Sedimentary Characteristics And Distribution Features In Sequence Stratigraphic Framework Of Subaqueous Fans In Nantun Formation, Ta'nan Depression

Ta'nan depression located in Tamuchage Basin which belongs to Mongolia and is in the same tectonic unit with Hailaer Basin, north of China. These two basins are part of the Central Asia—Mongolia aulacogen. Sedimentary layers in Tamuchage Basin include(from bottom to top) Budate Group of lower Jurassic system, Tamulangou Formation, Tongbomiao Formation, Nantun Formation, Damoguaihe Formation of Lower Cretaceous , Yimin Formation and Qingyuangang Formation of Upper Cretaceous. Syn-depositional faults active frequently during the period of Nantun Formation and the sediment supply was rich, so subaqueous fans developed abundantly in the basin. Some of the subaqueous fans display good industrial oil content, thus further research about the sedimentary and distribution features of subaqueous fans have both theoretical and practical values.Through the comprehensive analysis of well and seismic data, two sequence and eight systems tracts can be indentified in Nantun Formation. The bottom boundary of sequenceⅠis T3, the top boundary is T23-1, Lowstand systems tract(LST), transgressive systems tract(TST), Highstand systems tract(HST) and retrogressive systems tract(RST) are developed in the east, central and south part of the basin. Most of the western part of the basin lacks the presence of the sediments of sequenceⅠ. The bottom boundary of sequenceⅡis T23-1, the top boundary is T22, syn-depositional faults activities became less intence gradually. At the early stage of sequenceⅡ, under the influence of the general subsidence, sedimentation occurred in the whole depression and LST, TST, HST can be indentified during this stage. At the end of sequenceⅡ, faults activities had nearly came to an end, sedimentary layers are wide in range and uniform in thickness, subsidence centers are mainly located at the west and center of the depression.Subaqueous fans in Nantun Formation are classified into two types which include near-shore subaqueous fan and off-shore subaqueous fan. Near-shore subaqueous fan, which is a kind of deep water coarse clastic sediments that formed by terrigenous debris entering directly into lake, are mainly developed in shape of apron near the south and east fault areas of the basin. Through core description, the lithofacies in near-shore subaqueous fans are mainly composed of matrix-supported conglomerates, grain supported conglomerates, pebbly unequal particle sandstones, massive sandstones and turbidites. These lithofacies are interbeded with black mudstones in deep lake. Off-shore subaqueous fans are mainly developed in the central sub-depression. Except for the ones in the southern part of the basin, others are mainly formed by the removement of sediments. The lithofacies in off-shore subaqueous fans are mainly composed of matrix-supported conglomerates, pebbly sandstones, massive sandstones, siltstones and turbidites. On the aspect of sedimentary facies types, both of the two kinds of fans are devided into inner fan, middle fan and lower fan subfacies, the differences are that the inner fan undeveloped and middle fan developed in off-shore subaqueous fans. Lithofacies in the same subfacies of the two types of fans are also different. Distinction also appears in the seismic and well logging properties between near-shore subaqueous fan and off-shore subaqueous fan. The distribution features of subaqueous fans in sequence stratigraphic framework are concluded as follows: In sequenceⅠ, the range of development of subaqueous fans is small in LST, bigger in TST, smallest in HST and small still in RST. In sequenceⅡ, subaqueous fans also developed in LST, TST and RST, but undeveloped in HST and the average range of development of subaqueous fans in sequenceⅡis small compare to that in sequenceⅠ.The development of subaqueous fans in Nantun Formation of Ta'nan depression is influenced by sediments supply, tectonic activity and topography etc. Conclusions are made through the comprehensive analysis of seismic wave-impedance and heavy minerals etc, during the sequenceⅠperiod, near-shore subaqueous fans are mainly supplied by sediments from east and south and deposited at the footwall of the syn-depositional faults. Off-shore subaqueous fans in the south are mainly supplied by sediments from south and maybe underwent a long distant transportation, off-shore subaqueous fans in the central sub-depression may mainly formed by the slumping of the front of near-shore subaqueous fan. During the sequenceⅡperiod, near-shore subaqueous fans are supplied by sediments from east, off-shore subaqueous fans in LST and TST are supplied by the slumping of near-shore subaqueous fan in the east, in RST by the slumping of the delta in the north.