Triassic Depositional History of the Yangtze Platform and Great Bank of Guizhou in the Nanpanjiang Basin of South China

Abstract

The Nanpanjiang basin occurs in the southern margin of the south China plate. Marine sedimentation dominated from the Late Proterozoic to the Late Triassic when siliciclastic turbidites filled the basin and sedimentation regionally shifted to fluvial deposition. Permian and Triassic carbonate strata record a long history of platform evolution and include diverse architectures and evolutionary histories that reflect the impact of local depositional environments, rates of siliciclastic flux and accelerating tectonic subsidence as the basin experienced tectonic convergence and foreland basin development in the Triassic. The Triassic margin of Yangtze platform that rims the basin extends in a sigmoidal SW/NE trend fromYunnan through Guizhou. Several isolated platforms, including the Great Bank of Guizhou (GBG) and Chongzuo-Pingguo platform, occur within the basin in southern Guizhou and Guangxi. The basin expanded in the Late Permian during a regional transgression. The Yangtze platform and isolated platforms evolved from low-angle ramps with oolite margins in the Early Triassic to steepening Tubiphytes reef margins in the Middle Triassic (Anisian). Basin-wide shift from ramp to steepening-margins was stimulated by the evolution of Tubiphytes and other organisms that stabilized platform margins. The western Yangtze platform (Guanling and Zhenfeng) and northernmost isolated platform (GBG) aggraded and developed steep reef rimmed margins in the Anisian. During the Ladinian the Yangtze platform at Guanling aggraded and intertongued with basin filling clastics, while the margin at Zhenfeng developed a tectonically backstepped morphology controlled by faults. At the same time the eastern sector of the Yangtze platform (Guiyang) evolved from an erosionally collapsed margin to a progradational margin that advanced basinward at least 600 m over basin filling clastics. Unlike the Yangtze platform the northernmost isolated platform (GBG) evolved during the Ladinian from an aggradational reef rim to a high-relief erosional escarpment with a starved basin margin. The western Yangtze platform was drowned and buried by turbidites in the Late Triassic (Carnian) whereas shallow-water carbonate sedimentation continued until burial by siliciclastics in the eastern sector. The isolated platforms exhibit a south to north pattern of step-backed margins and pinnacle development and earlier drowning and burial by siliciclastics in the south versus greater longevity and later drowning in the north. Like the western sector of Yangtze platform, the northernmost isolated platform (GBG) drowned and was buried by clastic sediments in the Later Triassic (Carnian). These differences resulted from faster subsidence rates in the southern part of the basin caused by tectonic convergence along the southern margin of the south China plate. GBG has the longest history of isolated carbonate platforms in the basin. A faulted syncline exposes a continuous two-dimensional cross section through the platform interior and margins, thus facilitating a detailed assessment of its architecture and depositional history. Conformable Permian-Triassic boundary sections, and thick, continuously exposed sections through the Early to Middle Triassic biotic recovery interval make this platform an ideal area for evaluating the marine environments and biotic conditions that operated during the end-Permian extinction and its aftermath.