Discovery of Cryogenian interglacial source rocks in the northern Tarim, NW China: Implications for Neoproterozoic paleoclimatic reconstructions and hydrocarbon exploration

Abstract The Neoproterozoic Earth witnessed major global glaciation events with significant impact on paleoclimate and life evolution. The Tarim Craton in China preserves the records of four glaciation events during the Neoproterozoic which were correlated with the global glaciations, the nature and impact of these with respect to Neoproterozoic paleoclimatic–paleogeographic reconstructions remain unresolved. Here we report the discovery of a suite of source rocks from northeastern Tarim in which the strata formed during 655–635 Ma, corresponding to the interglacial period between the Sturtian and Marinoan diamictites. These source rocks are dominated by black shales and mudstones of up to 300 m thickness, and are characterized by high content of organic matter with TOC (total organic carbon) of 0.46%–3.5% (average 1.64%), vitrinite reflectance Ro of 1.28%–1.60%, and kerogen carbon isotope δ13C value between −28.58‰ and −31.89‰. Biomarker compounds indicate that the organic matter in these saprolite source rocks are made up of microorganisms such as algae and bacteria. The Pr and Ph values indicate a weak reducing–oxidizing environment, and most values of CIA (Chemical Index of Alteration) are >68, suggesting an interglacial temperate paleoclimate. The La/Th–Hf and Co/Th–La/Sc relationship suggests that the provenance of these rocks is mainly mixed felsic/mafic rocks. In the Tarim basin, these source rocks comprise an area of up to 90,000 km2 within Cryogenian rifts as inferred from seismic reflection profiles. Based on zircon U Pb ages of volcanic rocks underlying the shale units, it is inferred the source rock formed during the temperate Sturtian glaciation events with subsequent extensive biotic recovery and high productivity.