Main factors controlling the organic matter enrichment in the Lower Cambrian sediments of the Sichuan Basin, SW China

Abstract

The Lower Cambrian Qiongzhusi Formation contains more than 200 m of marine sediments mainly consisting of mudstone and black shales, which are widely accepted as the most important hydrocarbon source rocks of the Ediacaran–Cambrian giant gas field in the Sichuan Basin. The geochemical compositions of biomarkers and trace elements obtained from black shales at the bottom of the Qiongzhusi Formation were used to reconstruct the palaeoproductivity and palaeoenvironment and to determine the main factors controlling the deposition and preservation of this organic‐rich shales. Based on varieties of steroids and hopanoids compositions, multifarious palaeobios such as pelagophyceae, dinoflagellate, diatom (or its ancestor), sponges, and aerobic/anaerobic bacteria have been proved to evolve in the Early Cambrian in the Sichuan Basin. In addition, a relative high concentration of Cubio compared with modern oceanic palaeoproductivity proxies indicate a moderate to relatively high palaeoproductivity, which is favourable for the organic matter accumulation of this source rocks. Moreover, the evidence from biomarkers and trace element ratios suggest that the Lower Cambrian sediments were deposited under a non‐sulphurized anoxic condition, which provided a favourable preservation condition. The covariation among Porg, Cubio, and TOC shows that the palaeoproductivity is the main controlling factor for the high organic content of the black shales of the Qiongzhusi Formation.

Geochemical Characteristics and Their Marine Environmental and Organic Source Implications for the Lower Cambrian Shales in Guizhou Province, South China

“…The TOC contents > 2 % are mainly within depth ranges of 290 m to 360 m, and the Є1p shales display a higher average TOC content than the Є1n shales. The TOC values in this study are comparable to those of the Є1q shales (0.87-7.21%) in the Sichuan Basin [7].…”

“…But there is no consensus on the controlling factors of organic matter enrichment in this region. The primary reasons include (1) spatiotemporal differences caused by paleogeographic, paleoclimatic, and eustatic sea-level variations [6,7,14,20,21] and (2) the lack of comprehensive studies on organic and inorganic geochemical evidence [27]. Thus, the reconstruction of primary productivity and preservation conditions on the basis of multidisciplinary evidence in different regions of South China is critical for understanding the formation of organic-rich black shales in the Early Cambrian.…”

“…The Early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes, i.e., "Cambrian Explosion" [1,2]. During this critical time interval, organic-rich black shale was deposited during global sealevel rise and oceanic anoxic events [3][4][5][6][7]. The formation of organic-rich black shales is a complicated issue due to the interplay of various atmospheric, oceanographic, and biological factors and therefore has been an important research target for understanding life evolution and geochemical conditions of the Early Cambrian ocean.…”

“…They concluded that anoxic-ferruginous and euxinic bottom waters with moderately strong restriction resulted in high production and good preservation of organic matter. On the other hand, Wang et al [7] proposed that paleoproductivity was the main factor controlling the high organic content of the Є1q black shales in the Sichuan Basin due to the covariation among P org , Cu bio , and TOC. Liu et al [22] also proposed that productivity was the dominant factor controlling organic matter preservation within the black shales of South China.…”

Depositional Environment and Organic Matter Enrichment of Early Cambrian Niutitang Black Shales in the Upper Yangtze Region, China

“…Taking the Early Cambrian Niutitang black shale as an example, some researchers have reported that an anoxic paleoenvironment is the most significant factor affecting organic matter accumulation [27,28], while other researchers argue that a high productivity is the most important factor [14,29]. Still, others have concluded that the organic matter preserved in this black shale was accumulated by a combination of ideal preservation conditions and a high productivity [30,31]. It is worth noting that during the deposition of the Early Cambrian Niutitang black shale, the Upper Yangtze Region mainly included the intraplatform basin, slope, and deep basin settings [32], and the redox conditions, hydrothermal activity, and amount of terrigenous flux in these settings were different [6,26,33,34].…”