2000 GSA Annual Meeting -- Reno, Nevada
Author(s): DEIBERT, Jack, E., Department of Geology and Geography, Austin Peay State University, P.O. Box 4418, Clarksville, TN, 37044
Keywords: eolian, volcaniclastic, Knoll-Mountain, Nevada, Miocene
Ancient eolian volcaniclastic deposits are rare and small in scale because of rapid weathering of volcanic particles and reworking by other processes. However, a large eolian volcaniclastic sand dune complex is preserved in the Miocene Humboldt Formation near Knoll Mountain, Nevada. The complex is 35 m-thick and covers at least 2 square km in area and is dominantly trough cross-stratified with individual sets up to 11 m-thick. The sand is medium- to fine-grained and is composed mostly of angular volcanic glass shards. Evidence that the cross-strata formed by eolian rather than pyroclastic surge processes includes: the well-sorted nature of the sand, lack of coarse-grained material, dominance of high-angle subcritically-climbing cross-strata, multidirectional paleocurrents, fine-grained interdune deposits with mudcracks, and no primary pyroclastic deposits directly above or below the cross-strata. The eolian deposits are intercalated with alluvial deposits and together these fill an incised valley. The valley is incised into alluvial and marginal lacustrine deposits, and is overlain by alluvial deposits. The valley is interpreted to have formed during a relative fall in local base level along the margin of an extensional basin. Loose volcanic ash was transported by wind a short distance into the valley and accumulated into dunes adjacent to alluvial deposits during a relative rise in local base level. Continued alluvial deposition completely filled and overlapped the valley. Eolian strata are rarely preserved outside the valley, suggesting the incised valley played a critical role in the accumulation and preservation of the volcaniclastic eolian strata.
GSA Home Page
© Copyright 2000 The Geological Society of America. All rights reserved.