Lithology Characteristic and Sedimentary Environment of the Member 3of Dengying Formation,in Yangba Area,Northeast Sichuan

Nanjiang Yangba,is located in the northeast edge of upper Yangzi platform where the member 3 of Dengying Formation is mainly composed of dolomites.Based on studies in rock characteristics and sedimentary facies of Yangba Profile,and also combined with results of previous studies,indicate that the member 3 of Dengying Formation,in this district,can be divided into three lithologic sections:the lower part is mainly consisted of the silt-micritic and mud dolomicrites;the middle part is mostly composed of dolarenites with local brecciated mud dolomicrites;the major composition of the upper part is the silicified fine-microcrystalline dolomites,silicified stromatolithic dolomites with local brecciated silt-micriticdolomites and finely crystalline dolomite.Most of the rocks have laminated,banded,stromatolithic and stylolitic structure;and considered that the member 3 of Dengying Formation in this area is deposited in the environment of carbonate platform-the epicontinental ocean,and there are three sedimentary subfacies:lagoon,shallow-water carbonate sand beach,tidal flat.The entire profile is a transgressive-regressive sedimentary cycle.