Basin Wide Controls on Carbonate Platform Evolution: The Triassic Nanpanjiang Basin of South China

Lehrmann, Dan; Minzoni, Marcello; Payne, Jonathan; Enos, Paul; Yu, Meiyi; Wei, Jiayong; Kelley, Brian; Schaal, Ellen; Meyer, Katja; Li, Xiaowei; Montgomery, Paul

The Nanpanjiang Basin (NPJB) occurs within the south China plate bordered by Precambrian uplifts on the northeast and west and by a Triassic suture zone 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 2-D transects through the YP and IPs provide exposure along spatial and temporal gradients in tectonic subsidence rate, clastic input, and antecedent topography.

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 essentially maintained its Permian position. The northernmost IP (the 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, margin sector collapse and delivery 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 clastic turbidites in the Carnian while the eastern YP continued shallow-marine deposition until burial by clastics 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 or starved-basin conditions impacted the evolution of platform-margin geometries.

Timing and rates of subsidence and antecedent topography 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 #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013