University of California, Santa Cruz, United States

Integrated Ocean Drilling Program (IODP) Expedition 308 is the first stage of a two-component program dedicated to the study of overpressure and fluid flow on the Gulf of Mexico continental slope. Drilling at six sites revealed an active hydrodynamic environment and provided insight into geological processes near the seafloor. We tested a multidimensional flow model by examining how physical properties, pressure, temperature, and pore fluid composition vary within low-permeability mudstones that overlie a permeable and overpressured aquifer. We drilled, logged, and made in situ pressure and temperature measurements in Brazos- Trinity Basin IV off the Texas Gulf Coast where low sedimentation rates and normal pressures were predicted. We contrasted these observations with experiments in the Ursa region off the Mississippi Delta where rapid Pleistocene sedimentation occurred. At Ursa, multiple pore pressure penetrometer measurements recorded values that lie halfway between the hydrostatic pressure and the lithostatic pressure. Porosity-depth profiles suggest that these overpressures are maintained by compaction disequilibrium. Log, core, and seismic data illustrate that this overpressured region was subject to multiple events of slope instability, which generated mass transport deposits. A surprising result is that mudstones beneath Brazos-Trinity Basin IV are undercompacted, and most likely overpressured, relative to mudstones at the basin margin. Interbedded sands and mudstones within Brazos-Trinity Basin IV record a fascinating stratigraphic history that relates turbidite formation to eustatic sea level change. Postcruise science on both areas investigated will combine theoretical modeling and laboratory analysis to further illuminate coupled processes of flow, sedimentation, and deformation on passive continental margins.