A REVISED BIOSTRATIGRAPHY OF THE UPPER BLACKHILLSIAN STAGE (LOWER ORDOVICIAN, WESTERN LAURENTIA): NEW FAUNAS, NEW ZONES, AND HIGHLY RESOLVED REGIONAL CORRELATION

The uppermost shelly fossil zone of the Laurentian Lower Ordovician (Ibexian Series) has traditionally been recognized as "Zone J" in the lettered scheme devised by Ross and Hintze in the middle part of the last century. It was named the Pseudocybele nasuta Zone in 1997. Although the zone has been widely cited and recognized, the basis for this has been unclear, as the name bearer has not been revised since its original description in 1951, and virtually no work has been done on any of its faunal elements in its type area since 1953. Few of its constituent species are truly interpretable by modern standards, and like many traditional Laurentian trilobite-based zones, usage has depended more on community consensus than rigorous documentation. New collections from the original sections and others in western Laurentia reveal that the interval contains at least three strikingly distinct, stratigraphically successive assemblages which we recognize as three new zones. Pseudocybele itself is repesented in these faunas by at least five different species. All three zones can be recognized in the upper Fillmore Formation and Wah Wah Formation in the southern Confusion Range, western Utah. Other known occurrences of "Zone J" faunas can be directly correlated with one of the newly recognized zones: "Zone J" as represented in the Garden City Formation in northern Utah correlates entirely with the lowermost zone, whereas "Zone J" faunas in the Ely Springs Range of eastern Nevada represent the upper zone. The threefold increase in resolution is similar to that obtained in a recently proposed replacement zonation of the Tulean and lower and middle Blackhillsian stages, and completes a revision of the shelly fossil biostratigraphy of the upper half of the Ibexian Series.