Late Ordovician-Early Silurian sedimentary facies and palaeogeographic evolution and its bearings on the black shales in the Middle-Upper Yangtze area

During the Late Ordovician-Early Silurian, the Middle-Upper Yangtze area appeared as a shallow shelf surrounded by foreland uplifts, where the sedimentary environment was controlled by the Wuling-Xuefeng and central Guizhou foreland upwarped zones. The stagnant and reducing conditions in the inter-foreland back-uplift basins once provided vast accommodation spaces for the deposition of the black shales. The Katian black shales of the Late Ordovician in the Wufeng Formation are thin in thickness, and contain graptolite biostromes and abundant Radiolarias, suggesting a shallow-marine deep-water basin environment. The Rhuddanian black shales as the black graptolite shales in the lower part of the Lower Silurian Longmaxi Formation are considered as the inherited deposits of the Wufeng Formation. The overlying Aeronian Longmaxi Formation displays the shallowing-upward depositional sequences composed of the thin interbeds of black silty shales and grey siltstones, and records the transition from subtidal to intertidal and finally to tidal-flat environments. The space-time evolution of the depositional sequences was constrained by the surrounding palaeouplifts. The space-time range of the foreland uplifts was relatively small and gentle during the Late Ordovician. Till the Rhuddanian (Early Silurian), the steady expansion of the foreland uplifts led to the structural separation, development of the depressions nearly parallel to the foreland uplifts, and considerable variations in the thickness of the black shales in the Longmaxi Formation. Finally during the late Aeronian, the relative falling of sea levels and shallowing of sea water marked the termination of the deposition of the black shales in the Middle-Upper Yangtze area.