Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico, offshore Louisiana

摘要

We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon usingn3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 IntegratednOcean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations innpore water show that sulphate–methane transition zone, at 74 and 94 m below seafloor, are amongst the deep-nest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channeln(Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, prevent-ning normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic andnfocused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes atnthe seabed. This may also favour seawater circulation and we interpret the deep sulphate–methane transitionnzones as a result of high downward sulphate fluxes coming from seawater that are about 5–10 times above thosenmeasured in other basins. The results show that geochemical reactions within shallow sediments are dominatednby seawater downwelling in the Mars-Ursa basin, compared to other basins in which the upward fluid flux is con-ntrolling methane-related reactions. This has implications for the occurrence of gas hydrates in the subsurface andnis evidence of the active connection between buried sediments and the water column.