Abstract：The Middle Jurassic Formation in the Pingquan basin is a very thick sequence of red detrital rocks formed under inland dry climatic conditions.From below upward the sequence can be divided into four lithological members;the first member at the base is mainly marked by an alluvial fan depositional system,the second member is a lake depositional system,the third member is a river system,and the fourth member is a composite alluvial fan and river system.The above filling sequence reflects that at the time of deposition of the Tuchengzi Formation the basin experienced three evolutionary stages,i.e.:incipient subsidence,lake basin spreading and basin contraction.