The sedimentary characteristics of the lower part of the Yanchang Formation(Chang 10—Chang 8 oil bearing intervals)are quite different from the middle and upper parts of the Yanchang Formation(Chang 7—Chang 1 oil bearing intervals). During the deposition of the lower part of the Yanchang Formation，the western and southwestern areas of the basin were characterized by a gentle topography and the deposition was dominated by the braided fluvial，braided fluvial deltaic and coastalshallow lacustrine facies. The content of chemically unstable components was relatively higher in clastic rocks. The depositional framework changed greatly during the deposition of Chang 7 interval of the Yanchang Formation. The sedimentary systems changed to alluvial fan and fan delta in the west and southwest. The size of the downwarping depression increased quickly and semideep and deep lakes covered an area of over 10×10.4 km2. Simultaneously，the depocenter of the Ordos Basin was shifted tens of kilometers southwestwards. The component maturity of the clastic rocks increased and the rock combination was obviously different in the west and east dispersal tracts of sediments. Furthermore，analyses of  the distribution of the tuffs in the  Yanchang Formation indicate that the tuffs are steadily distributed in the basal part of Chang 7 interval of the Yanchang Formation and they were derived from the syndepositional volcanism. Their thickness decreased from the southwest to the northeast. These analyses indicate that the latest depositional period of Chang 8 interval of the Yanchang Formation is an important transitional stage in the basins sedimentary evolution. A strong tectonic movement occurred in the western margin and southwestern margin of the basin and volcanic activities took place  frequently.  These events represent the first episode  of the Indosinian Movement.