CONODONT SEQUENCE BIOSTRATIGRAPY OF PERMO-CARBONIFEROUS BOUNDARY STRATA IN THE EAST-CENTRAL ANTLER FORELAND BASIN, WESTERN UTAH

Middle Pennsylvanian through Early Permian strata of western Utah comprise 11 depositional sequences assigned to the upper Ely Limestone (EU-1) and Mormon Gap Formation (MG-1 through MG-10). Microfacies and cycle stacking patterns indicate that the Ely shelf (east-central Antler Foreland) was continuously flooded during deposition of the upper Ely and upper Mormon Gap Formations, but was only intermittently submerged during deposition of the intervening lower Mormon Gap Formation. The upper Ely and upper Mormon Gap Formations are comprised of regionally extensive, shoaling upward limestone parasequences containing little evidence of subaerial exposure. By contrast, lower Mormon Gap cycles are comprised of restricted marine dolomudstones capped by photozoan skeletal limestones. Sequences are thin and regionally discontinuous. Exposure features (breccias, oxidized surfaces, blackened crusts) are common on cycle tops.

Conodonts and fusulinids permit series-level age assignments of selected depositional sequences. The upper Ely Limestone (sequence EU-1) is given a middle to late middle Desmoinesian age based upon the occurrence of Wedekindellina euthysepta and Idiognathodus amplificus. The presence of Idiognathodus nodocarinatus indicates a latest Desmoinesian age for the overlying sequence MG-1. High-frequency sequences MG-3 and MG-4 yield specimens of Virgilian Streptognathodus pawhuskaensis and Streptognathodus brownvillensis?, respectively. Sequence MG-9 is assigned to the early Asselian stage on the basis of Streptognathodus cristellaris, Streptognathodus constrictus, and Sweetognathus sp. A hiatus representing much of the Sakmarian separates MG-9 from the overlying Artinskian sequence MG-10, which contains Sweetognathus whitei, Sweetognathus bucaramangus, and Mesogondolella bisselli.

Conodont sequence biostratigraphy of the Ely Limestone and Mormon Gap Formations provides a detailed framework for understanding the stratigraphic and depositional history of the eastern Antler Foreland basin and for correlating western Utah sequences to the global sea-level curve.