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 and west and by a suture zone of possible Triassic convergence to the south. During the Permian and Triassic the NPJB embayed the Yangtze Platform (YP) and contained several isolated carbonate platforms (IPs).
The NPJB presents an unparalleled natural laboratory for evaluating controls on carbonate platform-margin and slope architecture. Multiple 2D transects through the YP and IPs provide exposure along spatial and temporal gradients in tectonic subsidence rate, clastic input, and antecedent topography. Platform development across the end-Permian extinction allows assessment of the impact of global change from a basin-wide perspective.
During the latest Permian and Early Triassic the NPJB expanded from a narrow embayment to drown the eastern margin of the YP, a backstep > 100 km, while the western margin maintained its Permian position. The northernmost IP, the Great Bank of Guizhou (GBG), nucleated above the Permian margin of the YP.
The YP and IPs evolved from ramps and low-relief banks with oolite margins in the Early Triassic to steepening Tubiphytes-reef rimmed platforms in the Middle Triassic. During the Middle Triassic the YP developed extreme lateral variability resulting from differential subsidence and timing of basin filling-clastics. The GBG evolved a high-relief margin late in the Middle Triassic, akin to the southwestern margin of the YP. Asymmetric development of the GBG reflects antecedent topography and timing of easterly sourced clastic flux.
The western sector of the YP and the GBG drowned under pelagic carbonates followed by siliciclastic turbidites in the Carnian while the eastern YP continued shallow-marine deposition until burial by prograding siliciclastics in the late Carnian. The southerly IPs have backstepping geometry, terminal pinnacles, and earlier drowning and burial by clastics. Basin-margin intertonguing, or lack thereof, demonstrate that earlier basinal clastic influx and lateral concurrent starved-basin conditions impacted the evolution of platform-margin geometries.
Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, back-step geometries, and pinnacle development. Timing of clastic basin fill dictated 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 margins in the Middle Triassic reflects changes in carbonate-factory type following the end-Permian extinction.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California