Regional tectonic deformation of the northern Oregon coast as recorded by Pleistocene marine terraces

Pleistocene marine terraces of the northern Oregon coast are an important factor in understanding the tectonics and paleoseismicity of the central Cascadia subduction zone. The lowest marine terrace, tentatively correlated to 80,000 year old Whiskey Run terrace of southern Oregon, is intermittently exposed in the present day sea cliff along an 80 km section of coastline between Tillamook Head and Cape Kiwanda. Terrace sediments consist largely of fine material such as clay, silt and fine sand with several locations containing large amounts of gravel derived from nearby headlands and steep bedrock hills. The terrace sediments are interpreted to be deposited in back-barrier marine environments, such as a bay, very similar to the bays which presently exist on the northern Oregon coast. Interbedded with terrace sediments are peat horizons which represent buried marsh or forest surfaces. These peat horizons have gradational lower contacts and abrupt upper contacts with terrace sediments indicating that the marsh or forest surfaces formed gradually above sea level and were suddenly downdropped below sea level to be buried by bay sediments. Such features are consistent with a seismically active Cascadia subduction zone which produces interseismic coastal uplift and coseismic coastal subsidence