Page 72 - Sedimentology and Stratigraphy

26.2.2009 8:16pm Compositor Name: ARaju
Final Proof page 59
Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
Waves 59
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Fig. 4.21 The formation of wave ripples / -
in sediment is produced by oscillatory
motion in the water column due to wave
ripples on the surface of the water. Note
that there is no overall lateral movement
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of the water, or of the sediment. In deep
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water the internal friction reduces the
oscillation and wave ripples do not form
in the sediment.

At low energies rolling grain ripples form and sand of all grades. If the wave energy is high
(Fig. 4.22). The peak velocity of grain motion is at enough wave ripples can form in granules and peb-
the mid-point of each oscillation, reducing to zero bles, forming gravel ripples with wavelengths of sev-
at the edges. This sweeps grains away from the mid- eral metres and heights of tens of centimetres.
dle, where a trough forms, to the edges where
ripple crests build up. Rolling grain ripples are char-
acterised by broad troughs and sharp crests. At higher 4.4.3 Distinguishing wave and current ripples
energies grains can be kept temporarily in suspension
during each oscillation. Small clouds of grains are Distinguishing between wave and current ripples can
swept from the troughs onto the crests where they be critical to the interpretation of palaeoenviron-
fall out of suspension. These vortex ripples ments. Wave ripples are formed only in relatively
(Fig. 4.22) have more rounded crests but are other- shallow water in the absence of strong currents,
wise symmetrical. whereas current ripples may form as a result of
water flow in any depth in any subaqueous environ-
ment. These distinctions allow deposits from a shal-
4.4.2 Characteristics of wave ripples low lake (10.7.2) or lagoon (13.3.2)tobe
distinguished from offshore (14.2.1) or deep marine
In plan view wave ripples have long, straight to gently environments (14.2.1), for example. The two different
sinuous crests which may bifurcate (split) (Fig. 4.23); ripple types can be distinguished in the field on the
these characteristics may be seen on the bedding basis of their shapes and geometries. In plan view
planes of sedimentary rocks. In cross-section wave wave ripples have long, straight to sinuous crests
ripples are generally symmetrical in profile, laminae which may bifurcate (divide) whereas current ripples
within each ripple dip in both directions and are over- are commonly very sinuous and broken up into short,
lapping (Fig. 4.24). These characteristics may be pre- curved crests. When viewed from the side wave rip-
served in cross-lamination generated by the ples are symmetrical with cross-laminae dipping in
accumulation of sediment influenced by waves both directions either side of the crests. In contrast,
(Fig. 4.25). Wave ripples can form in any non-cohe- current ripples are asymmetrical with cross-laminae
sive sediment and are principally seen in coarse silts dipping only in one direction, the only exception