THE STRATIGRAPHIC DIVISION OF THE TRIASSIC IN THE MINJIANG RIVER VALLEY IN THE WESTERN QINLING MOUNTAINS

On the basis of the results of 1:50000 regional geological surveys,the Triassic distributed in the Minjiang River valley in Yanchang County,Gansu Province,may be divided from below upwards into five formations.The lower part is called the Choujiashan Formation,consisting mainly of dolomite and dolomitic limestone.Although no fossils have been collected yet,it may be stratigraphically and lithologically correlated with the Maresongduo Formation in Zoige of Sichuan Province and Tewo of Gansu Province,and its age may be assigned to the Olenekian Stage （late Early Triassic）.The Guojiashan Formation in the middle is composed of limestone with sandstone and shale,yielding abundant fossils of brachiopods,lamellibranchs,gastropods and conodonts,and its age is considered to be Anisian （early Middle Triassic）.Theupper part is successively represented by the Huashiguan,Daheba and Xiamalong Formations, dominated respectively by limestone,quartz sandstone and marls.The last two formations yield plant fossils and sporopollen and their age is assigned to the Ladinian Stage （Middle Triassic）.