Episodes of tidally-forced swale erosion on the inner shelf interspersed with millennial fluviodeltaic progradational interludes: Insights from northern Bohai Bay, China

Erosional swales are common features on the seafloor with vigorous bottom flow(s). On continental shelves with energetical tidal currents and limited river discharge, the initiation and evolution of tidally-induced erosional depressions and bedforms have been extensively discussed, but little is known about the evolution of such features when interspersed with active deltaic progradation over millennial timescales. A recent boomer seismic investigation revealed a swale erosional complex within and proximal to the subaqueous delta in northern Bohai Bay (NBHB), China, providing us with an opportunity to investigate such features. Here, using the same seismic dataset, we analyze the morphology, reflective characteristics and evolution of the erosional and depositional structures. The erosional swale complex consists of two episodes of tidally-induced incised channels within a northern progradational system (NPS; located in the northernmost Bohai Bay and prograde to the southeast), followed by an episode of extensive trough-and-bump formation (negative-and-positive relief) within the middle and northern NBHB, which are also erosional features. These features are all highly lineated, oriented WNW-ESE; we attribute their formation to the similarly-oriented tidal flows in this region. The multi-episode evolution of the swale erosional complex is predominately associated with progradation of the NPS, whose lobes cap the pre-existing erosional features and provide substrate for new erosion. The trough-and-bump complex was subject to long-term erosion, and may represent an evolution from the previous linear, tidally-forced marine incisions. The complex was subsequently buried in part by the step-wise progradation of a southern progradational system (SPS; developed within the southern 2/3 Bohai Bay and prograde to the north/northeast). In addition, we also observed clusters of longitudinal bedforms with convex internal reflectors superimposed on the erosional remnant (bumps) coeval with the erosion-dominated environment. They are observable on both sandy and muddy substrates.