Abstract

The Permian stratigraphic sequence in the Leach Mountains, northeastern Nevada, is more than 3,275 m thick, and consists primarily of carbonate rock with subordinate chert and fine-grained clastic rock. This Permian sequence is latest Wolfcampian, Leonardian, and Guadalupian in age, and includes the Third Fork Formation, the Badger Gulch Formation, the Trapper Creek Formation, the Grandeur Formation of the Park City Group, the Meade Peak Phosphatic Shale Tongue of the Phosphoria Formation, and the Murdock Mountain Formation and Gerster Limestone of the Park City Group. The Permian strata disconformably overlie the Mississippian–Pennsylvanian Diamond Peak Formation, and disconformably underlie undifferentiated Triassic clastic and carbonate rocks lithologically similar to the Dinwoody Formation and the Thaynes Limestone. The Permian rocks are lithologically and temporally correlative with those in the Cassia Mountains to the north and to those in the northern Pilot Range and Hogup-Terrace Mountains to the east.

Permian rocks in the Leach Mountains were deposited in a variety of marine. environments that developed during three major marine cycles: (1) Rocks beneath the upper Grandeur represent a regressive sequence in which the sediments were deposited successively in subtidal, intertidal, and supratidal marine environments. (2) Following a marine transgression that began during deposition of the uppermost Grandeur Formation, sediments of the Meade Peak and Murdock Mountain accumulated during a depositional phase characterized by repetitive onlap and offlap episodes. (3) Subsequently, open-marine conditions were restored with the deposition of shallow-subtidal sediment of the Gerster Limestone.

The Permian sequence has been deformed by both low-angle and high-angle faults. Post-Triassic thrust faulting in the southern Leach Mountains emplaced Permian rocks southeastward and eastward over Triassic rocks. In the north-central part of the range thrust faulting emplaced Mississippian–Pennsylvanian rocks eastward over Permian rocks. Post-Triassic younger-over-older low-angle faulting possibly related to the thrusting juxtaposed Triassic and Permian rocks in the north-central part of the range and juxtaposed fault slices of Permian rocks in the southern part of the range. Numerous high-angle normal faults cut the Permian, Triassic, and overlying Cenozoic rocks in the Leach Mountains. Most of the faults resulted from late Cenozoic Basin-and-Range extension.

Phosphate, which occurs in the Meade Peak Phosphatic Shale Tongue of the Phosphoria Formation, is the most significant mineral resource in the Leach Mountains.