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ABSTRACT

This guide describes a three-day field trip to the Paleogene sedimentary and volcanic rocks exposed between the Straight Creek–Fraser River and Entiat faults in the central Washington Cascades. These rocks record a history of deposition, deformation, and magmatism that can be linked to tectonic events along the North American margin using a robust chronology coupled with detailed sedimentological, stratigraphic, and structural studies. These events include deposition in a large sedimentary basin (Swauk basin) that formed in the forearc from <59.9–50 Ma; disruption and deformation of this basin related to the accretion of the Siletzia oceanic plateau between 51 and 49 Ma; the initiation, or acceleration of right-lateral, strike-slip faulting and the development of at least one strike-slip sedimentary basin (Chumstick basin) starting ca. 49 Ma; and the re-establishment of a regional depositional system after ca. 45–44 Ma (Roslyn basin) as strike-slip faulting was localized on the Straight Creek–Fraser River fault. These events are compatible with the presence of the Kula-Farallon ridge near the latitude of Washington ca. 50 Ma and its southward movement, or jump, following the accretion of Siletzia. This trip visits key outcrops that highlight this history and links them to regional studies of sedimentation, faulting, and magmatism to better understand the geologic record of this tectonic setting.

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