Origin of Overpressure and Slope Failure in the Ursa Region, Northern Gulf of Mexico

Abstract

Sedimentation-fluid flow-stability models predict the evolution of overpressure (pressure above hydrostatic) and stability of sediments in the Ursa region, northern Gulf of Mexico (IODP Expedition 308). Two-dimensional models across the Ursa region from 65–0 ka simulate overpressure up to 2 MPa in the shallow subsurface at Site U1324 and near 1 MPa at Site U1322, which is consistent with measurements. Overpressure history is controlled by sedimentation rate and distribution. Today overpressure is decreasing, but past high sedimentation periods were important overpressure sources. Infinite slope stability analyses indicate the slope is stable. Overpressure increase and stability decrease is most prevalent along the boundary between mud-rich mass transport deposits and silt-rich channel-levee deposits between Sites U1323 and U1322. This is controlled by high overpressure driving fluids laterally along permeable sediments to where overburden is thin. Even with high sedimentation rates at Ursa and lateral flow, unstable conditions are not simulated. Seismic and core observations, however, document recurring slope failures in the mud-rich deposits. We conclude that sedimentation-driven overpressure and lateral transfer of pressure created a marginally stable slope, but additional driving forces, such as extremely high sediment inputs or seismic accelerations created by salt tectonic-related earthquakes were required to initiate failure.