EVOLUTION OF A CARBONATE PLATFORM DURING THE LATE CAMBRIAN IN UTAH AND NEVADA: FROM SUB-AERIAL EXPOSURE TO CYCLIC AGGRADATION AND HIGHSTAND SHEDDING

Upper Paibian-lower Jiangshanian strata exposed in the Great Basin of Utah and Nevada record the early evolution of an upper Cambrian carbonate platform. Using newly measured and described stratigraphic sections and gamma-ray profiles constrained with biostratigraphic correlations, a sequence-stratigraphic model spanning lower Elvinia to Irvingella major and lower Taenicephalus zones was constructed. Analysis of carbon isotopes for this interval is underway, as rocks were deposited shortly following a globally correlated positive carbon-isotope excursion known as the SPICE event. A disconformable paleokarst surface in lower Elvinia zone strata of Utah marks the base of the interval. This surface displays a distinctive shift in gamma-ray values and characteristics of subaerial exposure and is interpreted as a sequence boundary. Overlying this surface, a regionally extensive shale blanket represents siliciclastic input during lowstand of sea-level. As sea level rose, sedimentation switched to aggradation of carbonate cycles recognized in the Sneakover Limestone of Utah and correlative Barton Canyon Member of Nevada. A trilobite biotic crisis known as the Pterocephaliid-Ptychaspid biomere boundary occurs in the middle of this interval and was used as a datum. While cyclic sedimentation continued upward in the Sneakover Limestone and overlying Hellnmaria Formation in nearshore settings, distal areas record deep-water seemingly non-cyclic sedimentation. In Nevada, base-level rise was rapid enough to inhibit carbonate production, forming a drowning unconformity. Once highstand conditions were reached, available accommodation was filled with deposition of cyclic strata in the Hellnmaria in Utah. As the rate of base-level rise decreased with time during highstand, the volume of carbonate sediment exceeded the amount of available accommodation, and sediment was shed to the deep-water environment, where a succession of carbonate turbidites accumulated in the Catlin Member of the Windfall Formation in Nevada. The sequence stratigraphic framework provided here was conservatively correlated to composite global sequences and the surface of sub-aerial exposure potentially correlates with the Sauk II-III sequence boundary and subsequently, a major transgression across Laurentia.