Landslide Ages and Implications for a Marine Terrace at Rialto Beach, WA

Abstract

Holocene landslides and marine terraces at Rialto Beach, on the Olympic coast of Washington state, may provide clues about the response of the Pacific coastline and adjacent hillslopes to Cascadia subduction zone (CSZ) earthquake activity. This study uses seven 14C samples from four distinct locations within a low-elevation marine terrace at Rialto Beach, WA to date landslides and constrain age of formation of the terrace. Two distinct landslides have been explored in detail: landslide 1 that occurred 800 years cal BP, and landslide 3, that occurred 150-300 years cal BP. The causes and triggers of these slope failure events were explored with new detailed field mapping and timing constraints. Observed landslide evidence includes hillslope geomorphology, soil composition of hillslopes, low-elevation marine terrace composition, and presence of slickenside bedrock-colluvium contacts within a drainage system. On landslide 1 I found a buttress unconformity on the hillslope containing interglacial peat and organic soil layer dated >48-27.5 cal ka, causing extensive zones of perching water that increases pore pressure and lowers slope stability. These geological layers are assumed to span the Rialto hillslope region. Dendrochronology to determine the age of trees on landslide 1 suggests that this slope was partially denuded during an intense winter storm in 1921 AD, potentially causing slope instability. Landslide 3, 150-300 yrs cal BP sits atop the potentially uplifted marine terrace which contains beach deposits older than 500-600 years cal BP. Dating suggests that the slide and uplift may be coseismic with the 1700 AD (270 BP) Cascadia subduction zone rupture. This study adds constraining dates of two distinct landslides that sit on top of the low elevation marine terrace, contributing to studies of coseismic uplift and landsliding on the Olympic coast.