Upper Middle Cambrian Stratigraphy of Western Utah

Abstract

The Wheeler, Marjum, and lower Weeks Formations of western Utah contain from 1600 to 2000 feet of limestone and shale in one of the thickest, best exposed, and most fossiliferous successions of upper Middle Cambrian rocks in North America. Three laterally interfingering lithofacies belts, viz. inner detrital, middle carbonate, and outer detrital, are recognized. Rocks of the inner and outer belts are characterized by a high content of detrital argillaceous and arenaceous material, whereas rocks of the middle belt are composed of relatively clean carbonates. Lithotope boundaries shifted extensively, but sedimentation appears to have been continuous from middle Middle to early Late Cambrian time. The biostratigraphic boundary between the Bathyuriscus-Elrathina and Bolaspidella Assemblage Zones coincides with the lithostratigraphic boundary between the Swasey Limestone and Wheeler Shale, and the biostratigraphic boundary between the Bolaspidella and Cedaria Assemblage Zones occurs in the lower part of the Weeks Formation. Interpretations based on physical and biological evidence indicate that faunal change at the base of the Bolaspidella Zone resulted from simultaneous widespread shift of lithotopes and biotopes, whereas faunal change at the top of the Bolaspidella Zone resulted from evolution in situ while lithotopes were relatively stationary.