Structure of the Roberts Mountain Allochthon in the Three-Bar Ranch and Tonkin Summit Quadrangles, Roberts Mountain, Eureka County, Nevada

The Roberts Mountains allochthon (RMA) and the underlying allochthon, structurally juxtaposed in the Antler orogeny, as well as post-orogenic strata were mapped in the Three-Bar Ranch Quadrangle and a small part of the Tonkin Summit Quadrangle, Eurkea County Nevada. Previous maps of this area show the RMA as comprising only the Ordovician Vinini Formation. Recent mapping demonstrates that the Ordovician Vinini Formation is structurally juxtaposed over the Devonian Woodruff Formation, which, in turn, structurally overlies the Mississippian Webb Formation, considered to be parautochthonous. Along the southwestern side of the Roberts Mountains, the Woodruff Formation exhibits a greater degree of folding than the lower member of the Vinini Formation that overlies it. To the northeast, the thrust sheet with the lower member of the Vinini Formation is structurally overlain by several imbricated thrust sheets of upper Vinini and Woodruff formations, which exhibit closed, plunging folds. To the east of this area, high-angle normal faults juxtapose the lower plate, autochthonous plate of the Roberts Mountains thrust with rocks of the RMA, and massive outcrops of jaserpoid are located along these faults.

The structural relations between the Webb, Woodruff, and Vinini formations is consistent with the hypothesis that the RMA was emplaced in the Roberts Mountains on two, major, telescoping thrust surfaces. The closed, plunging, north-trending folds observed in outcrops of limestone and chert beds within thrust sheets of Woodruff Formation and upper units of the Vinini Formation indicated that imbricated thrust faulting occurred within the allochthon during emplacement.

The results of our recent mapping have important implications evaluating oil and gas potential of the thick, organic rich black shale within the Vinini and Woodruff formations. There is a demonstrated potential for deeply buried source rocks within the thick, structurally down-dropped source rocks and a possible vertical stacking and imbrication of source and reservoir rocks.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California