MECHANISMS CONTROLLING GIANT SECTOR COLLAPSE (SCALLOPED MARGIN) OF THE YANGTZE PLATFORM MARGIN, NANPANJIANG BASIN, SOUTH CHINA

The south facing margin of the Triassic Yangtze Platform (YP) in southern Guizhou contains several large (14-28 km) concave basinward sector collapse scars that are spectacularly displayed in satellite images because of the contrast in erosional style between karsted carbonates and basin turbidites. The features occur along a linear ENE trend grossly aligned with a scalloped margin (collapse scars up to 5 km across) on the Great Bank of Guizhou (GBG), an isolated platform.

Mapping demonstrates the YP reef margin is preserved in promontories but has been completely removed within collapse scars, exposing interior facies. Mass waste deposits in the basin contain megabreccia with blocks up to 20 m across of Anisian Tubiphytes boundstone and a variety of Anisian to Ladinian margin and interior litholofacies. Breccias contain a chaotic assemblage of angular, massive and bedded clasts with inter-clast pores filled with marine or late burial cements. Intraclastic and skeletal grainstone locally accumulated as pore fill and in bedded units intercalated with and overlying megabreccia units.

Episodic collapse caused avalanche deposition of large breccia bodies. We estimate erosion of 198 km³ from the largest collapse scar and 38 km³ from the smallest. Grainstone intercalations indicate continued highstand shedding from the carbonate platform, with material possibly funneled down the scar. Timing of collapse is constrained by age of breccia clasts and onlapping siliciclastic turbidites.

The collapse scars in the YP are an order of magnitude larger than those described in modern and ancient platforms from sidescan and high resolution seismic data. However, these features are on a similar scale to the larger scallops described by Mullins and Hine (1989) from regional maps of the Bahamas. The extensive megabreccia and grainstone lobes associated with the collapsed margins of the YP present a potential analogue for slope debris hydrocarbon reservoirs.

The alignment of the collapse structures along a structural trend across the basin indicates a fault control of the margin and potential for seismic triggering of margin collapse. The occurrence of the structures only along the south facing margins indicates they may have developed over high-relief antecedent topography and maintained by faulting or prolonged basin starvation.