The Hornbrook Formation is a Cretaceous overlap assemblage that rests unconformably on accreted terranes and plutons of the Klamath Mountains in southern Oregon and northern California. The combined results of sandstone petrography, detrital zircon U-Pb age and Hf isotopic systematics, and whole-rock Nd analysis document an abrupt change in sediment sources for the Hornbrook Formation during the Late Cretaceous. The lower members of the Hornbrook Formation record provenance in the Klamath Mountains and the Sierran Foothills belt that is characterized by detrital zircon age distributions with large Jurassic and Early Cretaceous peaks (170–130 Ma) and positive zircon Hf and whole-rock Nd values. In contrast, upper members of the Hornbrook Formation include abundant sediment derived from the Cretaceous Sierran Batholith that is characterized by large Cretaceous peaks (120–85 Ma) in detrital zircon age distributions and less positive zircon Hf and whole-rock Nd values. A similar Late Cretaceous provenance shift is present in the Great Valley Group of California, which likely formed the southern continuation of the Hornbrook basin during deposition of the upper Hornbrook members. These provenance results may reflect changing plate kinematics along the U.S. Cordi margin during the Late Cretaceous, including extension and subsidence in the Klamath Mountains and Blue Mountains regions followed by rapid uplift of the main Sierra Nevada Batholith. Thus, the detailed provenance signature for the Hornbrook Formation presented here records regional tectonic events in the mid- to Late Cretaceous U.S. Cordillera, and suggests that the Hornbrook Formation and Great Valley Group shared similar sources but remained separate basins until mid- to Late Cretaceous time.

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