Late Holocene relict shorelines of the southwestern Louisiana chenier plain contain molluscan assemblages that vary greatly in taxonomic composition and bioclast preservation. Taxonomic composition varies with ridge age: older ridges are oyster rich, whereas younger ridges are dominated by infaunal bivalves. Taphonomic features can be separated into those caused by biostratinomic and those caused by pedogenic processes. Pedogenic alteration generally increases as ridge age increases, whereas biostratinomic alteration reflects the prevalence of reworked bioclasts in assemblages. These molluscan assemblages are extensively time averaged, causing temporal overcompleteness of depositional units (i.e., amount of time averaging for bioclasts within a unit is much greater than the time it took for that unit to form). Chenier-plain progradation over the last 3,000 years both caused and preserved the observed trend in community composition. This trend was caused by community replacement related to changing substrate stability and by changes in the source of reworked bioclasts, both of which operated in response to progradation. Net progradation also allowed this trend to be preserved because time averaging occurred episodically and shorelines were effectively separated into discrete generations. Although coastal deposits are not typically viewed as ideal sites for high-resolution paleoenvironmental studies, millennial-scale community trends can be detected in this setting.