--> Environmental Carbonate Stratigraphy and Cyclic Deposition of Smoky Member, Type Nopah Formation (Upper Cambrian), Nopah Range, Southern Great Basin, by Kirk F. McCutcheon and John D. Cooper; #91024 (1989)

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Environmental Carbonate Stratigraphy and Cyclic Deposition of Smoky Member, Type Nopah Formation (Upper Cambrian), Nopah Range, Southern Great Basin

Kirk F. McCutcheon, John D. Cooper

Carbonate rocks of the Smoky Member of the type of Nopah Formation consist of three cyclically developed megafacies whose light, medium, and dark gray colors impart a distinctive banded outcrop appearance to the 340-m thick section. The light gray bands are 30 to 80 m thick, consist predominantly of digitate cryptomicrobial doloboundstones with associated mud mound characteristics, and are generally devoid of megafossils and current-bedded features. Small-scale cyclic sequences are expressed by the recurrence of digitate cryptomicrobial and planar stromatolitic doloboundstones. The dark bands are 15 to 70 m thick, contain abundant current-bedded structures, and include bioclastic dolowackestones, bioclastic, oncolitic, and intraclastic dolopackstones, peloidal and oolitic dolograinstones, and a variety of cryptomicrobial doloboundstones. Recurrence of specific lithologies produces small-scale cyclic packages within the megafacies. The medium-gray bands are 5 to 25 m thick, contain abundant cryptomicrobial doloboundstones, and occur in vertical sequences of light, medium, and dark gray.

The light-gray megafacies is interpreted as a shoaling barrier mud bank system stabilized by cryptomicrobial structures. The dark-gray megafacies is interpreted as deposits of restricted peritidal environments shoreward of the shoaling barrier mud bank system. In this context, burrow-mottled bioclastic dolowackestones represent subtidal ponds and lagoons, and oncolitic, peloidal, and intraclastic dolopackstones to grainstones represent deposits of tidal flats and tidal channel levees. Cross-bedded dolograinstones and steep-sided cut-and-fill geometries reflect deposition in tidal creeks and subtidal surge channels. The medium-gray megafacies expresses subtidal paleoenvironments transitional between barrier and back barrier.

This tripartite megafacies assemblage is repeated three times in the Smoky Member. Each megacycle is interpreted to represent a shallowing sequence deposited in response to fluctuations in relative sea level.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.