1. Department of Geology,Northwest University,Xi'an,Shaanxi 710069,China;
2. State Key Laboratory of Continental Dynamics,Northwest University,Xi'an,Shaanxi 710069,China;
3. Research Institute of Northwest Company,Si-nopec,Urumqi,Xinjiang 830011,China
Abstract：An imbricateed progradational reflection caused by low velocity shale is found in Upper Triassic thick massive mudstones.It flows westward and consists of three irregular planar oval shaped lobes,each with 15~20m thick and 30~45km2 sizes.Three borehole data reveal that it is dark grayish pure shale and sandwiched within a background of dark grayish and brown-grayish calcareous claystone.The progradational reflection indicates a clinoform of 2.1 degree of ancient depositional surface.As the depositional background is deeper lacustrine environment,the progradational reflection may be muddy gravity flow deposits.Another similar southward progradational reflection occurred in the same horizon.It is close to down-thrown block of Lunnan Fault,a tectonic fault as the southern boundary of Saya uplift.Well YQ2 located at the southern frontier of the reflection reveals that the progradtional reflection is also caused by a low velocity of shale.Despite seen rarely,shale progradations are seldom found on outcrops.Lack of coarse sediment may be consequence of fine lilthology of eroded source area.
