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Origin of reverse-graded bedding in air-fall pumice, Coso Range, California

January 1, 1979

The origin of reverse grading in air-fall pyroclastic deposits has been ascribed to: (1) changing conditions at an erupting vent; (2) deposition in water; or (3) rolling of large clasts over smaller clasts on the surface of a steep slope. Structural features in a deposit of air-fall pumice lapilli in the Coso Range, California, indicate that reverse grading there formed by a fourth mechanism during flow of pumice. Reverse-graded beds in this deposit occur where pumice lapilli fell on slopes at or near the angle of repose and formed as parts of the blanket of accumulating pumice became unstable and flowed downslope. The process of size sorting during such flow is probably analogous to that which sorts sand grains in a reverse fashion during avalanching on the slip faces of sand dunes, attributed by Bagnold (1954a) to a grain-dispersive pressure acting on particles subjected to a shear stress. In view of the several ways in which air-fall pyroclastic debris may become reverse graded, caution is advised in interpretation of the origin of this structure both in modern and in ancient deposits.

Publication Year 1979
Title Origin of reverse-graded bedding in air-fall pumice, Coso Range, California
DOI 10.1016/0377-0273(79)90031-3
Authors W. A. Duffield, C. R. Bacon, G.R. Roquemore
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Volcanology and Geothermal Research
Index ID 70012501
Record Source USGS Publications Warehouse
USGS Organization Volcano Science Center