Depositional Sequence Stratigraphy of Lower Cambrian Grand Cycles, Southern Great Basin, U.S.A.

Abstract

The Lower Cambrian of the southern Great Basin records third-order cyclical alternation between siliciclastic- and carbonate-dominated sedimentation in a rapidly subsiding passive margin. These alternations, termed Grand Cycles, are a prominent feature of Cambrian stratigraphy on the Laurentian continent and have been the subject of study for more than 30 years. A sequence-stratigraphic analysis of Lower Cambrian Grand Cycles in the southern Great Basin indicates that the alternating siliciclastic- and carbonate-dominated depositional systems record cyclical 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 sequence boundaries lie within the siliciclastic half-cycle. Thick depositional sequences contain well-developed lowstand wedge, transgressive, and highstand systems tracts. Thinner depositional sequences lack a well-developed lowstand systems tract. At least five (and as many as seven) third-order cycles of accommodation change, superimposed on a long-term, second-order cycle, are preserved in the Lower Cambrian of the southern Great Basin. This study, like most studies, supports the view that Grand Cycles, and their associated depositional sequences, are a product of cyclical eustatic changes in sea level. However, this does 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, whereas periods of dry/warm climate promoted carbonate half-cycles.