The in-depth analysis of the development process of the delta front is of great significance for the judgment of the sedimentary environment,and it is necessary to deeply explore the formation characteristics and mechanism of the delta front from the perspective of hydrodynamics. In this study,the vertical distribution of the flow velocity in river water bodies and the development characteristics of delta front sedimentary bodies are investigated,from multiple perspectives of hydrodynamics,flume experiments,and underwater geomorphological observations. The results show that the phenomenon of river flow velocity,exhibiting a monotonic decrease with depth,has not been given sufficient attention. The terrain undulation of the middle and lower reaches of the riverbed increases with the flow direction,resulting in the underwater sand waves perpendicular to the flow direction,while the terrain undulation of the river section perpendicular to the flow direction becomes more and more gentle. During the process of rivers entering the sea,there will be a mainstream channel without branching channels underwater. Additionally,the originally undulating riverbed will gradually be filled up,changing from a sharp V-shaped or W-shaped to a U-shaped. It can be concluded that most of the front edge of the delta are sheet-like or fan-shaped,rather than a strip of erosion surface along the flow direction. Microfacies such as underwater distributary channels and bays do not develop in the delta front subfacies,which cannot be found in the modern delta front subfacies and are not supported by flume experiments. Furthermore,a water depth velocity interaction model for river channel and delta sedimentation is proposed.

The upper Oligocene Huagang Formation is crucial for oil and gas exploration in the central and southern Xihu sag of the East China Sea Shelf Basin. However,there has been ongoing debate on the sedimentary system,and limited research has been conducted on the characteristics of sand body architecture and the controlling factors. This paper studied the sedimentary environment,microfacies types,sand body architecture,and controlling factors of the upper Oligocene Huagang Formation through a comprehensive analysis of core,logging,and seismic data. The upper Huagang Formation in the central-south part of Xihu sag shows shallow-water delta plain sedimentation,with distributary channels being the predominant type of sand body. These distributary channels are categorized into three types based on their curvature: braided(curvature=1~1.05),meandering(curvature>1.15),and braided-meander transition(curvature=1.05~1.15). The sand bodies found in braided distributary channels are wide and thick,with intertwined wide bands. Channel bars in these channels are arranged in a speckled pattern. The sand body’s vertical stacking style is mainly characterized as incised stacking. In contrast,the sand bodies in meandering channels are narrow and thin,appearing as either single strip type or bifurcation-merging type in the planar direction. Thick sand layers are distributed as point bars in the channel margin,and the vertical stacking style is mostly isolated. The braided-meandering type of diversion channel is a transitional type between braided and meandering channels. It is characterized by moderate-sized channel sand bodies containing inner channel bars and point bars. The thickness and width of these distributary channels are positively correlated in a logarithmic manner. The ratio of width to thickness follows the pattern of braided type>transitional type>meandering type. The architecture characteristics of distributary channels are primarily influenced by paleogeomorphology and changes in base levels. Paleogeomorphology controls changes in channel types within shallow water delta plain,and low curvature braided distributary channels are more likely to develop in steep terrain. As the long-term base level increases,the size of distributary channels decreases,while a rise in mid-term base level has resulted in the transition of distributary channel sand bodies from incised to superimposed and isolated stacking types. This research enhances our understanding of the sedimentary structure of shallow water deltas and provides valuable guidance for predicting and developing distributary channel reservoirs in underground shallow water deltas within oil and gas fields.