PROVENANCE AND STRATIGRAPHIC ANALYSIS OF THE STEWART VALLEY, NEVADA

Field work in the Stewart Valley, located in western Nevada, has shown a need for expansion on research concerning sedimentary provenance and its changes through time. The valley is characterized by lacustrine and volcanic sediments from the Miocene Stewart Valley (SV) Group. The Savage Canyon Formation, with a generalized thickness of ~250 m, makes up the lower part of the SV Group and is dominated by deep lacustrine deposits. The upper Granny Goose Formation, with a generalized thickness of ~270 m, has deposits interpreted as forming in shallow or marginal lake environments. We measured three stratigraphic sections and collected 3 samples to document sediment source changes using sediment provenance analysis and clast counts in the lower, middle, and upper strata of the SV Group.

In the lower part of the SV Group are laterally discontinuous, very poorly sorted, conglomerates with bedsets up to 0.7 m thick, and large chute and pool structures, possibly indicating incised-valley filling channel deposits. Clasts are composed of andesite, andesite porphyry, rhyolite, granite, meta-sandstone, sandstone, and limestone. The middle section displays a coarsening upwards sequence with a matrix-supported breccia and clasts, ~2 to ~4 cm, which we characterized as a debris flow deposit. The upper part of the section has two clast-supported conglomerate layers, with clasts ranging in size from ~1.8 cm to ~20 cm. We interpret the upper section as a channel bed or cobble bar deposit. Clast counts in the middle and upper sections show rounded to subrounded andesite and andesite porphyry. The transition from silica-rich clasts to intermediate igneous clasts shows a difference in source origins, with the lower SV Group coming from a more mature source, while the middle and upper SV Group appear to originate from nearby volcanic sources. Ongoing mineralogical analysis of the supporting matrices in each measured section will help to further identify distinct sediment sources, and document temporal changes in provenance.