Geology Reference
In-Depth Information
Sea, Germany. Of similar diagnostic potential are
organisms that only occur in intertidal environments,
especially when preserved in live position such as the
bivalve
Mya arenaria
in Fig.
10.16b
. More difficult to
assign to a tidal flat setting are organisms that also live
subtidally. In such cases, additional evidence is
required to diagnose an intertidal setting. The poly-
chaete
Lanice conchilega
in Fig.
10.16c
is a case in
point. It is only in conjunction with other diagnostic
criteria such as bird tracks that an intertidal setting can
be allocated with some degree of confidence. The same
applies to the cockle or heart mussel (here
Cerastoderma
edule
) that lives just below the sediment surface
and that can be recognised by the bumpy surface or
the scars produced by slightly protruding shells
(Fig.
10.16d
). Similarly, while the presence of the poly-
chaete
Heteromastus filiformis
is betrayed by the occur-
rence of small (black or gray) fecal heaps on the surface
of modern tidal flats (Fig.
10.16e
), it would be the bird
tracks on the same bedding plane that would identify
the depositional environment as being intertidal in the
rock record. In principle this also applies to the lug-
worm
Arenicola marina
(Fig.
10.16f
), here in commu-
nity with
Heteromastus
. Circular resting hollows of
rays or feeding hollows of wading birds (Fig.
10.16g
,
here seagulls) and feeding hummocks created by fla-
mingos (Fig.
10.16h
, here in Langebaan Lagoon, South
Africa) complete the picture.
Considering that large-scale exposures of fossil
bedding planes are relatively rare in comparison to
vertical sections, it is inherently difficult to identify
intertidal settings with confidence in the rock record
on the basis of surface structures alone unless addi-
tional unequivocal diagnostic evidence is available.
Such evidence includes late-stage emergence runoff
features and traces of organisms restricted to the inter-
tidal, including the tracks and feeding structures of
wading birds. In cold climates, such evidence would in
addition encompass tool marks induced by moving ice
floes. In all other cases, it would be the association of
a multitude of features which, by application of the
exclusion principle, could eventually justify a decision
in favour of a particular environment.
that display features characteristic of very shallow
water and late-stage emergence. Prominent among
these are extensive sheets of symmetrical and asym-
metrical wave ripples (Fig.
10.17a
). Because ripples
also occur in subtidal environments, one should in
addition look for evidence of late-stage emergence.
Such features include ladderback ripples (Fig.
10.17b
),
especially where smaller trough-bound wave ripples
aligned perpendicular to the larger ripple crests are
associated with water-level marks (Fig.
10.17c
). The
small wave ripples in the troughs in Fig.
10.17c
formed
when the water level had dropped below the crest level
of the larger ripples but before the water-level marks
formed which later dissected the crests of the small
ripples where they merge with the steep slopes of the
larger ones.
Features formed during late-stage run-off are par-
ticularly diagnostic for emergence at low tide. Among
these are narrow streams of linguoid current ripples
dissecting wave-rippled surfaces, the crests of the
latter having in this case been flattened just before
emergence (Fig.
10.17d
). Shallow, laterally migrating
intertidal creeks are commonly paved by shell beds
(Fig.
10.17e
), in this case overlain by narrow sand rib-
bons formed during upper-plane-bed flow shortly
before emergence. It should be noted here, however,
that shell concentrations can also result from the bur-
rowing activity of intertidal organisms, in particular
Arenicola marina
(van Straaten
1952
). Near steeper
channel margins, such creeks display a multitude of
late-stage runoff features such as scour pits around
shells, grooves, rill marks, microbars and small fan
structures (Fig.
10.17f
).
Wave- and current-generated ripples are frequently
observed in muddy sediments upon exposure at low
tide (Fig.
10.17g
). This contradicts the common per-
ception that such bedforms do not form in fine-grained
sediments. Their occurrence has been explained by the
aggregated nature of the mud during transport and
deposition, the aggregates and also fecal pellets initially
responding to waves and currents as non-cohesive par-
ticles would, similar to the fine or very fine sand to
which they are hydraulically equivalent (Schieber and
Southard
2009
). In contrast to rippled mud, tidal flat
surfaces may locally become draped by thin blankets
of fluid mud (Fig.
10.17h
) that often display erosional
windows revealing the underlying sediment together
with any surface structures on them (in this case wave
ripples). An analogous feature can be observed in
10.4.2 Physical Surface Structures
Prominent physical surface structures frequently
observed on tidal flats include wave and current ripples
