The Preliminary Sequence Stratigraphy Research Of Modern Luanhe River Delta Since The LGM

Delta is located in interaction between land and sea. It is highly sensitive to climate change, tectonic movement, sea level fluction and sediment supply. All the reasons above make it is the best area for sequence stratigraphy research. Luanhe River Delta is an important place as an environment link of land and sea in Bohai Sea, which is south to Bohai Bay and backed by Liaodong Bay. The sequence stratigraphy research of this area has great significance for the Luanhe River Delta system even for the entire Bohai Sea.In this paper, we divided sedimentary facies of boreholes H2 and H3 using analysis of lithlogy, grain-size, micropaleontology and 14 C dating. The sedimentary faies in this area since the LGM(Last Glacial Maximum) are composed by fluvial facies, estuary facies, shallow sea facies, prodelta facies, etc. Combined the core with shallow seismic profiles, we made the identification of sequence stratigraphy and analyzed the age, thickness and seismic facies of sequence unit. On this basis, we made a preliminary sequence stratigraphy research of the Luanhe River Delta since the LGM.The sduty showed that Luanhe Riever Delta strta since the LGM is a sixth-order Ⅰtype sequence stratigraphy. At 2.3ka BP, the sea level dropped to 135~150m water depth, and the entire Bohai Sea was exposed, when incised-valley developed widely. At that time, wind-blown deposits deposited between the incised-vally, gravel coarse sand filled incised-vally in the shore region when gravelly sand filled incised-vally in the fluvial plain. The bottom of this fluvial sediment is the SB interface(Sequence Boundary). The underlying strata of SB interface are shallow facies formed in Xianxian Transgression which age is earlier in core H3. However, the strata in H2 are continential facies.After 9.5 cal kyr BP, the headbay delta facies deposited in both core H2 and H3 which are the beginning of transgression. Core H3 deposited a quite perfect composed of headbay delta-river dominated estuary-barrier. The TS interface(Transgressive Surface) is the bottom of headbay delta and separated the TST(Transgressive System Tract) from LST(Low System Tract) in core H3. As LST is not existed in core H2, the SB interface is TS interface.Since 7 cal kyr BP, the sea level reached the highest, core H3 is lack of HST(High System Tract) on account of the distance from provenance. Core H2 deposited shallow sea facies-shoreface facies-prodelta facies, and separated from TST with MFS interface(Most Flooding surface). The three interfaces mentioned above separated the strata into LST, TST, HST.LST: LST is located at SB interface and TS interface, and it is composed of taupe lightly red gravel coarse sand which is formed before 9.5cal kyr BP. LST can not be identificated in the shallow seismic profiles. LST has an uncomfortable contact with the underlying shallow sea deposition in core H3 which identificates the erosion of incised-valley. The lithology of LST in fan delta is coarser than in the subqueous area. The former sediment was rivers detainment sedimentation while the later was deposited by flux when sea level fluenced it.TST: TST is located at SB interface and MFS, which age is about 9.5~7.0cal kyr BP. Shallow seismic profiles show clutter reflection phase. headbay delta facies, estuary barrier facies were recognized in core H3 and estuary barrier facies in core H2. The depositional rate of TST in core H2 is 0.287cm/yr.HST: Coastal HST is primary deposited by Luanhe River which is dominated by progradational reflection configuration. It is only recognized in core H2 and upper 13 m water depth in shallow seismic profiles. The thickness of strata decreases in the direction of sea. Depositional rate at 290~155cal yr BP is 2.68cm/yr, and at 155~0cal yr BP is 3.48cm/yr. Offshore HST is Holocene shallow sea facies which was uncomformable contacted with underlying TST. The offshore HST were recongnized, which thickness is 1~4m.This study carried preliminary study on the sequence stratigraphy of modern Luanhe River, and identified delta deposition, estuary seposition and channel depositions, which has some guiding meaning to the sequence stratigraphy, sedimentary process and the possibly marine sand ranks.