Ordovician conodont biostratigraphy, diversity and biogeography in deep-water radiolarian cherts from Kazakhstan

Abstract

Studies of conodonts from the ribbon-banded radiolarian cherts and siliceous siltstones in the Burubaital Formation in the Buruntau area (SW Balkhash region, Kazakhstan) made it possible to characterize the conodont fauna from deep-water environments in the Ordovician. The conodont assemblages from chert successions at times of high diversity document at least 53 taxa (39 species and 14 taxa identified in open nomenclature) for the entire Ordovician. Conodont biodiversity in the Floian and lower Darriwilian is comparable to the diversity of conodonts inhabiting the neritic zone of epicontinental basins and continental shelves. In the Dapingian and from the middle Darriwilian to the end of the Ordovician, the taxonomic diversity of deep-water conodonts in Kazakhstan is lower and comprises no more than five–six species in total. Stratigraphic distribution of conodonts in the Burubaital Formation sections along with their distribution in the previously studied chert localities in Kazakhstan were used as a base to estimate the succession of conodont zones in deep-water siliceous deposits. The resulting conodont zonation is based on cosmopolitan conodont species and allows broad correlation with deposits of several other paleocontinents.The pelagic habitat of the Early Ordovician oceans is subdivided into two latitudinal biogeographical units - tropical, recorded in Siberia, Laurentia and adjacent terranes, and temperate to cold climate zone, recorded in Baltoscandia, Avalonia and Western Gondwana. The low gradient between the climate of high and low latitudes confirms that the climate in the Early Ordovician was warmer than in the Middle Ordovician.