Synorogenic Quartz Sandstone in the Jurassic Mobile Belt of Western Nevada: Boyer Ranch Formation

Abstract

The province of Lower Mesozoic layered rocks in western Nevada contains a diversity of marine lithologies whose deposition continued locally as late as Middle Jurassic time. At places in the province, mature quartz sandstone constitutes all or part of the highest stratigraphic units in sections of non-volcanic rocks and is believed to record the last deposition of terrigenous sediments before complete effacement of the marine basin by widespread orogeny. The sandstone was deposited early in the erogenic episode and at least locally in troughs created by folding of subjacent rocks. The sands are anomalously mature with respect to co-deposited clastic components and to coarse detrital materials in nearly all earlier Mesozoic rocks of the province. The problems are the source of the quartz sand and the reasons the sand was deposited synorogenically. The name, Boyer Ranch Formation, is formally proposed for a lithesome of homogeneous Jurassic quartz sandstone and basal conglomerate and limestone in the Dixie Valley region which is approximately in the northern third of the outcrop area of Jurassic quartz sandstone in western Nevada. The Boyer Ranch Formation contains up to 500 ft of sandstone, largely fine-grained calcareous quartz arenite, whose granulometric properties suggest eolian sorting, but whose bedding indicates quiet-water deposition. The sandstone lies above limestone and carbonate-pebble conglomerate with interstitial quartz sand. The inferred early Mesozoic geographic and tectonic histories in the Dixie Valley region suggest that the sands of the Boyer Ranch Formation accumulated at the eastern shoreline of the Mesozoic basin in western Nevada in late Early or Middle Jurassic time. Until the onset of orogeny, the sands remained unlithified, probably owing to eolian saltation, and followed a generally westerly regression of the shoreline. Postulated strong wave action prevented seaward movement of the sand. The late Early Jurassic or Middle Jurassic (or both) orogeny created local troughs which the sea reinvaded and provided an irregularly configured and low-energy shoreline environment such that movement of the sand into the water was no longer impeded. The sands may have evolved locally through the action of water and wind at the beach of the Early Mesozoic sea in western Nevada, or they may have been largely co-derived from a distant source with sands in Jurassic rocks of the eastern Cordillera and the Colorado Plateau.