CONTROLS ON CARBONATE FACTORIES IN THE TRANSITION FROM RAMP TO REEF-RIMMED PLATFORM IN THE HONGYAN SECTION OF THE TRIASSIC YANGTZE PLATFORM: PRELIMINARY RESULTS

The Nanpanjiang Basin (NPJB) occurs within the south China plate bordered by the Yangtze Platform (YP). The goal of this study is to test whether changes in seawater redox and carbonate saturation state affected differences in carbonate factory distribution and margin architecture.

At Hongyan the YP margin architecture is preserved in the western part of the NPJB. A syncline exposes a continuous two-dimensional cross section through the margin. During the Induan the YP developed a broad ramp with ~1.5 ^o slope. The ramp consists of with prograded ooid-shoals that change basinward to lime mudstone with slump folds and debris flow breccia. In the Olenekian a more abrupt bank profile developed with a barrier of ooid shoals and a restricted lagoon and peritidal interior. During the Middle Triassic, the platform developed a progressively steepening Tubiphytes microbial-cement reef-rimmed margin that reached up to 250m relief with slope clinoforms ~35 ^o. Slope facies changed from debris-flow breccia to talus and calciturbidites. Later in the Anisian and Ladinian the platform aggraded as Tubiphytes microbial-skeletal reefs developed at the margin, debris flow breccias intertongued with siliciclastic turbidites in the basin and peritidal cyclic facies developed across the flat-topped interior.

Hongyan section traverses the basin, platform margin and interior. Preliminary analysis of spectral gamma-ray logs and elemental geochemistry (U, Mo, V) show onset in basin anoxia in the Early Triassic, maintenance of an anoxic basin with redox fluctuations in the end of the Early Triassic, and oxic conditions in the Anisian. Carbonate factory types shift from skeletal in the Upper Permian to abiotic (oolite and micrite) in the Induan and Olenekian to microbial and abiotic (Tubiphytes, microbial crusts, cement) in the Anisian and microbial, abiotic and skeletal in the Ladinian. The shift to a more abrupt bank profile in the Olenekian indicates that the change 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. Quantitative petrographic analysis will allow us to further test whether shifts in basin redox and carbonate saturation affected changes in diagenesis and porosity preservation/ development.