GEOLOGIC MAP OF THE SPRUCE MOUNTAIN, NEVADA 7.5’ QUADRANGLE WITH AN EMPHASIS ON MIDDLE PENNSYLVANIAN TECTONISM

Multiple tectonic hypotheses have been invoked to explain the orientation of Ancestral Rocky Mountain (ARM) uplifts, and these depend in part on the nature of the tectonic boundary between Laurentia and Panthalassa. This study builds upon work in northeastern Nevada to elucidate the nature of the western Laurentia tectonic boundary during the Middle Pennsylvanian.

Middle Pennsylvanian strata in northeastern Nevada include the Hogan Formation that records deposition in relatively deep water above a section of Lower Pennsylvanian cherty shelf and platform limestones. To resolve the regional picture of facies changes of the Middle Pennsylvanian we mapped the Spruce Mountain 7.5’ quadrangle, which is about 40 km to the southwest of the deeper water Hogan Formation section in the Pequop Mountains. The geology of the Spruce Mountain quadrangle is complex. Strata are Paleozoic carbonates ranging in age from Ordovician to early-to-middle Permian. These strata are segmented and translated by multiple generations of normal faults with some of the early faults occurring at a low angle to primary bedding. These strata are also crosscut and overlain by igneous intrusive and extrusive deposits.

We have found that the Middle Pennsylvanian deep-water facies exposed in the Pequop Mountains are not present in the Spruce Mountain quadrangle. The Middle Pennsylvanian strata are a relatively thin succession of fossiliferous wackestones, packstones and grainstones, and there is no strong evidence for angular unconformities trimming the section, which are present in the section to the west. These results help constrain the continental stress field during ARM uplift. First, the accumulation of the Middle Pennsylvanian strata around Spruce Mountain required low rates of tectonic subsidence. Second, deep water facies of the Middle Pennsylvanian shallow-out moving southwest from the Pequop mountains towards Spruce Mountain indicating a bathymetrically positive area separating deep water facies in the Pequop Mountains from deep water facies documented at Buck Mountain and the White Pine Range, ~150 km to the southwest. These outcomes make the system difficult to reconcile with an east verging foreland basin and reduce the likelihood of a purely compressional Laurentia-Panthalassa boundary during ARM uplift.