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

Journal of Volcanology and Geothermal Research

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Abstract

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. ?? 1979.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Origin of reverse-graded bedding in air-fall pumice, Coso Range, California
Series title:
Journal of Volcanology and Geothermal Research
Volume
5
Issue:
1-2
Year Published:
1979
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Volcanology and Geothermal Research
First page:
35
Last page:
48
Number of Pages:
14