Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the Lower Silurian Longmaxi Formation, Southern Sichuan Basin, China

Abstract

Based on element geochemical and total organic carbon (TOC) analyses of core samples from the Silurian Longmaxi Formation black shales in the Sichuan Basin, this study tried to figure out the temporal changes of palaoenvironmental settings for the shale deposition. The redox-sensitive element ratios, such as U/Th, V/Sr, V/Cr, Ni/Co and V/(V + Ni) are useful indicators to define the redox condition of marine black shale deposition. Element geochemical analysis of Longmaxi Shale samples taken from well W201 and Z106 in the southern Sichuan Basin shows that the organic-rich shales at the bottom of the Longmaxi Formation were deposited in an anoxic environment, while the upper part organic-poor shales were deposited in an oxic environment. Meanwhile, the sedimentary and paleontological characteristics show that pyrite and microfossils are concentrated mainly in the lower organic-rich black shales, indicating also an anoxic environment. All these geochemical, sedimentary and paleontological criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Longmaxi Formation shales. Furthermore, geochemical indicators should be combined with sedimentary and paleontological features in the study of palaeoenvironment conditions for shale deposition.