Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Yu-Ying Zhang,Shu Jiang,Zhi-Liang He,Shuang-Fang Lu,Jian-Hua Zhao,Guo-Hui Chen,Dian-Shi Xiao

doi:10.1007/s12182-018-0262-x

Yu-Ying Zhang, Shu Jiang + Show 5 more

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https://doi.org/10.1007/s12182-018-0262-x

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Abstract

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon (TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze (Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio (biogenic molybdenum), Babio (biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nC18–nC25, and odd–even priority values are closed to 1 (0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation: (1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms; (2) high productivity provided the material foundation for organic generation; (3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

Highlights

Mineral components of the lower Cambrian strata in southeastern margin of the Upper Yangtze (SMUY) are dominated by quartz, followed by clay minerals, with a little feldspar, carbonate, pyrite and anhydrite (Table 1)
The SMUY was located in a deep shelf–deep basin environment during the early Cambrian (Steiner et al 2001; Zhu et al 2003) far from the western ancient land (Fig. 1), where the depositional interface was under wave base or storm-wave base
Content of organic matter in the lower Cambrian black shale in SMUY is controlled by the following factors: (1) Quartz in samples of the Niutitang Formation in Longbizui and Yanbei was of biogenic origin indicated by excess Si content, and there is a positive correlation between the content of quartz and total organic carbon (TOC) and a negative correlation between the content of clay minerals and TOC

Summary

Introduction

The early Cambrian recorded substantial changes in global ocean geochemical conditions and biological features compared with late Ediacaran (Knoll and Carroll 1999; Edited by Jie HaoPetroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, ChinaEnergy and Geoscience Institute, University of Utah, Salt Lake City, UT 84102, USAKimura and Watanabe 2001; Wille et al 2008): a rapid large-scale transgression occurred during the early Cambrian resulted in the global ocean transforming to an anoxic environment from an oxic environment in the late Ediacaran (Fike et al 2006; Jiang et al 2009; Babcock et al 2015); a biological event known as the ‘Cambrian Explosion’ happened represented by the abundance and species of fossils increasing abruptly (Brasier 1992; Marshall 2006). The LCBS in Upper Yangtze has been well analyzed, focusing on sedimentary environment assessment (Zhu et al 2003, 2006; Wang et al 2015a), redox conditions (Guo et al 2007; Lehmann et al 2007; Jiang et al 2009; Wang et al 2012, 2015b; Xu et al 2012; Och et al 2013) and hydrothermal activity (Lott et al 1999; Steiner et al 2001; Jiang et al 2006, 2007; Chen et al 2009). The controlling factors of TOC in the LCBS are still unclear, especially the control from multiple conditions in the sedimentary process

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