Chaochuan Fm

Its conformable basal part is differentiated from the grayish green moderately thick-bedded fine-grained sediment tuff of the underlying Guantou Fm by the appearance of purple red moderately thick-bedded gravel-bearing tuffaceous feldspar lithic sandstone with thin-bedded break red mudstone.

Its top is bounded by the occurrence of purple gray moderately thick- and thick-bedded sandy conglomerate. It is in conformable contact with the overlying Fangyan Fm strata.

The formation is extensively distributed in Zhejiang, especially in its southeast part. In the Wuyi-Zhuji area, there are a few volcanic partings in the red clastic sedimentary rocks, which differ from different basins in some extent. In the Laozu basin of Lishui there is a basalt intercalation in the lower part. In the Songyang basin, trachyte-andesite-porphyry and amygdaloidal andesite and basalt are intercalated in the formation. Southward to Hengshan, alkaline rhyolitic breccia ignimbrite is intercalated. In the Gushi area, augite andesite porphyrite occur. In the Liucheng basin the basalt and amygdaloidal andesite basalt are extensively distributed. In the Hushan basin red clastic rocks are occasionally intercalated with andesite porphyrite and breccia tuff. Near Guantantou in the southwest part of Longyou crops out the purple gray silstone of the formation, yielding gastropods Lioptacodes aff. choinoky, Viviparus onogoensis; ostracods Plicatounio (P.) sp., Nakamuranaia chingshanensis, Trigonioides (T.) sinensis, Nippononaia zhejiangensis; ostracods Cypridea (Morinina) heauensis, C. (M.) houyelingensis, Cypridea (Bisulcocypridea) oblonga, etc. The Chaochuan Formation is generally 500-832.9 m thick and up to 1353.9 m at maximum (Houling-shan) in the area. In the Wencheng-Zhenhai area volcanic rocks are even developed in the volcanic-tectonic basins, where the formation varies considerably in lithology. In the Lujiang-Sixi depressed area of Taishun, the formation is mainly represented by red macroclastic sedimentary rocks, intercalated with acidic and small amounts of moderately acidic pyroclastic rocks. In the Wencheng volcanic area, intermediate volcanic rocks are relatively developed; there exists relatively thick andesitic agglomerate near Shuiyinjian; in the Zhoudun area there occur basic volcanic rocks including olivine basalt porphyrite and amygdaloidal basalt, which are not persistent in thickness; In the Fanshan volcanic depressed area, the red beds are intercalated with a lot of acidic volcanic rocks. In the Wangzhou caldera there exist intermediate volcanic rocks including andesite, andesitic agglomerate, tuffaceous breccia and tuff, where the formation varies considerably in thickness, for example, 880.6 m in the Shanmen volcanic depressed area, only 190.7 m in Hengkeng and up to 1083.1 m in the Fanshan area.