Abstract

Gradational coarsening-upwards or fining-upwards sequences (order of meters thick) in overbank environments can be produced by progressive crevasse-splay/levee progradation or abandonment while the channel belt remains unchanged in position. In contrast, meter-scale facies sequences related to periodic channel-belt avulsion may show relatively abrupt changes in facies and paleocurrent direction. Models suggest that the number of such avulsion-related facies changes in any overbank sequence bounded above and below by channel-belt deposits is on the order of 1 to 10; this is confirmed using an ancient example. In this example the average thickness of each avulsion-bounded overbank facies sequence was 3 m: assuming an average recurrence interval for avulsions of about 1,000 years, the mean overbank deposition rate was about 3 mm per year.--Modified journal abstract.

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