Sediment sources and Holocene sedimentation history in Tillamook Bay, Oregon; data and preliminary interpretations

Abstract

Surface and core sediments from Tillamook Bay, Oregon, have been analyzed to determine modern and Holocene sediment sources and sedimentation history. Heavy mineral analyses established three sediment sources: (1) the five major rivers draining the volcanic and associated sedimentary rocks of the Coast Range, (2) small streams draining the sedimentary uplands that form the shoreline adjacent to Tillamook Bay and erosion of the shoreline by waves and currents, and (3) marine sediments carried to Tillamook Bay by longshore drift from drainage basins north or south of the bay. Stratigraphic and radiocarbon analyses show that the Holocene fill in Tillamook Bay began to accumulate sometime before about 9,000 years ago in deep parts of prefill river valleys. The rate of accumulation generally coincided with the rate of world-wide sea-level rise--faster (greater than about 3 meters per 1,000 years) up to about 7,000 years ago and slower (less than about 2 meters per 1,000 years) since that time. The vertical rate of accumulation at one core site seems to have been about the same from 3,300 years B.P. (before present) to the present, as it was from about 5,200 years B.P. to 3,300.