Global and regional controls on marine redox changes across the Ordovician-Silurian boundary in South China

Abstract

The Ordovician-Silurian (O-S) transition coincided with significant environmental and biological changes. In South China, the Yangtze Platform experienced both global and regional events at this time, including sea-level fluctuations, tectonic movements, volcanic eruptions, mass extinction, and widespread anoxia. The O-S transitional strata of the Yangtze Platform comprise organic-rich black shales that are an important oil source rock. To explore the evolution of watermass chemistry and its relationship to organic matter accumulation, we conducted an integrated Fe-S-C geochemical study (i.e., Fe-speciation, δ 13 C org , δ 34 S py , pyrite-S and TOC) of the O-S boundary sections at Datianba (Chongqing Municipality) and Shuanghe (Sichuan Province). These sections were located in the restricted inner Yangtze Sea, in contrast to the previously studied Wangjiawan section, which was deposited in a more open setting of the outer Yangtze Sea. In contrast to the well-oxygenated conditions at Wangjiawan, the Datianba and Shuanghe sections record persistently anoxic, and episodically euxinic, conditions during the Hirnantian, which can be explained by the unique paleogeography and restricted hydrography of the inner Yangtze Sea. We propose that the regional Kwangsian Orogeny, which was driven by collision of the Yangtze and Cathaysia blocks, played a key role in basin development, watermass chemistry changes, and accumulation of Wufeng Formation black shales within the Yangtze Sea. The more widespread black shales of the Lower Silurian Lungmachi Formation were linked to the post-Hirnantian global marine transgression.

Upper Ordovician continuous lithological succession in outer‐shelf facies, Yangtze Platform, South China: Facies changes and oceanographic reconstruction up to the Late Ordovician Hirnantian glaciation

“…A packstone lens comprising fine‐grained and highly broken bioclastic debris of crinoids, sponge spicules, and brachiopods, with fine sand‐size quartz, occurs interbedded with the graptolitic black shales from the middle of the formation (Figure b). The black shale of the Wufeng Formation was deposited in a dysoxic, stagnant and semi‐closed environment (X. Chen, ; X. Chen et al, ; X. Chen & Qiu, ; Y. Liu et al, ; Mu et al, ; Rong & Chen, ; Yan, Chen, Wang, & Wang, ), and occurs in most parts of the Yangtze epicontinental sea (X. Chen et al, ). Coarse quartz and bioclastic debris within the lenses shown in the Figure are likely of distal origin, derived by gravity flows and finally deposited in deep stagnant and dysoxic marine floor occasionally.…”

“…Biotic and geochemical evidence based on high‐resolution stratigraphic schemes from the South China Block shows that sea‐level fluctuation, climatic shifts, and tectonic movement were key factors in controlling litho‐ and biofacies (e.g. X. Chen et al, ; X. Chen, Rong, Li, & Boucot, ; Cheng & Wang, ; Feng, Yu, Fang, & Bian, ; Y. Li & Kershaw, ; Y. Liu, Li, Algeo, Fan, & Peng, ; Munnecke, Zhang, Liu, & Cheng, ; Rong, ; Rong & Chen, ; Rong, Chen, & Harper, ; Yan, Chen, Wang, & Wang, ; Yang et al, ; T. S. Zhang, Kershaw, Wan, & Lan, ; L. Zhou et al, ).…”

“…Chen, Rong, Li, & Boucot, 2004;Cheng & Wang, 1991;Feng, Yu, Fang, & Bian, 1993;Y. Li & Kershaw, 2003;Y. Liu, Li, Algeo, Fan, & Peng, 2016;Munnecke, Zhang, Liu, & Cheng, 2011;Rong, 1984;Rong & Chen, 1987;Rong, Chen, & Harper, 2002;Yan, Chen, Wang, & Wang, 2010;Yang et al, 2016;T.…”

“…Letters inside photos: B, burrow; Br, brachiopod; Cr, crinoid; G, gastropod; O; ostracod; T, trilobite (X Chen, 1984;X. Chen & Qiu, 1986;Y. Liu et al, 2016;Mu et al, 1981;Rong & Chen, 1987;Yan, Chen, Wang, & Wang, 2012),.…”

Development of Upwelling during the Sedimentary Period of the Organic-Rich Shales in the Wufeng and Longmaxi Formations of the Upper Yangtze Region and Its Impact on Organic Matter Enrichment

“…Previous scholars have conducted a large number of studies on the formation mechanism of this series of organic-rich shales. It is generally believed that the organic-rich shales in the Wufeng and Longmaxi Formations formed under the sedimentary background of an anoxic environment and high productivity [2][3][4][5][6][7][8], and that the semi-closed environment or rapid transgression was the main reason for the anoxic environment [2][3][4][5]. Studies on the cause of high paleo-productivity are scarce in the current literature.…”