Recursive Failure Of The Gulf Of Mexico Continental Slope: Timing And Causes

Seismic and multibeam data have shown the occurrence of Plesitocene large-scale Mass Transport Deposits (MTDs) and Holocene failure events in Ursa Basin at ~1000 m depth in the eastern levee of the Mississippi Canyon, northern Gulf of Mexico. During IODP Expedition 308 Sites U1322 and U1324 were drilled adjacent to the Recent failures and through several MTDs of Holocene and Pleistocene age. A complete suite of logging, sedimentological and geotechnical data were acquired to reveal the factors controlling initiation of past sediment failures and to characterize the hazard potential of future slope instabilities. Fluid overpressure estimated from a variety of direct and indirect methods indicates that the vertical effective stress is 50 to 70 % lower than if hydrostatic conditions existed. Overpressure in Ursa Basin most likely results from deposition of fine-grained sediments with rates at least 1 m ky-1 with peaks up to 25 m ky-1. The thickest and most widespread MTDs occur in periods of highest sedimentation rate. Considering the very low seismicity experienced by the margin it is most probable that the changes in sedimentation rate might be the primary driving force of slope failure. At Sites U1322 and U1324 a total of 14 MTDs were identified, which leads to a frequency of emplacement of 1 MTD/4.5 ka. Considering only the thickest MTDs the recurrence interval reduces to 1 MTD/10 ka.