GEOCHRONOLOGY, PALEONTOLOGY, AND RECOGNITION OF POSSIBLE TSUNAMI DEPOSITS AT FOSSIL POINT, COOS BAY, OR

The aptly named Fossil Point in Coos Bay, Oregon has been producing fossils for nearly 150 years, a fairly long history for a west coast fossil site. Yet despite this long history not only is little of the fossil material published, but the geochronology and environmental assessment of depositional setting is notably lacking. With a team on community college students were aiming to remedy these shortcomings. One of the biggest initial surprises has been the connection to subduction zone earthquakes and the tsunamis triggered by them. The South Oregon Coast holds many memories of Cascadia quakes, both the relatively recent 1700 quake and older events disappearing into obscurity. Records of older events are relatively rare owing to either erosion or changing sea levels. Previously the Coos Conglomerate of Fossil Point was assigned to the underlying Empire Formation, dating to roughly sometime during the Miocene based on biostratigraphy. However, some 1950s work on marine invertebrates noted the Coos Conglomerate contains a mix of Miocene and Pleistocene fossils. Upon new investigation of the Coos Conglomerate we propose it actually represents a Cascadia megathrust earthquake driven tsunami deposit from the mid to late Pleistocene. The narrow area is a back flow with imbricated clasts showing a current direction towards the ocean rather than inland like a typical storm deposit. The invertebrate fossils are a mix of Pleistocene and Miocene taxa as previously noted, but unlike limited past work, some of the described or collected vertebrates from the deposit represent definitively Pleistocene taxa. We’ve collected new cetacean, pinniped, chondrichtheys, and salmonid fossils. This high-energy deposit is in distinct contrast with the shallow water lagoonal deposits of the Empire Fm or the moderate energy rocky beach deposits of the overlying Whiskey Run Pleistocene terraces.