CONTROLS ON FACIES ARCHITECTURE OF A LARGE TRIASSIC CARBONATE PLATFORM - THE GREAT BANK OF GUIZHOU, NANPANJIANG BASIN, SOUTH CHINA

The Great Bank of Guizhou (GBG) is an exceptionally well exposed isola ted Triassic platform in the Nanpanjiang Basin of South China, The pla tform is exhumed with its depositional profile preserved and is dissec ted by a faulted syncline that exposes a complete and uncomplicated cr oss section providing a unique opportunity to evaluate mechanisms invo lved in its birth, evolution, and demise. The GBG formed near the sout hern margin of the Yangtze Platform during a deepening event that expa nded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian, Initial accumulation of the GBG began in tbe la test Permian with small reef mounds and open-marine skeletal packstone s. Following the end-Permian extinction, cyanobacterial boundstones gr ew over the bank top, During the Early Triassic the GBG developed a lo w-relief bank profile with mobile oolite shoals at the margin, shallow -subtidal and peritidal deposits in the interior, and gentle slopes do minated by pelagics, debris-flow deposits, and turbidites at the basin margin, In the Middle Triassic (Anisian) the GBG developed a progress ively steepening profile rimmed with massive Tubiphytes reefs, The pla tform was hat topped with tidal-flat deposits across the interior, Bas in-margin deposition was dominated by turbidites and debris flow depos its but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose, In the Middle Triassic (Ladin ian) an erosional escarpment up to 1700 m high developed at the margin , Platform-margin strata are bedded packstones similar to interior str ata, whereas breccias at the basin margin contain coral-boundstone cla sts suggesting erosion of reefs from the escarpment, A restricted subt idal lagoon formed in the interior, producing an atoll-like morphology , Later, a Bat-topped profile was restored as tidal Bats spread across the interior, In the beginning of the Late Triassic deepening contrib uted to termination of the GBG before siliciclastic turbidites end sha les were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundan t muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture, The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries, The GBG's larger size i ncreased mud production and protected it from extensive winnowing of m ud, which in turn resulted in off-bank shedding of muddy sediments tha t were stable on relatively gentle, basin margin slopes which progress ively steepened and ultimately led to avalanche deposits and a high-re lief erosional escarpment, In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their a ngle-of-repose geometries.