Gulf of Mexico Hydrocarbon Seep Communities V. Biofacies and Shell Orientation of Autochthonous Shell Beds below Storm Wave Base

Clam and mussel assemblages associated with petroleum seepage on the Louisiana continental slope form the only substantial shell accumulations below storm wave base on the northwestern Gulf of Mexico shelf and slope. Four distinct biofacies are present at the seeps, dominated respectively by mussels, lucinid clams, vesicomyid clams and tubeworms. Each primary seep site is typically composed of a series of not necessarily contiguous, autochthonous beds dominated by one biofacies. Mussels and tubeworms often co-occur, but neither normally co-occur with lucinid or vesicomyid clams. Lucinid and vesicomyid clam beds have the best chance of preservation. Despite essentially undisturbed accumulation in quiet water below storm wave base, concavity ratios rarely differ from 1:1 and frequency of articulation may be low. Dominantly concave-up valves previously reported in quiet water may result from man's fishing activities. Significant variability in shell orientation, frequency of articulation and concavity ratio between adjacent samples indicates that many individual stratigraphically-equivalent samples should be used in any taphofacies analysis of assemblages formed in low-energy environments. Lucinid beds which form below the sediment surface and vesicomyid beds which form on the sediment surface differed significantly in shell orientation and articulation frequency. Assemblages forming below storm wave base in low-energy environmentsmore » may comprise a wide variety of taphofacies depending upon whether formation occurs primarily beneath the sediment surface or on the sediment surface, despite contemporaneous formation under similar environmental conditions.« less