Conodonts as biostratigraphic tools for redefinition and correlation of the Cambrian–Ordovician Boundary

Abstract

AbstractThe Cambrian–Ordovician Boundary is recognized at different horizons on various continents by utilizing several fossil groups. Conodonts are abundant, diverse, and less provincial and facies-controlled than other fossils in this interval; many species are widespread and some are cosmopolitan. Strata representing the Cambrian–Ordovician Boundary interval from Asia, Australia, and North America can be correlated easily when those strata are from cratonal or shelf environments. Strata from slope facies have fewer conodonts, fewer taxa, and taxa may have different ranges compared with cratonal and shelf facies. It may be easier to correlate intercontinentally within cratonal/shelf facies than to correlate intracontinentally from cratonal/shelf facies to slope facies.A new hierarchical arrangement of conodont interval zones and subzones is proposed for western North America. These include theProconodontus tenuiserratusZone, theP. posterocostatusZone (new), theP. muelleriZone (new), theEoconodontusZone (new, withEoconodontus notchpeakensisandCambrooistodus minutusSubzones), theCordylodus proavusZone (emended, withHirsutodontus hirsutus, Fryxellodontus inornatus, andClavohamulus elongatusSubzones), theCordylodus intermediusZone (new, withHirsutodontus simplexandClavohamulus hintzeiSubzones), theCordylodus lindstromiZone (new), andCordylodus angulatusZone (new).The boundary point for the base of the Ordovician System will be chosen so as to coincide with a correlatable conodont zonal boundary; other fossil groups will support correlation of this boundary point. Three alternative horizons are being considered. The base of theCordylodus proavusZone is the most distinctive but is regarded by some as older than is appropriate for the base of the Ordovician. The base of theCordylodus intermediusZone is recognizable by faunal changes in several evolutionary lineages at a level that is slightly younger than the presently recognized boundary in Australia, North America, and parts of Asia but slightly older than the base of the Tremadoc Series of Europe. The base of theC. lindstromiZone can be recognized in most areas by the lowest occurrence of the nominate species, which is of questioned taxonomic validity and uncertain biostratigraphic utility; this horizon is closest to the base of the Tremadoc Series of Europe.Choice of a stratotype section has been narrowed to the Cow Head Group in Newfoundland, eastern Canada; and to the Fengshan and Yehli formations in Jilin Province, northeastern China. The Cow Head Group was deposited on and near the base of the continental slope. Erosion by debrisslide breccias resulted in a significant hiatus near the boundary interval in the Broom Point sections, a condition contrary to guidelines for choice of boundary sections. This may be less of a problem in other sections, such as Green Point. The Dayangcha section in China was deposited on the outer part of a continental shelf and has acritarchs, conodonts, graptolites and trilobites. Additional study of the China section is needed to document fully the ranges of critical conodont taxa.