ABSTRACT

Hydrocarbon seepage in both liquid (crude oil) and gas (principally methane) forms has been amply documented over the past decade from submersible dives on the northern Gulf of Mexico seafloor overlying salt diapirs. These seepage sites are inhabited by a remarkably diverse chemosynthetic fauna and are associated with massive carbonate buildups formed through bacterially-mediated processes of hydrocarbon oxidation coupled with sulfate reduction. This study addresses questions concerning the timing and longevity of seepage from four representative sites in the Green Canyon area (27°50'N; 91°30'W) on the basis of radiometric dating assays of massive carbonates that act as time keepers of hydrocarbon seeps.

²³⁰Th dates from GC-140 and GC-184 blocks place the initiation and termination of massive seepage there at 195 ±25 Ka and 13.3 ±2.7 Ka, respectively, and are in agreement with the chronology of the salt dome emplacement at shallow depth during mid to late Pleistocene low sea-level stands. The prolific seepage activity to the southeast in GC-185 (Bush Hill) and the ²³⁰Th dates of 3.2 to 1.4 Ka are attributed to a recent episode of subsidence caused by salt withdrawal which created late normal faults. When multiple dates, subsurface imaging of the salt domes by 3-D seismics, and high resolution subsurface chronostratigraphy are available from the same site, a link is apparent between the incidence of low sea level stands, salt diapirism, and enhanced hydrocarbon seepage.

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