University of California, Santa Cruz, United States

Integrated Ocean Drilling Program Expedition 308 is an abbreviated form of Proposal 589-Full 3 entitled "Overpressure and Fluid Flow Processes in the Deepwater Gulf of Mexico: Slope Stability, Seeps, and Shallow Water Flow". We will explore the coupling of overpressure, flow, and deformation in passive margin settings. We will test 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. Drilling, logging, and in situ measurements will be performed in the Brazos-Trinity #4 minibasin and in the Ursa region of the northern Gulf of Mexico. These basins are 300 km apart and have experienced very different Pleistocene sedimentation histories. Brazos-Trinity #4 will serve as a reference location because its low sedimentation rate generated little overpressure. In contrast, Ursa experienced extreme sedimentation rates, has high overpressures, and will serve as a type location to study overpressure and flow. Drilling and consequent postcruise studies will illuminate controls on slope stability, seafloor seeps, and large-scale crustal fluid flow. Two key components of the experimental plan are to take substantial whole-core geotechnical samples for later shore-based analysis and to deploy a tapered penetrometer, the T2P probe (developed jointly between the Massachusetts Institute of Technology [MIT; USA], the Pennsylvania State University [USA], and IODP) to measure in situ pressure and temperature. Expedition 308 science will meet many of the objectives proposed in the original IODP Proposal 589-Full3 and will provide the foundation to implement long-term in situ monitoring experiments in the aquifer and bounding mudstones in a future expedition to meet the full objectives of IODP Proposal 589-Full3.