REVISITING STRATIGRAPHY OF THE EOCENE TO MIOCENE SEDIMENTARY PERIPHERAL ROCKS ON THE OLYMPIC PENINSULA, WASHINGTON

Recent studies suggest that Siletzia is an oceanic plateau that accreted to the margin of the Pacific Northwest at ~50 Ma while the plateau was still under construction. Following accretion, there was rapid subsidence, and formation of a regional basin on top of Siletzia basalts and accumulation of a thick section of Cenozoic sedimentary strata. These basin deposits are exposed on the northern and southern periphery of the Olympic Peninsula and record a sequence of interbedded siltstone and sandstone deposited as turbidites in a deep marine environment. Collectively these strata are known as the peripheral sequence of the Olympic Peninsula.

Strata are thought to be part of the regional Tofino-Juan de Fuca forearc basin and the age is defined by previous work utilizing fossil assemblages and lithofacies analyses. However, limited geochronologic data has been collected, and questions remain on not only how basal strata correlate along the northern Olympic Peninsula, but also how strata from the north correlate to age-equivalent strata in the densely forested southern Olympic Peninsula. For example, the Aldwell Formation represents the first sediments that were deposited on top of Siletzia basalts and is characterized by massive basaltic siltstone with a minor basal conglomerate. However, there are unnamed Eocene units underlying the Aldwell Formation that are not put into context of the overlying peripheral rock stratigraphy. Additionally, recent geochronologic data indicates the Blue Mountain Unit (BMU) is <45 Ma and age-equivalent to Eocene peripheral rocks. This is significantly younger than previous interpretations that suggested it was deposited coeval with lower Crescent Formation basalts and that these rocks were thrust underneath Siletzia basalts <45 Ma.

We integrate previous provenance and stratigraphic data with new lithofacies mapping of the peripheral strata and BMU, conglomerate clast counts, and measured stratigraphic settings to re-evaluate basin formation in context of Siletzia accretion. We aim to better understand basin formation following accretion of a young, buoyant, and hot oceanic plateau to a continental margin. Detailed regional stratigraphic correlations are critical for reconstruction of the Eocene—Miocene regional and basin formation following collision of Siletzia.