Page 181 - Sedimentology and Stratigraphy

168 The Marine Realm: Morphology and Processes

show a bimodal (two main directions of flow) and
bipolar (two opposite directions of flow) pattern. It
should be noted, however, that these opposing palaeo-
flow directions are not seen in all tidal sediments: first
the flow in one direction (either the ebb or flood tide)
may be much stronger than the other, and second the
two flows might be widely separated and only one
may have been active in the area examined (Dalrym-
ple & Choi 2007). Under favourable circumstances,
bipolar cross-stratification may be seen in a single
vertical section produced by alternating directions of
migration of ripples or dunes. This is known as her-
ringbone cross-stratification (Figs 11.6 & 11.7)
and it results from a tidal current flowing predomi- Fig. 11.7 Herringbone cross-stratification in sandstone
nantly in one direction for a period of time, probably beds (width of view 1.5 m).
many years, followed by a change in the pattern of

tidal flow that results in another period of opposite

flow. This pattern of alternating directions should not
be interpreted as a diurnal pattern as this would imply
unrealistically high rates of sediment accumulation.
The herringbone pattern is characteristic of tidal sedi-
mentation, but is not found in all instances.

Mud drapes on cross-strata
At the time of high or low tide when the current is
changing direction there is a short period when there
is no flow. When the water is relatively still some of

the suspended load may be deposited as a thin layer
of mud. When the current becomes stronger during
the next tide, the mud layer is not necessarily
removed because the clay-rich sediment is cohesive
and this makes it resistant to erosion (4.2.4). Mud
drapes formed in this way can be seen in wave and
current ripple laminated sands deposited in shallow
water in places such as tidal mud flats (13.4): the
heterolithic beds formed in this way display flaser or

lenticular lamination depending on the proportion
$

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%
of sand and mud present (4.8). Mud drapes can also
occur within cross-beds: a lamina of sand is deposited
on the lee slope of the subaqueous dune during strong
tidal flow but as the tide changes direction mud falls
out of suspension and drapes the subaqueous dune
(Dalrymple 1992). There are circumstances where
mud drapes can form in other depositional regimes,
Fig. 11.6 Features that indicate tidal influence of transport
for instance in rivers that have only seasonal flow, but
and deposition: (a) herringbone cross-stratification; (b) mud
drapes on cross-bedding formed during the slack water they are most common in tidal settings: abundant,
stages of tidal cycles; (c) reactivation surfaces formed by regular mud drapes are a good indicator of a tidally
erosion of part of a bedform when a current is reversed. influenced environment (Figs 11.6 & 11.8).