Tectonic Implication of Facies Patterns, Lower Permian Dry Mountain Trough, East-Central Nevada
Type of Culminating Activity
Master of Science in Geology
Dr. Walter S. Snyder
Dr. Claude Spinosa
Dr. Paul K. Link
Tectonic sequence boundaries, identified during facies analysis of an east-west transect across the northern portion of the Dry Mountain Trough (DMT), document differential subsidence and uplift within this Early Permian basin. The DMT in east-central Nevada, is one of several Early Permian basins that developed along the continental margin of western North America. The tectonic activity associated with the development of sequence boundaries within the DMT can be correlated with similar episodes elsewhere, and provides evidence for Late Paleozoic continental margin tectonic episodes.
These tectonic episodes are not associated with terrane accretion and are younger than the Antler orogeny and older than the Sonoma orogeny. The correlation of these and similar tectonic episodes in lower Paleozoic strata (Trexler and Cashman. 1991) resulted in the definition of tectonic events and phases. Events are tectonic episodes with effects visible at outcrop scale, such as angular unconformities, pervasive deformation and stratigraphic evidence for uplift or subsidence. Phases are defined as single or multiple episodes with regional effects, but of short duration, reflecting a characteristic tectonic style and may encompass one or more events. The following phases are recognized in the DMT - the Humboldt. Late Pennsylvanian to earliest Permian, the Dry Mountain. Early Permian, and the Ishbel, Late Permian.
Generally. Wolfcampian to Leonardian deposition within the DMT reflects three distinct north-south trending facies packages: (1) a shallow marine carbonate to fan/braid-delta facies = western margin facies; (2) a deeper water basin facies and an eastern carbonate ramp = central basin facies; and (3) an eastern shallow shelf facies = eastern shelf facies. Two stratigraphic sections were measured in detail: McCloud Spring in the Sulphur Springs Range, within the western margin facies; and Portuguese Springs in the Diamond Mountains, within the central basin facies.
Analysis and correlation between the two measured sections and facies relationships suggest pulses of tectonic activity that are evidenced by sequence boundaries. The first event preceded development of the DMT and was a shallow water transgression over Devonian Slaven Chert at McCloud Spring and over Pennsylvanian (Morrowan to Atokan) Ely Limestone at Portuguese Springs. This transgression is a poorly understood event within the Humboldt phase. The second event initiated the development of the DMT, with basinal sedimentary units deposited unconformably over the surface of the first event shallow water carbonate. Interpretation of this second event surface as an unconformity is based on: (1) the abrupt facies transition from a shallow shelfal environment to a basinal environment at McCloud Spring; (2) the undulatory and altered character of the boundary surface at McCloud Spring and; (3) conodont data which at both localities suggest an abrupt change from Late Pennsylvanian-Early Permian elements to Early Permian elements. The tectonic activity that developed the DMT marks the beginning of the Dry Mountain phase, Wolfcampian (Early Permian); this phase ended in medial Leonardian time. Localized subsidence and uplift occurred within the DMT during the Dry Mountain phase and is demonstrated by abrupt facies transttions and the erosion and inclusion of ?Wolfcampian clasts within the DMT conglomerate. Marked uplift and subsidence defines the initiation of the Ishbel phase within the DMT during late Leonardian time. This tectonic episode within the central basin facies (Portuguese Springs), caused a paleogeographic reversal. Portuguese Springs previously more basinward than McCloud Spring, became a subaerial topographic high, whereas, subaqueous deposition continued at McCloud Spring.
Gallegos, Dora M., "Tectonic Implication of Facies Patterns, Lower Permian Dry Mountain Trough, East-Central Nevada" (1991). Boise State University Theses and Dissertations. 742.