CENOZOIC MARINE AND TERRESTRIAL ICHNOFOSSILS IN WESTERN WASHINGTON STATE

Western Washington State was dramatically changed with the Farallon plate subduction and the accretion of Siletzia onto North America. Mid ocean ridge basalts and bioturbated ocean sediments were uplifted to form the Crescent formation on the Olympic Peninsula over the course of the early Cenozoic. At the same time, arc magmatism altered the landscape further inland forming the Cascade Range, on the flanks of what is now Mount Saint Helens, there is evidence of past volcanic activity interbedded with fossiliferous sediments. Lithological specimens have been collected for comparison of grain size, mineralogy, and sedimentary structures via microscopy for each site. The goal of this research is to utilize ichnology and sedimentology to reconstruct depositional setting and paleoenvironment at both of our sites.

Our marine ichnofossil site is a collection of outcrops within the Crescent Formation in the southern most portion of Olympic National Forest. Each area contains a turbidite horizon with a range of marine worm burrow fossils. Preliminary data from the specimens collected suggests up to six different burrows types based on diameter, shape, and depth of the burrows. Further work will be done to determine if some of the burrows are from juveniles and adults of the same species. We hope to develop an ichnofacies based on our observations of these fossils to estimate water depth at the time of deposition.

Terrestrial fossils occur at our Mount Saint Helens site within sandstones, interbedded with volcaniclastic conglomerate layers up to 150 cm thick that likely originated as lahars. Within the sandstone layers there are examples of crossbedding which has led us to interpret the site as fluvial. We have recovered a diverse assemblage of plant fossils dominated by coniferous and deciduous tree fossils as well as portions of a trackway. Initial observations indicate that is of a mammalian origin, and from an animal that has very similar fore and hind feet, with the gait pattern of a diagonal walker. The footprints are small, only 1.5 cm across, but have clear paw pad and nail impressions. The trackway fossils will be cast and further studied to estimate body size and determine the genus of the mammal that left them. Similar plant fossils have been found in other areas near by this site, and have been dated to the early Miocene; we will be collecting additional samples of the volcanoclastic conglomerate at our site to locate zircons and obtain a date range on our fossil assemblage as well.