The Ordovician Conodonts And The Cambrian-Ordovician Boundary At The Platform Facies In The Tarim Basin, China

The collection for conodont samples of the Ordovician was carried out in the platform facies region of the Tarim Basin. Sixteen outcrop sections were sampled systematically and 29 well drillings were collected at the important intervals. More than 600 samples were acidized, and thousands specimens were obtained. Forty eight characteristic species belonging to 27 genera are described herein, and 4 new conodont species are established. Based on the conodont data, the evolutionary sequence of the Ordovician conodonts in the platform facies of the Tarim Basin was cleaned, one assemblage and 7 biozones are revised. After the cleaning, there are 1 assemblage and 20 biozones across the Ordovician in ascending order in the platform facies region of the Tarim Basin. Furthermore, these 21 biostratigraphic units can be directly correlated with the two major conodont biogeographic provinces (North Atlantic biogeographic province and North America Mid-continent biogeographic province) in China and all around the world, which may make the Tarim Basin, be a bridge of direct correlation between the two major biogeographic provinces.Four phases of the biogeographic attribution can be divided in the Ordovician period,â‘ the phase of pan-earth genera in the Tremadocian age,â‘¡the phase of mixture by South China and North China genera from the Floian to Dawanian age,â‘¢the phase of South China type from the Darriwillian to the middle Sandbian age,â‘£the phase of North China type from the late Sandbian to the Hirnantian age. In this paper, a detailed comparison is made between the four phases in the Tarim Basin and the corresponding faunas in South China and North China. After analyzing the paleogeographic reconstruction and the paleo Tethys Ocean flow surrounding the Tarim plate, a point was made that the alteration of conodont faunas of the Ordovician on the Tarim plate was jointly caused by the plate tectonics and ocean currents. It deepens the previous view that the alteration of conodont faunas was controlled by water depth and water temperature.Based on the obtained biostratigraphical results, carbon isotope stratigraphy was carried out in Kalpin region. Biostratigraphically constrained sequences at the Wushi Yingshan and Kalpin Cement Plant sections (Kalpin Region) were densely sampled for geochemical studies. Carbonates across the Cambrian-Ordovician boundary of both sections mainly record negative carbon isotope values. Stable isotope curves show four negative and four positive excursions appearing alternately at the Wushi Yingshan section and five negative alternating with five positive excursions at the Kalpin Cement Plant section. The carbon isotope logs of these two sections are correlated with the international Cambrian-Ordovician boundary key sections:â‘ Dayangcha section in China,â‘¡Green Point section (GSSP) in Canada,â‘¢Black Mountain section in Australia andâ‘£Lowson Cove section in USA. These correlations suggest that the Cambrian-Ordovician boundary of the Wushi Yingshan section and the Kalpin Cement Plant section can be placed within a particular horizon that also corresponds to the observed biostratigraphic units.