236 / 2022-03-23 17:04:25

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

Ordovician-Silurian transition,black shale,sedimentary difference,paleotectonism,paleoclimate,sea-level change,Yangtze area

During the Ordovician–Silurian (O–S) transition, the Yangtze area experienced significant geological environmental changes, which have been recorded by the widely distributed black shale of the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations. However, the geochemical and petrological analyses of the O–S black shale in the well JY1 (eastern Sichuan) and the well YY2 (western Hubei) drill cores show that there are some significant differences in the lithology, thickness, and geochemical characteristics of the O–S black shale despite the long-term similar variation trend in petrology and geochemistry, which may suggest some major geological environment changes and their differential influence on black shale deposition in the Yangtze area. Based on the analysis of lithology, total organic carbon (TOC), Mo_EF, and U_EF, four long-term sedimentary cycles have been identified in the Well JY1(one for Wufeng Formation and three for Longmaxi Formation), whereas only three long-term sedimentary cycles can be recognized in the Well YY2 (one for Wufeng Formation and two for Longmaxi Formation). Similar geochemical and petrological variations of the Late Ordovician Wufeng Formation in JY1 and YY2 wells indicated that the two studied areas may experience similar environment changes and no significant differences in paleogeomorphology were noted during the Late Ordovician. Different from the gradual increase of the geochemical proxies (TOC, Mo_EF, Ni_EF, etc.) followed by an abrupt decrease in the Wufeng Formation, these proxies of the Silurian sedimentary cycles in both JY1 and YY2 wells exhibit an abrupt increase at the base followed by a gradual decrease, showing that controlling mechanisms for Silurian black shale deposition in the study areas are different from that for Late Ordovician black shale. However, the black shale in each Silurian cycle in JY1 has a much larger thickness than that in YY2. Compared with the thick silty shale developed in JY1 during the Rhuddanian–Aeronian transition, there is a stratigraphic gap in YY2 except for the thin calcareous dolostone formed in a strongly restricted environment. These differences indicate that paleotectonism and glacial eustasy should be the key factors for black shale development, but they have varying influences on the deposition of the Silurian black shale in the eastern Sichuan and western Hubei. Since the Early Rhuddanian stage, continuous tectonic subsidence in the eastern Sichuan promoted the development of thick black shale, while a persistent tectonic uplift in western Hubei led to the formation of thin black shale. Meanwhile, the study area experiences a significant sea-level drop related to intensified glaciation during the Rhuddanian–Aeronian transition, which is recorded by low chemical index of alteration (CIA) values in YY2 and JY1 wells. This resulted in the formation of a stratigraphic gap in the western Hubei and thick silty shale in the eastern Sichuan. Moreover, abrupt increases in CIA, TOC, Mo_EF, and Ni_EF observed at the lower part of the Aeronian shale in the two studied wells suggest that the eastern Sichuan and western Hubei experienced a significant sea level rise during the Early Aeronian stage, which may be related to global warming event. Subsequently, the eastern Sichuan and western Hubei both suffered a significant tectonic uplift related to Kwangsian Movement leading to the termination of Aeronian black shale. However, the thickness variations of the Aeronian black shale in the two study areas indicate that remarkable paleogeomorphic differences in the eastern Sichuan and western Hubei were persistent until the Middle Aeronian stage. Thus, this study not only reveals the differences of the O–S black shale in the eastern Sichuan and western Hubei but also interprets the formation mechanism of these differences, which provide new data and perspectives to understand global paleoclimate and sea-level changes during the O–S transition.