Harry H. Roberts¹
(1) Louisiana State University, Baton Rouge, LA

Abstract: Fluid and gas expulsion on the Northern Gulf of Mexico Slope: Effects of frequency and magnitude on seafloor impacts with special emphasis on gas hydrates

A decade of repeated direct observations and samplings of hydrocarbon seep/vent-related features on the upper continental slope of the northern Gulf of Mexico has qualitatively determined that the magnitude and frequency of expulsion events directly impacts the variability of seafloor geology and biology. Observational data collected by manned submersible and age-dating of shell debris trapped in hydrocarbon seep/vent- derived sediments and authigenic carbonates suggest long time scales perhaps modulated by sea level change (10 years), mesoscale frequencies (interannual) forced by fault activation associated with continual salt adjustment to sedimentary load, and short-term changes associated with dissociation of surface or near-surface gas hydrates forced by water temperature changes at tidal (diurnal) to Loop Current frequencies (intraannual). Seafloor response to hydrocarbon seepage/venting is highly influenced by expulsion magnitude as well as frequency. Expulsion events characterized by high flux rates are usually mud-prone and result in either the development of mud volcanoes or mudflows. At the other end of the spectrum are slow flux rate events that generally result in mineral-prone seafloor responses such as mounded or chimney-like authigenic carbonates or other less impressive forms of seafloor lithification (hardgrounds, slabs, nodules). Most of this lithification is by Ca and Mg-rich carbonates, but more exotic minerals like barite are also important in localized areas. Neither mud-prone or mineral-prone end members of the expulsion spectrum support viable and wide-spread chemosynthetic communities. Intermediate flux cases, like surface or near-surface gas hydrates, however, support dense and widespread chemosynthetic communities as well as spatially variable local seafloor conditions (small-scale fluid mud expulsion to zones of lithification). Trophic resources stored and periodically released by local gas hydrates associated with formation-decomposition cycles accounts for the high level of local variability in biologic as well as geologic response.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana