Rapid sedimentation, overpressure, fluid flowand slope instability at the Gulf of Mexicocontinental margin

Integrated Ocean Drilling Programme (IODP) Expedition 308 studied overpressure and fluid flow on the Gulf of Mexico continental slope. The scientific program examined how sedimentation, overpressure, fluid flow, and deformation are coupled in a passive continental margin setting. The expedition investigated the model of how extremely rapid deposition of finegrained mud leads to rapid build-up of pore pressure in excess of hydrostatic (overpressure), underconsolidation and continental slope instability. Expedition 308 tested this model by examining how physical properties, pressure, temperature, and pore fluid compositions vary within low-permeability mudstones that overlie a permeable, overpressured aquifer. Three sites were drilled in the Ursa Basin off the Mississippi Delta, using the research drillship R/V JOIDES RESOLUTION (Fig. 1). In the Ursa Basin rapid, late Pleistocene sedimentation was known to be present. Drilling documented severe overpressure in the mudstones overlying the aquifer. The most important achievement of IODP Expedition 308 is to have successfully recorded in situ formation pressure and temperature in an overpressured basin. This is the first time that a coherent data set of such measurements has been obtained.