Date of Award


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Thesis open access


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 while all the other Precambrian zircons from the Hornbrook Formation resemble northern Sierra Nevada terranes. 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.

In the Mesozoic detrital zircon age distribution, the reduction in the Early Cretaceous peak and the appearance of a Late Cretaceous peak suggests increasing input from an active magmatic arc other than a Klamath Mountain source. This age signature is supported by the variability of the sandstone petrography, the decreasing volcanic input, and the decreasing amount of zirconium, which demonstrate either the dissection of a magmatic arc source and/or multiple source regions. Furthermore, the members of the Hornbrook Formation correlate with the combined detrital zircon distributions of the Great Valley Basin more closely than with the combined distributions of the Methow Basin. This data supports past interpretations of the Hornbrook Basin as part of an extensive forearc basin that included the Great Valley Basin and possibly the Ochoco Basin where more research is needed. 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 sediment input from the Klamath Mountains activity and from an adjacent Jurassic-Cretaceous magmatic arc.