Yuefeng Shen , Fritz Neuweiler, Adrian Immenhauser

Bulk-rock based carbon-oxygen chemostratigraphy should be combined with a detailed understanding of depositional facies (mineralogy, porosity), its 2D-chronostratigraphic architecture, and diagenesis. The Ordovician of the western Tarim Basin recorded a peculiar litho-biostratigraphic succession. The Darriwilian Yijianfang Formation formed part of a carbonate ramp dominated by filter feeders. Toward its top, there is condensation succeeded by a multi-Myr hiatus. The hiatus correlative succession is a black-shale (Darriwilian to early Sandbian Saergan Formation) preserved in slope-to-basin settings. A marine red-bed interval (Sandbian Tumuxiuke Formation) diachronously succeeded toward a more basin-wide record. Finally, the late Sandbian to Katian Lianglitag Formation re-established a shallow-water carbonate factory (ramp-to-platform), but this time being highly productive and hosting a diversifying assemblage of benthic primary producers. By exploring diagenesis associated with a first component-specific data-set of δ13C - δ18O values and by integrating and filtering respective literature bulk-rock data, a synoptic chemo-chronostratigraphic sequence is presented. It displays segments lasting from tens of Myrs to several 100 kyrs. There is a long-term trend of increasing δ13C values culminating in the early Katian at 3.2 ‰ followed by a steady decrease. This tipping point is associated with a precursory baseline shift that interferes with the short-term Guttenberg carbon-isotope excursion presumably associated with a positive shift of δ18O values. The baseline shift was driven by regional effects of photosynthesis and a boosting (dasycladacean-related) production of aragonite along the Sandbian-Katian boundary interval. There is a medium-term (Darriwilian to earliest Sandbian) negative δ13C excursion coinciding with both a basal positive δ18O excursion and the demise of the Darriwilian carbonate ramp (Suecicus-Event, new term). It might represent the effects of volcanism/SO2-outgassing during the switch from a passive to an active continental arc. This event masquerades the elsewhere recorded middle Darriwilian carbon-isotope excursion. Caution is needed to consider the Tarim realm for global Ordovician chemostratigraphy.