PROVENANCE ANALYSIS OF THE CRETACEOUS HORNBROOK FORMATION OF NORTHERN CALIFORNIA AND SOUTHERN OREGON: EVIDENCE FOR A NON-KLAMATH CRETACEOUS ARC SOURCE

The provenance of the Cretaceous Hornbrook Formation, deposited unconformably on the eastern terranes of the Klamath Mountains of Northern California and Southern Oregon, may prove critical to reconstructing the complex Cretaceous paleogeography of this region. Sandstone petrography from each of the five members of the Hornbrook Formation indicates a variable source region with elements from basement uplift, transitional continent, recycled orogen, and dissected arc sources. A conglomerate clast count from the Klamath River Conglomerate member is broadly consistent with sandstone petrography, but yielded more quartzite and fewer volcanic clasts than the sand-sized fraction. This reflects the different susceptibility to weathering and diagenesis of these rock types. Likewise, Klamath River Conglomerate quartzite clasts contain a Precambrian detrital zircon age distribution very similar to that found in the Antelope Mountain Quartzite within the Yreka subterrane of the Eastern Klamath Mountains, but these Precambrian ages are absent in the sandstone sample from the same member. This discrepancy suggests that the quartzite was not broken down into sand-sized grains during transport but rocks from plutonic arc sources were disaggregated. In addition, detrital zircon age spectra from sandstones throughout the strata of the Hornbrook Formation clearly show that the provenance of the Hornbrook Formation changed with time and was not limited to the Klamath Mountains, contrary to past assumptions. While an Early Jurassic age peak occurs in the basal and upper members of the Hornbrook, it is absent in the middle strata. A Late Jurassic-Early Cretaceous peak, prominent in the basal three members, is minor in the fourth member and disappears altogether in the youngest member of the Hornbrook. Completely absent in the lower members, a Late Cretaceous peak dominates the fourth member and nearly disappears in the upper member. With the exception of the quartzite clasts in the basal member, Precambrian ages are rare, and Paleozoic ages, common in the Klamath Mountains, occur only in the youngest member of the Hornbrook Formation. Thus, the changing provenance of the Hornbrook Formation likely reflects activity in an adjacent Jurassic-Cretaceous magmatic arc with minor sediment input from the Klamath Mountains.