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Fluvial Bars Reconstructed from a Deep, Straight Channel, Upper Carboniferous Coalfield of Northeast England
R. Stuart Haszeldine(2)
Fluvial sandstones from the northeast England coal field can be defined from subsurface information to be elongate, low-sinuosity ribbons. In excellent coastal exposures the sandstones are dominated internally by downcurrent-descending planar-tabular cross beds with unimodal paleocurrents and no obvious signs of lateral accretion.
The Seaton Sluice Sandstone is typical of these ancient rivers. Deposition was dominated by interdigitating lobate-crested sandwaves at least 20 m wide, which descended the leeside of a single 10-m-high, 200-m-wide bar and deposited planar cross beds. Lunate megaripples and dunes transported sediment along the top of these sandwaves and deposited trough cross beds. Paleocurrents became more complex up through the bar as low-stage component flows, directed towards the river bank, influenced the topographically high areas of the bar. Winnowing of the bar head (top) produced a coarsening-up of sand grain size. Using analogies with modern rivers, this Seaton Sluice river is interpreted as an ancient medial bar which migrated westward along a 1.9-km-wide channel and accreted onto the north rn river bank to form a lateral (bank-attached) bar. There is no evidence of bar emergence, and flow-stage flunctuations were probably not pronounced.
This type of detailed facies reconstruction enables accurate estimates to be made of channel depth, width, and flow variation. Channel processes were not related to local climate, but to bank stability, flow character and sediment grain size.
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