02511nas a2200229 4500008004100000245015100041210006900192300001200261490000700273520179100280653001502071653001302086653001402099653001902113653002002132653001802152653000902170100001602179700002002195700001902215856004702234 2011 eng d00aThe Lawsonian Stage - the Eoconodontus Notchpeakensis FAD and HERB Carbon Isotope Excursion Define a Globally Correlatable Terminal Cambrian Stage0 aLawsonian Stage the Eoconodontus Notchpeakensis FAD and HERB Car a621-6400 v863 a

The best definition for the base of the terminal Cambrian (Stage 10) is the conodont Eoconodontus notchpeakensis FAD ± onset of the HERB carbon isotope excursion. These horizons allow precise intercontinental correlations in deep marine to peritidal facies. The agnostoid Lotagnostus americanus (Billings, 1860) FAD has been suggested as a Stage 10 base, but restudy of types and typotypes shows that the species occurs only in Late Cambrian (Sunwaptan) debris flow boulders in Quebec (Westrop et al., this volume). Non-Quebec reports of "L. americanus" are an amalgum of small samples of often poorly documented specimens with effaced–highly furrowed cephala and pygidia and with or without a highly trisected pygidial posteroaxis. Many of these occurrences have local species names, but no evidence suggests that they record intraspecific variation of a globally distributed taxon. They are not synonyms of L. americanus. Lotagnostus, largely a dysoxic form, does not allow precise correlation into oxygenated platform facies. Another proposal used the conodont Cordylodus andresi FAD as a Stage 10 base, but other work shows this FAD is diachronous. An unrealistic approach to L. americanus’ systematics and the correlation uncertainty of C. andresi are overcome by defining a Stage 10 base at the globally recognizable E. notchpeakensis FAD, with the C. andresi FAD a useful proxy on cool-water continents. The "Lawsonian Stage", named for Lawson Cove in western Utah, has a basal GSSP at the E. notchpeakensis FAD and replaces informal Stage 10. The Lawsonian, ~150 m-thick in western Utah, underlies the basal Ordovician Iapetognathus Zone.

10aagnostoids10aCambrian10aconodonts10aHERB excursion10aLawsonian Stage10aUnited States10aUtah1 aLanding, E.1 aWestrop, S., R.1 aAdrian, J., M. uhttp://dx.doi.org/10.3140/bull.geosci.1251