Sedimentation and evolution of the Mesoproterozoic Kunyang Group in the Huili-Huidong region, Sichuan and its adjacent areas

The Mesoproterozoic Kunyang Group strata were laid down in the Huili-Huidong region, Sichuan and its adjacent areas. They may be arranged, from bottom to top, into the Huangcaoling, Fuliangpeng, Dalongkou and Meidang Formation in Yunnan, and Limahe, Fengshanying and Tianbaoshan Formations in the Huili region, Sichuan. Seven sedimentary facies have been distinguished, in terms of palimpsest sedimentary structures, lithologic features and depositional cycles, for the Mesoproterozoic Kunyang Group, including the fluvial facies, littoral facies, shallow marine shelf facies, tidal flat-lagoon facies, mixed shelf facies, stromatolitic reef facies and carbonate platform facies. The fluvial facies consists of the conglomerates at the base of the Limahe Formations, representing the early sediments in the study area. The littoral facies is made up of the quartzose sandstones with wedge-shaped cross beddings in the Limahe Formation and of the quartzose sandstones and mudstones with ripple marks in the Huangcaoling Formation. The shallow marine shelf facies widely spread in the study area is composed of siltstones and mudstones with horizontal lamination. The tidal flat-lagoon facies is found in the Tianbaoshan, Limahe and Meidang Formations and composed of sandstones, limestones and mudstones with well-developed lenticular beddings, flaser beddings, ripple beddings, herribone cross beddings, interference ripple marks and mud cracks. The mixed shelf facies is mostly confined to the Meidang Formation and composed of siltstones, mudstones and limestones. The stromatolitic reef facies and carbonate platform facies are of wide-spread occurrence in the Fengshanying and Dalongkou Formations. The former is represented by stromatolitic reefal limestones and dolostones. The latter consists of thin-bedded limestones with horizontal lamination, lenticular beddings and flaser beddings, dolomitic limestones and dolostones. The mixed shelf facies occurs in the Meidang Formation and is assembled by siltstones, mudstones and limestones.