DEVELOPMENT OF A SEQUENCE STRATIGRAPHIC FRAMEWORK FOR THE UPPER NOPAH FORMATION (UPPER CAMBRIAN, EASTERN CALIFORNIA AND WESTERN NEVADA)

The Halfpint and Smoky Members comprise the upper Nopah Formation and are easily recognizeable in the western Great Basin as a banded cliff, sandwiched between the slope-forming Dunderberg Shale Member below and the recessive beds of the Ordovician Pogonip Group above. Constructing a sequence stratigraphic model is critical for evaluating the broad depositional patterns for the Cambro-Ordovician interval. However, development of a detailed model is hampered by pervasive late-stage dolomitization obscuring original textures and the lack of a robust invertebrate fossil record.

Here we provide the preliminary results of a new sequence stratigraphic framework for the upper Nopah Formation using microbialite biostratigraphy, facies stacking patterns, and outcrop gamma-ray profiling. Five microbialite biozones are recognized from the Halfpint Member up through the lower Pogonip Formation. In stratigraphic order, the biozones are dominated by small domal stromatolites ('form A') and thrombolites, large branching thrombolites, large columnar thrombolites, small domal stromatolites ('form B'), and small columnar stromatolites. Lithofacies are divided into oolitic and oncolitic packestone/grainstone, silty grainstone, flat-pebble conglomerate, peloidal wackestone and grainstones, mottled wackestone, and microbial boundstone. All of the units are affected by late-stage dolomitization. Gamma-ray profiles were collected with a hand-held standard scintillometer at 1.5 meter intervals, using a total counts function for 10 seconds.

The data suggest recognition of missing zones within the upper Nopah Formation as one moves from the middle ramp toward the craton margin. These breaks in the section are related to higher-order sea level fluctuations. In some stratigraphic levels, the sequence boundary zones are recognized by intraformational conglomerates or breccias. The disconformity between the Nopah Formation and the overlying Pogonip Group is easily recognized on the gamma-ray profiles and microbialite biostratigraphy but less so in the lithostratigraphy.