Triassic Yangtze Platform margin: Evolution, internal architecture, and death of a large, attached carbonate platform, Guizhou Province, China

Abstract

The stratigraphy and geometry of the Triassic Yangtze Platform margin in south China demonstrate the fundamental role of differential tectonic subsidence and timing of basinal siliciclastic influx in the development and termination of a vast, attached carbonate platform.

The Yangtze Platform developed as a carbonate ramp following drowning in latest Permian, but evolved into a low-relief platform during the Olenekian (late Early Triassic). During early Middle Triassic (Anisian) the platform developed a progressively steeper profile with cyclic peritidal carbonates and an interior lagoon forming in the lee of Tubiphytes reefs. In the Ladinian (late Middle Triassic) platform-margin cyclic peritidal sand flats extended into the interior, defining a flat-topped morphology. The southwestern sector of the platform drowned at the beginning of Late Triassic (Carnian); deep-water lime mudstone and black shale resting unconformably on faulted peritidal deposits signal tectonic control. The platform-to-basin transition records repeated aggradation, fault-controlled backstepping, and progradation prior to final drowning. Shallow-water carbonate deposition continued to the northeast until burial by shallow-water siliciclastics later in the Carnian.

Stratal geometries of three platform-to-basin transects reveal significant along-strike variability in the platform. The southwestern sector aggraded and stepped back during Middle Triassic and drowned at the beginning of Carnian. The northeastern sector retreated in the Anisian, prograded markedly in Ladinian and early Carnian, before burial by siliciclastics. Comparisons indicate that tectonic subsidence, local faulting, and rate of basinal deposition controlled the evolution and architecture of the platform margin and slope.

Evolution and time of drowning of four isolated platforms in the south-adjacent Nanpanjiang Basin indicate that rapid subsidence and basin fill started earlier in the south and progressed northward. This pattern suggests a foreland setting with collision along the southern perimeter of the basin.

Evolution and large-scale architecture of the Yangtze Platform presents an analog for lateral variability in seismic-scale characteristics of carbonate platforms. Given subsidence history and source of siliciclastic input, basin-wide, seismic-scale evolution and termination of carbonate accumulations may be predictable.