Middle-Upper Cambrian Boundary in North America

Abstract

The position of the Middle-Upper Cambrian boundary in the North American faunal province has been uncertain because of inadequate criteria for correlation with type-Cambrian rocks. Various proposals place the boundary as high as the middle of the Conaspis Zone and as low as the base of the Cedaria Zone. Newly described agnostid trilobites from Utah are identical or closely related to species that occur in the Acado-Baltic faunal province in Scandinavia, which contains the generally accepted reference section for the Middle-Upper Cambrian boundary. These agnostids are associated with a large assemblage of nonagnostid trilobites typical of the North American faunal province. Comparisons based on occurrence of identical species, relative generic range zones, and level of evolutionary development among agnostid genera indicate that the Bolaspidella Zone of Utah correlates with the interval that includes the Ptychagnostus atavus to Lejopyge laevigata Zones of the Paradoxides Series of Sweden. The Bolaspidella Zone is therefore the uppermost zone of Middle Cambrian age in North America. This paper proposes that the Middle-Upper Cambrian boundary in North America be stabilized between rocks that contain faunas of the Bolaspidella and Cedaria Zones.