FACIES AND MICROFOSSIL STUDY OF THE MECHANISMS AND TIMING OF TERMINATION OF THE HESHAN CARBONATE PLATFORM, NANPANJIANG BASIN, SOUTH CHINA

This study evaluates mechanisms involved in the termination of the Triassic Heshan platform in the Nanpanjiang Basin of south China. Shallow-marine carbonate-platforms are biological systems that flourish in shallow, photic, marine-waters with low nutrient and siliciclastic mud levels. Proposed mechanisms of termination of carbonate platforms include 1)global sea-level rise, 2) accelerated tectonic-subsidence, and poisoning of carbonate producing organisms by nutrient or siliciclastic flux. The Heshan platform is one of several carbonate platforms in the Nanpanjiang basin. Mapping at two sections shows an upward change from shallow-water facies (eg. oolite) to deeper-water facies (eg. nodular bedded lime mudstone) followed by shale. The shift to shale marks termination of carbonate sedimentation. Nodular facies suggest a shift to a deeper regime, which can result from siliciclastic influx, effectively reducing water clarity, slowing down the rate of carbonate accumulation, and therefore resulting in the deepening of the water column over the platform. Conodont biostratigraphy has revealed that the Heshan platform terminated earlier in the Triassic than the Pingguo platform 50 km to the east. Differential timing of termination rules out global sea-level rise and supports localized basin subsidence and siliciclastic flux as mechanisms causing the termination of the two platforms.