Positive carbon isotope excursions: global correlation and genesis in the Middle–Upper Ordovician in the northern Tarim Basin, Northwest China

Stable carbon isotope ratio (δ13Ccarb) analysis has been widely applied to the study of the inter-continental or global marine carbonate correlation. Large-scale Cambrian–Ordovician carbonate platforms were developed in the Tarim Basin. But research on fluctuation characteristics and global correlation of δ13Ccarb is still weak. Based on conodont biostratigraphy and whole-rock δ13Ccarb data in the Tahe oil–gas field of the northern Tarim Basin, the global correlation and genesis of positive carbon isotope excursions in the Darriwilian—Early Katian was examined. Three positive excursions were identified in the Tahe oil–gas field including the middle Darriwilian carbon isotope excursion (MDICE), the Guttenberg carbon isotope excursion (GICE), and a positive excursion within the Pygodus anserinus conodont zone which is named the Early Sandbian carbon isotope excursion (ESICE) in this paper. Furthermore, these positive excursions had no direct relation with sea level fluctuations. MDICE and GICE could be globally correlated. The Middle–Upper Ordovician Saergan Formation source rocks of the Kalpin outcrops were in accordance with the geological time of MDICE and ESICE. GICE had close relationship with the source rock of the Lianglitag Formation in the basin. Massive organic carbon burial was an important factor controlling the genesis of these positive excursions.