Mud mounds on the continental slope, northwestern Gulf of Mexico and their relation to hydrates and seafloor instability

High resolution seismic data delineate acoustically amorphous mounded structures on the continental slope of the Northwest Gulf of Mexico. The thermogenic gas hydrates reported from Green Canyon Block 184 are associated with one of these mounds. This particular mound occurs in a highly faulted area and overlies an extensive chaotic seismic facies. The chaotic facies indicated underconsolidated, overpressured sediments. Gas and fluids are migrating up the fault plane to the sea floor. The relief of the mound is partially the result of the throw of the associated faults, but also to a combination of other factors such as the upwelling of gaseous muds, in-place expansion of the hydrates and the generation of diagenic carbonates.;The acoustic signature of the known hydrate accumulation of Green Canyon Block 184 was applied to high resolution data in Mississippi Canyon Block 798 and 799 in an attempt to predict the occurrence of hydrates. Piston cores within an acoustically amorphous mound in Block 798 encountered extremely gassy sediments with sizeable chunks of white, presumably biogenic hydrates. The geologic setting in Block 798 was similar to Green Canyon 184 with complex faulting and subsurface sequences of thick chaotic facies.;Numerous other examples of acoustically amorphous mounds occur in areas of Mississippi Canyon, Green Canyon, Ewing Bank, Garden Banks and East Breaks. The acoustic signature of mounds along the upper slope in water depths too shallow for hydrate formation show no appreciable difference from the deep water varieties. The venting of gas and fluids from overpressured zones appears to be the common factor for all mounds thereby making them mud diapir/mud volcano type structures. However, the possibility of pingo-type formation cannot be entirely ruled out.;No evidence of seafloor instability was directly related to the presence or possible presence of gas hydrates. However, if it can be established that massive hydrate formation exist on the continental slope, then a substantial lowering of sea level such as occurred in the late Pleistocene, would result in decomposition of the hydrates and trigger mass wasting. Such a mechanism could explain abundance of buried erosional scars along the continental slope.