Dating the initiation of the Antler orogeny: Evidence from Middle Devonian strata in the Monitor Range, Eureka County, Nevada

In Whiterock Canyon, central Nevada the contact between the carbonate strata of the Rabbit Hill Limestone and overlying radiolarian cherts and the shaly limestone has been mapped as the Roberts Mountains thrust correlate with a relationship that occurs in other mountain ranges of north-central Nevada. Generally the contact is covered and not well exposed. However, excavation of the contact in Whiterock Canyon and its detailed study indicate that it is stratigraphic and not structural.; The deep basinal, radiolarian chert and thinly laminated lime mudstone of the Ryegrass Formation (named here for the first time) is underlain by relatively shallow-water and relatively thin-bedded carbonate strata of the miogeoclinal succession. The age of the facies change at the contact, constrained by fossils, ranges from Pragian to Givetian and corresponds to a time of relative sea-level lowstand as well as contemporaneous shallow water carbonate sedimentation immediately to the east. Thus, a eustatic rise in sea-level cannot explain the abrupt facies change from relatively shallow water to a deep basin. Instead the facies change that is interpreted as a abrupt deepening of the depositional environment must record tectonic subsidence. The contact displays possible evidence of karstification, which would indicate, uplift and erosion before tectonic subsidence and deposition of the Ryegrass Formation. Broad regional uplift and emergence of the continental shelf during the earliest initiation of the emplacement of the Roberts Mountains allochthon might explain the subaerial exposure of the Rabbit Hill Limestone, which would allow for dissolution of the limestone. This was followed by very rapid deepening which is marked by the very distinct facies change from the Rabbit Hill Limestone to the Ryegrass Formation.; This evidence indicates that the Rabbit Hill-Ryegrass contact records a possible passage of a peripheral bulge and then subsidence of a foreland basin during inboard migration of the Roberts Mountains allochthon. This, in turn, is evidence of a late Pragian to early Givetian age for the onset of the Antler orogeny. This age is significantly older than previously interpreted, significantly extending the duration of this orogeny.