Cordilleran Orogen Arc and Ophiolite-derived sands: Sources, petrology, geochemistry, and downstream evolution in Oregon and California

Abstract

In a Mediterranean climate, sands in drainages of the composite Cordilleran Orogen in the Sierra Nevada, Klamath Mountains, Cascade Mountains and Coast Ranges of Southern Oregon and Northern California are derived from multiple orogenic source domains as varied as oceanic crust, continental crust and magmatic arc. Quantitative petrographic data shows the carbonate-poor sands commonly contain <25 % quartz; those with greater quartz abundance include admixed detritus from quartzose sedimentary rocks or quartz-rich Sierran alluvium. Geochemistry specifically identifies much of the detritus as mafic to intermediate composition with mg* values of 45 to 55, Th/Sc <0.3 and Cr/Th > 20 with Cr/V > 1, Rb/Ni < 1 and Y/Ni < 1. Within the Klamath area streams, in ultramafic-rich source areas, sands are >50 % ultramafic with mg* > 60. Significant quantities (5–45 %) of ultramafic + mafic grains with high abundances of MgO, Cr and Ni are present in downstream sands. In other cases, downstream, with mixing of various less mafic detritus, ultramafic grains are <5 %, mg* < 50, with some high Cr due to detrital chromite. To the south, sands from several major sources contribute to the Sacramento River. From Coast Ranges streams significant schist-rich siliceous and ophiolite sand are added to volcanic-rich, basalt-andesite (Cascades arc source) upstream Sacramento River sands. Feldspathic sand is contributed from uplifted dissected arc basement Sierra Nevada and Klamath Mountains streams. Along the river course, in a reactive landscape, there is variable mixing of sands from different sources due to river flow levels at normal and flood stages and by erosional reworking of river basin sand deposits.