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Influence of Seawater Chemistry and Syndepositional Faulting on the Drowning of the Yangtze Platform, Nanpanjiang Basin, Guizhou Province, South China


The Yangtze Platform formed a vast carbonate platform that stretched across south China and bordered the Nanpanjiang Basin. During its long Triassic history, the platform evolved from ramp to rimmed margins and developed extreme variability in architecture in the Middle Triassic. In the northeast periphery of the basin the platform developed a progradational margin in which the platform advanced basinward over siliciclastic turbidites. In the western area the margin aggraded vertically and intertongued with turbidites at Guanling and backstepped and collapsed at Zhenfeng. Differences in architecture can be explained largely as the result of differential tectonic subsidence and timing of siliciclastic turbidite fill at the basin margin. In the northeast, lesser subsidence and earlier turbidite fill of the basin margin proximal to the detrital source resulted in reduced accommodation and progradation. In contrast, to the west, high rates of subsidence, syndepositional faulting and later arrival of turbidites, distal to the source area, locked the platform into aggradational and backstepping mode. The development of progressively higher relief locked the platform into a positive feedback of extreme upbuilding; the lack of basin infilling resulted in margin instability and gravitational sector collapse. Late Triassic termination of the Yangtze Platform also differed dramatically between the two areas. In the northeast termination occurred as shallow-marine siliciclastics overwhelmed carbonate sedimentation resulting in an eventual upward shift to marine and fluvial siliciclastics. In the west the platform drowned with a shift to pelagic limestones, organic-rich marls and black shale. Syndepositional faults are overlapped by the deep-marine sediments indicating that tectonic subsidence was a major factor leading to the drowning of the platform. The presence of bioturbation and minor benthic fossils over vast areas and only local development of black shale indicates widespread bottom-water oxygenation and localized areas of anoxia during the initial drowning. The succeeding widespread shift to black shale deposition indicates that seawater anoxia or excess nutrients may also have played a role sealing the demise of the platform. Analysis of spectral gamma ray signatures and concentrations of redox and nutrient trace elements will be used to evaluate the degree of seawater anoxia or excess nutrient levels that contributed to platform termination.