PALYGORSKITE IN PALEOSOLS FROM THE MIOCENE XIACAOWAN FORMATION OF JIANGSU AND ANHUI PROVINCES, PR CHINA

Strata associated with Tertiary basalts of the Xiacaowan Formation, in the North Jiangsu Basin of eastern China, contain mudstones which are rich in palygorskite and montmorillonite, with lesser concentrations of dolomite and CT-opal. Massive montmorillonite clays may have develo ped as vertisols by deep meteoric weathering of underlying basalt flow s in a sub-humid to semi-arid climatic setting with pronounced seasona lity. Laminated montmorillonite clays, higher in the sequence, accumul ated in low-lying areas in inland lakes with poor external drainage. C limatic fluctuations resulted in periods of increased aridity, during which falling lake levels exposed montmorillonite-rich vertisols, lake and playa sediments, to further pedogenesis. Lowering of the water ta ble and increased rates of evaporation allowed leaching of sodium and calcium from the soils; this, in conjunction with the introduction of silica and magnesium by rising ground water, led to the neomorphic dev elopment of extensive stratiform palygorskite deposits.