SEDIMENTARY CHARACTERISTICS AND EVOLUTION OF LOWER CRETACEOUS DUHONGMU FORMATION IN BAIYINCHAGAN SAG,ERLIAN BASIN IN INNER MONGOLIA

Sedimentary facies characteristics and evolution of the Duhongmu Formation of Lower Cretaceous in Baiyinchagan Sag were thoroughly studied using core observation data,well logging data,regional geologic data,and previously published data. The study shows that the sedimentary facies are dominated by the fan delta,braided-river delta,turbidite fan and lacustrine facies. Fan delta and turbidite fan were deposited at the steep northern slope,and braided-river delta deposits occur at the southern gentle slope. In addition,at the border of fan delta,braided-river delta as well as the centre of sag developed lacustrine facies mainly occurs in the center of the sag. The Baiyinchagan Sag transformed evolved from a strongly faulted basin to a subsiding depression sunken during the time interval when the deposition of Duhongmu Formation was deposited. Sedimentary evolution of the Duhongmu Formation can be divided into early,middle and late stages. From early to late stage,the area distribution of fan delta facies and braided-river delta facies gradually reduced,and backward about retreating by 3 km and 4-11 km,respectively. At the same time,the distribution of lacustrine facies expanded and became the main sedimentary facies.