Cambrian-Ordovician Craton Margin Section, Southern Great Basin: A Sequence Stratigraphic Perspective

COOPER, JOHN D., California State University, Fullerton, CA, and JEFFREY C. EDWARDS, San Francisco State University, San Francisco, CA

The Upper Cambrian through Upper Ordovician cratonal to miogeoclinal transition section in southern Nevada and eastern California accumulated on a gentle homoclinal ramp and includes a variety of peritidal and subtidal carbonate facies. This section consists of a westward-thickening wedge assigned to the Nopah and Mountain Springs formations and can be related to one type 1 and at least four type 2 sequences.

The basal part of the section is the Dunderberg Shale Member of the Nopah Formation, which is composed of meter-scale cyclical shale-carbonate bundles. Carbonate interbeds signify a variety of peritidal to deep subtidal paleoenvironments that comprise shelf margin and transgressive systems tracts. The regionally abrupt contact between the uppermost shale of the Dunderberg and the overlying carbonates of the upper Nopah is not a classic Grand Cycle boundary because it does not represent a major paleoenvironmental shift.

The progradation of a thrombolitic bank complex in the overlying upper Nopah may reflect emergence on the craton, evidenced by the Sauk II-Sauk III cratonic disconformity. In the craton margin section, this disconformity has a subtle signature in a condensed section with the Dunderbergia-Elvinia biozone interval where it represents a type 2 sequence boundary. Most of the upper Nopah carbonate section is developed as microbial boundstone and genetically related carbonates of a major progradational microbial bioherm complex that represents a Sauk III highstand systems tract.

The overlying "A" member of the Mountain Springs Formation rests in sharp and, in places, slightly discordant contact with the top of the Nopah. This contact is interpreted as a marine flooding surface and type 2 sequence boundary. Member A consists of Lower Ordovician transgressive and highstand systems tracts comprised of oolitic shoal and back-shoal to intertidal carbonates. At least one, and possibly more intramember exposure surfaces are suggested by lithoclast breccias and subtle microkarst, and may represent type 2 sequence boundaries. The type 1 unconformity that separates member A from overlying "B" member is the top of the first-order Sauk Sequence. The "B" member is a thin stratal interval of late Middle to Late Ordovician age and consists of dark, burrow-mottled skeletal w ckestone and mudstone that is part of a transgressive systems tract. It is bounded above by

another type 1 unconformity and therefore represents the first-order Tippecanoe sequence of Sloss.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)