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WITHDRAWN Age and Origin of a Relict Marine Terrace Along the Pacific Coast of Olympic Peninsula, Washington State, USA

Description: 

WITHDRAWN We identified an elevated shoreline terrace at 12 sites along ~150 km of western and northern Olympic Peninsula coastlines using airborne LiDAR data. At three sites we excavated 9 test pits down to bedrock or indurated abrasion surfaces (to depths up to 1.5 m, or in a few cases we then augered down to refusal). Four gouge cores from two sites and in-situ tree stumps at another also constrain Relative Sea Level (RSL) histories. The terrace deposits consisted of an organic-rich soil (silt loam - sandy silt), underlain by silty fine-medium sand with parallel lamination, thin pebble layers, and shell debris. The sand overlaid a boulder/cobble layer deposited on an abrasion surface. Radiocarbon ages of charcoal and shells collected from terrace sand exposed in the test pits ranged from 1690-1415 cal yrs BP. Ages of modern shells collected from the beach at low tide were <165±20 14C yrs BP so reservoir effects on local intertidal clams is minimal. Two archaeological sites help constrain the terrace age. At Cape Alava, well preserved wood houses suggest that the Makah built a village on an ca. 800-year-old shoreline terrace. At the Hoko River mouth, the river incised through ~3m of fluvial deposits in the last 2.9 ka, suggesting at least a 3 m drop in late Holocene RSL. Ages from gouge cores of terrace wetland deposits and in-situ tree stumps from the lower intertidal zone at the Ozette River ranged from 6890 – 5060 cal yrs BP. The terrace deposits and radiocarbon ages suggest the following: 1) mid-Holocene RSL rise inundated the sites between 6500-5000 years ago; 2) continued RSL rise created an abrasion surface and deposited overlying beach deposits; and 3) rapid or abrupt uplift after ~1400 cal yr BP. Abandonment of the terrace appears to have happened suddenly, pointing to a tectonic rather than glacioistostatic origin. Rapid uplift of marine abrasion surfaces could have happened multiple times in the late Holocene, only to be subsequently removed by coastline erosion. The single terrace that we observe records the last instance of rapid uplift, which could be eliminated by ongoing coastal retreat.

Session: The Landscape Record of Earthquakes and Faulting [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Brian

Student Presenter: No

Invited Presentation: 

Poster Number: 12

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