Influence of Ocean Redox Conditions and Carbonate Saturation State on Carbonate Factories and Platform Architecture: Examples From the Permian and Triassic Nanpanjiang Basin, South China

The Yangtze Platform (YP) and the Great Bank of Guizhou (GBG) formed a prominent shelf and an isolated platform, respectively, in the Nanpanjiang Basin (NPJB) of south China during Late Permian through Middle Triassic time, an interval characterized by large changes in seawater redox conditions. Continuous exposures from platform to basin across the YP and GBG allowed high-resolution mapping of their stratigraphic architecture through time. Parallel histories of platform architecture in the YP and GBG include: 1) an Upper Permian abrupt, high-relief, reef rimmed margin; 2) a basal Triassic (Induan) ramp with prograding ooid shoals that change basinward to a lime-mudstone-dominated outer-ramp; 3) a Lower Triassic (Olenekian) progressively steepening, elevated ooid shoal margin aggrading up to 500 m above the basin with slope angles up to 35o; and 4) a Middle Triassic steep, prograding Tubiphytes reef margin. Spectral gamma-ray logs and elemental geochemistry (U, Mo, V) indicate the onset of protracted basin anoxia during earliest Early Triassic time, subsequent fluctuations in redox conditions, followed by a final return to oxic conditions at the end of the Early Triassic. Carbonate factory types at the platform margins shifted from skeletal animals, foraminifera, and algae in the Upper Permian to abiotic or microbially mediated (oolite and micrite) in the Induan and Olenekian, followed by a return of higher skeletal content in addition to microbially mediated micrite and early marine cements during the Anisian. The shift to an aggraded, steep oolite bank profile in the Olenekian predated the return of the skeletal carbonate factory. Stratigraphic forward modeling of the Lower Triassic architecture of the GBG indicates that a low sediment diffusion rate, presumably resulting from a high carbonate saturation state and pervasive early cementation, is required to explain the Lower Triassic transition to a steep platform margin. As oolites are usually related to ramps or are perched on steep antecedent topography that they did not construct, these Lower Triassic examples appear to reflect a new carbonate platform morphotype that can develop during intervals characterized by high saturation state, even in the absence of skeletal reef builders