Mechanism of organic matter accumulation in black shale of the Datangpo Formation: Insights from paleo-environmental variation during the Cryogenian non-glaciation

Abstract

Mechanism of organic matter (OM) accumulation in the Cryogenian Datangpo Formation of the Nanhua Basin is poorly understood, restricting shale-gas exploration in these non-glacial deposits. To understand mechanism of OM accumulation, we carried out high-resolution analyses of geological and geochemical data from shales of the lower three members (I.II.III) of the Datangpo Formation across the basin. The results show that OM distribution in the basin is heterogeneous: (1) stratigraphically, total organic carbon (TOC) concentrations vary from moderate values in Member I, peaks in Member II, to lowest values in Member III; and (2) spatially, TOC concentrations in Member I increase with increasing depositional water depths, whereas they are high and low throughout Member II and Member III across the basin, respectively. The heterogeneity of OM accumulation in shales is interpreted to be controlled by complex paleo-environmental variations in water-mass exchange, paleoclimate, seawater redox state, and hydrothermal activities. Specifically, (1) ferruginous-euxinic conditions, intensive hydrothermal activities, and pronounced water-mass exchange between the basin and the open ocean favored OM accumulation in Member I; (2) expanded euxinic conditions, dramatic chemical weathering, and reduced water-mass exchange resulted in OM enrichment in Member II; and (3) less extensive anoxic conditions, reduced chemical weathering, and stagnant water-mass exchange led to OM depletion in Member III. This integrated conceptual model for OM accumulation in shales of the Datangpo Formation shows the influences of both primary productivity and redox conditions in the Nanhua Basin.