ERSC 2P16 Lecture 21: Bed forms and stratification under unidirectional flows pt 2
7 Aug 2015
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Bed forms and stratification under unidirectional flows 2: washed-out dunes: as flow strength increases dunes become longer and lower, washing out into the next bed form, upper plane bed: a flat bed with intense sediment transport. Regular relief as flow parallel mounds a few grain diameters high (termed current lineations). Grain long axes in upper plane bed deposits are distinctively flow parallel and imbricate upstream (10. To 30 from bedding is the normal range). Flow parallel a-axes orientation results in parting-step lineation(p-sl) on bedding plane surfaces that are parallel to current lineation (cl). The alignment of a-axes causes the sandstone to preferentially break along that direction. Heavy mineral shadowsmay be present when opaque heavy minerals are included in the sand bed (as little as 3%opaque heavy minerals is sufficient for shadows to form). These are paleocurrent indicators: flow is towards the sharply defined side of the shadow, parallel to current lineations.
