USING CORDILLERAN ORTHOQUARTZITES TO TRACK PALEOGENE RIVERS

Mesoproterozoic-Cambrian Cordilleran orthoquartzites from the interior intermountain west were an important source of clasts in Maastrichtian-early Miocene fluvial conglomerates deposited along the continental margin in California and Oregon. Howard (2000), Miller et al., (2016), and Sabbeth et al., (2019) have demonstrated the utility of using resistant orthoquartzite clasts to study Cordilleran paleodrainage and landscape evolution that complements regional detrital mineral and volcanic provenance studies (e.g. Jacobson et al., 2011, Henry et al., 2012, Sharman et al., 2014, Shulaker et al., 2018). Between 2015-2022 undergraduate and graduate students sampled orthoquartzite-rich fluvial conglomerates for clast composition, paleocurrents, and sedimentology. Statistical and sampling methods used to quantify composition, paleocurrents, and provenance correlations are summarized in Howard (1993). Orthoquartzite-rich conglomerates nonconformably overlie Cretaceous and older plutonic and metamorphic rocks in NE Oregon, northern Sierra Nevada, California Coast Ranges, southern Sierra Nevada and the Mojave Desert. Orthoquartzite clasts are abundant within discrete stratigraphic horizons of Maastrichtian conglomerates in the California Coast Ranges where they correlate with high percentages of Precambrian detrital zircons in matrix sandstone. A distinctive compositional assemblage of orthoquartzite clasts is found in the late Paleocene upper Goler Fm of the Mojave Desert. These clasts were recycled into Paleogene conglomerates of the California Coast Ranges including late Paleocene-early Miocene Witnet Fm, non-fluvial, mid-Eocene Butano Fm, and late Eocene-early Miocene Tecuya Fm. In the Elkhorn Mtns of Oregon and the Tehachapi Mtns of California Paleogene conglomerates contain abundant boulder-sized orthoquartzite clasts up to 60cm from presumed primary sources that are now >200 kilometers to the east. At Old Dad Mtn in the NE Mojave, orthoquartzite conglomerate with SW-flowing paleocurrents was deposited in a paleochannel cut in gneissic basement. This channel was subsequently filled with tuffaceous conglomerate and inferred Peach Springs Tuff (18.7 Ma) and is offset by mid-late Miocene dextral strike-slip faults.