Raised Shorelines Along the Pacific and Juan De Fuca Coasts of the Olympic Peninsula, Washington State, USA

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We identified elevated shoreline terraces at 12 sites along ~150 km of western and northern Olympic Peninsula coastlines using LiDAR-based elevation profiles. All of the terraces lie in formerly glaciated areas and thus are all post-glacial in age. Modern Mean Higher High Water (MHHW) levels range from ~2.2 m to 2.5 m. Raised shoreline angles approximate the elevation of the former MHHW and lie between ~3 m to ~5.1 m above present day MHHW. Multiple shoreline terraces at several sites suggest at least two punctuated drops in relative sea level. At one site, an elevated notch in a bedrock sea stack corresponds closely to the elevated terrace back edges nearby.

Two sites afford an estimate of terrace age through materials unearthed in past archaeological excavations. At Cape Alava near Lake Ozette, well-preserved wood houses and textiles suggest that the Makah built a village on an 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 relative sea level in the late Holocene. These fluvial deposits underlie preserved beach and estuarine landforms above modern MHHW suggesting that sea level transgressed the site before river mouth incision.

Possible explanations for these elevated shoreline terraces include ongoing isotatic response to deglaciation of the Cordilleran Ice Sheet (GIA) and slow/rapid tectonic uplift. We discount a GIA explanation for the elevated terraces because the young age estimates and amount of possible uplift would require faster uplift rates than current estimates based on GIA. Areas to the east and north of the western Olympic Peninsula experienced much thicker ice volumes and show evidence of progressive rebound until about 11 ka and by about 7.5 ka sea levels were rising faster than rebounding areas. We consider tectonic uplift as the most viable explanation for these terrace elevations and plan on future work to attain better age control to better constrain uplift rates.

Presenting Author: Brian L. Sherrod

Authors