EASTERN SLOPE FACIES OF THE HAVALLAH-SCHOONOVER BASIN, NEVADA: TRANSITIONAL ZONE BETWEEN THE ANTLER HIGH AND DEEP MARINE BASIN IN THE LOWER PART OF THE GOLCONDA ALLOCHTHON

The 500 km long Golconda allochthon (GA) in Nevada preserves remnants of the Late Devonian to Permian Havallah-Schoonover Basin. The deep marine basin lay west of the Antler orogenic high, the long-lived welt of deformed basinal strata of the Roberts Mountain allochthon (RMA). The GA was imbricated and thrust over the RMA and its sedimentary overlap in middle Permian to Triassic time. South of Winnemucca and Battle Mountain, the Havallah assemblage includes a structurally lower stack of Eastern Slope facies and an overlying stack of deeper Mid-Basin facies. For at least 80 km along strike, from the Battle Mountain area to the Northern Shoshone Range, the Eastern Slope facies is characterized primarily by Pennsylvanian to middle Permian argillite and siltstone deposited at intermediate depths and by Permian sponge spicule chert containing faunas redeposited from a shallower source. The source was probably the structurally underlying, autochthonous Antler sequence. Similar sponge assemblages characterize a belt of middle Permian spiculite deposits that runs from the Phosphoria Basin in Idaho along the Antler High as far south as the Northern Shoshone Range in central Nevada. We infer that the Eastern Slope facies of the GA represents a transitional zone between the rim and deep basin based on its apparent depositional links to the Antler High, inferred intermediate depths of deposition, and structural position. The detached basement of the Eastern Slope facies may have been extended crust of an Antler arc or the RMA. In contrast, probable basement slivers of the Mid-Basin facies have a rifted back-arc or ocean basin character that includes pillow basalt, radiolarian chert, and hydrothermal manganese deposits. Slight differences in the ages of the youngest Havallah in the Mid-Basin facies (late early Permian) and strata of the Eastern Slope facies and Antler sequence (middle Permian) suggest that the basin collapsed from west to east during the Permian. Shoaling of the deforming package of Mid-Basin strata is constrained by the unconformably overlying latest Permian to Early Triassic age Koipato sequence, a volcanic and volcaniclastic section. Because the Koipato is not known to overlap the Eastern Slope facies or the Antler High, the final emplacement of the Mid-Basin facies over the Eastern Slope facies may be younger.