STRATIGRAPHIC ANALYSIS OF HOLOCENE DELTAIC DEPOSITS ON THE WESTERN MARGIN OF THE PUGET LOWLAND, HOOD CANAL, WASHINGTON

Holocene deltaic deposits on the western margin of the Puget Lowland in Washington State record the retreat of the Cordilleran ice sheet, rise in eustatic sea level, and the exhumation of the Olympic Mountains. Following glacial retreat and subsequent sea-level rise, modern depositional systems began to develop on the perimeter of the fjord complex in the lowland. The western-most fjord, the Hood Canal, is fed by the Duckabush and Hamma Hamma watersheds on its western shoreline. These watersheds neighbor each other and have similar drainage size and deltas. Both record Holocene deposition, marine transgression and anthropogenic influences. Their proximity to each other provides an opportunity to identify the depositional signatures of regional geologic events, and to characterize the smaller-scale historic events specific to each watershed.

Vibracores of Holocene sediment collected at variable depths in the upper and lower portions of the deltas display an overall fining upward trend. Pebble to cobble-grade gravel is found in the lower portions of the cores at depths ranging from ~.5 to ~1.5 meters. Depth of coring depended on the depth of coarse gravel, which exceeds the 4-inch diameter of the vibracore pipe.

Gravel at the base of the cores is overlain by fine-grained sediments that are organized into fining-upward units of silty-sand to mud. The gravel is likely glacial in origin, and the transition to fine-grained sediments probably records post-glacial, eustatic sea level rise. The origin of fining-up units in the younger deposits is uncertain but may be related to lateral migration of deltaic channels. The most recent changes in the deltas are in part related to the construction of Highway 101 causeways. Future analysis of the core sediments by C-14 and cesium age dating will provide a time scale for the development of the Holocene deltas. Identification of the gravel clasts found in the bottom of the cores will allow us to determine their origin.