Roberts Mountains thrust relationships in a critical area, northern Sulphur Spring Range, Nevada

Abstract

At the orogenic front of the Antler orogeny, the Roberts Mountains allochthon lies on Lower Mississippian Dale Canyon Formation and consists of Middle and Upper Devonian Woodruff Formation, slivers of Silurian to Lower Devonian Roberts Mountains Formation, and Ordovician Vinini Formation, in structural succession. Devonian Pilot Shale, mapped unconformably below Dale Canyon flysch, was mapped previously as allochthonous Woodruff Formation. The Vinini recognized here was also mapped previously as Woodruff. The Dale Canyon was mapped previously as Chainman Shale. Recognition of Pilot Shale here necessitates the elimination of a previously postulated thrust plate in which Woodruff was shown thrust upon Devonian carbonate rocks; it also makes more consistent the structural sequence within the Roberts Mountains allochthon along its frontal margin. Assignment of fine-grained siliciclastic turbidites in the flysch trough of the Antler foreland to the Dale Canyon Formation eliminates anomalous use of Chainman Shale as a stratigraphic term and allows its useful retention for delta-slope deposits distal to the Diamond Peak molasse. The so-called Dry Creek fault, previously interpreted to cross the mapped area, cannot be recognized. Some outcrops of Devonian carbonate rocks, previously mapped as in structural windows below Woodruff Formation, are actually Quaternary landslide blocks.