Abstract: The Late Mexozoic strata in the Senjitu-Sichakou area of northern Hebei is a sequence of pyroclastics, intermediate lavas and lacustrine clastic rocks. This sequence of deposits has a total thickness of nearly 6400 m and may be divided in ascending order into the Upper Jurassic Chapeng Formation and Huajiying Formation and Lower Cretaceous Qingshila Formation. Chapeng Formation It is composed of tuffaceous conglomerate, sandstone, siltstone and limestone, containing abundant fossils such as conchostracans Nestoria and fish Peipiaosteus pa-ni. Its upper part is intercalated with marl and andesite（with a K-Ar age of 136.2 Ma）. The total thickness is 1011 m. Huajiying Formation It is represented by five sedimentary-eruptive cycles made up of clastic rocks（conglomerate, siltstone and limestone） and intermediate lavas（andesite, andesite agglomerate, basaltic andesite and trachyandesite）. The clastic rocks yield the Rehe fauna——the Eosestheria（conchostracans） -Lycoptera（fish） -Ephemeropsis（insects） assemblage as well as bivalves and plant fossils. The andesite has K-Ar ages of 114.88—101.37 Ma. The total thickness is 4451 m. Qingshila Formation It consists of shale, siltstone and fine sandstone. Its upper part is intercalated with low-quality coal beds, yielding abundant plant and insect fossils, The total thickness is 442 m.