Petrological and geochemical characteristics of the Ordovician–Silurian black shale in eastern Sichuan and western Hubei, South China: Differential sedimentary responses to tectonism and glaciation

Abstract

During the Ordovician–Silurian transition period, the Yangtze area experienced significant environmental changes, which were recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations. However, the petrological and geochemical analyses of the Upper Ordovician-Lower Silurian black shale in Well JY1 (eastern Sichuan Basin) and Well YY2 (western Hubei Province) drill cores show that there are some significant differences with respect to lithology, thickness, and geochemical characteristics despite the long-term similar variation trend in petrology and geochemistry. This may suggest some major environmental changes and their differential influence on black shale deposition in the Yangtze area. Based on the analyses of lithology, total organic carbon (TOC), Mo_EF, and U_EF, four long-term sedimentary cycles (one for the Wufeng Formation and three for the Longmaxi Formation) were identified in Well JY1. In Well YY2, only three long-term sedimentary cycles can be recognized (one for the Wufeng Formation and two for the Longmaxi Formation). Different from the gradual increase of the geochemical proxies (such as TOC, Mo_EF, and Ni_EF) followed by an abrupt decrease in the Wufeng Formation, these proxies of the Silurian sedimentary cycles in both wells exhibit an abrupt increase at the base followed by a gradual decrease, showing that controlling mechanisms for the Lower Silurian black shale deposition in the study areas were different from those of the Upper Ordovician black shale. However, the black shale of each Silurian cycle in Well JY1 is much thicker than that in Well YY2. Compared with the thick silty mudstone developed in Well JY1 during the Rhuddanian–Aeronian transition, there is a stratigraphic gap in Well YY2 except for a thin calcareous dolostone formed in a strongly restricted environment. These differences indicate that tectonism and glacial eustasy should be the key factors for black shale development, but their influences on the deposition of the Lower Silurian black shale varied between eastern Sichuan Basin and western Hubei province. Since the early Rhuddanian stage, continuous tectonic subsidence in eastern Sichuan Basin promoted the development of thick black shale, while a persistent tectonic uplift in western Hubei Province led to the formation of thin black shale. Meanwhile, the study areas experienced a significant sea-level drop related to intensified glaciation during the Rhuddanian–Aeronian transition, which is recorded by the low chemical index of alteration (CIA) values in both wells. This resulted in the formation of a stratigraphic gap in western Hubei Province and the deposition of thick silty mudstone in eastern Sichuan Basin. Moreover, abrupt increases in CIA, TOC, Mo_EF, and Ni_EF observed in the lower part of the Aeronian black shale in the two wells suggest that eastern Sichuan Basin and western Hubei Province experienced a significant sea-level rise during the early Aeronian stage, which may be related to a global warming event. Subsequently, eastern Sichuan Basin and western Hubei Province both suffered a significant tectonic uplift related to the Kwangsian Movement leading to the termination of the Aeronian black shale formation. However, the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable geomorphological differences persisted in these two areas until the middle Aeronian stage. Thus, this study not only reveals differences of the Upper Ordovician-Lower Silurian black shale in eastern Sichuan Basin and western Hubei Province, but also interprets the causes of these differences, which provide new data and perspectives for the understanding of global paleoclimate and sea-level changes during the Ordovician–Silurian transition.