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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 900

Last Page: 900

Title: Origin of Thin, Siliceous Beds in Monterey Shale, Elk Hills Field, California: ABSTRACT

Author(s): Robert R. Berg, D. G. Kersey

Article Type: Meeting abstract


The Miocene Monterey Shale consists of thinly interbedded black shale and siliceous beds in a section 1,800 ft (550 m) thick on the western anticline, Elk Hills field. Selected full-diameter cores were examined through the upper section west of, and partly equivalent to, the Stevens Oil Zone sandstones. The siliceous beds are commonly 1 to 5 cm, and rarely 8 to 10 cm, thick. The beds are generally structureless or contain a few indistinct laminae. Bases are in sharp contact with underlying shale, and some tops are gradational to overlying shale. In a few beds, the uppermost parts show curved laminae that represent low-amplitude ripples. Therefore, the beds seem to be distal turbidites composed of common, massive A divisions and rare, rippled C divisions.

Many beds have a fine granular, graded texture with a thin basal zone of coarser detritus. The beds are composed of finely-crystalline, siliceous material, in some places partly replaced by dolomite(?). Petrographic study shows a significant content of fine sand to silt-size detritus. In a typical graded sequence, grains of quartz, plagioclase, and rock fragments form a thin lag at the base where they comprise more than 50% of the rock and have an average size of 0.13 mm. Detrital grains decrease upward to less than 3% at the top, and average size decreases to 0.05 mm.

The thin, regularly bedded nature of the section, significant detrital content, and graded texture suggest that the siliceous beds are turbidity-current deposits. The siliceous component was probably pelagic, diatomaceous sediment from the basin floor that was incorporated in turbidity flows, transported a short distance, and redeposited with terrigenous detritus in massive A divisions of the turbidite sequence. Alternatively, subsequent recrystallization destroyed original lamination and produced the structureless beds.

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