Geologic characteristics and lithologic member division in the Late Jurassic Manketou'ebo Formation in Wulanhade area, Dongwuzhumuqinqi, Inner Mongolia, China

According to the achievement of the project named “1∶50000 Areal Geology Survey in Huiyin Obo, Daletingchagan, Gulabosaihan Obo, Wulanhade Farm”, it is suggested that the volcanic apparatus type for Late Jurassic Manketou′ebo Formation in the Wlanhade, Dongwuqi, Inner Mongolia is caldera , from the early stage to late, There are four evolution stages: initial eruption stage, large-scale eruption stage, collapse deposition stage of caldera and regrowth of caldera (subvocanic rhyolite), as same as the typical caldera evolution model, basically. In the same stage, the rock associations have the same eruptive strength. So, the former three stages which belongs to volcanic stratum can be devided into three members, respectively. The first Member is tuffaceous conglomerate with tuffite; the second Member with the main rhyolitic ignimbrite, welded rhyolitic breccia tuff and rhyolitic tuff, with local have little rhyolite; the third Member is breccia tuffite and tuffite. This division has a uniform standard, and it is easy for operable mapping in the field. Each lithologic member expresses the strong or weak features of the caldera activity, recording the evolution stage of the caldera.Thus, the divison integrate the lithologic member division with the study of the volcanic evolvement organically, and it is good for building correct stratigraphic sequence and stratigraphic correlation in the large area.