2022-10-15 Erin Donaghy, Revisiting stratigraphy of the Eocene to Miocene sedimentary peripheral rocks on the Olympic Peninsula, Washington

The Lecture:

The role of oceanic plateau collision in the geologic development of the Olympic Peninsula: testing geologic connection between Wrangell-St. Elias (AK) and Olympic (WA) National Parks.

The Pacific Northwest (PNW) has a long geologic history of seismic activity. Recent studies suggest the PNW experienced a collision ~50 million years ago (Ma, early Eocene) which resulted in an enormous oceanic plateau attaching itself to the west margin of the continent. That plateau is known fondly, as Siletzia.

Erin Donaghy will discuss what occurred after the accretion of Siletzia. She will describe the interbedded siltstone and sandstone turbidites in the deep marine environment. These rock strata are known as the peripheral sequence of the Olympic Peninsula, and are thought to be part of the regional Tofino-Juan de Fuca forearc basin. Correlation of Eocene deposits on the north side of the Olympic Peninsula and those on the south side is still not firmly established.

Erin and colleagues have been using new mapping and zircon geochronology of the north Olympics’ Blue Mountain Unit to re-evaluate the basin formation in the context of Siletzia accretion. She will explain how these new data suggest that continentally derived sediment flooded into the basin immediately before and after a major period of basaltic volcanism. Erin will offer new interpretations of how the Blue Mountain Unit reflects a period of marginal rifting after accretion. She will also discuss the depositional setting for the Aldwell, Hoko River, and Makah Formations, which are younger than the docking event.

Erin and colleagues conducted more sampling this past summer (2022) in Wrangell-St. Elias National Park, AK, and the Olympic National Park, WA (September 2022). Ongoing high-precision uranium-lead zircon geochronology of interbedded volcanic rocks will provide more precise depositional ages for the sequences. Geologists think the Yakutat terrane, currently colliding with southeastern Alaska, may have originated as part of the Siletzia plateau offshore the PNW. She will discuss the significance the changing provenance of Eocene sedimentary rocks in the Tofino-Juan de Fuca basin within the context of a rifting oceanic plateau, migrating triple-junctions, and the possible northward translation of the Yakutat terrane during this time.


The Speaker:

Erin Donaghy is currently at Purdue University (West Lafayette, IN), soon completing her PhD at the Department of Earth, Atmospheric, and Planetary Sciences.

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