Article: Late Ordovician graptolites from the North American Midcontinent
Daniel Goldman and Stig M. Bergström
Although relatively uncommon in the dominantly calcareous upper Ordovician (Cincinnatian) sedimentary rocks in the North American Midcontinent, graptolites have been recorded from many localities during some 150 years of study. A revision of these faunas, the first general review for half a century, is based on new collections from about 20 key localities as well as on museum specimens. These epicontinental faunas are locally rich in individuals but diversity is low, with only about 20 species recognized herein. Widely recorded, but poorly known and commonly misidentified species, such as Amplexograptus manitoulinensis (Caley), Climacograptus putillus (Hall) and Rectograptus peosta (Hall), are redescribed based on type and topotype material. Some stratigraphically important species, such as Climacograptus nevadensis Carter, Dicellograptus complanatus Lapworth, and D. gravis Keble and Harris, previously unknown from the Midcontinent, are also described along with a few other, more common and well-known species. Several Cincinnatian species are shown to have a previously unrecognized biostratigraphical utility. The graptolite zonal succession recognized by Riva in New York-Quebec can be applied readily to many of the sections studied, and a bipartite subdivision of the A. manitoulinensis Biozone appears feasible. Conodonts make it possible not only to classify the sections in terms of Atlantic conodont zones but also to establish ties between graptolite and conodont zones. Most Midcontinent graptolite faunas represent the endemic Laurentian Biofacies that differs significantly from the cosmopolitan Oceanic Biofacies of marginal areas of Laurentia. Overlap between these in Oklahoma allows correlation between separate graptolite zonal schemes. The new biostratigraphical data are used for regional correlations and for assessment of sequence stratigraphy.