Geologic Controls on the Distribution of Hydrocarbon Seeps and Chemosynthetic Communities in the Northwest Gulf of Mexico Continental Slope: Case Studies

James F. Reilly II, Ian R. Mac Donald, C. S. Lee, W. W. Sager, E. K. Biegert, James M. Brooks

Extensive, active hydrocarbon seepage on the continental slope of the northwest Gulf of Mexico has been documented over the last 10 years with the presence of these seeps influencing the near seafloor geology and the seafloor biomass. Seafloor modification includes hydrocarbon charging of the sediment, the formation of clathrate hydrates, precipitation of authigenic calcium carbonate and the development of seep-related chemosynthetic communities in a normally depauperate environment. To date approximately 45 chemosynthetic sites have been identified where individual communities may consist of a single species of organism or may be comprised of a astronomically diverse assemblage which may reflect ecological conditions forced by fluid type of temporal constraints on seep a tivity. Further, a seep site may fail to support more than a very sparse assemblage indicating a likely external influence on the development of a viable community. In this study a comparison is made of the geologic conditions present at several seep sites where both single species and composite assemblages of chemosynthetic organisms are present. In addition, sites where active seepage would suggest an environment conducive to the development of vibrant communities but display a limited or lack of colonization are described. Visits by manned submersibles documented the presence of four separate, distinct communities in the Green Canyon OCS region and only sparsely single occurrences of tubeworms and mussels within two sites in Garden Banks and one site in Green Canyon.

Detailed structural mapping illustrates distinctly different histories of structural development at the different sites. These differences may indicate a structural component likely constrains colonization at the seeps possibly by controlling hydrocarbon flux at the seafloor. Orbital imagery acquired over these sites would support a temporal constraint on seepage as extensive perennial slicks are present at the Green Canyon sites where vibrant communities are present and not at the Garden Banks sites. Repeat coverage by manned submersible supports this model as seeps seen actively discharging fluids at the seafloor during one visit were not active on subsequent visits.

Discrimination of likely seep sites where the probability of the development of significant chemosynthetic communities is high requires a collaborative examination of the seafloor and subsurface structure. By calibrating the geophysical data with direct investigation of the seafloor environment combined with a detailed structural history of the seep site allows for an evaluation protocol to be developed for examination of likely seep sites and possible chemosynthetic community development.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995