FORMATION OF CARBONATE CYCLES AND STACKING PATTERNS IN THE LATE CAMBRIAN CARBONATE PLATFORM, CENTRAL NEVADA AND WESTERN UTAH

Several mechanisms such as allocyclic, autocyclic, combined allocyclic and autocyclic progradation models, and random facies models have been proposed to explain the formation of peritidal carbonate cycles and their temporal stacking patterns. However, allocyclic models remain the most prominent idea in carbonate cyclo and sequence stratigraphic interpretation. This idea is problematic for carbonate successions that developed during supergreenhouse times, such as in the Late Cambrian when levels of atmospheric C0₂ were noticeably the highest (~4000-7000ppm; Berner, 2001) compared to the other Phanerozoic time intervals. Such high atmospheric CO₂ would have prevented the development of polar ice sheet, thus limiting and minimizing glacio-eustatic sea-level fluctuations. Five main sections in central Nevada and western Utah were logged to reveal the cycle types, stacking patterns, facies associations and possible forming mechanisms of the Late Cambrian carbonate successions. Peritidal to shallow subtidal cycles dominate the cycle types in central Nevada sections, where as deep subtidal and peritidal cycles occur in the lower and upper part of the western Utah section, respectively. Carbonate successions in Shingle Pass sections, central Nevada consist of peritidal carbonate complex, shallow subtidal, and sand shoal facies associations that are laterally discontinuous and have different cycles stacking from section to section. Cycles stacking in the lower part of central Nevada sections are different regionally with cycles stacking in the Orr Ridge section, western Utah, meanwhile cycles stacking are similar and correlatable regionally at the upper part of both Nevada and Utah sections. Those stacking of cycles comprise of peritidal and shallow subtidal cycles. These findings indicate that the lower part of the Late Cambrian carbonate successions were deposited in two different settings : shallow water platform setting for central Nevada section and deep ramp setting for western Utah section which was bounded by a steep trough in between these two sections. It is also interpreted that interactions of post-rift thermal subsidence, local tectonic activities and carbonate production rates were the major control of carbonate cycles and stacking patterns formation in this successions.