Abstract

In the area from the Colorado Rocky Mountains to central Nevada the Ordovician System is represented by an unparalleled display of carbonate facies with some intertonguing quartz sandstones. Both lithologically and faunally the Ibexian and Mohawkian rocks in Colorado are the near shore facies of the eastern interior seas, not of the seas west of the Transcontinental Arch. The Arch lay in westernmost Colorado and east of the Pavant Thrust in Utah.

Rocks of the four standard North American Ordovician series (Ibexian, Whiterockian, Mohawkian, and Cincinnatian) are present in the Basin Ranges, but the Whiterockian is missing in Colorado, Wyoming, and the Dakotas. This absence indicates an unconformity of almost 30 million years duration.

The stratotype sections for the Ibexian and Whiterockian are in the Basin Ranges. Their biostratigraphic zonation provides not only a standard for the first 40 million years of Ordovician history in Laurentia, but also a satisfactory standard for international correlation. Because of the complexity of tectonic disruption, understanding these zonal standards is essential to the proper interpretation of lateral facies relationships and recognition of the progradation dominant in both the carbonate and quartzitic formations in Utah and Nevada. Late Ibexian alkalic olivine basalts in the Vinini Formation suggest that intraplate rifting led to correlative soft sediment slumping in the southern Toiyabe Range and breccia flows in the Hot Creek Range.

Although no petroleum has been produced from any Ordovician unit in the Basin Ranges, potential reservoirs and source beds are present. Among the most promising reservoir beds are the Eureka Quartzite (particularly where it is poorly cemented above the Kanosh Shale), Waulsortian-type carbonate mounds like that so well exposed at Meiklejohn Peak, Nevada, and other types of carbonate mounds composed of Calathium and algal-sponge assemblages. Structural deformation and intense fracturing also give many of the dense, non-porous dolomite formations some reservoir potential. The most promising source beds in the Ordovician section are the Vinini Formation and Kanosh Shale (which apparently once contained as much organic matter as some of the oil-prone Devonian and Mississippian shales in the region, but is thermally overcooked across much of Nevada), and peritidal deposits in the Antelope Valley Formation.