FACTORS CONTROLLING DIFFERENTIAL GROWTH, MARGIN GEOMETRY AND DROWNING OF AN ISOLATED PERMIAN-TRIASSIC PLATFORM IN THE NANPANJIANG BASIN, SOUTH CHINA

A well-exposed isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin of south China, was developed from the latest Permian to the earliest Late Triassic. Platform strata are dissected by two faulted syncline exposing two complete cross sections through the interior, margins and flanks, enabling a detailed assessment of depositional controls.

Previous studies portrayed the platform as having a relatively symmetrical architecture even though much of the former work was focused on the platform interior and northern margin-basin transition. Our recent research reveals significant lateral variability in evolution of margin architecture along depositional strike of the margin and differences in the timing of drowning and termination of the platform. The northern margin evolved from a steepening to progradational architecture, with an additional pulse of retreat, basinally restricted clastic wedge and later progradation developed in the east. Preliminary data indicates drowning also occurred earlier in the east. The earlier drowning and differences in margin architecture in the east are interpreted to reflect earlier clastic flux from the Jiangnan uplift, and differential tectonic subsidence. The southern margin exhibits evidence of large scale gravitational collapse (scalloped margin or sector collapse) and local subaerial unconformity development. These features are interpreted to represent oversteepening over antecedent topography inherited from the Permian reef margin and reactivation of faults that controlled the position of the margin.

Additional studies are being conducted of the petrography, conodont biostratigraphy, spectral gamma ray assays and trace element geochemistry/chemostratigraphy of the platform drowning succession to isolate specific mechanisms (subsidence, clastic flux, nutrient loading, anoxia) that contributed to differential timing of drowning.