2000 GSA Annual Meeting -- Reno, Nevada

Abs. No. 50118


Author(s): McKELVEY, Matt, A., and DEIBERT, Jack, E., Department of Geology and Geography, Austin Peay State University, P.O. Box 4418, Clarksville, TN, 37044, deibertj@apsu.edu

Keywords: eolian, volcaniclastic, Knoll-Mountain, Nevada, Miocene

Large-scale cross-stratified sandstone units are recognized in the Miocene Humboldt Formation near Knoll Mountain, Nevada. These deposits are unusual because they are composed almost entirely of volcanic glass shards. The cross-strata may represent primary pyroclastic surge deposits or pyroclastic material reworked into eolian dunes. The sandstone units are up to 35 m-thick and are composed of tabular to lenticular trough cross-stratified sets that average 3 m-thick and are as thick as 11 m. Maximum length of sets is at least 200 m-long in both transverse and longitudinal directions. The sets are composed of 1 mm- to 5 cm-thick cross-strata that typically have angles of 15 to 25 degrees with a maximum of 30 degrees. All strata observed are subcritically climbing suggesting a relatively low deposition rate. Paleocurrents are multidirectional, but have a dominant flow to the northeast (N70E). The sand units are dominantly composed of fine- to medium-grained (1 to 2.5 phi), well sorted, angular glass shards with 1% volcanic crystals and nonvolcanic lithic fragments. Rare lags of granule-sized nonvolcanic fragments occur along set-bounding surfaces and lower portions of troughs. Stratification features observed in these deposits are more consistent with those found in eolian dune deposits rather than pyroclastic surge deposits. We interpret this deposit to be formed by wind transporting loose volcanic ash over a short distance and forming large barchanoid dunes. Deposits of this type and size are not well documented, thus these stratification features may serve to help recognize other eolian volcaniclastic deposits.

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