--> 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

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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

Abstract

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). The goal of this study is to test whether changes in seawater redox and carbonate saturation state affected variation in carbonate factories and margin architecture. The margins of the YP and GBG are preserved in synclines that expose continuous 2-d cross sections. Platform 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 Upper 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 Lower Triassic (Olenekian) progressively steepening margin with a barrier of ooid shoals, aggrading up to 500 m relief with slope reaching 31°, and a dolomitized platform interior, and 3) a Middle Triassic (Anisian) steep prograding Tubiphytes reef margin which maintained relief of 400-500 m and slopes up to 31°. Spectral 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 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 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 carbonate margins reported from the modern and ancient are 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, abrupt redox shifts, or high carbonate saturation states (e.g. Proterozoic, or episodes following mass extinction). Such systems may have significant potential as hydrocarbon prospects.