Paleoecologic Implications of Chemoautotrophic Molluscan-Dominated Assemblages on the Louisiana Continental Slope

CALLENDER, W. RUSSELL, and ERIC N. POWELL, Texas A&M University, College Station, TX

Dense chemoautotrophically based molluscan communities are common at sites of active petroleum seepage in the Green Canyon and Garden Banks lease blocks on the Louisiana upper continental slope. The presence of dense molluscan aggregations has important implications for paleoecology because the continental slope is generally thought of as a geologic setting depauperate of macrofossils. Liquid and gaseous petroleum seepage, primarily along faults associated with salt diapirism, provides localized nutrient input which facilitates high molluscan productivity. Authigeneic carbonate, formed due to methane oxidation, occurs at and below the sediment-water interface in areas of molluscan concentrations.

Trophic structure of these bivalve-dominated communities is based on three separate carbon pathways: chemoautotrophy, utilizing either H(2)S or CH(4), or autotrophy, utilizing organic food sources. The largest quantity of preservable molluscan biomass is produced by infaunal lucinid and thyasirid clams which utilize H(2)S. Thyasirid and lucinid clams form extensive three-dimensional shell beds which can extend for several tens of meters. Second and third in terms of preservable biomass are vesicomyid clams and mytilid mussels which utilize H(2)S and CH(4) respectively. Vesicomyid clams form shell pavements at a smaller scale then the lucinid and thyasirid shell beds. The least amount of preservable biomass is produced by autotrophs including trochid and nerite gastropods, limid bivalv s, crabs, and starfish. Energy flow requirements for benthic communities on the oligotrophic continental shelf and slope settings of the northern Gulf of Mexico require a localized nutrient enrichment, such as a petroleum seep, to support dense concentrations of large individuals. This relationship of nutrient enrichment to enhanced biomass productivity probably also was a dominant factor in the formation of similar autochthonous benthic assemblages in the fossil record.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)