Recognition of deep-water benthic assemblages in the fossil record: Taphonomy and community characteristics of Louisiana continental slope petroleum seep assemblages

hemoautotrophic benthic assemblages associated with petroleum seepage form the only substantial shell accumulations below storm wave base on the northern Gulf of Mexico shelf and slope. Five distinctive biofacies are associated with petroleum seepage, dominated respectively by vestimentiferan tubeworms, lucinid, thyasirid and vesicomyid clams and mytilid mussels. The taphonomy of petroleum seep death assemblages includes dissolution as the most pervasive mode of shell alteration. The degree of fragmentation is high and is likely caused by biological breakage and extreme dissolution. The dominant species in each assemblage generally reflect the overall taphonomic signature of the assemblage in which they dominate. The taphonomic attributes of petroleum seep death assemblages are very similar to those of ancient autochthonous benthic assemblages. The differences that exist are due primarily to the incompleteness of taphonomic processes at the seeps and secondarily to the fact that interpretations of ancient autochthonous assemblages have been based on conceptual models that have not been adequately tested by comparison with modem analogs. Autochthonous assemblages show substantial small-scale spatial variability in all taphonomic attributes.;Preservable paleoecological characteristics representative of cold seep assemblages include: high density-low diversity molluscan assemblages dominated by large individuals, high molluscan biomass concentrations aligned in linear trends which may be associated with subsurface faults, carbonates with depleted ;The Campanian Tepee Buttes also share many paleoecological characteristics with recognized ancient seep assemblages. Methane and hydrogen sulfide-rich fluids from underlying strata were transported along fault conduits to supply a localized nutrient source for lucinid-dominated benthic communities. The Tepee Butte assemblage occurred in isolated, isotopically light carbonate masses associated with shale that contained a sparsely populated benthic fauna. The Tepee Butte assemblages were dominated by dense accumulations of Nymphalucina occidentalis (Lucinidae) with moderate to high articulation frequencies. Shell preservation was generally poor. The size-frequency distribution was principally unimodal and dominated by large individuals. Biomass production was extremely high, especially compared to the surrounding areas. Based on taxonomic uniformitarian comparisons with modern lucinids, the Tepee Butte lucinids probably used...