Late Paleozoic deformation in the Osgood Mountains and Dry Hills, northern Nevada
Creators: name: Siebenaler, Samuel A.
Publisher: name: University of Nevada, Las Vegas
Year: 2010

description: Pennsylvanian (IP) and Permian (P) deposits (Etchart Formation) and structures in the northern Osgood Mountains and Dry Hills are identified and interpreted to reconstruct a late Paleozoic tectonic history for the time between the Antler and Sonoma orogenies. Mapping, field observations, fusulinid-based unit ages, and structural analysis are used to identify three fold and fault sets, a fault set, a fold set, and at least two angular unconformities. The faults consist of N to S striking normal faults (fault set 1) that are cross cut by WSW-ENE striking thrust faults (fault set 2). Fault sets 1 and 2 are cross cut by imbricated SW-NE striking thrust faults interpreted to be the Golconda Thrust (fault set 3). Fault set 3 is offset in the southern Dry Hills by an oblique right-lateral normal fault zone (fault set 4) interpreted to be the Getchell Fault. Recognition of this offset allows for estimates of right-lateral offset of the Getchell Fault ranging from 1.7 km to 8 km and 600 m of throw, since the emplacement of the Golconda Allochthon. 
Etchart members contain at least four fold sets. The oldest fold set (F1) is SW-trending, gentle, upright, sub horizontal to gently SW-plunging folds constrained to Missourian time. F2 folds are WSW-to-ENE trending and gently plunging, open, upright folds. F3 folds are gentle to open, upright to steeply east inclined, NNW-trending and gently plunging. F2 and F3 folds are both interpreted to be Wolfcampian to Leonardian in age based on the age ranges of the unit to which they are restricted. F4 folds are NE-trending and gently plunging, open, upright to steeply inclined folds. The Etchart Formation contains at least two angular unconformities: the C6 and P1 angular unconformities of Trexler et al. (2003). Using crosscutting relationships and parallelism, the structures are interpreted to be the result of at least six deformational events during or after IP and P time. Timing of the oldest four of these deformations is constrained to between Atokan and Leonardian stages based on unit ages, stratigraphic relationships, and structural overprinting. 
The unconformities along with additional lithologic evidence warrant division of the Etchart Formation into three separate informal members: the Upper, Middle, and Lower Etchart members. Interbedded Permian age conglomerates contain Atokan age clasts, require reevaluation of the presence of Battle Formation in the area, and indicate Permian uplift and erosion of Pennsylvanian age deposits. Lithologic characteristics of the Etchart members imply that increasing amounts of siliclastic material was shed into the Dry Hills basin from the IP to the early P, coinciding with increased frequency of tectonism and increasingly shallow depositional environments. 
Correlation of units in the Dry Hills to those at Edna Mountain, Carlin Canyon, and the Pequop Mountains are used to fit the local structural history of the Dry Hills into the regional tectonic history. The structural and depositional history of the Osgood Mountains and Dry Hills are plotted along with six other locations to fit a regional scale late Paleozoic deformational history. 
This analysis suggests relatively major regional shortening events in the Atokan and Missourian, resulting in largely parallel structures, and the C6 and P1 unconformities. Several other lesser contractional events occur in the region during the Desmoinesian, Wolfcampian, and Leonardian with scattered orientations.