DEATH IN GUIZHOU - LATE TRIASSIC DROWNING OF THE YANGTZE CARBONATE PLATFORM

The Yangtze platform in south China formed a stable palaeogeographic e lement from the Late Proterozoic to the end of the Middle Triassic wit h deposition of shallow-water carbonates during much of this time. A p ortion of the Yangtze platform in south-central Guizhou drowned at the transition from Permian to Triassic, as the south-adjacent Nanpanjian g basin encroached about 100 km northward, but a new, stable platform margin was established that persisted through the Early and Middle Tri assic. This long history as a stable carbonate platform ended at the t ransition from the Ladinian to the Carnian. The latest Ladinian rocks, the Yangliujing Formation, are 490 m of shoaling-upward carbonate cyc les of grapestone and bioclastic grainstone, fenestral limestone, and stromatolitic dolomudstone, commonly overprinted by extensive subaeria l diagenesis. The beginning of the Carnian is marked by a rapid transi tion to medium-dark-grey, nodular lime mudstones containing ammonoids, conodonts and thin-shelled bivalves, the Zhuganpo Formation. The uppe r part of this thin pelagic limestone contains many muddy intraclasts, some slightly bored and encrusted, indicating incipient cementation. The overlying Wayao Formation is a condensed black shale with thin int erbeds of dark-grey, manganiferous lime mudstone near the base. Ammono ids, conodonts, thin-shelled bivalves, and articulated crinoid stems a re abundant. Fine-grained,greywacke with sole marks forms prominent bu ndles within grey, calcareous shale in the overlying Laishike Formatio n. Ammonoids and thin-shelled bivalves occur sporadically in this 810- m-thick unit. Calcareous shale with thicker-shelled bivalves and packa ges of cleaner, coarser-grained sandstone characterize the Banan Forma tion, 460 m thick. The sandstone units generally coarsen and thicken u pward, with ripples, medium-scale trough cross-beds, and rare U-tube b urrows. Quartzose, coal-bearing siliciclastics 690 m thick form the ov erlying Huobachong Formation. Thick-bedded, cross-stratified sandstone and conglomerates are amalgamated into thinning- and fining-upward in tervals separated by blocky mudstones. This fining-upward motif contin ues into the overlying Erqiao Formation, but coals are lacking. At the beginning of the Late Triassic (Carnian) the previously stable Yangtz e platform, on which peritidal limestones were forming, was drowned an d covered by dark lime mud that was cemented into intraclasts and nodu lar lime mudstone. Black shale and manganiferous pelagic limestone for med a condensed interval, recording maximum submergence. Turbidite san dstone and shale of the Laishike flysch filled the accommodation space of 800 m created during drowning of the Yangtze platform, leading to deposition of shoaling-upward shelf and paralic sandstones and shales, but without significant carbonate production. The succeeding fining-u pward siliciclastics are interpreted as braided-stream deposits with c oals that mark minor marine incursions. The shallow-shelf and braided- stream deposits form a molasse 1500 m thick. It was apparently derived from the west, in contrast to the underlying flysch where palaeocurre nt directions are from the north or northeast. The entire Yangtze plat form became emergent during the Late Triassic and was never submerged again. Subtle local differences in the drowning sequences indicate dif ferential subsidence and suggest that tectonics played a role in the d eath of the Yangtze platform. (C) 1998 Elsevier Science B.V. All right s reserved.