Page 182 - Sedimentology and Stratigraphy

Wave and Storm Processes 169

same way. At the highest tides the current is strongest
enabling more transport and deposition of sand on the
bedforms in the flow. When the difference between
high and low tide is smaller the current will transport
a reduced bedload or there may be no sediment move-
ment at all. A cyclical variation in the thickness of
foreset laminae in cross-beds may therefore be attrib-
uted to variations in flow strength in the neap–spring
cycle and these are called tidal bundles. In an ide-
alised case, the laminae would show thickness vari-
ations in cycle in multiples of 7 or 14 (Yang & Nio
1985), but there is often no sedimentation or bedform
Fig. 11.8 Cross-bedded sandstone in sets 35 cm thick with migration during the weaker parts of the tidal cycle,
the surfaces of individual cross-beds picked out by thin so this ideal pattern is rarely seen.
layers of mud. Mud drapes on cross-beds are interpreted as
forming during slack water stages in the tidal cycle.
11.3 WAVE AND STORM PROCESSES
Reactivation surfaces
The depth to which surface waves affect a water body
In places where there is one dominant direction of is referred to as the wave base (4.4.1) and on con-
tidal current the bedforms migrate in that direction tinental shelves two levels can be distinguished
producing unidirectional cross-stratification. These (11.1). The fair weather wave base is the depth to
bedforms can be modified by the reverse current, prin- which there is wave-influenced motion under normal
cipally by the removal of the crest of a subaqueous weather conditions. The storm wave base is the depth
dune. When the bedform recommences migration in waves reach when the surface waves have a higher
the direction of the dominant flow the cross-strata energy due to stronger winds driving them. Below the
build out from the eroded surface. This leaves a storm wave base the sea bed is not normally affected
minor erosion surface within the cross-stratification, by surface waves.
which is termed a reactivation surface (Figs 11.6 &
11.9) (Dalrymple 1992).
11.3.1 Storms
Tidal bundles
Storms are weather systems that have associated
The strength of the tidal current varies cyclically and
1
strong surface winds, typically in excess of 100 km h ,
hence its capacity to carry sediment varies in the
and they may affect both land and marine envi-
ronments. In continental settings they are important
in aeolian transport of material (8.1), which includes
the transport of airborne sediment out into the
oceans. Large storms have a very large impact in
shallow marine environments and storm-related pro-
cesses of sedimentation are dominant in most shelf
and epicontinental seas. There are three components
to the effects of storms on shelf environments. The
strong winds drive currents in the oceans that move
water and sediment across and along continental
shelves. They also generate large waves that affect
much deeper parts of the shelf than normal, fair
weather waves: these waves rework the sediment on
Fig. 11.9 A reactivation surface within cross-bedded sands the sea floor generating characteristic sedimentary struc-
is a minor erosion surface truncating some of the cross-beds. tures (14.2.1). Finally, the high-energy conditions