Continental slope limestones of Lower and Middle Triassic, South China

Abstract

Continental slope limestones of the Lower and Middle Triassic in South China are composed of hemipelagic limestones, gravity-flow limestones, and contour-current limestones. The hemipelagic limestones are dark coloured and thin bedded, and contain substantial amounts of fine-grained terrigenous materials, volcanic materials, and sponge spicules. Gravity-flow limestones are developed in all stages of the Middle and Lower Triassic in South China. Five fundamental types of gravity-flow limestones are recognized: slide limestones, debris-flow limestones, grain-flow limestones, turbidity-current limestones and rockfall limestones. They formed five types of amalgamated beds: slide–debris-flow limestones, slide–debris-flow–turbidity-current limestones, slide–debris-flow–grain-flow–turbidity-current limestones, debris-flow–turbidity-current limestones, and debris-flow–grain-flow–turbidity-current limestones. The first two types were formed mainly in the Early Triassic slopes. The Middle Triassic slopes were characterized by the widespread rockfall limestones. Growth faults, storms, earthquakes, and oversteepened slopes are probable triggers of gravity flows. Contour-current limestones are isolated lenses or thin, ripple-laminated beds of grainstones intercalated in hemipelagic argillaceous limestones and lime mudstones. They were formed at the base of the slopes. Palaeocurrent data indicated that the contour currents flowed perpendicularly to the slope. The contour-current limestones are not as common as the gravity-flow limestones, but they are important in the reconstruction of the palaeogeography and palaeotectonic setting in South China.