Two new types of sandbars in channels of the modern Ganjiang Delta, Poyang Lake, China: Depositional characteristics and origin

摘要

Two new types of sandbars are found in distributary channels of the modern Ganjiang Delta,Poyang Lake,China,respectively named as branching-point bar (BPB) and merging-point bar (MPB).BPB is defined as sandbar formed at the branching point of a channel where one channel branches into two.MPB is defined as sandbar formed at the merging point of two channels.BPB results from current velocity decrease at the branching point.BPB,arrowhead-shaped and growing towards upstream through upstream accretion,can be divided into bar head,middle and tail.Bar head refers to the tower part of the bar below average low water level,submerges under water whether in rainy seasons or dry seasons,and no plants grow on it.Bar middle refers to the middle part of the bar between average low and average high water levels,and sparse plants grow on it.Bar tail refers to the upper part of the bar above average high water level,and plants grow densely on it.From bar head to bar tail,sediment becomes finer and sorting degree becomes poorer;plant fossils increase in number,and silting-mud layers increase both in number and thickness.Vertically BPB shows a fining-upward sequence.BPB can be classified into 3 types:baby BPB with only bar head,child BPB with bar head and middle,and adult BPB with bar head,middle and tail.Generally from the lower delta plain to upper delta plain,BPB becomes more and more mature.If a channel is suddenly abandoned,BPB at different stages may be preserved.MPB is very similar to BPB in depositional characteristics,and main differences lie in its origin and growth direction.Discovery of BPB and MPB has important implications in finding remaining oil in oilfields.Previously,geologists considered point bars on convex bank of meandering channels or longitudinal bars and transverse bars in the middle of braided channels as favorable locations for finding remaining oil.Our study shows such locations as branching points and merging points of channels are also significant because BPB and MPB develop at these locations.Remaining oil has been found at branching points of channels in Pubei Oilfield,Songliao Basin,Northeast China.