Multiple fluvial styles in Late Miocene post-rift successions of the offshore Bohai Bay Basin (China): Evidence from a seismic geomorphological study

During the Late Miocene, river systems exhibiting different planform styles caused accumulation in the post-rift offshore Bohai Bay Basin in China; these changes in the fluvial style have profound implications for the evolution of the Bohai Sea shelf and the Late Miocene monsoon-controlled climate. Using three-dimensional (3D) seismic data and well and mud logging data, seismic- and well-based fluvial sequence stratigraphic analyses have been untaken. The lower Minghuazhen Member can be subdivided into four fourth-order sequences (SQm1 to SQm4), each of which contains low and high accommodation systems tracts. Five fluvial styles, including wide trunk incised valleys, narrow-medium incised tributary valleys, medium trunk channels, narrow-medium low-sinuosity channels, and narrow high-sinuosity channels, are recognized based on seismic expressions and well-log patterns. The seismic geomorphological study of selected stratal slices indicates the temporal change in the fluvial styles from low-to high-sinuosity channels between LAST and HAST. As shift in drainage direction from SSW to SEE occurs between SQm1 and SQm2, which may be attributed to the lateral migration of the Late Miocene paleo-Luan River and the paleo-Qinglong River that originated from Yanshan Mountain to the north. The development of wide incised braided trunk channels together with some narrow-medium incised tributary systems at the base of SQm3 and SQm4 can be interpreted as a response to climate-induced enhanced sediment supply. In addition, the paleoclimate reconstruction based on the palynological analysis reveals that the Late Miocene fluvial transition from a meandering river to a braided river occurring at SQm3 and SQm4 could be linked to multiple periods of short-lived intensification of the East Asian Summer Monsoon (EASM) during the late Late Miocene within an overall declining trend; this might have been conducive to triggering seasonal braided rivers under the higher monsoon rainfall.
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