Study On Squcncc And Sedimentary Fades Of A’ershan Forming In A’nan Depression Of Erlian Basin

The A’nan depression is located at the east of Magni org in Erlian basin. It is aminor faulted lake basin. The main target sedimentary intervals in this paper are theArershan formation of the Lower Cretaceous. During the period of Arershanformation, the depression is at the early development age of Erlian basin, when theenviorment are with humid climate, lower lake level and sufficient sediments supply.In the study rea, the main tectonic belts include the A’nan anticline, Hanan buried hill,Menggulin anticline and Shannan sag, and A’nan slop. These tectonics control thesediment distribution, change of the base level, as well as the types of sequence andthe sediment.According to theory of continental sequence stratigraphy, two third-ordersequences in the A’ershan formation of A’nan depression are divided with examiningcore, log and seismic data. These sequences includ SQ1(layer A1+A2) and SQ2(layer A3+A4). The unconformable contact between SQ1and SQ2is recognizedaround structural high part of the Menggulin Fault. The unconformable contactbetween SQ2and overlying strata is discovered widely in the places including mostsmall uplift in this depression and slope belt of the southern boundary and upliftedside of the A’ershan Fault. The unconformable contact was also found between SQ2and overlying strata, and it has a more massive scale than those of SQ1-SQ2. Inaddition, the sand and sandy conglomerate in the A’ershan Formation and the thickermud in the overlying strata are developed. Therefore, the contact between SQ2andoverlying strata is also boundary between LST and TST in the process of basin filling.In the process of indentifying sediment type, the data of cores, logs and seismicfacies were used. The sediment environment of each period was also analyzed. Thesedimentary facies such as fan delta and near shore subaqueous fan were identified.Among the sedmentary facies, the fan delta is formed along the steep slope withsufficient sediment supply, and it is composed of sandy conglomerate with the grainacting as supportion. In the fan delta, scour surface and braid channel are widelydeveloped. The near shore subaqueous is formed near downthrown side of growthfault in the deep lake of rapid subsidence.According to the results of sediment facies analyses, the distribution of threesediment facies of A’ershan Formation was mapped. In the study area, fandelta are mainly developed in close to the main source area, the inter space of thevalley and the structural transform zone. The nearshore subaqueous fan are mainlydeveloped in close to the secondary source area or at the end of sand body. Due to thesuddenly increase of accommodation space, the nearshore subaqueous fan is formedand its scale is smaller than that of fan delta.