CYCLIC DEPOSITIONAL SEQUENCES AND INTERPRETATION ABOUT THEIR ORIGIN ON PENNSYLVANIAN MARINE DEPOSITS FROM EASTERN GREAT BASIN, NEVADA (U.S.A)

Dott (1958) proposed for the first time the existence of cyclic depositional sequences in Pennsylvanian deposits in western North America. The origin of these cycles was related to eustatic sea level changes, establishing an analogy to the current hypothesis to explain the cause of cyclothems in the Midcontinent area. Dott did not present strong evidence to support the existence of these cycles and less about their eustatic origin. Nevertheless, Dott’s idea has a considerable value because any global explanation such as eustatic sea level change for these cyclothems requires describing equivalent cycles in all type of facies. The origin of these cycles has been recently related to two global effects such as variation in Milankovitch parameters and fluctuations in ice volume linked to a glaciation on Gondwana during the Upper Carboniferous (Langhorne and Read, 2000).

Preliminary studies have been developed in the Ely Limestone of the Lower Pennsylvanian of White Pine County, Nevada, to determine the existence of cyclic depositional sequences. The combination of a sedimentological and paleontological approach has resulted in the identification of cycles within the Ely Limestone. These cycles are subtidal carbonate cycles with a shallowing-upward organization developed in a carbonate ramp. Two types of cycles, a shallow to mid-ramp and a deep-ramp, have been identified according to the definitions of Osleger (1991).

To test whether or not these cycles are related to eustatic variations an alternative approach based on geochemistry is proposed. Pristine conodonts (CAI~1-1.5) have been collected at the top of these cycles. If these cycles are eustatic there should a characteristic isotopic signal to compare with standard eustatic curves and geochemical models for 87Sr/86Sr ratios (e.g. Deninson et al., 1994). Additionally, 144Nd/143Nd analyses using conodonts for the timeframe or Nd/Sm using correlation through isochrons might give an absolute age between successive cycles. This technique should show whether these cycles have Milankovitch cyclicity, thus linking eustatic change to climatic change, possibly forced by the fluctuation of ice volume on Gondwana during the Upper Carboniferous.