COMPARATIVE ANALYSIS REVEALS CONTROLS ON SEISMIC-SCALE ARCHITECTURE OF CARBONATE PLATFORM MARGINS: TRIASSIC NANPANJANG BASIN, SOUTH CHINA

Here we present a comparative analysis of platform evolution demonstrating the impact that laterally-variable tectonic subsidence, rate of basinal clastic deposition at the toe of slope, and changes in carbonate factory type have on the evolution, large-scale architecture, and termination of carbonate complexes.

The Triassic Nanpanjiang basin (NPJB), south China, presents an exceptional natural laboratory for evaluating controls on carbonate platform margin and slope architecture. Multiple two dimensional transects through the Yangtze Carbonate Shelf and several isolated platforms provide exposure along spatial and temporal gradients in tectonic subsidence rate, siliciclastic input, antecedent topography, and oceanography. Platform development across the end-Permian extinction and evolving seawater chemistry allow assessment of the impact of carbonate factory change from a basin-wide perspective.

Our analysis suggests that timing and rates of subsidence controlled along-strike variability, timing of drowning, back-step geometries, and pinnacle development. Timing of clastic dispersal in the basin explains differences in platform-margin geometries such as slope angle, relief above basin floor, and progradation at basin margins. Shift to ramp profiles with oolite margins in the Early Triassic and back to steep-sided, microbial-cement margins in the Middle Triassic reflects changes in carbonate-factory type following the end-Permian extinction and related geochemical perturbations. Eustasy, in contrast, had very little influence on platform morphology and large-scale architecture.

Process-based depositional models derived from the NPJB can aid in the prediction of facies distribution and architectural styles at the basin scale in other systems, particularly in areas of active tectonism and temporal variations in oceanographic conditions, such as the prolific Tertiary carbonates reservoir province of southeast Asia and the Cretaceous pre-salt carbonate play offshore Brazil.