13-4 Discovery of the “Missing” Late Cretaceous Forearc Basin in SW Oregon: Implications for Subduction and Collision Tectonics

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

Presenting Author:

Authors:

Abstract:

The Cretaceous-Paleogene tectonic evolution of the PNW Cordilleran system is poorly understood in part due to scarcity of Late Cretaceous strata and enigmatic differences between forearc basin strata in northern California and SW Oregon. The Great Valley Group (GVG) in the Sacramento Valley, CA, is up to 10-12 km thick and preserved mostly in the subsurface with ages ranging from Early through Late Cretaceous (~143 – 66 Ma), while the Hornbrook Formation (Fm) in SW Oregon and northernmost California is exposed in a ca. 1-km thick dipping section of Late Cretaceous strata (~100–72 Ma). The Hornbrook Fm resembles the relatively thin eastern part of the GVG where Late Cretaceous forearc-basin strata onlap Sierra Nevada granitic rocks, but there is no known equivalent of the thick western GVG in Oregon. The Dothan Fm is a wide belt of deformed marine turbidites and minor volcanic rocks in SW Oregon that is traditionally interpreted as the Late Jurassic – Early Cretaceous equivalent of the Franciscan subduction-accretionary complex in California. In the prevailing model, crystalline rocks of the Klamath Mountains (KM) were emplaced over the Dothan Fm in a regional thrust sheet preserved today as tectonic nappes and klippen in the Snow Camp Mt area. Our new results show that the Dothan Fm instead represents the missing thick western facies of the Cretaceous forearc basin in SW Oregon. Detrital zircon U-Pb data yield maximum depositional ages of ~100–88 Ma and a shared provenance with coeval strata of the Hornbrook Fm and GVG. Field mapping shows that the Early Cretaceous Myrtle Group and Late Cretaceous Dothan Fm rest in depositional nonconformable contact on KM crystalline rocks, and the crystalline rocks are exposed in the cores of south-plunging anticlines. The Dothan Fm is deformed by thrust faults, compressive duplex structures, and tight to isoclinal folds with axial-planar slaty cleavage that together record post-depositional tectonic shortening, thickening, and sub-greenschist facies metamorphism. Geologic cross sections reveal a 10-km thick SE-dipping imbricate thrust stack that represents an unknown combination of original stratigraphic thickness and regional tectonic thickening. These data and observations require major post-72 Ma shortening, burial, crustal thickening, and mountain building in the northern Klamath Mts that likely occurred during Early Eocene collision of the Siletz terrane with western North America.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.