Fluvial herring-bone cross-stratification in a modern tributary mouth bar, Coonamble, New South Wales, Australia |
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| Authors: | M MUSTAFA ALAM KEITH A W CROOK GRAHAM TAYLOR |
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| Institution: | Geology Department, Australian National University, GPO Box 4, Canberra 2601, Australia; School of Applied Science, Canberra College of Advanced Education, PO Box 1, Belconnen 2616, Australia |
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| Abstract: | Herring-bone cross-stratification occurs in tributary mouth bar sediments less than 150 yr old in Warrena Creek near its confluence with the Castlereagh River some 2000 river kilometres from the sea in northern New South Wales. These streams have low gradients, with straight to anastomosing channels which become sinuous and distributive downstream. Channel beds are sand but banks are almost exclusively mud which is burrowed and extensively penetrated by roots. Herring-bone cross-stratification results from flow reversals in Warrena Creek during flood events. Flow direction depends upon discharge and stage in the creek relative to that in the adjacent river. The lithofacies resemble inter-tidal deposits and could easily be misidentified on the basis of herring-bone cross-stratification in an ancient sedimentary sequence. Herringbone cross-stratification should be regarded as diagnostic of depositional environments in which current directions are principally determined by reversals of water surface gradient, rather than by regional slope. Flow reversal phenomena may be a characteristic of very low gradient fluvial systems. |
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