Invertebrate trace fossils from Paleogene fluvial strata in Western Washington, USA

Abstract

Fluvial strata in the Paleogene Chuckanut Formation contain a wealth of trace fossils that include both vertebrate and invertebrate ichnofossils. Vertebrate traces have received careful scrutiny, but invertebrate traces have not previously been described. Trace fossils occur in two sediment types. Mudstones deposited in subaqueous environments include highly bioturbated layers that are dominated by horizontal meniscate burrows. Siltstone and fine-grained sandstone beds primarily contain vermiform trails on bedding surfaces, with sparse vertical traces. Some of these sandy beds also contain vertebrate tracks. Chuckanut Formation beds commonly show rapid facies changes in sediments that were deposited along a meandering river during a time of semitropical climate. Transitions from subaqueous to subaerial conditions may have been related to changing water levels caused by variations in seasonal precipitation or episodic flood events. Other possible factors include ponds created by abandonment of channels, overbank sediment s deposited by floods, and breaching of levees to form crevasse splay deposits. Trace fossil assemblages show local variations in the diverse depositional environments of the Chuckanut Formation. Many of the ichnofossil suites clearly represent Scoyenia Ichnofacies, but some occurrences may involve overprinting of Scoyenia and Mermia ichnofacies.