Stratigraphy and structure of metamorphosed upper Paleozoic rocks near Mountain City, Nevada

Abstract

Upper Paleozoic rocks near Mountain City, Nevada, consist of a marine sequence of Late Mississippian age unconformably overlying deformed Ordovician rocks of the Roberts Mountains allochthon. This is the oldest and thickest Mississippian Antler “overlap sequence” in Nevada. At its base, clastic rocks of the Grossman Formation are overlain by a fining-upward sequence of the Meramecian Banner Formation. Limestone at the top of the Banner Formation is overlain by basaltic lava, tuff, and minor limestone of the Nelson Formation. The Nelson Formation is, in turn, overlain by the Chesterian Chainman Shale. This Upper Mississippian sequence records subsidence and rifting or strike-slip tectonics that affected the continental margin of western North America soon after thrusting associated with the Late Devonian–Early Mississippian Antler orogeny. The occurrence of the Chainman Shale in the upper part of this Antler overlap sequence both near and to the east of Mountain City seems to record a westward migration of the Antler foredeep basin across a foundered portion of the Antler orogenic belt. The occurrence of submarine basalt flows of Meramecian age in the Nelson Formation, in correlative rocks to the west, and in exposures of the Golconda allochthon south of Mountain City suggests that Late Mississippian basaltic volcanism was widespread, affecting the continental margin of North America as well as the offshore Golconda basin. Block faulting and uplift during Pennsylvanian-Permian time caused disruption and erosion of Upper Mississippian sequences deposited elsewhere on the Antler orogenic belt. A metamorphosed, folded, and thrust-faulted assemblage of chert-argillite, basalt, gabbro, siliciclastic and limestone turbidites, fine-grained dolomitic quartz arenite, and minor siliceous tuff of late Paleozoic(?) age rests in thrust contact above the Mississippian overlap sequence. These basinal rocks, referred to as the “Reservation Hill sequence” (RHS), were probably deposited in deep water on the continental slope and rise of an ocean basin receiving chert-rich quartzose and carbonate detritus from North America. The RHS is lithologically similar to Mississippian-Permian rocks of the Golconda allochthon. Prior to its southward thrust emplacement during the Mesozoic, the RHS was probably deposited on oceanic or transitional crust in an east-west–trending re-entrant or embayment of the late Paleozoic continental margin in the area of the present Snake River Plain. During the late Mesozoic, rocks in the vicinity of Mountain City were regionally metamorphosed, folded, and cut by south-verging imbricate thrust faults. This episode of compressive deformation occurred prior to intrusion of a quartz diorite stock, which has a minimum age (K/Ar hornblende) of 134 Ma. Other similar dioritic stocks in northern Elko County that have K/Ar ages on biotite of ∼153–155 Ma also post-date thrusting and metamorphism of rocks as young as Triassic, suggesting that the age of deformation is post-Triassic and pre– (or syn–) Late Jurassic. Mesozoic folds and thrusts near Mountain City represent considerable north-south shortening and form part of a belt of south-verging folds and thrust faults trending east-west across northern Elko County. This belt may be an eastward deflection or splay of the Jurassic and Cretaceous Winnemucca, Luning, and Fencemaker fold and thrust belts recognized farther to the south and is subparallel to other major Cordilleran lithotectonic belts in northeastern Nevada, including the edge of the Precambrian sialic crust. The wholesale eastward deflection of Cordilleran trends in northeastern Nevada may reflect either the presence of a long-standing indentation in the continental margin of western North America or an oroclinal bend of post-Triassic and, possibly, post–Late Cretaceous age.