The Structural Characteristics Of Shallow Strata In The Northwest Of Bohai Bay

The northwest Bohai Bay, lieing in the west of Bohai Sea, is characterized by very shallow water (0~15m in depth) and surficial muddy sediments on the intertidal and shallow sea srea.The shallow stratigraohical structure on the northwest continental shelf of the Bohai Bay has been analyzed by using more than 400km sub-bomttom profiles. Four seismic units Ua, Ub, Uc and Ud can be recognized through stratigraphic analysis, and every two seismic units separate by a regional backscatter suface, furthermore, seismic units Ua and Ub were divided into sub-seismic units Ua1, Ua2 and Ub1, Ub2 by two local backscatter suface respectively. We compared these seismic units and sub-seismic units with the characteristics of lithology, micro-fossile, and dates in three bore cores, to decipher the sequence pattern and sedimentary evolution of the shallow strata in the northwest of Bohai Bay.The seismic unit Ua was formed during the late Holocene about 2 to 0 ka BP. The Ua1, showing a parallel or transparent structure, is composed by the modern aggradational sediments. The Ua2 exposing a prograding reflection pattern with eastward-stepping reflectors, is likely made up by Haihe River delta.The seismic unit Ub,formed during the middle Holocene about 6 to 2ka BP. The Ub1, exhibiting a prograding structure with southwestward-stepping reflectors, is probably driven by Luanhe River delta. The Ub2, displaying an aggradational or northeastward wake-prograding structure, is possibly the Yellow River-derived sediments.The seismic unit Uc was formed during the early Holocene about 10 to 6ka BP, comprises the sediments of estuary tidal and subtidal sediments, characterized by landward upset structure with subparallel reflectors.The seismic unit Ud was formed from the late Pleistocene to early Holocene about 20 to 10ka BP, shows a blank or subparallel backscatter structure, and contains some land river and shallow lake sediments.An entire I-type sequence can be defined in the northwest of Bohai Bay:(1) Lowstand system tract, corresponding to the seismic unit Ud, was formed from the late Pleistocene when the sea-level fallen down to the lowermost position, to early phase of the early Holocene when the sea-level rose and occupied the northwest of Bohai Bay.(2) Transgressive system tract, corresponding to the seismic unit Uc, was formed during the early Holocene when the sea-level rose rapidly to the maximum flooding position in the west coast of Bohai Bay.(3) Early highstand system tract, corresponding to the seismic unit Ub, was caused by aggradational-progradational delta sediments during the middle Holocene.(4) Late highstand system tract, corresponding to the seismic unit Ua, was caused by progradational delta sediments during the late Holocene.