REDEFINED LATE CRETACEOUS DEPOSITIONAL AGE FOR THE DOTHAN FORMATION: IMPLICATIONS FOR FOREARC EVOLUTION AND TERRANE COLLISIONS ALONG THE CORDILLERAN MARGIN IN SOUTHWEST OREGON

The scarcity of Late Cretaceous strata in the Pacific Northwest represents a critical gap in our understanding of the tectonic evolution of the North American Cordillera, particularly in the context of terrane collisions and translations. The Dothan Formation (Fm) has long been interpreted as part of the Jurassic–Early Cretaceous subduction-accretion complex in SW Oregon (OR) and correlated to the well-known Franciscan complex (Yolla Bolly terrane) in northern California (CA). However, the depositional age of the Dothan Fm is uncertain, and much of it consists of relatively unmetamorphosed and non-mineralized marine sandstone and shale that lack the characteristic blueschist facies and mélange fabrics of the Franciscan complex. We present new geologic mapping and detrital zircon (DZ) U-Pb geochronology that challenge the classic Dothan–Franciscan correlation and provide new insights into the depositional history and tectonic significance of the Dothan Fm. New and published DZ data from 12 sandstone samples reveal unexpectedly young maximum depositional ages (MDA) of ~100–88 Ma for the Dothan Fm, requiring a revised Late Cretaceous depositional age for this unit. Statistical analyses of DZ data support a shared provenance and correlation of the Dothan Fm with the Hornbrook Fm in SW OR, Montgomery Creek Fm in northern CA, and coeval strata in the Ochoco basin (OR) and Great Valley Group (CA). Mapping near Snow Camp Mountain (OR) reveals that the Dothan Fm unconformably overlies Jurassic metamorphic and ultramafic rocks of Klamath Mountains (KM) affinity. The KM rocks, previously interpreted as klippen of a large Early Cretaceous overthrust nappe, are reinterpreted as structural windows of older rocks exposed in the cores of S-plunging anticlines. Our results suggest that most of the Dothan Fm represents the previously unrecognized thick (>700 m) sedimentary fill of the Late Cretaceous Great Valley-Hornbrook-Ochoco forearc basin system in SW OR. A Coniacian MDA and slaty cleavage in the Dothan Fm imply major contraction and tectonic burial after ~88 Ma, possibly during early Eocene collision of the Siletzia composite terrane. Detailed mapping will aim to distinguish mélange-type Dothan Fm along the coast from inboard forearc basin facies, and potential structural boundaries between these domains.