Page 68 - Sedimentology and Stratigraphy

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Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
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Flows, Sediment and Bedforms 55

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Fig. 4.15 Migrating straight crested dune bedforms form
planar cross-bedding. Sinuous or isolated (linguoid or lunate)
dune bedforms produce trough cross-bedding. (From Tucker
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1991.)
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Fig. 4.14 Graphs of subaqueous ripple and subaqueous
dune bedform wavelengths and heights showing the absence
of overlap between ripple and dune-scale bedforms. (From
Collinson et al. 2006.)

vortex is well developed creating a counter-current at
the base of the slip face that may be strong enough
to generate ripples (counter-flow ripples), which
migrate a short distance up the toe of the lee slope
(Fig. 4.17).
A further effect of the stronger flow is the creation Fig. 4.16 Subaqueous dune bedforms in a braided river.
of a marked scour pit at the reattachment point. The
avalanche lee slope advances into this scoured trough
Constraints on the formation of dunes
so the bases of the cross-beds are marked by an undu-
lating erosion surface. The crest of a subaqueous dune Dunes range in size from having wavelengths of about
formed under these conditions will be highly sinuous 600 mm and heights of a few tens of millimetres to
or will have broken up into a series of linguoid dune wavelengths of hundreds of metres and heights of
forms. Trough cross-bedding (Fig. 4.15) formed by over ten metres. The smallest are larger than the
the migration of sinuous subaqueous dunes typically biggest ripples. Dunes can form in a range of grain
has asymptotic bottom contacts and an undulating sizes from fine gravels to fine sands, but they are less
lower boundary. well developed in finer deposits and do not occur in