The lower Upper Devonian rocks in southwestern Utah--the Guilmette Formation and equivalents--represent a final regressive pulse of the major Late Devonian marine inundation of the Western Interior of the United States and record marine carbonate deposition on a wide continental shelf. They consist primarily of limestone, dolomite, and quartz arenite deposited in a shallow north-trending miogeosyncline, which constituted a single major basin of accumulation on this shelf.
The Guilmette Formation and equivalents were deposited in shallow normal to hypersaline marine waters. The environments of deposition include: a moderate- to high-energy intertidal environment, a moderate-energy subtidal environment, a lower energy, deeper subtidal environment below effective wave base, and a high-energy environment in local shallow areas of mud mounds and bioherms.
The carbonate deposition of the Guilmette Formation and equivalents was interrupted periodically by the deposition of quartz arenites. These may represent the breaking up of the miogeosynclinal-cratonic pattern of deposition. In most areas, the Guilmette and equivalents are overlain by a thin transgressive marine quartz arenite deposit--the Cove Fort Quartzite and basal Leatham equivalent.
Previous paleontologic evidence indicated a general Middle to Late Devonian age for the Guilmette Formation. The present study narrows this range and suggests that the age of the Guilmette Formation and its equivalents is late Middle Devonian (Stringocephalus brachiopod zone) to early Late Devonian (Uppermost Palmatolepis gigas conodont zone).
Available subsurface data suggest that the petroleum possibilities of the Guilmette Formation and equivalents in southwestern Utah are poor. Several tests have penetrated .the interval with only minor shows of oil in rocks with low porosity and permeability. Nevertheless, many outcrop samples of the same interval, appear to have excellent porosity and permeability and a strongly fetid odor,