Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada

Preliminary paleontologic data from the northern Independence Mountains, Nevada, suggest that at least part of the Schoonover sequence is time equivalent to the lithologically and structurally similar upper Paleozoic Havallah sequence. The base of the Schoonover is a major thrust fault that juxtaposes paleogeographically distant but, in part, time-equivalent rocks. Along the west side of the range, the Schoonover sequence structurally overlies Late Mississippian nonmarine to shallow-marine rocks that unconformably overlie deformed Ordovician strata of the Roberts Mountains allochthon. This nonmarine to shallow-marine sequence is correlative to overlap assemblage sequences elsewhere in Nevada. Along the east side of the range, the Schoonover is thrust over shallow-marine to slope-facies Permian rocks that unconformably overlie deformed lower Paleozoic miogeoclinal rocks as well as the Roberts Mountains allochthon. The allochthonous Schoonover sequence is a heterogeneous assemblage of deformed deep-water sedimentary rocks and greenstone units. Isoclinally folded chert in the structurally lower half of the Schoonover yields ages based on radiolaria that range from earliest Mississippian to possibly latest Devonian at the very base of the sequence to as young as Early Pennsylvanian in structurally higher parts of the sequence. Clastic rocks within the lower part of the sequence contain debris derived from erosion of Roberts Mountains allochthon-type rocks. Rare volcaniclastic and volcanogenic units in the Schoonover indicate an andesitic-dacitic source. The structurally higher part of the allochthon contains a thick succession of silty-limestone turbidite deposits that represent yet a third, carbonate source terrane for the Schoonover sequence. Structural data indicate southeast-directed thrusting of the Schoonover sequence. Thrusting postdates deposition of Late Permian rocks and is inferred to predate intrusion of Late Jurassic plutons present to the north of the map area. Similarity of structural style and constraints on the timing of thrusting in the Independence Mountains strongly suggest that the Schoonover sequence may represent the northeasternmost exposures of the Golconda allochthon. The age of at least some of the pelagic sediments in the Schoonover sequence may overlap the age of the youngest rocks known in the Roberts Mountains allochthon as well as the age of the Antler Orogeny. This suggests that the Schoonover sequence was deposited, at least in part, in a location that was paleogeographically distant from the edge of the Nevadan portion of the North American shelf during the Antler Orogeny. No volcanism occurred on the continental shelf during the emplacement of the Roberts Mountains allochthon; therefore, the presence of tuffaceous turbidites within the Schoonover also suggests deposition far away from the continental margin. On the other hand, the composition of coarse clastic rocks within the Schoonover is compatible with their derivation from the eroding Roberts Mountains allochthon, suggesting proximity to the North American shelf. If sedimentary rocks from dual sources (volcanic arc and Roberts Mountains allochthon) are indeed interbedded in the Schoonover, rather than tectonically interleaved, then this might argue in favor of deposition of the Schoonover-Havallah sequences in a back-arc basin setting.