TAPHONOMIC RATES OF MOLLUSCAN SHELLS PLACED IN AUTOCHTHONOUS ASSEMBLAGES ON THE LOUISIANA CONTINENTAL-SLOPE

A mixed assemblage of lucinid and mussel shells were placed in mesh ba gs and left at a site of autochthonous death assemblage formation in a petroleum seep community on the Louisiana upper continental slope for a period of 3 yr. Upon recovery, the shells were assessed for taphono mic alteration and compared to a control assemblage of unaltered shell s. The data verify a basic assumption of taphofacies analysis; that ev idence of taphonomic processes preserved with the assemblage does in f act document the primary taphonomic processes that biased the assembla ge from the original assemblage of living preservable organisms. Signi ficant variability in taphonomic rates existed between shells from loc ations 10 m apart, as is typical of autochthonous assemblages, so that small-scale variability in the taphonomic process was important. Muss els were more severely altered than lucinids. Mussels were more heavil y dissolved, had more altered edges, were more prone to fragmentation and exhibited greater weight loss (=carbonate loss) than did the lucin ids. The estimated residence time for mussels on the continental slope is approximately 3 to 15 yr. Lucinids may be indefinitely preserved. The observed discrepancy between the dominance of lucinids at fossil s eeps and the frequent dominance of mussels at recent seeps can be expl ained by biased preservation favoring the lucinids.