Alluvial to Marine Facies Transition in the Antler Overlap Sequence, Pennsylvanian and Permian of North-Central Nevada

Abstract

Autochthonous sedimentary rocks of the Antler overlap sequence indicate regional subsidence of north-central Nevada during Pennsylvanian and Early Permian time as an inland sea gradually transgressed over coarse alluvium eroded from remnants of the Antler orogenic highlands. Alluvial conglomerates and sandstones of the Lower Atokan (Middle Pennsylvanian) Battle Formation (up to 240 m thick) rest unconformably on folded and faulted lower Paleozoic strata that were deformed by emplacement of the Roberts Mountains allochthon during the Antler orogeny. Paleocurrents and stratigraphic relations suggest that the Battle Formation represents deposits of several paleotributaries of a single major paleovalley draining to the southwest. The Battle Formation and overlying limestones constitute a conformable transgressive sequence. In ascending order, the lithofacies and corresponding depositional environments of the Battle Formation are: 1) massive clast-supported conglomerate deposited in proximal portions of a braided-stream system, 2) very sandy conglomerate with rare foreset cross-stratification deposited in the middle reaches of a braided-stream system, 3) cross-stratified conglomeratic sandstone deposited in the distal part of a braided-stream system, 4) fine conglomerate and fine-grained, commonly muddy, sandstone deposited near the terminus of a braided-stream system, and 5) interbedded calcareous sandstone, calcareous conglomerate, and mudstone of tidal and deltaic origin. The Battle Formation is overlain by Pennsylvanian carbonates (Etchart Formation, Highway Limestone, and Antler Peak Limestone) deposited in shallow-marine and/or marginal-marine environments. Paleontologic evidence indicates roughly contemporaneous deposition of 1) open-marine carbonates within a shallow inland sea, 2) mixed terrigenous-carbonate sediments around the margins of the shallow sea, and 3) conglomerates and sandstones in alluvial environments farther inland.