Trexler, J.H., Jr.¹, Cashman, P.H.¹, Davydov, V.I.², and Snyder, W.S.², (1) Department of Geological Sciences and Engineeringt, MS 172, University of Nevada, Reno, NV 89557, (2) Department of Geosciences, M.S. 1535, Boise State University, Boise, ID 83725
The Antler orogeny, recorded by the formation of a foreland basin in latest Devonian time, was followed by a series of distinct late Paleozoic orogenic events that affected the western margin of North America. These events can be recognized by angular unconformities which locally trim folds or thrust faults, and which are commonly overlain by orogenic conglomerates. The timing of the deformation can be precisely determined biostratigraphically, by dating rocks above and below the unconformities. The late Paleozoic unconformities (and tectonic events they record) have been little recognized, in part due to broad application of stratigraphic terminology throughout Nevada. For example, in some cases a single formation name has been applied to rocks both above and below an unconformity. Revision of late Paleozoic stratigraphic terminology is therefore in order. The evidence for the Antler orogeny is a foreland basin containing latest Devonian - early Mississippian clastic sediments derived from the west (Poole, 1974). Although widespread folding of early Paleozoic rocks in Nevada has also been attributed to the Antler orogeny, we are not aware of any field relationships that require the folding to be late Devonian - early Mississippian. Instead, the strata that unconformably overlie the Antler allochthon and associated structures, called the "Antler Overlap Sequence", are late Pennsylvanian or younger. Mid-Mississipian deformation is developed in early and mid-Mississippian rocks of the Antler foreland basin, and therefore post-dates the Antler orogeny. The folded and faulted foreland basin rocks are unconformably overlain by Chesterian (late Mississippian) rocks, thus confining the deformation to mid-Mississippian (Trexler and others, 2003). Structures formed at this time include east- to southeast-vergent folds and thrust faults in the Piñon Range (Silberling et al, 1997; Tosdal, unpublished mapping) and in the northern Adobe Range. This deformation is represented by an angular unconformity throughout the Adobe Range, Pinyon Range and Diamond Mountains. Mid-Mississippian deformation is also recorded in southern Nevada. On the Nevada Test Site, it is expressed as an abrupt change in clast composition in the submarine fan deposits (Trexler and Cashman, 1997). Stratigraphic names like "Chainman Shale" and "Diamond Peak Formation" have been applied to rocks both above and below the mid-Mississippian unconformity, obscuring the evidence for mid-Mississippian "C2" deformation. Atokan and Morrowan (early Pennsylvanian) rocks are folded and faulted, then unconformably overlain by late Missourian to Virgillian rocks, recording mid-Pennsylvanian "C6" deformation. This deformation is expressed as northwest-vergent, folds and imbricate thrust faults in the southern Adobe Range (Trexler et al. 2004) and as southwest-vergent folds at Edna Mountain, near Golconda (Villa et al., 2007). In both places, mid-Pennsylvanian deformation is the most pervasive of several late Paleozoic deformation events. The type section of the "Battle Formation" post-dates the mid-Pennsylvanian deformation, but this term has also been applied to an early Pennsylvanian conglomerate that is involved in the southwest-vergent folding at Edna Mountain; this stratigraphic usage, too, must be revised. Biostratigraphic age control, well-constrained field relationships, and use of well-defined stratigraphic terminology are necessary to distinguish "Antler" from the different late Paleozoic deformation events in Nevada. In areas of complex deformation, detailed knowledge of the kinematics associated with each deformation event is necessary to understand the structural history.
References Cited
Poole, F.G., 1974, Flysch deposits of the Antler foreland basin, western United States, in Dickinson, W.R., ed., Tectonics and Sedimentation, Volume Special Publication 22: Tulsa, SEPM, p. 58-82.
Silberling, N.J., Nichols, K.M., Trexler, J.H., Jr., Jewell, P.W., and Crosbie, R.A., 1997, Overview of Mississippian depositional and paleotectonic history of the Antler foreland, easter Nevada and western Utah, in Link, P.K., and Kowalis, B.J., eds., Geological Society of America Fieldtrip Guidebook, Volume BYU Geology Studies, 42, 1: Provo, Brigham Young University, p. 161-196.
Theodore, T.G., Moring, B.C., Harris, A.G., Armstrong, A.K., and Finney, S.C., 2003, Geologic map of the Beaver Peak Quadrangle, Elko and Eureka Counties, Nevada: Reno, NV, Nevada Bureau of Mines and Geology.
Trexler, J.H., Jr., and Cashman, P.H., 1997, A southern Antler foredeep submarine fan: the Mississippian Eleana Formation, Nevada Test Site: Journal of Sedimentary Research, v. 67, p. 1044-1059.
Trexler, J.H., Jr., Cashman, P.H., Cole, J.C., Snyder, W.S., Tosdal, R.M., and Davydov, V.I., 2003, Widespread Effects of Mid-Mississippian Deformation in the Great Basin of western North America: Geological Society of America Bulletin, v. 115, p. 1278-1288.
Trexler, J.H., Jr., Cashman, P.H., Snyder, W.S., and Davydov, V.I., 2004, Late Paleozoic tectonism in Nevada; timing, kinematics, and tectonic significance: Geological Society of America Bulletin, v. 116, p. 525-538.
Villa, D.E., Cashman, P.H., Trexler, J.H.Jr., Davydov, V.I., and Taylor, W.J., 2007, Late Paleozoic deformation at Edna Mountain, Humboldt County, Nevada (abs.): Geological Society of America Abstracts with Programs.