Abstract

The Leppy Hills area has been considered by many workers to be less disrupted structurally than other ranges in the eastern Basin and Range Province. Consequently, these workers have used measured sections from the area to define regional stratigraphy. However, geologic mapping by the author at 1:24,000 indicates major stratigraphic and structural complexities are present, suggesting caution in the use of these measured sections.

Structural relations of more than 2,000 m of Devonian through Permian age miogeoclinal strata with a series of felsic volcanic flows, which have a K-Ar age of 11.9 m.y., provides the basis for dating Tertiary deformation in the Leppy Hills. Faulting ended by Pleistocene time, for Quaternary deposits overlap the faults. The amount of extension in the Leppy Hills, calculated along the simplest cross section, is at least 70 percent.

Facies changes present in the 150-m-thick Pennsylvanian and Mississippian Chainman Shale, Undivided, in the Leppy Hills and southern Pilot Range record the last stages of trough-filling that transcend the Mississippian and Pennsylvanian system boundary. The upper part of the Chainman Shale, Undivided, is predominantly a shallow-water carbonate facies that grades into the overlying Ely Group and accumulated in either a filled or very slowly subsiding eastern part of the Antler foreland basin.

Lenticular conglomerate bodies present in the upper part of the 70-m-thick Permian Ferguson Mountain and Stratheam Formations, Undivided, in the northern part of the Leppy Hills area represent a series of nearshore bars, shoals, and beaches. Southward these rocks grade into marine, silty biomicrites characteristic of the Ferguson Mountain Formation. This intertonguing relationship suggests that the Strathearn Formation in the Leppy Hills area is of Wolfcampian age.

The 600-m-thick Devonian Guilmette Formation contains lenticular, calcite-breccia masses along faults oriented subparallel to bedding. These faults have a distinctive concave upward spoon-shaped geometry. Breccia fragments are commonly covered with concentric bands of fibrous calcite that indicates cavity filling. The breccia masses developed during extensional faulting of Tertiary age by collapse of the wall rock into cavities along irregular fault surfaces.

In three localities, the upper part of the Chainman Shale, Unidivided, is in subhorizontal fault contact with the Guilmette Formation, indicating that at least 200 m of section (the normally intervening Pilot Shale and Joana Limestone) have been cut out over a lateral distance of 1 km. Where present, the Pilot Shale is separated from the underlying Guilmette Formation by a silicified breccia zone. This breccia, and those within the Guilmette Formation itself are inferred to result from listric/normal faults of Tertiary age.

As the Chainman Shale and older units are thinned primarily by faulting, all paleogeographic reconstructions based upon variations in formation thickness are uncertain. No direct evidence for thrust faulting of Paleozoic or Mesozoic age was found in the Leppy Hills. Rather, all the observed field geometries can be explained by east to northeast-trending Miocene and (or) Pliocene extension. Thus, the strata exposed in the Leppy Hills were probably passively rafted eastward throughout the Mesozoic in the upper plate of more deeply seated thrusts.