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Abstract

The long-lived Yangtze platform (YP) drowned abruptly and was buried by pelagic facies and siliciclastic turbidites in western Guizhou Province during the Late Triassic (Carnian). The uppermost carbonate platform facies are peritidal cyclic limestone and dolostone containing a restricted biota and having fenestral laminate caps. Equivalent margin facies consist of intraclastic, grapestone, oolitic grainstone, and lenses of coral-Tubiphytes algal boundstone indicating high-energy shoals and patch reefs.

The drowning horizon is a laterally variable sharp surface or gradational shift to dark, nodular-bedded, pelagic lime mudstone to wackestone. The contact lacks hardgrounds, phosphatized, or glauconitic surfaces that would indicate drowning by excess nutrient flux. Uppermost platform carbonates have a tropical photozoan biota and lack siliciclastic content, indicating neither climate cooling nor clastic flux played a role in drowning. Rare bioturbation and benthic biota in the lower part of the drowning interval indicate dysaerobic conditions with an upward shift to anoxic conditions.

Syndepositional faults had a significant impact on the evolution of the western sector of the Yangtze platform and controlled three local accommodation cycles. Faults developed during the last accommodation cycle tip out at the drowning horizon and include a flower structure upon which a pinnacle reef developed as the rest of the platform drowned. Lateral variability in the drowning horizon and thickness of the post-drowning pelagic facies point to differential tectonic subsidence causing sinking of the platform into deep water along faults.

Magnetic susceptibility and paleomagnetic reversal correlation demonstrates that the western sector of the platform drowned while shallow marine mixed carbonate-siliciclastic sedimentation continued in the eastern sector to be terminated later in shallow water by increasing rates of clastic flux. Starved black shale horizons in the basin indicate persistent water stratification and bottom water anoxia; elevated trace metal concentrations indicate dysaerobic to anoxic conditions and enhanced preservation of organic matter. Tectonic subsidence likely submerged the western sector into deep, toxic waters of the stratified basin causing the killing of benthic marine carbonate production.

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