Stratigraphic and three-dimensional morphological evolution of the late Quaternary sequences in the western Bohai Sea, China: Controls related to eustasy, high sediment supplies and neotectonics

摘要

The Bohai Sea is a shallow, semi-closed gulf (basin) at the proximal position of the very wide (one to several thousand kilometers) eastern Chinese margin and was persistently impacted by tectonic frameworks. Its late Quaternary stratigraphy provides an excellent natural laboratory for research on the sedimentary processes and evolutionary patterns on flat proximal continental margins with high sediment inputs and how these processes respond to synsedimentary tectonic activity. However, due to the scarcity of comprehensive sequence stratigraphic studies, our understanding of the above processes is limited. Here, we conducted a high-resolution sequence stratigraphic study using boomer seismic data with pseudo-3D coverage. The seismic data revealed seven seismic units (SUs A-G) within the late Quaternary stratigraphy, including (1) three marine prodeltaic units (SUs A, C, G) characterized by dipping/oblique internal reflections and/or lobate structures, (2) two terrestrial (fluvial/lacustrine) units (SUs B and E) dominated by heterogeneous reflections, (3) an erosional remnant unit (SUs D) with semitransparent reflectors and a lenticular geometry, and (4) a ravinement-attributed aggradational unit (SU F). Based on the extrapolation between the seismic stratigraphy and the onshore sedimentary architecture and well-established sequence stratigraphic models, our preferential interpretation for the four sets of SUs is that they correspond to the highstand, lowstand, falling-stage and transgressive system tracts, respectively. The seismic geomorphology suggests that the bulk of the accommodation is dominated by the former two, which is consistent with the alternative terrestrial-and-marine succession revealed by previous borehole studies. At such a proximal gulf, although the accumulation of thick highstand units is not surprising, the two thick lowstand units that are bounded above and below by two different subaerial unconformities are exceptional. Our interpretation indicates that abnormally thick accumulation occurred under subaerial environments during the lowstand period, which most likely corresponds to alluvial/floodplain progradation/aggradation given the western Bohai Sea is connected to the alluvial North China Plain. The lower subaerial unconformities of the lowstand units are associated with intensive subaerial (most likely fluvial) excavation. Erosion under SU E leads to the formation of incised and eroded areas that can be tens of kilometers wide and tens of meters deep, cutting into SU A or even into SU B, which are several times greater than the last glacial maximum incised valley at the top of SU E. Such intensive erosion appears to be a crucial contributor to the thick lowstand units, as it created more accommodation. The great scale of the subaerial incisions is presumably attributed to the fact that the bulk of them evolved from tectonically triggered depressions because their locations are consistent with the NE-SW- and NW-SE-striking fault populations. The tectonic forces also controlled the accumulation by resulting in high-relief basement blocks for the pre-Holocene sedimentation. However, despite such strong tectonic controls, the subsequent deposition filled and leveled the areas between the high-relief, steep-sided basement blocks and eventually resulted in a relatively flat substratum for post-LGM deposition, suggesting that the geomorphic threshold for geological control at such a proximal gulf was passed because of the high-supply environment.