Microbial Reefs on Carbonate Ramp: Case Study from Western North America with Global Perspective

Karen Griffin

Thrombolite and stromatolite bioherms ranging from 0.3 to 3.0 m in height are the main components of microbial reefs that developed on the western margin of North America during the Late Cambrian. These microbial reefs occur in the Smoky Member of the Nopah Formation and are well exposed in eastern California and western Nevada. Four lithofacies are recognized within the Smoky Member, including (1) microbial boundstones composed of stromatolite, thrombolite, stromatolitic thrombolite, and undifferentiated fabrics, (2) peloidal carbonates composed of grainstones, packstones, wackestones, and mudstones, (3) ooid carbonates composed of oolites, and (4) mixed siliciclastic carbonates composed of carbonate mudstones and siltstones. These units were deposited on a homoclinal ra p where agitated conditions allowed for the migration of active peloid and ooid shoals across the microbial reefs. Thrombolite and stromatolite reefs also occur at this time in several areas of Gondwana, Laurentia, China, and Baltica. Several common features that may have been important to these microbial reef systems are relatively shallow-water agitated conditions, relatively low input, and variable metazoan influence.

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