Effect of sedimentary environment on the formation of organic-rich marine shale: Insights from major/trace elements and shale composition

Abstract

Sedimentary environment and redox conditions play a significant role in the formation of organic-rich shale. The vertical variations of major/trace elements, TOC (total organic carbon) and mineral compositions were investigated for the Lower Silurian Longmaxi typical marine shale in the Upper Yangtze Platform, South China, to decipher sedimentary environment (redox conditions, detrital input, sedimentation rate (SR), paleoproductivity and watermass restriction). This gives insight to the effects of variations in sedimentary environment on the formation of organic-rich shale. The results show that the Lower Longmaxi Formation is composed of three systems tracts: TST (transgressive systems tract), EHST (early highstand systems tract) and LHST (late highstand systems tract). From TST to LHST, the contents of TOC, quartz and pyrite decline steadily, whereas clay minerals and carbonate show an increasing trend. Thus, organic-rich siliceous (ORSS), organic-moderate mixed (OMMS) and organic-lean argillaceous (OLAS) shales are dominant for TST, EHST and LHST, respectively. Due to a continues fall of the relative sea-level, a deep-water shelf environment with anoxic condition, high paleoproductivity and low detrital input in TST gradually evolves into a semi-deep-water environment with dysoxic-oxic conditions, moderate paleoproductivity, and moderate detrital influx in EHST, further to an environment of oxic condition, low paleoproductivity, and high detrital flux in LHST. As evidenced by Mo-TOC and MoU patterns, ORSS was deposited in a moderately restricted watermass, weaker than Black Sea, which progressively evolved into an enhanced restriction degree during the deposition of OMMS and OLAS due to a fall of relative sea-level. Major/trace elements and excess silica (EX-SiO2) concentrations illustrate that origins of quartz in the Lower Longmaxi shale are primarily terrigenous, biogenic and authigenic (smectite illitization). The average proportions of EX-SiO2 (biogenic and authigenic quartz) are 52.34%, 24.86% and 6.57% for ORSS, OMMS and OLAS, respectively. Biogenic quartz is the dominant contributor to EX-SiO2, while the contribution of authigenic quartz is limited, and the formation of the former is earlier than the latter according to the diagenetic transformation temperature. As both biogenic quartz and organic matter are firmly related to the abundance and preservation of paleoorganisms, anoxic condition, high paleoproductivity, low detrital flux and moderate restriction collectively control the high enrichment and preservation of organic matter and biogenic quartz, while oxic condition, low paleoproductivity, high detrital flux and stronger watermass restriction are unfavorable for the enrichment of biogenic quartz and organic matter.