Floodplain formation and sediment stratigraphy resulting from oblique accretion on the Murrumbidgee River, Australia

Oblique accretion is a significant process of deposition along low-energy, mixed-load and suspended-load Australian rivers. Previously described as accretionary bank deposits sandwiched between well-developed point bars of sand and gravel and muddy over-bank deposits, fine-grained oblique-accretion deposits dominate the floodplain stratigraphy of many inland Australian rivers. They contribute more than 65% of floodplain sediments along the Murrum-bidgee River and almost all of the floodplain formed by bend migration on the suspended-load channels of the Darling and Cooper basins. Deposits consist mainly of alternating thin beds of sand and mud (inclined heterolithic stratification), with some plant litter, that form as drapes on the prograding bank. These beds dip mostly channelwards and quickly wedge out as they grade up and onto the floodplain. Because oblique accretion traps nearly all of the sediment deposited from suspended load near the channel margin, vertical accretion on distal areas of the floodplain is minimal. Where oblique accretion is associated with scroll formation, the resulting deposits are more complex, sometimes including a component that slopes away from the channel on the distal side of the first floodplain scroll. A model is presented showing how, with point bars or scrolls either present or absent, oblique accretion can make a significant contribution to the preservation of fine-grained within-channel deposits in contemporary floodplains. The examples presented here demonstrate that analogues to ancient point-bar deposits containing alternating sandstone and shale sequences are common in the low-energy fluvial environments of inland Australia.