HIGH-RELIEF OOLITE PLATFORM MARGINS IN THE NANPANJIANG BASIN OF SOUTH CHINA RESULTED FROM EXTREME SEAWATER CARBONATE SATURATION IN THE EARLY TRIASSIC AFTERMATH OF THE END-PERMIAN MASS EXTINCTION

Recent mapping in the Nanpanjiang Basin (NPJ) reveals impressively thick (900m), steep-sided (35-65⁰), high-relief (>750m) Lower Triassic platforms with oolite grainstone margins. Such architecture occurs in the attached Yangtze Platform (YP) that borders the NPJ, and in the Great Bank of Guizhou (GBG), an isolated platform within the basin.

The east-facing margin of the YP at Guanling and the north-facing margin of the GBG at Bianyang and Xiliang developed over a low-relief Permian substrate as a ramp that rapidly aggraded and steepened, forming a high-relief margin. The south-facing margins of the YP at Anlong and the GBG at Bangeng developed on a high-relief Permian barrier reef margin that faced the deep basin to the south.

The Lower Triassic margins are composed of oolitic grainstone that is variably dolomitized and contain giant ooids (up to 1 cm) and composite coated grains (>1cm). Steep slope clinoforms contain transported ooids. South-facing margins of the YP and GBG have greater amount of giant ooids, indicating higher energy from waves and tidal currents and large gravitational collapse scars (scallops). North and south facing margins of the GBG contain megaclasts of cemented oolite shed to the basin. Clasts contain bladed cement indicating marine lithification and erosion of blocks from the margins. 2

Giant ooids are bimineralic and contain radial or micritic cortices (calcite), alternating with coarse pseudospar (aragonite), with distorted ooids formed by brittle compaction of micritic cortices around voids that are interpreted to have been dissolved aragonite. Many ooid nuclei are fragmented shards of other ooids, and giant ooids and composite grains contain numerous abrasion surfaces, indicating both high carbonate saturation and high current energy reaching a threshold of physical breakage and abrasion. Stratigraphic forward modelling of the GBG with input of subsidence rates from backstripping analysis indicates oolite production rates of 800-3000 m/My and successfully simulates the observed architecture.

Our results indicate an extreme rise in seawater carbonate saturation after the end-Permian extinction decimated skeletal production, resulting in a shift to abiotic, oolitic carbonate production and marine cementation yielding thick, steep and high-relief margins.