HR: 0800h
AN: OS31A-0160    [Abstracts]
TI: Anomalous sand deposit associated with evidence of late Holocene uplift near Bremerton, Washington
AU: * Martin, M E
EM: memartin@u.washington.edu
AF: Earth and Space Sciences University of Washington, Box 351310, Seattle, wa 98195, United States
AU: Maxcia, C
EM: claudia_maxcia@yahoo.com
AU: Gerardi, F
EM: f.gerardi@unict.it
AF: Dipartimento di Scienze Geologiche Università degli Studi di Catania, Corso Italia 55, Catania, 95129, Italy
AU: Bourgeois, J
EM: jbourgeo@u.washington.edu
AF: Earth and Space Sciences University of Washington, Box 351310, Seattle, wa 98195, United States
AB: The origins of a puzzling sand unit at the head of Sinclair Inlet likely include a Puget Sound tsunami and its return flow but may also include sand volcanoes, debris flows, or both.. The inlet, a branch of Puget Sound 22 km west of Seattle, contains at its head an intertidal mudflat that is fringed by partly urbanized tidal marshes and alder swamps. Cores and pits in these remnant wetlands reveal an abrupt upward change from tidal flats to freshwater forest that probably resulted from uplift during the Seattle fault earthquake of A.D. 900-930 or during slip of similar age on the Tacoma fault. The puzzling sand unit formed at or about the time of this uplift. The unit consists of silty fine sand commonly 1 m in thickess. It has a sharp contact with shelly mudflat deposits below and a sharp to gradational contact with overlying peat. The lower part of this peat contains the growth-position roots of western red cedar (Thuja placata), and the upper part forms the soil of modern salt marshes. We found the sand unit, 1.1 m thick in areas, along 0.5 km of the upper inlet's shoreline, beneath modern tidal marshes and alder swamps. The unit contains at least three parts: a basal subunit that fines upward from medium and coarse sand to fine sand and silt; a thin (less than 1 cm thick) clay bed, commonly with flame structures; and a capping subunit up to 50 cm thick that is dominated by silty fine to medium sand but which also contains pebbles derived from rock and glacial outwash. Preliminary diatom analysis shows the unit contains diatoms from a brackish environment. A possibly related sand unit lies 0.8 km inland, up the valley of Gorst Creek. Its thickness is in the range 0.2-1.0 m as observed thus far in creek banks and in pits at a nearby park. Unlike the sand that borders the inlet, it contains trough cross-bedding. We are investigating the relationship between this deposit and the silty sand deposit in the tide flats. Preliminary diatom analysis did not find any diatoms in this deposit.
DE: 4558 Sediment transport (1862)
DE: 4564 Tsunamis and storm surges
DE: 7221 Paleoseismology (8036)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting