Physical, chemical, and biological controls on the stratigraphic evolution and spatial variability of an isolated carbonate platform [electronic resource]
Keywords: Geological and Environmental Sciences.

Isolated carbonate platforms can provide substantial hydrocarbon reservoirs because they develop significant depositional relief and are commonly buried by relatively impermeable basin-filling sediments. Consequently, there is ongoing interest in the mechanisms that control their morphology and facies distributions. Because the deposition of carbonate sediments is influenced by a complex interaction of environmental conditions that vary both temporally and spatially across depositional systems, the relative importance of controls on platform morphology is difficult to constrain. The key to developing a better understanding is to recognize causal links between changes in oceanographic conditions and variability in resulting geometries. Unfortunately, most studies of exposed carbonate platforms limit the ability to discern influences beyond fluctuations in relative sea level, because they are based on a single cross section of a platform and record deposition during periods of relative stasis in ocean chemistry and biotic evolution. In this study, however, I examined cross-sectional exposures from three geographic sectors of the Late Permian to Late Triassic Great Bank of Guizhou (GBG) in the Nanpanjiang Basin of south China. Deposition on the GBG spanned the tumultuous transition from Paleozoic to Mesozoic oceans, providing a rare opportunity to evaluate the influence of changes in carbonate factory type during a period of significant global change. In addition, I compared multiple exposures of the platform to determine the differential impact of basin-wide controls such as external sediment supply and antecedent topography on the evolution of platform geometries. The integrated analysis of satellite imagery, field mapping, chemostratigraphy, biostratigraphy and petrography shows that chemical and biological controls associated with end-Paleozoic extinction and environmental disturbance led to changes in carbonate factory type that contributed to significant temporal variability in the platform geometry. The GBG initiated with small patch reefs in the latest Permian, was dominated by microbial carbonates in the immediate aftermath of extinction, progressed to a low-relief bank with oolite shoal margins in the earliest Triassic when skeletal organisms were rare, developed a high-relief morphology with steep slopes when transport was limited by rapid cementation, and later established a reef-rimmed margin as environmental conditions allowed for the return of abundant skeletal organisms in benthic environments. In contrast, physical controls, including antecedent topography and external sediment supply, produced significant spatial variability in the GBG. In the northwestern sector, over-steepening led to the development of an erosional escarpment and bypass margin following an initial interval of Early Triassic progradation over a shallow basin floor. In the northeastern sector, a similar pattern was disrupted by Middle Triassic influx of siliciclastic sediments that raised the basin floor, reduced topographic relief, and provided a structural foundation for multiple episodes of progradation and the retention of an accretionary margin. In the southwestern sector, an adjacent deep basin established significant topographic relief, eliminating the structural support for progradation and leading to large-scale sector collapse of the platform margin. This study provides causal links between depositional environments and resulting geometries and shows that changes in carbonate factory type, differences in antecedent topography, and the pattern and timing of external sediment supply have a significant impact on the stratigraphic evolution of isolated carbonate platforms.