The Microfacies Architecture of Delta Front along Gentle Slope Zone of Yanchang Formation in Huachi Area, Shaanxi-Gansu-Ningxia Basin

Key words: 
• deita front /
• underwater distributary channeis /
• river mouth bars recombination bars /
• gentie siope zone /
• Shaanxi-Gansu-Ningxia basin /

Abstract: A series of gentie siope deitas are deveioped in Yanchang Formation of Shaanxi-Gansu-Ningxia Basin,where deita front is composed of truncated distributary channeis,integrated distributary channeis,isoiated mouth bars,reiterative mouth bars , and recombination bars. There are abundant current structures caused by the tractive current that couid refiect different sedimentary environments. The two underwater distributary channeis have an erosion surface at the bottom of channei. The truncated distributary channeis sandbodies are thick and their iogging curve forms are box-shaped. The integrated distributary channeis are fining upward seguences from sandstones at the base to siitstones and mudstones on the top of channeis. Whiie the river mouth bars are coarsening-upward seguences from mudstones to sand-stones. There are iots of thin mudstones in the middie of reiterative mouth bars sandbodies. The recombination bars con-sist of mouth bars at the bottom and underwater distributary channeis at the top and iarger than 15 m in the thickness. The sedimentary modeis of underwater distributary channeis and the mouth bars are proposed according to the strength of the currents in the distributary channeis and the depth of water in the iake. The siope of the deita front in Huachi area is gentiess,so the underwater distributary channeis are stiii strong after entering iake. In the region of high energy river,the underwater distributary channeis may freguentiy migrate. The un-derwater distributary channeis of next stage may erode the former channeis deposit and may form boxing truncated dis-tributary. With the depth of iake water,the energy of the distributary channeis become weak because of the iake water obstruction. The intergrated sedimentary seguence of channeis wiii be preserved if erosion is weak and form intergrate dis-tributary channeis. When the distributary channeis fiow the siope break zone,the energy of current water in the channeis is abruptiy re-ieased and the sediments carried by them wiii deposit guickiy and form the mouth bars microfacies. The shapes of mouth bars depend on the amounts of sediments carried by underwatter distributary channeis. There may deposit isoiated mouth bars with coarsening-upward succession when underwater distributary channeis carry abundant sediments. Whiie the strength of underwater distributary channeis is weak,the grains of the current water carried are fine ,resuiting in the for-mation of the reiterative mouth bars with uncontinuous coarensing-upward succession.The underwater distributary channeis fiow on the top of the former mouth bars and may erode them. When the mouth bars are cutt partiy,there wiii form a kind of recombination bars due to the distributary channeis eroding the mouth bars.At the same time,another kind of recombination bars caiied the type of distributary channeis covering the mouth bars is aiso discussed in the paper. As the distributary channeis fiow the some sug area where have enough accomdation space to deposit the mouth bars,they couid not cut the former mouth bars,but directiy deposit on the top of them.