Peter Flemings, University of Texas at Austin (United States)
Brandon Dugan, Rice University (United States)
Derek Sawyer, University of Texas at Austin (United States)
Julia Schneider, University of Texas at Austin (United States)
Hilary Strong, University of Texas at Austin (United States)
Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress (λ* = (u-uh)/(σ'vh) ) in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone and sandstone. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. The average sedimentation rate from the seafloor to the top of the Blue Unit is 12 mm/year at Site U1324 and 3.6 mm/yr at Site U1322. Ursa mudstones have hydraulic diffusivities of 2 x 10-8 m2/s whereas siltstones have diffusivities greater than 2 x 10-7 m2/s. We interpret that at Site U1324, rapid deposition of fine grained low permeability mudstone in the upper 200 mbsf generated the high overpressures present.
Reduced overpressure at depth at Site 1324 suggests suggest lateral flow (drainage) whereas high overpressure at Site 1322, where the sedimentation rate is low requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2 degrees) offshore from the Mississippi delta.