University, 307 Deike Building, University Park, PA 16802 United States
AU: Behrmann, J
EM: jan.behrmann@geologie.uni-freiburg.de
AF: Geologisches Institut, Albert-Ludwigs-Universit„t Freiburg, Abertstr. 23-B, Freiburg im Breisgau, D-79104 Germany
AU: John, C
EM: cjohn@pmc.ucsc.edu
AF: Dept. of Earth Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 United States
AB: The Ursa Basin lies ~100km downdip of the present mouth of the Mississippi River, northeastern Gulf of Mexico, at ~1000 m depth. In this area, large-scale Mass Transport Deposits (MTD) of Holocene and Pleistocene age were drilled at Sites U1322, U1323 and U1324 during IODP Expedition 308. Recent large landslides also occurred 1 to 10 km NE of the drilled sites. To illuminate the factors controlling initiation of past sediment failures and to characterize the hazard from future slope instabilities, a complete suite of logging data and direct measurements of moisture and density and undrained shear strength were acquired during Expedition 308. Preliminary results indicate that Ursa Basin has very high sedimentation rates, at least 10m/ky with peaks up to 25 m/ky. This has resulted in a particular porosity profile, with a relatively rapid decrease from 80% at seafloor to 55% at ~50 mbsf. Then, a gentler gradient is observed down to the bottom of the hole, with the lowest porosities at ~45%. Fluid overpressure estimated from the porosity profile and from pore pressure penetrometers indicates that the vertical effective stress is 50 to 60% lower than if hydrostatic conditions existed. The trends in density and undrained shear strength are relatively smoother at Site U1324 compared to Site U1322. At Site U1322 density and strength measurements show significant increases (up to 0.1 g/cm³ and 50 kPa respectively) associated with MTDs. This may indicate higher consolidation of MTD, which probably results from shearing and dewatering of the sediments during the landslide process. High sedimentation rates are a crucial factor leading to overpressure and slope instability at Ursa. The location, depth, thickness and amount of shearing in MTDs will further influence slope stability.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3021 Marine hydrogeology
DE: 3022 Marine sediments: processes and transport
DE: 3036 Ocean drilling
DE: 3070 Submarine landslides
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005