DEPOSITIONAL SEQUENCE STRATIGRAPHY OF LOWER CAMBRIAN GRAND CYCLES, SOUTHERN GREAT-BASIN, USA

The Lower Cambrian of the southern Great Basin records third-order cyc lical alternation between siliciclastic-and carbonate-dominated sedime ntation in a rapidly subsiding passive margin. These alternations, ter med Grand Cycles, are a prominent feature of Cambrian stratigraphy on the Laurentian continent and have been the subject of study for more t han 30 years. A sequence-stratigraphic analysis of Lower Cambrian Gran d Cycles in the southern Great Basin indicates that the alternating si liciclastic-and carbonate-dominated depositional systems record cyclic al changes in accommodation volume along the Laurentian passive margin . In outer ramp/platform settings, type-1 sequence boundaries coincide with the top of the carbonate half-cycle; in mid-ramp settings, the s equence boundaries lie within the siliciclastic half-cycle. Thick depo sitional sequences contain well-developed lowstand wedge, transgressiv e, and highstand systems tracts. Thinner depositional sequences lack a well-developed lowstand systems tract. At least five land as many as seven) third-order cycles of accommodation change, superimposed on a l ong-term, second-order cycle, are preserved in the Lower Cambrian of t he southern Great Basin. This study, like most studies, supports the v iew that Grand Cycles, and their associated depositional sequences, ar e a product of cyclical eustatic changes in sea level. However, this d oes not rule out the possibility that allogenic forcing resulting from changes in climate also played an important role. Periods of wet/cool climate may have promoted siliciclastic half-cycle development, where as periods of dry/warm climate promoted carbonate half-cycles.