FOSSIL POINT, COOS BAY OREGON; A NEW INVESTIGATION INTO THE FOSSILS, CHRONOLOGY, AND RELATIONSHIP TO CASCADIA MEGATHRUST EARTHQUAKES

The aptly named Fossil Point on the outskirts of Coos Bay Oregon is a known fossil site producing a variety of marine invertebrates and the occasional whales and pinnipeds. Despite a collection history dating back over 100 years and fossils from this locality appearing in numerous museums, few specimens from the site have been formally described. Further complicating the issue is a notable lack of geochronology. The lowest exposed sediments, only accessible during low tides in a wave-cut platform, belong to the Empire Formation. The Empire has been previously assigned to the late Miocene to Pliocene based on invertebrates and one of the few described cetaceans from the site. Capping the sequence is a Pleistocene gravel with some tentative dates in the 100-150,000 range of the Pleistocene. Sandwiched in between the two formations is the Coos Conglomerate, a very high energy deposit with shells, rip-up clasts from the Empire sandstone, and cobbles to boulders of numerous other rock types. The basal contact is deeply excavated into the Empire Formation, implying some gap in time, yet previous workers have proposed the Coos Conglomerate as a unit of the older Empire Formation. Using a newly discovered baleen whale skull, sea urchins, and gastropods from the Coos Conglomerate we propose this unit is actually early Pleistocene in age and the high energy deposition of the sediments suggests a Cascadia Megathrust earthquake generated tsunami as the method of deposition. We also report on a reexamination of a walrus mandible from the capping gravel as further ago constraints on that unit. This locality may serve to extend our record of Cascadia events further back into the past, important both for constraining reoccurrence intervals, but also for understanding the impacts of a tectonically active coastline of the marine faunas inhabiting the region.