Relationship between the origin of organic-rich shale and geological events of the Upper Ordovician-Lower Silurian in the Upper Yangtze area

Abstract

Abstract Based on detailed sedimentological observations and geochemical analyses, four geological events have been identified, and a genetic depositional model has been established within a sequence stratigraphic framework. The organic-rich siliceous shale interbedded with dense bentonite layers in the transgressive systems tract 1 (TST 1) is attributed to the first stage of high-frequency volcanism (which started at ∼447.62 Ma) associated with the collision of the Cathaysia and Yangtze Blocks. The presence of the organic-fair lime mudstone/shelly limestone indicates that Hirnantian Glaciation in the highstand systems tract (HST). The occurrence of organic-rich siliceous shale interbedded with scarce bentonite layers in the transgressive systems tract 2 (TST 2) is attributable to the second stage of low-frequency volcanism (which started at ∼443.83 Ma) and to postglacial effects. The formation of organic-fair mixed shale with silty laminae is attributed to bottom-current activity in the early highstand systems tract (EHST). The occurrence of organic-poor clay-rich shale is due to terrigenous input in the late highstand systems tract (LHST). Our research also shows that the high marine productivity caused by volcanic ash and a high burial rate in an anoxic depositional setting led to organic matter and siliceous mineral enrichment in the TST 1 and TST 2. The favorable organic matter preservation conditions and the postglacial biotic recovery played significant roles in the formation of the organic-rich shale in the TST 2.