ALLOCHTHONOUS CONCRETIONS CONTAINING FOSSIL CRABS FROM TERTIARY MARINE FAN CONGLOMERATES, WASHINGTON STATE, USA

Reworked concretions containing fossils Crustacea have been recovered from mid-Cenozoic conglomerate deposits in western Washington State, U.S.A. The best known and most diverse of these crab assemblages is from outcrops of the late Eocene Hoko River Formation, northern Olympic Peninsula. Conglomerates containing abundant fossiliferous concretions occur in five other middle Eocene to late Oligocene units in western Washington: the Humtulips, Lincoln Creek and Pysht formations, the conglomerate at Point Ellice on the Columbia River, and in the ?Aldwell Formation at Pulali Point. In some of these deposits, the calcite-cemented concretions are lithologically similar even though the crabs are derived from a mix of paleo-habitats. In other strata, the crabs apparently represent a fauna from one habitat type but the concretions show lithological diversity. Entrainment of the conglomerates within debris flows transported them all into outer shelf/slope submarine fans of a very narrow continental shelf. Such modes of transport do not adequately explain the taxonomic composition of all of these faunal assemblages. In some instances, crabs from the same conglomerate differ significantly in geochronologic age and the most taxonomic diverse assemblages are the least likely to represent a single temporal grouping. Just as they are today, allochthonous concretions may be eroded from sedimentary strata (even from shoreline subaerial outcrops) and be redeposited multiple times. Each redeposition event adds more taxonomic diversity to the assemblage. Paleodepth analysis and taxonomic faunule descriptions from such crustacean collections have a high probability of being erroneous. We suggest that conglomerates in other marine strata worldwide may contain similarly mixed lithologies and mixed temporal fossil assemblages, and that this has been overlooked.