Abstracts
SEARCH Open Science Meeting
October 27, 2003
Seattle, Washington, USA
Geological and Geophysical Research into the Impact of Earthquakes on Prehistoric Coastal Occupation: the Mid-Holocene Occupation and Abandonment of the Tanginak Spring Site
Elizabeth Mahrt1, Bretwood Higman2, Joseph MacGregor3, Joanne Bourgeois4, Ben Fitzhugh5
1Earth and Space Science, University of Washington, Seattle, WA, 98195, USA, Phone 206-534-6686, bmahrt@u.washington.edu
2Earth and Space Science, University of Washington, Seattle, WA, 98195, USA, Phone 206-526-5389, hig314@u.washington.edu
3Earth and Space Science, University of Washington, Seattle, WA, 98195, USA, joemac@u.washington.edu
4Earth and Space Science, University of Washington, Seattle, WA, 98195, USA, Phone 206-543-0489, jbourgeo@u.washington.edu
5Anthropology, University of Washington, Box 353100, Seattle, WA, 98195, USA, Phone 206-543-9604, Fax 206-543-3285, fitzhugh@u.washington.edu
Since their earliest arrival, humans living on the tectonically active subarctic coastal margins of the northern Pacific have had to adapt to both gradual and rapid environmental changes. The research reported here documents the effects of dynamic geological processes on the settlement history of an ancient archaeological site on the Kodiak Archipelago, Alaska. The Tanginak Spring Site on Sitkalidak Island was occupied from 7500 BP until it was permanently abandoned about 6000 BP. We conducted geological and geophysical analyses of the surrounding area in order to elucidate environmental conditions which existed during the occupation and abandonment of this site. Volcanic ash layers permit correlation amongst the archaeological and geological sites. Our investigations suggest that during the period of occupation the area was experiencing gradual sea level rise, followed by an earthquake with associated uplift and tsunami at the time of abandonment.
The Tanginak Spring site is situated on an 8-m-tall bench above a salt marsh protected from the ocean by a series of beach ridges. We examined beach-ridge history with ground-penetrating radar as well as with some trenching. Near the salt marsh are hills bounding small peat bogs at varying elevations recording tephra and peat accumulations since deglaciation We excavated, described and sampled multiple excavations in the salt-marsh and freshwater peats in order to reconstruct environmental history before, during, and after Tanginak site occupation. Evidence for abrupt sea-level change (uplift) at or near the end of occupation includes: peat overlying beach deposits; sharp peat facies changes, and tsunami deposits implying co-seismic deformation.
The geology near the Tanginak Spring Site provides a basis for understanding why this site was occupied and abandoned. During occupation, sea level rise prevented a lagoon below the site from infilling and also rendered locations closer to sea level vulnerable to erosion and storms. The earthquake that we postulate marked the end of occupation was probably exceptionally damaging and followed within minutes by a large tsunami. Uplift associated with this earthquake permanently drained the harboring lagoon and left the new shoreline far from the site, leading to abandonment of the location.
Abstract Categories: Coastal Processes, Student Poster
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