Depositional controls on mineral assemblage and organic matter accumulation of upper Ordovician-lower Silurian mudstones in northwestern Guizhou area, China

The control of depositional conditions on sedimentary records spanning the Ordovician Wufeng-Silurian Longmaxi successions on the Yangtze Platform is not fully understood due to a lack of high-resolution faices analysis. This study aims to build a regional depositional model of this stratigraphic interval, to reveal the fundamental controls on the mineral assemblage and organic matter (OM) accumulation. Vertical lithofacies variability of two continuous cores in northwestern Guizhou area, south China, was analyzed at high resolution by RoqSCAN (an automated mineralogy tool) and field emission scanning electron microscope (FE-SEM) imaging. Based on multiscale sedimentary structure, mineral composition, total organic carbon (TOC) content, and geochemical characteristics, seven lithofacies were identified and interpreted as four facies associations (FA A-D) to characterize the spatio-temporal variation in these sedimentary environments. The deep-siliceous shelf FA-A is the richest in TOC content and biogenetic quartz, attributed to the strong anaerobic watermass, minimal terrestrial-flux, and enhanced organic-productivity during two global transgressions. The middle mixed shelf FA-B has moderate TOC and mixed minerals resulting from reworking processes during a fall of sea level caused by regional uplifts. Although the primary organic-productivity was promoted by increased volcanic ash fall, dilution due to increased terrestrial inputs made OM difficult to accumulate. The shallow muddy shelf FA-C and the shallow silty shelf FA-D were not favorable for OM accumulations because of the low organic productivity, strong oxygenated watermass, and high detrital dilution, through long-term sea level drops. The facies attributes of the target mudstones were remarkably influenced by regional tectonic-eustasy, in addition to global glacioeustasy.