Geology Reference
In-Depth Information
Fig. 13.11
Transverse cross-section of Shambles Bank in the
English Channel (see also Fig.
13.7
). The flat erosional surfaces
that constitute the main master bedding may be the result of ero-
sion by storm waves. The pattern is partly aggradational, because
the sand is trapped in the residual eddy that determines the
location of the bank. The large superimposed dunes produce
compound crossbeds with cosets over 6 m in thickness (After
Bastos et al.
2003
)
Fig. 13.12
Longitudinal and transverse sections through a
deep-shelf tidal ridge in the Celtic Sea. The units are:
1
flank
deposits of the early stage of ridge growth;
2
climbing, very
large compound dunes of the high-energy phase of ridge growth;
3
swatchway channel cut through the ridge by strong, cross-ridge
tidal flows when the ridge crest was shallowest; and
4
abandon-
ment deposits formed by destructive reworking of the ridge crest
by wave action after active growth ceased. Dominant current
was to the SW. See Fig.
13.6a
(area 6) for location (After
Reynaud et al.
1999b
)
Fig. 13.13
Sequential model of ridge evolution and splitting,
based on the Norfolk Ridges (see location in Fig.
13.6a
, ridge-
field 1). The process of ridge splitting is probably initiated by the
development of a cross-ridge swatchway (in steps 2 and 3 from
the
left
), which grows in size until the original ridge separates
into two parts (After Caston
1972
)
coarsen upward, because current speeds and wave
action are highest on the ridge crest. If, however, the
ridge becomes large enough that it impedes cross-ridge
flow, then there may be a tendency for finer sediment
to accumulate in the area of weaker currents on the
ridge crest. As we will see later, however, the sea-level
history during ridge growth and migration can be sig-
nificant, because ridges are so large that their lag time
is likely to be of the same order as the duration of a
high-frequency sea-level cycle.
