Geology Reference
In-Depth Information
Fig. 5.14
Surface
morphology (
a
) and cross
section (
b
) through a
compound dune in Cobequid
Bay. In (
a
), the compound
dune, whose profile is
outlined by the
dashed white
line
, is flood asymmetric,
whereas the superimposed
simple dunes are ebb oriented
at an oblique angle to the
crest of the compound dune.
In (
b
), the cross beds formed
by the superimposed simple
dunes have internal cross
bedding that dips in the same
direction as the master
bedding planes (
white dashed
lines
) that were formed as the
troughs of the simple dunes
migrated over the brink of the
compound dune
approximately as the square of dune size. Small simple
dunes can reverse partially or completely during each
half tidal cycle; thus, their facing direction records
only the most recent flow. By contrast, large to very
large compound dunes have lag times of months to
years and are a good indicator of the residual-transport
direction over such periods. In this case, seasonal
changes in river discharge can play a role in dune
reversal (Berné et al.
1993
).
The deposits of the elongate sand bars consist pre-
dominantly of cross beds (Figs.
5.10a
,
5.13
b and
5.14b
). Within simple dunes, reactivation surfaces and
tidal bundles (Visser
1980
; see also Chap. 3) are vari-
ably developed. In areas with relatively slow currents,
such as where 2D dunes occur, the reactivation sur-
faces are closely spaced (i.e. a few centimeters to deci-
meters apart; Fig.
5.13b
), but they can be as much as a
1-2 m apart in areas with strong currents; such is the
case with 3D dunes that migrate rapidly. In all dunes,
erosional removal of the dune crest during the passage
of a subsequent dune can make recognition of the reac-
tivation surfaces difficult. Compound dunes generate
compound cross bedding
(Dalrymple
1984, 2010b
), in
which gently dipping (typically < 10°) master bedding
planes separate smaller cross beds generated by the
superimposed simple dunes as they migrate down the
master surfaces (Fig.
5.14b
); see Dalrymple (
1984,
2010b
) and Dalrymple and Rhodes (
1995
) for more
detail. In general, the deposits of a compound dune
coarsen upward because the trough experiences lower
currents speeds than the dune's crest. Mud drapes are
not abundant in the deposits of the elongate sand bars
because the suspended-sediment concentration is low
(Fig.
5.3c
), but they are most common in relatively
