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

In the northern Pequop Mountains the Guilmette Formation was divided into four mappable units (from the base): yellow slope former, ledge forming interval, quartz arenite, and upper Guilmette. This subdivision reveals intervals of missing and repeated strata not previously recognized.

Mapping at 1:12,000 suggests the uppermost Guilmette Formation is attenuated and truncated by low-angle faulting and not by a stratigraphic unconformity as previously interpreted. Mississippian Joana Limestone overlies the Guilmette in the Pequops, but the absence of an intervening Mississippian Pilot Shale is interpreted here as a structural omission, which is supported by the presence of this formation in neighboring Goshute, Bald-Buck, Spruce, and Ruby Mountains. Although the Pilot Shale was not found stratigraphically overlying the Guilmette, it was mapped in stratigraphic contact below the overlying Joanna Limestone in the northern Pequops. This finding suggests that the timing of the Pilot Basin was synchronous with nearby areas, and discounts the hypothesis that the Antler orogeny promoted local non-deposition or erosion in the Pequops during the Devonian-Mississippian transition. In the Pequops, previous workers measured the Guilmette at 1,750 feet, but the complex structural history of the range, both from attenuation and omitted strata make a determination of true stratigraphic thickness unreliable.

The Eocene W-NW inferred transtensional “Wells Fault” System, theorized to collectively and dextrally offset time-equivalent Paleozoic platform sediments to the south and slope facies to the north by 60-70km, was not found in the Pequop Mountains. Rather, several normal faults, Basin and Range-aged, were mapped to follow W-NW-trends. Therefore, we believe the “Wells Fault” system is not the mechanism behind abrupt regional facies changes. Other hypotheses proposing different mechanisms are considered, such as basement architecture and Mesozoic thrust displacement. Because attenuation structures prevented thorough facies analyses of the Devonian-Mississippian strata, this study cannot directly support hypotheses proposing basement architecture influenced sedimentation patterns of NE Nevada. However, it provides alternate explanations to strata offset and omission by means of Mesozoic tectonism.