Climate change controls on extreme organic matter enrichment in Late Permian marine–terrestrial transitional shales in Guizhou, south China

Abstract

Black organic-rich shale is an important target for unconventional natural gas exploration. However, compared with marine shale, the mechanism of organic matter enrichment in shale deposited in the marine–continental transitional environment is controversial because of the strong heterogeneity. In this study, an integrated sedimentological and elemental geochemical investigation was conducted for the dark shales in the Lopingian coal-bearing strata of western Guizhou, South China. This study aims to clarify the detailed control mechanism of major geological events on the organic matter accumulation in the transitional facies. Sedimentological observations show that the Lopingian in the Liupanshui area is mainly composed of coal, tight sandstones, and dark mudstones, which were deposited in the tidal flat-lagoon and fluvial delta. Petrological and geochemical evidence suggests the provenance was mainly resulted from the Emeishan basalt weathering in the west of the study area. In addition, the geochemical proxies indicate that the Longtan shales are characterized by low primary productivity, oxic–suboxic bottom water, and a relatively low depositional rate. The paleoclimate index indicates that the climate was predominantly warm and humid with intense chemical weathering during the Lopingian period, and changed to an extremely hot climate in the Early Triassic. Combined with the previous research, this study proposes that the Large Igneous Province (LIP) played a critical role on the organic matter enrichment of the Lopingian across the western Yangtze Block. Firstly, the onset of Emeishan LIP created an essential tectonic background and paleogeographic pattern for massive terrigenous debris to gather here from the west. Secondly, the climate was significantly affected due to the continuous weathering of the Emeishan basalt plateau and the greenhouse gases emission from the Siberian LIP. During the Lopingian, the plants flourished under the tropical rainforest climate and supplied the terrigenous organic matter. Accompanying chemical weathering increased, massive reworked coal fragments were brought into the brackish tidal flat-lagoon environment. Particularly, global warming related to the Siberian LIP and regional transgression occurred in the late Lopingian, forming the black shales with the highest organic matter content (up to 26.39%) of member Ш of the Longtan Formation. However, both the production and supplementation of organic matter significantly decreased because of the end-Permian mass extinction, thus stopped the organic matter enrichment in the Early Triassic Feixianguan Formation.