Palaeochannel reconstructions from point bar deposits: a three-dimensional perspective

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ABSTRACT

Quantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel-bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three-dimensional (3-D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down-valley bend migration and/or increase in channel-bend sinuosity. Two-dimensional (2-D) cross-sections in varying orientations across these simulated deposits display lateral-accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation.

Most cross-sections show point bar deposits thickening away from the meander-belt axis due to a lateral progression from thinner bend-exit deposits to thicker bend-apex deposits (caused by down-valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down-valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross-section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3-D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedsets of laterally accreted sediment.

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