DETAILED PROVENANCE OF THE HORNBROOK FORMATION, OREGON AND CALIFORNIA: UNDERSTANDING A CRITICAL LINK IN CRETACEOUS CORDILLERAN PALEOGEOGRAPHY

­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, a region critical to understanding Cretaceous Cordilleran paleogeography. Previous studies documenting the stratigraphy, sedimentology, and tectonic framework of the Hornbrook Formation included conglomerate clast and sandstone petrography, as well as paleocurrent analysis, to conclude that the Hornbrook Formation sediment sources were primarily, if not exclusively, within the Klamath Mountains. However, our detailed provenance analysis generates a new picture of an evolving Hornbrook basin that received sediment from both the Klamath Mountains to the west and additional sources located southeast of the basin. The combined results of sandstone petrography, detrital zircon U/Pb age and Hf isotopic systematics, whole rock neodymium analysis, and major- and trace-element geochemistry throughout the Hornbrook strata indicate that the Klamath Mountains could not have been the only sediment source throughout deposition, as previously suggested. Instead, these provenance results record increasing sediment input from a Sierran-like source to the southeast and are consistent with the presence of a northeastern continuation of the Cretaceous Sierra Nevada batholith that shed sediment to the west and northwest during Cretaceous time. Moreover, these results suggest that provenance analysis based on sandstone petrography and detrital zircon age signatures may not record a mafic provenance signal that tends to be preserved in fine-grained strata. The detailed provenance signature for the Hornbrook Formation presented here documents the character of sedimentation in a basin “in place” with respect to North America, and can be used to compare with coeval sedimentation in other basins that may have undergone extensive translation, or to test tectonic models that place Cretaceous basins in new configurations and imply new correlations between them.