Page 67 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
54 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 54 26.2.2009 8:16pm Compositor Name: ARaju

distances) range up to 500 mm (Leeder 1999). The erally dunes are tens of centimetres high in water
ratio of the wavelength to the height is typically depths of a few metres, but are typically metres high
between 10 and 40. There is some evidence of a in the water depths measured in tens of metres (Allen
relationship between the ripple wavelength and the 1982; Leeder 1999).
grain size, approximately 1000 to 1 (Leeder 1999). It
is important to note the upper limit to the dimensions
Dunes and cross-bedding
of current ripples and to emphasise that ripples do not
‘grow’ into larger bedforms. The morphology of a subaqueous dune is similar to a
ripple: there is a stoss side leading up to a crest and
sand avalanches down the lee slope towards a trough
4.3.2 Dunes (Figs 4.15 & 4.16). Migration of a subaqueous dune
results in the construction of a succession of sloping
Beds of sand in rivers, estuaries, beaches and marine layers formed by the avalanching on the lee slope and
environments also have bedforms that are distinctly these are referred to as cross-beds. Flow separation
larger than ripples. These large bedforms are called creates a zone in front of the lee slope in which a
dunes (Fig. 4.13): the term ‘megaripples’ is also some- roller vortex with reverse flow can form (Fig. 4.17).
times used, although this term fails to emphasise the At low flow velocities these roller vortices are weakly
fundamental hydrodynamic distinctions between rip- developed and they do not rework the sand on the lee
ple and dune bedforms. Evidence that these larger slope. The cross-beds formed simply lie at the angle of
bedforms are not simply large ripples comes from rest of the sand and as they build out into the trough
measurement of the heights and wavelengths of all the basal contact is angular (Fig. 4.17). Bedforms that
bedforms (Fig. 4.14). The data fall into clusters which develop at these velocities usually have low sinuosity
do not overlap, indicating that they form by distinct crests, so the three-dimensional form of the structure
processes which are not part of a continuum. The is similar to planar cross-lamination. This is planar
formation of dunes can be related to large-scale tur- cross-bedding and the surface at the bottom of the
bulence within the whole flow; once again flow cross-beds is flat and close to horizontal because of
separation is important, occurring at the dune crest, the absence of scouring in the trough. Cross-beds
and scouring occurs at the reattachment point in the bound by horizontal surfaces are sometimes referred
trough. The water depth controls the scale of the to as tabular cross-bedding (Fig. 4.18). Cross-beds
turbulent eddies in the flow and this in turn controls may form a sharp angle at the base of the avalanche
the height and wavelength of the dunes: there is a slope or may be asymptotic (tangential) to the hori-
considerable amount of scatter in the data, but gen- zontal (Fig. 4.17). At high flow velocities the roller

Fig. 4.13 Dune bedforms in an estuary:
the most recent flow was from left to right
and the upstream side of the dunes is
covered with current ripples.