Quantitative Seismic Geomorphology Of Multiple Fluvial Patterns In The Post-rift Lacustrine Basin

Guided by the new principle of fluvial sequence stratigraphy,the integrated seismic-and well-based sequence stratifraphic study shows that the Neogene Lower Minghuazhen Member corresponds to one complete third-order sequence with four fourth-order sequence named as SQm~1,SQm~2,SQm~3 and SQm~4,each with low accommodation system tract(LAST)and high accommodation system tract(HAST).The increasing rate of base level generally slows down during the depositon of the Lower Minghuazhen Member,while the net-to-gross ratios in different sequences gradually increases.The trend in thickness of four sequences remain relatively constant in the study area,which is characterized by thiner part in the uplift area of the eastern Shaleitian area and the tendency of thickening towards eastern and western part.Meandering-and braided-river deposits predominated through the Lower Minghuazhen Member of the study area after a detailed inverstigation of regional geological settings as well as a comprehensive sedimentological analysis of lithofacies,seismic and logging facies.The paleo-flow direction of bypass fluvial systems in the Shaleitian area generally shifted from SSW to SSE during the deposition of the Miocene Lower Minghuazhen Member combined with the calibration of seismic reflection,seismic attribute and drilling lithology.The paleodrainage system could be attributed to Luan Paleo-river or Qinglong Paleo-river system originated from the mountain and hill of the North Hebei Plateau.Meandering rivers were well developed in SQm~1 and SQm~2,while braided rivers and meandering rivers could coexisted especially in the low accommodation system tract of SQm~3 and SQm~4.The sinuosity of meandering rivers in typical slices of the key Shaleitian area,which are transported by mixed-load sands,could vary from 1.18 to 1.83.The sinuosity of meandering rivers in the high accommodation system tract are overally higher than those in the low accommodation system tract.There are positive scalling relationships between meandering arc-height,channel wavelength and meandering belt width,and the ratio of meandering belt width to meandering arc-height usually ranges between 1 and 4.And there is also a strong power-law scaling relationship between channel bar length(0.33km?3.22km)and width(0.12km?3.61km).Geomorphological and hydrological study shows that both the difference of regional depositional paleo-slope gradient and the variation of bankfull paleodischarge are critical influence factors for fluvial transition in the Shaleitian area during the depositon of the Lower Minghuazhen Member.The depositional variation from braided-river dominated deposits in the Guantao Formation to meandering-river dominated deposits in the Lower Minghuazhen Member can be attributed to multiple episodic and rapid subsidence of the Bohai Bay Basin from late Miocene(12Ma?5.3Ma),and the occurrence of braided-river deposits can be interpreted as the relatively lower newly-created accommodation rate in the middle to late phase of late Miocene rapid subsidence episode.Quantitative paleoclimate study based on palynological assemblage from a key well indicates that moderate intensification of East Asian Moonsoon could be the immediate cause of well-developed braided rivers in the low accommodation system tract of SQm~3 and SQm~4within the late Miocene global cooling trend.Besides,the difference of vegetation coverage in the riparian zone could also exert a great influence on the vertical transition between meandering river and braided river.