Lower Cambrian Grand Cycles of the Southwestern Great Basin, U.S.A.
MOUNT, JEFFREY F., University of California, Davis, CA, LAURENCE R. GREENE, Unocal Corporation, Santa Fe Springs, CA, and DONNA L. HUNT and JENNIFER DIENGER, University of California, Davis, CA
Depositional Grand Cycles are a prominent feature of Cambrian strata of western North America. These cycles are traditionally viewed as unconformity bounded depositional sequences or parasequences that are correlative throughout the Cordillera. Consisting of a lower siliciclastic half-cycle and an overlying carbonate half-cycle, these sequences appear to record regional deepening-shoaling events on a broadly subsiding Cambrian passive margin shelf.
The Lower Cambrian Grand Cycles of the White-Inyo and Death Valley regions differ greatly from their Middle and Upper Cambrian counterparts throughout the Cordillera. These differences are manifested in the following ways: (1) Lower Cambrian siliciclastic half-cycles are relatively carbonate poor and were deposited primarily in deeper subtidal shelf settings; (2) siliciclastic half-cycle deposition is dominated by episodic storm processes, in contrast to the meter-scale, peritidal shallowing-upward cyclicity characteristic of younger Grand Cycles; (3) Grand Cycle boundaries are more diffuse and, in many cases, difficult to identify in more cratonward sections, and (4) trilobite zone boundaries appear to cross Lower Cambrian Grand Cycle boundaries, indicating that the cycles are not re ionally correlative, unconformity bounded sequences. Our depositional modeling indicates that these differences can be attributed to paleogeographic factors unique to the Early Cambrian. These include relatively rapid thermotectonic subsidence of the nascent Early Cambrian passive margin coupled with a high rate of eustatic sea level rise, high sediment yields off of the North American craton, and a relatively narrow Early Cambrian shelf.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)