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Depositional Environments of Permian Phosphoria Formation and Related Rocks, Leach Mountains, Northeast Nevada

MARTINDALE, STEVEN G., County of Orange, Santa Ana, CA

In the Leach Mountains, northeastern Nevada, the Phosphoria Formation is represented by phosphatic rocks of the late Leonardian Meade Peak Phosphatic Shale Tongue. The Meade Peak is overlain by dolomitic siltstones and chert, including black bedded chert, of the upper Leonardian(?) to lower Guadalupian Murdock Mountain Formation. The black bedded chert is related to the Rex Chert Member of the Phosphoria Formation.

There is little consensus regarding depositional environments of the Meade Peak. Also, little work has been presented on the origin of the black bedded cherts such as in the Murdock Mountain and Rex Chert.

Locally, a shallow subtidal to perhaps partly intertidal depositional model for the Meade Peak is based upon characteristics of reworked phosphatic clasts, a crowded and mixed shallow-water fauna, oolite beds, and stratigraphic position between shallow subtidal to supratidal rocks.

Supratidal deposition of dolomitic siltstones and units of black bedded chert in the Murdock Mountain is based largely on abundant cauliflower-shaped blebs of chalcedony with shapes resembling modern anhydrite nodules. Also, zebraic chalcedony in the black bedded chert is interpreted as a replacement product of evaporite deposits.

The setting in this southwestern part of the depositional basin of the Phosphoria Formation, in the late Leonardian, during the deposition of the Meade Peak, is interpreted as a network of shallow to very shallow marine basins and intervening, perhaps periodically emerged shoals. Subsequently, during the late Leonardian(?) to early Guadalupian, there was a marine regression and the supratidal environment of the Murdock Mountain Formation was established.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)