Paleoecological analysis of molluscan assemblages from the middle Eocene Cowlitz Formation, southwestern Washington

Abstract

Molluscan assemblages from upper middle Eocene deposits of southwestern Washington represent a regional biotic system that provides independent evidence of a range of depositional environments in a delta. Four marine molluscan assemblages reflect a subtropical fauna that inhabited an inner neritic to upper bathyal gradient down the prodelta slope. Comparisons with Recent molluscan taxa indicate changing trophic structures from dominantly infaunal filter feeding in the shallow, coarser-grained substrate to dominantly infaunal deposit feeding in the fine-grained mud. A very high diversity of carnivorous gastropods indicates that the biocenosis included a wide array of predominantly errant, soft-bodied invertebrate prey. The most shallow marine assemblages show evidence of dense Turritella patches within infaunal bivalve-dominated associations, indicating mosaics of distribution across a soft-sediment substrate. Low diversity marginal-marine and freshwater assemblages show faunal links with each other. They indicate interdistributary bay and marsh environments that are commonly part of a river-dominated delta.