Geology Reference
In-Depth Information
The box-core was recovered from a dune of the type
shown in Fig.
10.17k
. It is interpreted to show a vertically-
stacked spring-neap-spring sequence with clearly visible
tidal bundles in the crossbeds of the lower unit and
more faintly preserved ones in the upper unit. The two
units were formed during the ebb tide over successive
spring-tide periods and are separated by a bipolar,
current-rippled sequence formed over the intervening
neap-tide period. Due to the small width of the core, it
is not precisely clear how many bundles were actually
formed in each case, at least ten (representing 5 days)
having been identified in the lower unit.
In contrast to the rather rare occurrence of tidal
bundles in cross-bedded sand of back-barrier tidal
flats, tidal bedding represented by sand-mud couplets
is more frequently encountered. These are preferen-
tially formed along mobile intertidal creeks as long as
sufficient suspended matter is available to settle out at
high tide. However, as intertidal creeks are rather shal-
low, one rarely finds more than just a few sand-mud
couplets stacked above each other (Fig.
10.20
). Each
cycle begins on the rising tide as the tidal flat is inun-
dated and the flood current begins to move sand across
the sediment surface formed during the previous fall-
ing tide. Mud then settles out during the slack-water
period over high tide and is subsequently covered by a
sand layer in the course of the ebb tide. Each sand layer
may be composed of two opposing current-generated
ripple cross-stratified units, the thickness of each
depending on the relative dominance of one current
component over the other. In contrast to subtidal
(de Boer et al.
1989
) or intertidal estuarine rhythmites
(Dalrymple et al.
1991
), one would not expect large
numbers of stacked couplets or any clear evidence of
the daily inequality of the tide. At a larger spatial scale,
a characteristic depositional facies is the so-called
'inclined heterolithic stratification' (Thomas et al.
1987
). These form in the process of lateral channel
migration or meandering, and are identified on the
ground by what has also been called 'longitudinal' or
'lateral-accretion' bedding (Reineck
1958
; Bridges
and Leeder
1976
).
As sand content decreases and mud content
increases toward the mainland coast, the tidal flat grad-
ually transforms into an almost featureless muddy
plain, tidal channels or creeks being now restricted to
locations where freshwater streams drain the hinter-
land. This is in stark contrast to non-barred macrotidal
mud flats that are commonly sculptured into meandering
Fig. 10.19
A vertically stacked spring-neap-spring cycle with
tidal bundles preserved in intertidal dune cross-beds formed
over spring tide. The bundles are clearly visible in the
bottom
sequence
, but only faintly so in the
upper
one. Note that the
bundles are not separated by mud drapes, but instead by finer-
grained sand
to form so-called lenticular bedding (Fig.
10.18h
, cf.
Reineck and Wunderlich
1968
. Flemming
2003a
). Due
to the high water content of the mud, overburden pres-
sure can result in the formation of convolute bedding
in this environment (Fig.
10.18h
, bottom). Pure mud is
either completely homogenized or thinly laminated,
depending on the degree of local bioturbation. These
eventually grade into salt marshes, the laminated
deposits of which are usually intensely bioturbated by
root structures (Fig.
10.18j
).
The cores illustrated in Fig.
10.18
do not include
any evidence of tidal bedding such as sand-mud cou-
plets or tidal bundles associated with deposition in the
course of neap-spring cycles. Such rhythmic sedimen-
tary structures are well documented from subtidal
channels (Visser
1980
; Allen and Homewood
1984
)
and from estuarine mudflats in macrotidal settings
(Dalrymple et al.
1991
), but have rarely been reported
from back-barrier tidal flats. A cross-bedded example
from the Wadden Sea is presented in Fig.
10.19
.
