Geology Reference
In-Depth Information
unusual concentric tool mark is illustrated in Fig.
10.17i
where a protruding polychaete tube has excavated a
circular groove around its holdfast (diameter ca.
20 cm). Larger-scale current-generated features on
tidal flats are represented by 2D and 3D dunes
(Fig.
10.17k
) that, in back-barrier tidal basins, are usu-
ally best developed at spring tide. Upon emergence,
the dunes commonly display well-developed water-level
marks along their steep slipfaces. Finally, extensive
and often very selective shell beds swept together by
wave action, in contrast to current-generated lag deposits,
can be found locally on more exposed parts of inter-
tidal flats (Fig.
10.17l
).
in Fig.
10.18b
. The term 'herringbone' is strictly
reserved for sets of ripple cross-stratified beds displaying
opposing dip directions formed in the course of indi-
vidual ebb-flood or flood-ebb cycles. Not all opposing
cross-beds comply with this definition because indi-
vidual units may be separated by hiatuses of varying
duration. Furthermore, misinterpretations can result
where bidirectional currents are wrongly inferred from
trough cross-beds cutting each other at odd angles
(Reineck and Singh
1980
).
A typical stratification sequence found along shal-
low, migrating intertidal creeks is illustrated in
Fig.
10.18c
where the partly excavated and still articu-
lated shells of
Mya arenaria
in live position protrude
through a shell lag deposit that accrued as a tidal creek
migrated across it. The core reflects in a remarkable
way the subsurface conditions of a surface situation as
illustrated in Fig.
10.16b
. A second shell layer near the
bottom of the core indicates the depth of the tidal creek
during a previous crossing. The horizontally stratified
channel fill above this layer is partly obliterated by bio-
turbation. Quite different are the deposits found along
the margins of larger and deeper channels. Such tidal
flat margins are frequently composed of horizontally
laminated, partly waterlogged beds that, at depth, may
be deformed into convolute beds by sudden liquefac-
tion events (Fig.
10.18d
; cf. Wunderlich
1967
). In other
cases, channel-margin deposits comprise small current-
and/or wave-rippled cross-bedded sets (Fig.
10.18e
).
Convolute lamination has also been observed to form
as a result of entrapped air (de Boer
1979
). Processes
and concepts of convolute bed formation, also includ-
ing tidal flats, have been comprehensively described by
Williams (
1960
) and Einsele (
1963
).
Proceeding from sand flats to mixed flats, the degree
of bioturbation gradually increases (Fig.
10.18f, i
). At
low mud content (slightly sandy mud), ripple troughs
initially get draped by thin mud layers that, in cross-
section, produce the well-known flaser structures
(Fig.
10.18g
). As the mud content increases with
decreasing energy, the mud drapes get thicker and
eventually form interconnected wavy layers alternat-
ing with rippled sand layers to produce the characteris-
tic wavy bedding around the transition between
intertidal muddy sand and sandy mud facies. Finally,
as the sand content decreases (sandy mud to slightly
sandy mud facies), the internal sedimentary structures
are now dominated by thick mud drapes interrupted by
connected or disconnected sand lenses (starved ripples)
10.4.3 Internal Sedimentary Structures
Reconstruction of ancient depositional environments
is commonly based on the interpretation of internal
sedimentary structures, bedding types, and stratifica-
tion sequences observed in rock outcrops or cores. In
modern environments, internal structures are either
visualized by trenching and preparation of lacquer
peels or, where conditions prevent this, by coring and
preparation of relief casts using suitable resins (e.g.,
Bouma
1969
). Due to the water-saturated nature of the
sediments, coring is the only feasible procedure in
tidal flat research. While sedimentary structures are
well preserved in cores, they have the disadvantage of
only revealing narrow sections of laterally more exten-
sive structures.
In spite of this, the systematic preparation of both
short box-cores since the 1950s and longer vibro-cores
since the late 1970s has revolutionized our understand-
ing of tidal flat deposits (Reineck and Singh
1980
).
This is illustrated in Fig.
10.18
by a small selection of
relief casts ranging from exposed sand flats to pro-
tected salt marshes. As mentioned earlier, small sub-
aqueous dunes are best developed along the
ebb-dominated, outer tips of flood ramps on sand bod-
ies facing the inlet. This is exemplified by the cross-
bedding in Fig.
10.18a
where the flow was dominated
by the current flowing from right to left. The tangential
cross-beds are indicative of 3D dunes and hence rela-
tively strong flow as opposed to planar cross-beds
indicative of 2D dunes and weaker flow. Considerably
weaker and more evenly distributed bidirectional cur-
rents of uniform strength are reflected in the sequences
of vertically-stacked herringbone cross-stratified units
