GEOLOGIC MAP OF THE UPPER DEVONIAN GUILMETTE FORMATION, NORTHERN PEQUOP MOUNTAINS, ELKO COUNTY, NEVADA

The Upper Devonian Guilmette Formation was recently mapped in the northern Pequop Mountains and divided into four mappable units: Yellow Slope Former, Ledge Former, Quartz Arenite, and Upper Guilmette. This subdivision reveals missing and repeating strata not previously recognized, owing to north-trending low-angle normal faulting and thrusting within the formation. In addition, W-NW sinistral transtensional faults are mapped parallel to the dextral Wells Fault System, which offset north-trending Basin and Range extensional structures.

Results from mapping suggest the uppermost Guilmette Formation is sheared and truncated by low-angle normal faulting, and not by a stratigraphic unconformity as previously interpreted. Mississippian Joana Limestone overlies the Guilmette Formation in the Pequop Mountains, but the absence of an intervening Mississippian Pilot Shale is interpreted here as a structural omission, which is supported by the regional presence of this formation in neighboring ranges including the Goshute, Bald-Buck, Spruce, and Ruby Mountains. This finding suggests that the timing of the Pilot Basin was synchronous with nearby areas, and discounts the hypothesis that the Antler orogeny promoted non-deposition or erosion in the Pequop Mountains during the Devonian-Mississippian transition. In the Pequop Mountains, previous workers measured the Guilmette Formation as being 1,750 feet thick, but the complex structural history of the range complicates measurement of true stratigraphic thickness. Structural thinning, by fault omission and shearing of footwall rocks, suggests that earlier measurements significantly underestimated the thickness of the Guilmette Formation in the Pequop Mountains.