Attributes And Evolution Of Buried Channel Systems On Modern Yellow River Delta Since The Last Glacial Maximum

Shallow buried channel systems are one of the main disaster-inducing factors, and they are also playing an important role in the study of the sedimentary environmental evolution since late Quaternary. The sea level was130to155m lower than the modern value worldwide during the Last Glacial Maximum (LGM), which caused the exposure of the modern continental shelf of China. The modern Yellow River sub-delta was not submerged until the mid-Holocene. Togather with the fluctuating of sea-level and the rechanneling of Yellow River, a complex, marine-continental alternant strata and multi-phase paleo-channel systems inside were formed beneath the focus area. It was difficult and controversial to rebuilt the evolution pattern of the paleo-sedimentary settings and paleogeography in the study area because of the semienclosed environment of the Bohai Sea.To study these problems, funded by the Public Science and Technology Research Funds Project of Ocean (No.20100529-4) named "Study on the standard stratigraphy and geological disaster on the modern Yellow River delta", on the basis of the investigation funded by908project, in this paper, the sub-delta area in the vincity from the modern Yellow River Habour to the Mouth within20m in depth on the modern Yellow River delta was focused on. A total of650km of high-resolution shallow-seismic profiles were acquired with a CSP2200sparker sub-bottom profile system and a core named GYDY was obtained from2011to2012. According to the seismic stratigraphic, chronostratigraphic, lithostratigraphic, biostratigraphic and magnetostratigraphic analyses, the depositional framework of the study area since LGM and the attributions and distributions of the paleo-channel systems were revealed, furthermore, the evolution pattern of stratigraphy and paleo-channels was discussed.The geological and geophsical analyses of the strata were demonstrated that the stratigraphic succession can be divided into six depositional units (DU1-6) and five seismic units (SU1-5), which correspond to four sedimentary facies and correlate with other coeval successions in the Bohai Sea area, designated the delta facies, the first marine facies, the second continental facies and the second marine facies, respectively. The sub-delta channel system had a two-phase fluvial drainage, which existed on the upper interfaces of SU3and4(T3and T4reflecting interfaces, respectively), as evidenced by erosion of the two strata and chaotic seismic units that formed when the focus area was exposed subaerially.A two-phase channel systems were detected beneath the strata which was deposited since the LGM from the seismic profiles. The first-phase channels were of the eroding type, including three river courses, designted channel a-c, respectively. These channels were formed during the late Pleistocene (early deglacial) on the upper interface of SU3. Channel a and b formed beneath the focus area A with mid-scale, exhibiting low sinuosity characteristics. Channel were meandering course with central bar inside. The second-phase channels were the estuarine dendritic drainage system that was formed during the early Holocene (late deglacial), and they exhibit depositional channel characteristics and located on the upper interface of SU3.Based on the fluvial assumption and the paleo-flow estimates during the deglacial, it was confirmed that channel c of the first-phase drainage was the contemporaneous main stream of the Yellow River. Because of the large scale of the cross-sectional area, its average discharge was5.9times that of the modern Yellow River.(The minimum value was also larger than the modern Yellow River.) In addition, the mean discharges of channels a and b were1.2times and1.1times that of the modern drainage, respectively. The discharge of channel d, was far less than the values of the first-phase channels. However, because channel d could be merely part of the estuarine dendritic drainage, the total discharge ofthe drainage system can not be estimated, the range and mean value of the channel d discharges required verification and modification. Estimates of the hydrodynamic parameters revealed identical velocities of the four courses and large discharges that were characterized remarkably by deglacial channels, which demonstrated the probable correlation between them and the paleo-Yellow River system.Moreover, the evolutionary hypothesis of the buried channel and the sedimentary environments was discussed in the paper, which was in the response to the sea-level variation and the switching of the Yellow River since the Last Glacial Maximum (LGM). According to this pattern, the formation age of the paleo-channels was modified from the LGM to the last deglacial, which is supported by the disassembly theory of the Yellow River during MIS2. The first-phase channels were formed for the molten ice caps and were abandoned with the rechanneling of the Yellow River; they were buried by the fluvial deposits that were carried by the dendritic system above. However, after filling the first-phase channels, the second drainage was covered by seawater during Huanghua transgression with rapid sea-level rising. With short subaerial exposure, the deltaic stratum of DU4was small scale and had a lower gradient. Next, the seawater stably filled the modern sub-delta. Despite being unable to have incising of any channels, many buried tidal creeks and disturbed geologic bodies were discovered on the upper abandoned lobes (T2reflects interface in the shallow seismic profiles), which were filled with the5-m thick deltaic stratum of DU1.Due to the lack of micro-fossils data, the paleo-environemtns and the formation time of the sub-delta channels during LGM can not be comfirmed. More work should be done to fully understand the environmental information of the strata and the geological history of the whole Yellow River system.