Internal Architectural Patterns of Bar Fingers Within Digitate Shallow-Water Delta: Insights from the Shallow Core, GPR and Delft3D Simulation Data of the Ganjiang Delta, China

Abstract

Digitate shallow-water deltas are commonly found in modern lakes and bays, as well as within cratonic petroliferous basins. They develop one or multiple sinuous finger-like sands (i.e., bar fingers), including high-RSI (sinuosity ratio of distributary channel and bar finger ≥1) and low-RSI (RSI < 1) types. Bar fingers consist of four types of subenvironments, that is, distributary channels, point bars, mouth bars, and levees. However, the internal architecture within the above subenvironments is still unclear. This paper documents the internal architecture of a digitate delta based on the integration of shallow-core and ground-penetrating radar data from the Ganjiang Delta, China, coupled with Delft3D simulations. Our results show that multiple convex-up muddy-silty accretion beds are developed in mouth bars, which top lap the side of the distributary channels or point bars and down lap the bottom of the mouth bar. The accretion beds have low dip angles (<2°), which is slightly higher for the upper accretion beds. Point bars, unique to the high-RSI bar finger, develop multiple inclined silty drapes, which top lap the top of the point bar. The cohesive levee and backwater effect impede the migration of the distributary channel, resulting in silty drapes with high-dip angles (can be >10°) compared with those in the supplying river. This dip angle exhibits a negative relationship with downstream distance and a positive exponential relationship with lateral migration distance. Silty drapes become dense along the migration direction of the distributary channel. The levee develops multiple horizontal muddy accretion beds. The high-RSI bar finger develops a large number (>3) of accretion beds in mouth bars with high dip angles, and a large number of accretion beds in thick levees, compared with the low-RSI bar finger. The results of this paper provide insights into the prediction and development of cratonic digitate shallow-water delta reservoirs.