PRELIMINARY GEOLOGIC MAP OF THE SPRUCE MOUNTAIN 7.5’ QUADRANGLE, ELKO COUNTY, NEVADA

The Spruce Mountain 7.5’ quadrangle sits at the southwestern end of the Pequop Mountains in northeastern Nevada. The quad includes the Spruce Mining District, the southern portion of Spruce Mountain Spur, and most of Spruce Mountain. The initial project resulted in a 1:24,000-scale map covering most of the quadrangle. Current mapping builds on earlier 1:42,500 mapping and more recent, detailed mapping of the mining district. This study focuses on detailed analysis of late Paleozoic sedimentation and tectonism within the quadrangle and the surrounding area.

Rocks exposed within the quadrangle are carbonate dominated Lower Ordovician to upper Permian sedimentary units with no apparent angular unconformities, though parts of the section are disconformably missing and/or removed by faulting. The structural setting is governed by Cenozoic normal faulting. There are large-offset high-angle normal faults on the east and west sides and large-offset low-angle normal faults on the northern and southern ends of Spruce Mountain. Smaller normal fault networks accommodate extension. Map scale reverse faults associated with (presumably) Sevier deformation are rare or have been inverted by Cenozoic extension.

Three stratigraphic sections were measured and sampled through the Pennsylvanian Upper Ely and Hogan formations on the Spruce Spur. These measured sections document the extent of the Hogan Formation (Middle Pennsylvanian) calcilutite above the Ely Formation. Previous maps lumped the two formations. At Spruce Spur, the Hogan Formation is present and thick enough to be mapped at 1:24,000-scale. The thickness of the Hogan varies along the Spruce Spur and work during the 2023 field season will clarify how much of this is syn-depositional or due to faulting from Mesozoic-Cenozoic tectonism. The mechanism of thickness variation is significant because the lateral continuity of the Hogan Formation has bearing on the style of tectonism that occurred along the western Laurentian margin during its deposition. Intra-Ely Formation layer parallel shortening was also found and documented, but it is not presently known if this shortening is due to Paleozoic or later tectonism. Mesoscale analyses planned during 2023 address the timing of this layer parallel deformation.