CHANGE IN SEAWATER REDOX AND CARBONATE SATURATION STATE: A MECHANISM FOR BASIN-WIDE SHIFTS IN CARBONATE PLATFORM ARCHITECTURE AND CARBONATE FACTORIES: EXAMPLES FROM THE PERMIAN-TRIASSIC NANPANJIANG BASIN, SOUTH CHINA

The Nanpanjiang Basin occurs within the south China plate bordered by the Yangtze Platform (YP) and contains an isolated platform, the Great Bank of Guizhou (GBG). This study tests whether changes in seawater redox and carbonate saturation affected carbonate factories and margin architecture.

The margins of the YP and GBG are preserved in synclines that expose 2-D cross sections. Margin architecture was constrained by outcrop mapping aided with high-res. satellite images and stratigraphic sections. Parallel shifts in architecture of the YP and GBG include: 1) an U. Permian abrupt, high-relief reef rimmed margin, 2) a basal Triassic (Induan) broad ramp with prograded ooid shoals that changes basinward to lime mudstone, 3) a L. Triassic (Olenekian) progressively steepening margin with a barrier of ooid shoals, aggrading up to 500 m relief with slope reaching 31^o, and a dolomitized platform interior, and 3) a M. Triassic (Anisian) steep prograding Tubiphytes reef margin which maintained relief of 400-500 m and slopes up to 31^o.

Gamma-ray logs and elemental geochemistry show onset of basin anoxia in the Induan, redox fluctuations, and a return to oxic conditions in the end of the Olenekian and Anisian. Carbonate factories shift from a relatively high proportion (6%) of skeletal contents in the U. 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 biotics in the Middle Triassic may reflect the recovery from extinction and seawater anoxia. However, the shift to an abrupt aggrading oolite profile in the Olenekian indicates that the change in architecture preceded biotic recovery, suggesting that oxygenated waters and ultra-high carbonate saturation may have spurred the onset of a steep aggraded margin.

Oolitic margins occur in ramps with the exception of those that are formed upon antecedent topography. The development of a rapidly aggrading, steep, high-relief oolitic margin may be a new carbonate platform morphotype developed in systems lacking skeletal carbonate production with high carbonate saturation states (e.g. Proterozoic, or episodes following mass extinction). Such systems may have significant potential as hydrocarbon prospects.