PreCambrian

AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

REGIONAL PALEOGEOGRAPHY

UPPER PRECAMBRIAN

Initial rifting of the craton to form the passive margin which dominated the Early Paleozoic passive margin of western North America was accomplished by latest Precambrian (about 650 Ma). A westward thickening wedge of sediments was deposited as the eastward transgressing sea moved across the craton in the Upper Precambrian. Deposition continued essentially uninterrupted throughout the Upper Cambrian.

In northeastern Nevada, the Precambrian is represented by the siliceous and argillaceous metasediments of the McCoy Creek Group. Original lithologies include feldspathic and arkosic sandstones, siltstones, shales and mudstones, pebble, cobble and boulder conglomerate, and minor amounts of limestone and tuff(?). These sediments were regionally metamorphosed to quartzites, metasiltstones, argillite and phyllite, conglomeratic schists and gneisses, marble, and meta-tuff during a post-Jurassic lower greenschist facies event. The metamorphosed McCoy Creek Group sediments preserve sedimentary structures such as well-developed graded and convolute bedding, pebbly beds, and small-scale horizontal laminae and cross-bedding.

Very little attention has been given to protolith and depositional reconstruction for the cratonally derived McCoy Creek Group. Hose and Blake (1976) suggest that the McCoy Creek sediments were deposited in a nearshore shallow marine shelf to slope environments and that density currents may have played a role in the deposition of some of the siliciclastics. Clark and others (1985) have suggested that over 1,000 feet of massive greenstone, thinly bedded silty limestone, limey siltstone and argillite, and sandstone of the McCoy Creek Group in the Bull Run Mountains may represent a deep lacustrine rift basin. They interpret the overlying Lower Cambrian Prospect Mountain Quartzite as a fluvial system which overrode the rift basin and the succeeding Lower Cambrian Edgemont Formation siltstone, mudstone and limestones as the inital miogeoclinal subsidence following continental separation.