Discussion of Plants Fossils and Its Geological Ages of the Lower-Middle Jurassic in Mangya Area, Qinghai Province

The Jurassic in the Mangya area is the northwestern outspread part of the coal-bearing series of the northern Qaidam basin in Qinghai province.This area is located at the depression in front of the southern Altun mountain,and is named the Gus coalfield in the Qinghai coal geology zonation.The Lower-Middle Jurassic in this area is the only coal-bearing strata.The characteristics of the genera and species and their combinations of the plant fossil assemblage 1 and 2 were analyzed.Through biostratigraphic correlation with neighboring areas,the ages of two fossil assemblages were determined.Fossil combination 1 is analogous to the Cladophlebis Acme Zone,and its geological age is roughly equivalent to the Pliensbachian.Fossil assemblage 2 is analogous to Coniopteris Acme Zone,and its geological age is equivalent to the Bajocian-Bathonian.The coal-forming periods in this area is further discussed based on these analyses of basic stratigraphic sequence and fossil assemblages.