Sedimentary Evolution and its Controls on Coal Accumulation for the Late Permian in the Upper Yangtze Area

The study areas include Guizhou, southern Sichuan, and eastern Yunnan, where abundant coal resources are preserved in Late Permian strata. The Late Permian key fauna and flora and their combinations are studied and a scheme of stratigraphic multi-subdivision for Late Permian succession are established. Typical Changxingian ammonoids such as Sinocel ites sichuanensis and Pseudotirolites qianjiangensis have been found in C₁₂ coal seam of the Huoshaopu coal mine, Panxian County, western Guizhou, and a Changxingian Brachiopod fauna have been found in B4 marker limestone of the Yanbeihou section, Zhijin County, western Guizhou. This makes it possible to subcorrelate the Late Permian invast areas of southern China. For lithostratigraphic purposes, the upper Permian is subdivided into the Longtan Formation (Wuchiaping Formation, Middle and lower Members of Xuanwei Formation) and the Changxing Formation (Wangjiazai Formation, Dalong Formation, and upper Member of Xuanwei Formation). The Longtan Formation is subdivided into the lower Member and the upper Member. Synthetic studies suggest that the Late Permian coal-bearing sequences in study areas were mainly formed in five depositional systems, including braided river and braid delta system, alluvial plain fluvial system, deltatidal flat system, Lagoon-tidal flat system, and carbonate Platform System. The delta-tidal flat system is believed to be the most important system for the coal accumulation in western Guizhou. This system can be further divided into the fluvial-dominated upper delta plain, fluvial and tide influenced transitional delta plain, and tide-dominated lower delta plain and tidal plain environments. The overall paleogeographical units of the Late Permian in the study areas include non-marine alluvial plain dominated by fluvial channel, alluvial fan in eastern Yunnan, transitional paralic plain dominated by deltatidal flat system and lagoon-tidal flat system in westerm Guizhou and southern Sichuan, marine carbonate platform in eastern and southern Guizhou, eastern part of southern Sichuan, and southern part of eastern Yunnan, and deep water fault basin in the vicinity of Ziyun of southern Guizhou. Lithofacies paleogeographical maps of Early Longtanian stage, late Longtanian stage and Changxingian stage have been drawn based on analysis of various contour maps such as clastic and carbonate rocks ratio, thickness of sandstones, percentage of sandstones, thickness of coal seams, and thickness of strata.