Evidence for sea‐level falls in the Permian‐Triassic transition in the Ziyun area, South China

Abstract

The sedimentology of three sections showing the Permian-Triassic (P-T) transition on an Upper Permian reef in Ziyun, Guizhou Province, southwest China provides evidence of sea-level fall in this area. The P-T transition in the Gendan section, which covers a reef-front sequence, contains a subaerial exposure surface and various desiccation cracks (including rolled-up chips, in-chip cracks, pyramid cracks, vertical cracks and sheet cracks), indicating sea-level falls. The P-T transition in the Shitouzhai section, which covers a reef-core sequence, contains three micritic beds with abundant pseudomorphs of tabular gypsum crystals replaced by calcite. The underlying centimetres of deposits are composed of fine-crystalline dolostone, and have a sharp contact with the underlying reef-core facies. The association of gypsum pseudomorphs and fine-crystalline dolostone suggests an origin related to a supratidal evaporative environment. The P-T transition in the Tanluzhai section, which covers a back-reef sequence, is fine-crystalline dolostone, and shows a laminated algal structure indicating tidalflat environments. The middle part of the dolostone interval has become a collapse breccia composed of dolostone fragments and a matrix of micrites or crustose multi-generation calcite cements. The formation of the collapse breccia is interpreted to be related to the dissolution of the evaporite interbeds originally present between the thin dolostone beds. The formation of the evaporite deposits requires a supratidal condition.