TRIPARTITE DIVISION OF THE CRETACEOUS OF THE SICHUAN BASIN

The Cretaceous of the Sichuan basin is represented by non-marine red clastic rocks, up to 3000 m in thickness. Vertically it shows distinct tripartite characteristics. It overlies the Upper Jurassic disconformably and underlies the Lower Tertiary conformably. In recent years, through biostratigraphical work, abundant fossils have been collected; they include ostracods,bivalves, conchostracans,charophytes, spores and pollen. These fossils can be divided into four fossil assemblages. Most of them belong to the fresh-water type and a part belong to the brackish-water type. These fossils exhibit three development stages. They are of major significance in the division and correlation of the Cretaceous of China. According to three sequences of rocks, biotic successions and sedimentary cycles,the Cretaceous of the Sichuan basin may be divided into three series; i.e. the Lower Cretaceous: the Chengqiangyan Group and the Tianmashan Formation; the Middle Cretaceous: the Jiaguan Formation and the lower and middle parts of the Jiading Group （the Wotoushan and Daerdang Formations）; and the Upper Cretaceous:the Guankou Formation and.the middle and upper parts of the Jiading Group （the Sanhe and Gaokanba Formations）.