Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico, offshore Louisiana
Article first published online: 24 AUG 2010
© 2010 Blackwell Publishing Ltd This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Volume 11, Issue 1, pages 34–47, February 2011
How to Cite
GAY, A., TAKANO, Y., GILHOOLY III , W. P., BERNDT, C., HEESCHEN, K., SUZUKI, N., SAEGUSA, S., NAKAGAWA, F., TSUNOGAI, U., JIANG, S. Y. and LOPEZ, M. (2011), Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico, offshore Louisiana. Geofluids, 11: 34–47. doi: 10.1111/j.1468-8123.2010.00304.x
- Issue published online: 1 FEB 2011
- Article first published online: 24 AUG 2010
- Received 9 October 2009; accepted 1 July 2010
- focused fluid flow;
- seal bypass system;
- mud volcano;
- 3D seismic;
- IODP Expedition 308;
- Gulf of Mexico
We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon using 3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 Integrated Ocean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations in pore water show that sulphate–methane transition zone, at 74 and 94 m below seafloor, are amongst the deepest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channel (Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, preventing normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic and focused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes at the seabed. This may also favour seawater circulation and we interpret the deep sulphate–methane transition zones as a result of high downward sulphate fluxes coming from seawater that are about 5–10 times above those measured in other basins. The results show that geochemical reactions within shallow sediments are dominated by seawater downwelling in the Mars-Ursa basin, compared to other basins in which the upward fluid flux is controlling methane-related reactions. This has implications for the occurrence of gas hydrates in the subsurface and is evidence of the active connection between buried sediments and the water column.