| Located in the eastern part of Mongolia,Tanan Depression belongs to the central rift zone of Hailaer-Tamuchage Basin. It is a compound fault depression with fault in the east and overlap in the west, and contains west slope belt, west secondary depression, western buried-hill fault belt, center secondary depression, central buried-hill fault belt, east secondary depression and fault terrace zone. Lower Cretaceous mainly develop the stratas of Tunbomiao Formation, Nantun Formation, Damoguaihe Formation and Yimin Formation. Among which Damoguaihe Formation and Nantun Formation are the the focus of this paper. Based on sedimentology, sequence stratigraphy and cyclic stratigraphy, we used core, logging and seismic data to study the patterns of seismic reflection termination, which were applied to identify the sequence boundary, analysis base-level cycle and seismic facies. Ultimately, we established the sequence stratigraphic framework of fine separate-layer and revealed how the sequence stratigraphic framework controls the distribution of sedimentary system. It has important practical significance to the fundamental geology and the breakthrough of hydrocarbon exploration.The patterns of seismic reflection termination was first presented by Mitchum, etc in 1977. The patterns contains onlap, downlap, toplap and truncation. The patterns of seismic reflection termination were further perfected in 1989, and apparent truncation was presented. Meanwhile, the study of stratigraphic pattern was added to the means of identifing sequence boundary, and the geological significance of depositional termination and stratigraphic pattern were discussed.Logging data, conventional seismic data, instantaneous phase and wave impedance inversion of seismic reflection data were used to determine the patterns of seismic reflection termination. The key of identifing apparent truncation is judging the source direction and observing along the section in the direction of dip. The key of identifing truncation is finding where the denuded sediment is redeposited. Toplap develops in delta sedimentary. Onlap develops in shore-shallow lake sedimentary, and indicates the base-level rise. These patterns have a indicative significance to identify sequence boundary. However, seismic reflection termination caused by packing structure and sublacustrine fan don't indicative the exist of sequence boundary.Based on identifing correct patterns of seismic reflection termination, we studied the sequence boundary of Damoguaihe and Nantun Formation in the area. Three second order discontinuity including T3,T22 and T2 were founded in Tanan Depression. Nantun Formation corresponds to the upper part of fault super-sequence. Damoguaihe Formation corresponds to fault-depressed diversionary super-sequence. Two third order discontinuity including T222 and T201 were founded. Unconformity develope in basin margin. the sedimentary system above and below the third order sequence boundary have no change. Two maximum fiooding surface including Sq1-2 and T23 were founded. Above the maximum fiooding surface, the sedimentary area broaden, the water body deepen, and the stratigraphic pattern is different from these below.We analysed the change of base-level of Damoguaihe and Nantun Formation in Tanan Depression, and put forward some conclusion. In Tanan Depression, regional tectonic movements control the basin type and the development of Megasequence and Supersequence. In faulted period of Nantun Formation, the episodic movements of basin-controlling fault control the development of third-order sequence. Sequence style formed in intense fault period is developed in Sequence I of Nantun Formation, while Sequence style formed in late fault period is developed in Sequence II of Nantun Formation. In fault-depressed diversionary period of Damoguaihe Formation, the episodic movements of tectonic subsidence control the development of third-order sequence. The difference of subsiding extent causes varied gradient of foreset bed in delta front. Sequence style formed in depression period is developed in Damoguaihe Formation.Based on the seismic reflection markers of Tanan Depression, 21 seismic facies types including 8 main and 13 secondary types, were identified by using amplitude, continuity, internal structure and external form of seismic reflection wave. Wedge-filling facies with medium-intense amplitude and moderate continuity and sub-parallel facies with medium amplitude and high-moderate continuity develop in Nantun Formation. Foreset facies with medium-intense amplitude and moderate continuity and sub-parallel facies with medium-intense amplitude and high-moderate continuity develop in Damoguaihe Formation.Distribution of sedimentary system is controled by sequence stratigraphic framework. Supersequence controls the types of sedimentary system. Nantun Formation is characterized by nearshore subaqueous fan—semi-deep lacustrine facies sedimentary system, while Damoguaihe Formation is characterized by delta—lacustrine facies sedimentary system. Third order sequence controls the distribution location of sedimentary system. Nearshore subaqueous fan develops in southeast of sequence I and central close to basin-controlling fault of sequence II. Delta develops in northwest of sequence III. In sequence IV, there are two delta sedimentary systems, one located in northwest, and another located in northeast. System tract controls the stacking pattern of sedimentary system. TST develops retrogradation nearshore subaqueous fan. The scope of nearshore subaqueous fan is minimum in HST. RST develops progradation nearshore subaqueous fan, and the scope extend.
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