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Processes of Transport and Sedimentary Structures
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form cross-bedding the dune must migrate. Dune-
scale cross-bedding therefore cannot be generated by
short-lived flow events. Dunes are most commonly
encountered in river channels, deltas, estuaries and
shallow marine environments where there are rela-
tively strong, sustained flows.

<

4.3.3 Bar forms

Bars are bedforms occurring within channels that are
of a larger scale than dunes: they have width and
height dimensions of the same order of magnitude as
the channel within which they are formed (Bridge
* <
2003). Bars can be made up of sandy sediment, grav-
elly material or mixtures of coarse grain sizes. In a
sandy channel the surfaces of bar forms are covered
with subaqueous dune bedforms, which migrate over
the bar surface and result in the formation of units of
cross-bedded sands. A bar form deposit is therefore
typically a cross-bedded sandstone as a lens-shaped

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body. The downstream edge of a bar can be steep and
develop its own slip-face, resulting in large-scale
Fig. 4.17 The patterns of cross-beds are determined by
the shape of the bedforms resulting from different flow cross-stratification in both sandstones and conglom-
conditions. erates. Bars in channels are classified in terms of their
position within the channel (side and alternate bars
at the margins, mid-channel bars in the centre and
point bars on bends: Collinson et al. 2006) and their
shape (9.2).

4.3.4 Plane bedding and planar lamination

Horizontal layering in sands deposited from a flow is
referred to as plane bedding in sediments and pro-
duces a sedimentary structure called planar lamina-
tion in sedimentary rocks. As noted above, current
ripples only form if the grains are smaller than the
thickness of the viscous sublayer: if the bed is rough,
the small-scale flow separation required for ripple for-
Fig. 4.18 Planar tabular cross-stratification with tangential
mation does not occur and the grains simply roll and
bases to the cross-beds (the scale bar is in inches and is
saltate along the surface. Plane beds form in coarser
100 mm long).
sands at relatively low flow velocities (close to the
very fine sands or silts. This grain size limitation is threshold for movement – 4.2.4), but as the flow
thought to be related to the increased suspended load speed increases dune bedforms start to be generated.
in the flow if the finer grain sizes are dominant: the The horizontal planar lamination produced under
suspended load suppresses turbulence in the flow and these circumstances tends to be rather poorly defined.
flow separation does not occur (Leeder 1999). The Plane bedding is also observed at higher flow veloc-
formation of dunes also requires flow to be sustained ities in very fine- to coarse-grained sands: ripple and
for long enough for the structure to build up, and to dune bedforms become washed out with an increase