NEW UPPER MARJUMAN AND LOWERMOST STEPTOEAN (UPPER CAMBRIAN) BIOSTRATIGRAPHIC SCHEME FOR THE EASTERN GREAT BASIN, NEVADA AND UTAH

The Cedaria and Crepicephalus Zones of the upper Marjuman Stage have been widely used on the Laurentian craton, and they can also be correlated globally. Nevertheless, there have been few attempts to refine these zones since their establishment in the late 1950s.

Newly collected range data and new species from eastern Nevada and western Utah provide an opportunity to divide the uppermost Marjuman Crepicephalus Zone into two species-based zones. In addition, two new zones can be recognized for the lowermost Steptoean Stage. In ascending order, the new zones are the Kingstonia greatbasinensis-Coosina ariston, Tricrepicephalus beethoveni-Meteoraspis walcotti, Coosella paraperplexa and Aphelaspis hui Zones.

Both the K. greatbasinensis-C. ariston and T. beethoveni-M. walcotti Zones are characterized by the first appearance of more than 25 species and 10 species respectively, although many of these are inadequately known and are placed in open nomenclature. The C. paraperplexa Zones is defined by the first occurrence of the name bearer, but the occurrence of Cheilocephalus pattersonensis is also important in recognizing this zone.

The Marjuman-Steptoean extinction boundary (the base of the Glyptagnostus reticulatus Zone in northwest Canada, Queensland, China, Kazakhstan, and Siberia, and the base of the Aphelaspis Zone in southwest Canada, central Texas, upper Mississippi Valley, and southern Appalachians) can be correlated globally with its distinctive trilobite faunal turnover event. A stratigraphic position immediately above the Marjuman-Steptoean extinction and morphologic similarity between Coosella paraperplexa and C. perplexa make them the best reference for correlation with other Laurentian sections.