Controls on Carbonate Factories in the Transition from Ramp to Reef-Rimmed Platform in the Hongyan Section of the Triassic Yangtze Platform

At Hongyan the Yangtze Platform margin architecture is preserved in the western part of the Nanpanjiang Basin of south China. A syncline exposes a continuous 2-D cross section through the margin. Stratigraphic correlations with spectral gamma ray logs demonstrated that the margin evolved from an Upper Permian skeletal grainstone shoal to an Induan ramp profile with ~1.5° slope. The ramp consists of prograded ooid-shoals that change to bioturbated lime mudstone with slump folds and debris flow breccias and laminated lime mudstones on the mid to outer ramp. In the Olenekian, a more abrupt profile developed as the barrier of ooid shoals developed with and a restricted lagoon and peritidal interior. During the Middle Triassic (Anisian), the platform developed a progressively steepening Tubiphytes microbial-cement rimmed margin. The margin reached up to 250 m relief with slope clinoforms ~35°. Slope facies changed from debris-flow breccia to talus and calciturbidites, and siliciclastic turbidites of the Xinyuan formation buried the slope.

Spectral gamma-ray logs and geochemistry (U, Mo, V) shows evidence of basin anoxia in the Early Triassic and basal Anisian and shift to oxic conditions later in the Middle Triassic. Spectral gamma ray profiles show generally low values and low Uranium (U) content across the Permian-Triassic boundary and in the Lower Triassic (Induan). This notable Uranium depletion probably resulted from a global Oceanic Anoxic Event (OAE) related to the end-Permian mass extinction and depletion of seawater U due to widespread sedimentation of U in the world’s anoxic oceans.

Carbonate factories shift from a relatively high proportion (6%) of skeletal contents in Upper Permian to abiotic (oolite and micrite) (98%) in the Induan, to microbial and abiotic (11% and 54%) in the Olenekian, to a return of higher skeletal content (6%) in addition to microbial crusts and cement in the Anisian. This return of skeletal factories in the Middle Triassic may reflect the recovery from the end-Permian extinction and seawater anoxia. However, the shift to a more abrupt bank profile in the Olenekian indicates that the shift preceded the biotic recovery from the end-Permian extinction, suggesting that seawater redox conditions may have had a greater role on margin architecture than biotic evolution.