A FRESH LOOK AT THE DOTHAN FORMATION IN SW OREGON: TECTONICALLY THICKENED SEDIMENTARY FILL OF THE LATE CRETACEOUS FOREARC BASIN

A major tectonic boundary in SW Oregon separates Jurassic metamorphic (greenschist-amphibolite facies) and ultramafic rocks of the Klamath Mountains (KM) from relatively unmetamorphosed marine sedimentary and minor volcanic rocks of the Dothan Formation (Fm). Prior to this study, the Dothan Fm was correlated to the Franciscan complex in NW California and considered part of the Yolla Bolly terrane. In the widely accepted tectonic model (e.g., Blake at al., 1985), KM crystalline rocks are thrust over the Dothan Fm in a regional thrust sheet expressed as a series of tectonic nappes and klippen in the Snow Camp Mt. area. New field mapping for this study shows that the Early Cretaceous Myrtle Group and Late Cretaceous Dothan Fm instead rest in depositional nonconformable contact on the older KM crystalline rocks. Field relations and preliminary detailed mapping show that the older KM crystalline rocks are exposed as structural windows in the cores of S-plunging anticlines. Sheared sedimentary and volcanic rocks mapped as Dothan Fm by Dott (1971) in the Brookings area (Yolla Bolly terrane in newer maps) may be a continuation of the Franciscan subduction complex in NW California. Northeast of Snow Camp Mt. the Dothan Fm is exposed in two transects, along Cow Creek and NW of Galice. In both areas we observe common internal thrusts and duplex structures, and tight to isoclinal folds with axial-planar slaty cleavage that record substantial post-depositional tectonic shortening and thickening. Geologic cross sections along the Cow Creek and Galice transects reveal ca. 8-10 km structural thickness of the SE-dipping imbricated Dothan stack. We apply a range of shortening-to-thickening ratios commonly seen in thrust belts to estimate the original stratigraphic thickness of the Dothan Fm and magnitudes of tectonic shortening. We conclude that large areas of the Dothan Fm represent the thick fill of a Late Cretaceous forearc basin equivalent to the thinner Hornbrook Fm to the east, and correlates to the Ochoco (OR) and Great Valley (CA) forearc basin. Evidence summarized in this study and in a companion abstract (Darin et al., 2025) requires a period of major post-88 Ma contraction, tectonic burial, and mountain building that likely occurred during early Eocene collision of the Siletzia composite terrane with western North America.