Triassic tank: Platform margin and slope architecture in space and time, Nanpanjiang Basin, South China

The Nanpanjiang Basin (NPJB) is a large, complex basin within the south China plate bordered by Precambrian uplifts on the northeast, southeast, and west and by a Triassic suture zone to the south. During the Permian and Triassic, the NPJB formed an embayment in the Yangtze Platform (YP) and contained several isolated carbonate platforms (IPs), including the Great Bank of Guizhou (GBG) and the Chongzuo-Pinnguo Platform. The NPJB presents an exceptional natural laboratory for evaluating controls on carbonate platform margin and slope architecture. Multiple two-dimensional transects through the YP and IPs 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. The YP and IPs evolved from ramps and low-relief banks with oolite margins and mud-rich slopes early in the Early Triassic to steepening Tubiphytes-reef rimmed platforms with slopes progressively enriched in clast-supported breccias in the Middle Triassic. Despite differences in the slope angle and windward-leeward differences in grain size at the bank margin, the Early Triassic margin-slope systems have very similar characteristics throughout the basin. During the Middle Triassic, the YP and IPs developed extreme lateral variability in margin architecture due to differences in tectonic subsidence and siliciclastic basin fill at the toe-of-slope. The southwestern sector of the YP and the GBG drowned under pelagic carbonates followed by siliciclastic turbidites in the Late Triassic, Carnian, while the northeastern YP continued shallow-marine deposition until burial by prograding shallow-marine siliciclastics. The southerly IPs have backstepping geometries, terminal pinnacles, and earlier drowning and burial by siliciclastics. Differences in antecedent topography affected margin width and stability, resulting in changes from broad aggrading to prograding margins vs. high-relief and collapsed margins. Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, backstepping geometries, and pinnacle development. Timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and progradation at basin margins. Development of ramp profiles with oolite margins in the Early Triassic and subsequent development of steep-sided margins in the Middle Triassic reflects changes in carbonate factory type following the end-Permian extinction. 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, for example, in the prolific Tertiary carbonates reservoir province of southeast Asia.