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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 55 (1971)

Issue: 2. (February)

First Page: 356

Last Page: 356

Title: Stratification in Willow Creek Alluvial Fan, Eureka Valley, Inyo County, California: ABSTRACT

Author(s): Robert Q. Oaks, Jr., Emily C. Oaks

Article Type: Meeting abstract

Abstract:

Stratification in the entrenched main channel in the upper 5 mi (of 12) of Willow Creek fan was analyzed by means of continuous-strip photos and sections spaced 2,000 ft apart. Average and maximum bed thicknesses decrease, respectively, from 1.7 and 6.0 ft near the fan head to 1.0 and 3.9 ft downfan. Individual beds extend from 10 to > 500 (700?) ft, although few beds extend > 200 ft.

Gravel forms pack (framework), pack-float (transitional), and float ("puddingstone") fabrics. Most cobbles and boulders are subspherical and subround, and thus usually do not form distinctly oriented fabrics. Stream deposits exhibit distinct channel geometry, pack fabric, and little silt and clay. Mudflow fabrics vary from pack to float. Some are uniform; others grade laterally from float to pack within 25 ft. Mudflows show high length:thickness ratios; the majority of mudflows lack graded bedding. The ratio of mudflows to stream deposits exceeds 70% and increases downfan.

Float beds generally contain < 40% gravel and > 15% silt and clay, whereas pack beds usually contain > 60% gravel and variable amounts of silt and clay. All deposits are poorly to extremely poorly sorted, and range from strongly coarsely skewed to symmetrical. Silt forms most of the matrix of mudflows, whereas clay ranges from only 1% to 7%. In the silt-clay fraction, feldspars exceed quartz, and clay minerals are minor in amount, the result of semiarid climate (17 in. rainfall/year), dominance of granitic rocks with large feldspar phenocrysts in the drainage basin (> 60%), and predominantly mudflow transport.

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