Abstract: A Critical Revision of Event Stratigraphy of Middle Devonian Carbonate Reservoir Rocks, Nevada and Utah

Donlon O. Hurtubise, Charles A. Sandberg

Recognition of new Middle Devonian units in the stratigraphic interval between shallow-water carbonate-platform cyclical dolostones of the Simonson Dolomite below and the Guilmette Formation above provides a better understanding of dolomitized Devonian petroleum reservoir rocks in and around the producing fields of Railroad Valley, Nevada Recorded within this interval, which previously has been variously assigned to as many as four different formations owing to local overprinting of secondary dolomitization, is evidence for the Taghanic onlap of North America. Early studies of the Simonson in eastern Nevada and western Utah had placed the Simonson-Guilmette contact imprecisely at the local lithologic change from dolostone to limestone, generally corresponding to the highe t occurrence of the brachiopod Stringocephalus. Our new measured section data and conodont biostratigraphic and paleobiogeographic studies, however, reveal that a widespread, thick (as much as 200 m), cliffy sequence of limestone, named the Fox Mountain by Hurtubise in 1989, underlies an even more widespread unit of slope-forming predominantly yellow silty carbonate rocks and calcareous siltstone, as much as 50 m thick. This yellow slope-forming unit, which was depositionally continuous with the Maywood Formation in Montana and the Souris River Formation in the Williston basin, constitutes an easily recognized member that readily defines the base of the Guilmette Formation throughout Nevada and Utah.

The Fox Mountain Formation, which has been the target of gold exploration in some areas, is divisible into two members. The thicker, restricted-marine, sparsely to nonfossiliferous lower member, consisting mainly of lime-mudstone (pelmicrite and dismicrite) with interbeds of evaporite-solution breccia, evidences extensive Devonian karstification. The largely open-marine upper member, containing common brachiopods and gastropods and less common stromatoporoids and corals, includes a basal unit of crinoidal wackestone and lime-mudstone (biomicrite) that locally fills channels and sinkholes in the lower member. The upper member records a major transgression across the previously shallow Middle Devonian carbonate platform. The overlying yellow slope-forming unit at the base of the Guilmet e records an initial regression followed by a deepening event (renewed Taghanic onlap) that continued well into the Late Devonian.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada