Multiple controlling factors of lower Palaeozoic organic-rich marine shales in the Sichuan Basin, China: Evidence from minerals and trace elements

Abstract

Lower Palaeozoic organic-rich shales are widely distributed in southern China. In this study, the organic matter contents of Lower Cambrian, Upper Ordovician, and Lower Silurian shales were analysed, and a high degree of vertical heterogeneity was found. In the studied sections, the total organic carbon concentrations are high at the bottom and gradually decrease upward. The total sulphur contents of the studied shales display a similar trend in the vertical direction, and they are high in the lower part of the section. The carbon isotopes of organic matter (δ 13 C org ) present a distinct trend that is indicative of 13 C enrichment, which indicates that the total organic carbon variations between the different shale units in the Sichuan Basin may be related to changes in ocean biomass and the sedimentary environment. The mineral composition (specifically, the pyrite, quartz, and clay abundances) of these shales suggests that the deep-water sedimentary environment was conducive to the preservation of organic matter. Total organic carbon is positively correlated with quartz contents but negatively correlated with clay contents, suggesting an input of biogenic silica. The majority of the quartz was deposited via the slow settling and accumulation of recrystallised biogenic opal and silica-rich plankton. This study of the trace element geochemistry of the Lower Palaeozoic shales indicates that productivity, burial efficiency, redox environment, and hydrothermal activity all affected the accumulation of organic matter. The Cambrian Niutitang shale is rich in nutrient elements. The Longmaxi and Wufeng shales feature high Mo contents and total organic carbon correlation coefficients of 0.93 and 0.89, respectively. Efficient burial was essential for the enrichment of organic matter in the Wufeng and Longmaxi shales. The redox parameters suggest that the organic-rich shales (total organic carbon > 2.0%) were deposited in a strongly reducing environment. The U/Th ratios and Ni–Zn–Co correlations indicate that organic material was carried by deep hydrothermal fluids and enhanced the accumulation of organic matter.