DISCONTINUITIES AT THE BASE OF LATE PALEOZOIC SECTIONS-ANCESTRAL ROCKIES UNCONFORMITIES, TERTIARY DETACHMENTS, OR BOTH?

In northeastern Nevada, among a cluster of metamorphic core complexes, the Pequop Mountains contain a sheet of Permian Pequop Formation resting upon a gently dipping fault. To the west, a juxtaposition of the Pequop Formation above the crystalline “basement” in the Ruby Mountains is marked locally by contacts with strong foliation. In nearby ranges where the Late Paleozoic section and overlying Triassic beds comprise tilted sections, the basal contact is marked by structural truncation of beds dipping into a fault. In the central Pequop Mountains, Pequop Formation locally overlies Ordovician Eureka quartzite along a contact crudely parallel to bedding and distinguished by the absence of normally intervening formations such as Guilmette and Chainman. In the footwall Eureka quartzite crops out as coherent, thin, lithified, brecciated layers, whereas at other exposures, mainly where the hanging wall units are not well preserved, the quartzite is strongly dismembered into blocks among numerous brittle faults. The consistent presence of commonly unmetamorphosed Pequop Formation in the hanging walls of postulated detachments is striking.

In this paper, we choose to assess other options that may bear upon the current setting of the Pequop. Among these are the following:1) relation to Cretaceous Sevier or 2) Laramide thrust movement, 3) detachment related to Jurassic Elko or Nevadan contractional events, 4) Late Permian (Sonoma) thrust movement, or 5) widespread, enigmatic, contraction, during the transition from Pennsylvanian to Permian periods between ca. 305 to 295 Ma (Humboldt Orogeny).

The principal record of the deformational episode is contained by synorogenic Strathearn Formation that formed at the front of the Coyote thrust. According to Theodore et al. (2004), the lower Strathearn Formation of latest Missourian (ca. 303+Ma) through early Asselian (ca. 298- Ma) ages is overlain by conglomerate the beds of which are confined to the early Permian (latest Asselian, ca. >295 Ma through middle Sakmarian, ca. 290 Ma). The well-preserved nature of the Pequop outcrops is compatible with Tertiary emplacement although the source of the beds remains to be identified.