The Upper Triassic Yanchang Fm underwent denudation and incision of river valley to different degrees before the deposition of Lower and Middle Jurassic in Ordos basin due to the strong tectonic uplifting du-(ring) Indosinian movement

resulting in rolling paleotopography.Five major paleo-streams

including Gan-Shaan and Qingxi

were developed in early Jurassic'of which Gan-Shaan paeo-stream was the main stream channel.In the Gan-Shaan paleo-stream drainage area

the strata from Fuxian Fm to Yan'an Fm have different sedimentary and palaeogeographic characteristics

and they altogether reflect the structural and sedimentary evolution history of the basin.