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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1701

Last Page: 1701

Title: Depositional Environment of Neogene Mehrten Formation at Its Type Section: ABSTRACT

Author(s): Lori L. Summa, Jeffery F. Mount, Kenneth L. Verosub

Article Type: Meeting abstract

Abstract:

The Mehrten Formation represents a major period of Neogene nonmarine sedimentation in east-central California. The Mehrten Formation was originally defined as andesite-rich clastic sediments deposited in the Mokelumne River drainage basin of the Sierra Nevada foothills. The unit has since been extended to include volcanogenic sandstones, mudflows, and lava flows occurring throughout the foothills and in the Sierra Nevada itself. However, the precise depositional history and chronology of the Mehrten Formation have remained largely undocumented.

Preliminary studies undertaken in the area of the type section at Camanche Lake suggest that the andesitic sediments of that region were deposited in a braided fluvial system. Upward-fining sequences of coarse-grained andesitic sands and gravels cut by channeled conglomerates reflect this depositional environment. The occurrence of clast-supported gravels and sandy matrix-supported gravels, which grade laterally and vertically to planar and trough cross-bedded sands, further suggests that the braided fluvial system was located on the distal reaches of an alluvial fan. This particular alluvial fan was shedding coarse clastic debris to the west. Alluvial-fan deposits in the Mehrten Formation are tectonically significant because they record the onset of late Tertiary vertical tectonics i the Sierra Nevada. The andesitic composition of the sediments further reflects the convergent nature of active tectonism in California at this time.

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