Geology Reference
In-Depth Information
of rotary tides with no distinct slack-water period, and
because suspended-sediment concentrations are gener-
ally very low.
Large and very large dunes are typically covered by
smaller dunes and have a compound morphology. Such
dunes generate
compound crossbedding
, composed of
stacked, inclined, planar to trough crossbeds formed
by the superimposed smaller dunes (Fig.
13.5b, c
). The
lee side of compound dunes typically has a much lower
slope (commonly <10°) than that of simple dunes.
Flow separation does not occur, and the smaller, super-
imposed dunes migrate continuously down the larger
dune's lee side, from the crest to the trough. Flow
expansion and flow deceleration bring about deposi-
tion on the lee side of the larger dune, so that each
superimposed smaller dune leaves behind a crossbed
that gets preserved. The continuous accretion of such
crossbeds forms the master bedding of the compound
dune. The superimposed dunes may themselves be
compound, so that 'compound-compound' dunes can
occur (Anastas et al.
1997
). It is noteworthy that the
smaller dunes migrate in essentially the same direction
as the larger dune, forming an architecture termed
foreward accretion
. Upslope-climbing ripples or
smaller dunes formed by the subordinate current are
likely to be preserved, forming herringbone cross-
stratification. Tidal-current reversals are generally not
capable of generating master-bedding surfaces in large
to very large dunes, because the time required for such
large dunes to reverse greatly exceeds the duration of a
tidal cycle. For example, the time needed for a 4 m-high
dune to reverse is about 200 days of continuous bed-
load transport, based on an average rate of transport
typical of tidal environments (Dalrymple and Rhodes
1995
). Longer-term flow reversals, such as those asso-
ciated with seasonal changes in the wind regime or
ocean circulation, could, however, cause dune reversal
and the creation of master-bedding planes. Similarly,
high-energy storms with greatly increased sediment-
transport rates could also bring about dune reversal
(Houthuys et al.
1994
; Le Bot and Trentesaux
2004
).
The storm-wave activity can also erode the crest of the
dune, generating horizontal erosion surfaces (e.g.
McCave
1971
; Dalrymple
1984
; Berné et al.
1991
).
The vertical succession of structures produced by a
compound dune generally coarsens upward, and the
individual cross beds in it commonly become thicker
upward (Fig.
13.5
). The upward-coarsening trend is
caused by the fact that the shear stress exerted by
the flow is higher at the crest than in the trough of
the compound dune. The downward thinning occurs
due to the fact that the superimposed dunes become
smaller as they migrate down the larger lee face
because they are losing sediment to the cross bed that
they leave behind (Rubin
1987
). The bottomset region
of compound dunes is the area where muddy deposits
are more likely to occur; bioturbation is also greater
there than elsewhere.
13.5
Offshore Tidal Ridges
Tidal-current ridges are widely developed on tidal
shelves and comprise a significant fraction of the total
volume of all sandy deposits in the modern. They are
the largest bedforms that exist, reaching 200 km in
length, 10 km in width and 50 m in height. Offshore
tidal ridges generally occur as fields of regularly
spaced, parallel
en echelon
ridges (Off
1963
) that can
cover tens of thousands of square kilometers, as in the
Celtic Sea, the North Sea, and the East China and
Yellow seas (Fig.
13.6
). They may also occur as iso-
lated ridges in the lee of islands and capes (in this case
they are called
banner banks
). Offshore tidal ridges
are made up by the accretion, at the largest scale, of
the crossbeds formed by dunes.
13.5.1 Ridge Morphodynamics
Unlike dunes, ridges are not the expression of the tur-
bulence of the primary flow at the seabed and the
related local advection of sediment. The ridges occur
in rotary 'cells' within tidal-transport paths where the
residual transport retains a higher proportion of the
sediment that is in transit. Within these cells, ridges
grow in place of flat sand sheets: sediment moves in
opposite directions on either side of the ridge crest,
with the potential for sand to circulate around bank ter-
minations (McCave and Langhorne
1982
; Howarth
and Huthnance
1984
). Near the coast, these cells can
correspond to the eddies that are generated by a head-
land (Fig.
13.7
; Pingree and Maddock
1979
). Further
from the shore, on the open shelf where linear ridges
develop, the rotary circulation is the result of a positive
feedback between the reversing tidal flow and the
topography of the ridge (Zimmerman
1978
; Pan et al.
2007
). This rotary residual transport of sand is driven
