Geology Reference
In-Depth Information
Fig. 3.5
Example of a lug
worm,
Arenicola
, burrow
from an intertidal fl at on the
Wadden Sea of Germany
3.2.2.1
Surface Structures
The sediment surface may show a wide range of con-
fi gurations produced by physical processes. Bedforms
are by far the most common and diverse of the sur-
face structures, but desiccation features can also be
common.
Those sediments that have even a modest amount of
cohesiveness contain enough mud so that exposure to
the atmosphere leads to desiccation. The result is the
formation of mud cracks (Fig.
3.6
). These features are
easily recognized on bedding planes and may also be
recognized from the stratigraphic perspective (Fig.
3.7
).
Typically the surface of them is slightly concave
upward, especially at the margins of each “crack”.
Desiccation features can develop on intertidal surfaces
but may also form in various other environments that
have no tidal infl uence such as fl uvial fl ood plains.
They are not the result of tidal processes but of alter-
nating wet and dry conditions. Such alternations may
coincide with tidal cycles, e.g. the spring high-tide
position on a tidal fl at. Here the sediment might be
deposited during spring high tide and is then followed
by several days of exposure when desiccation can take
place. The alternations of wet and dry can be non-cyclic
also, especially on wind-tidal fl ats. In summary, desic-
cation features can be associated with tidal conditions
and with tidalites but they do not themselves, serve as
distinctive tidalite signatures.
Bedforms
Interaction of fl uid motion and the sediment substrate
develops regularly undulating surfaces called bed-
forms. Tidal fl ux produces currents of varying veloci-
ties over time and space that produce a spectrum of
scales and geometries of these bedforms. Scale, that is
wave length and height, is dependent on current
strength, water depth and grain size. Morphology is
also infl uenced by current strength. Some bedforms
are linear and others are so-called three-dimensional
bedforms (Fig.
3.8
).
Bedforms are produced in many environments and
in subtidal, intertidal and non-tidal environments. They
are commonly preserved in the ancient record and are
displayed on bedding planes (Fig.
3.9
). None of this
necessarily means tidal processes are required to pro-
duce such features; they are not. All conditions where
fl uid fl ow moves over the sediment substrate at speeds
in excess of the threshold of sediment movement pro-
duce bedforms and many depositional environments
experience such conditions.
3.2.2.2
Three-Dimensional Structures
A special aspect of bedform development and preser-
vation is commonly associated with tidal conditions.
Those bedforms that develop on intertidal fl ats and
subtidally in estuaries and deltas may accumulate fi ne
sediment in their troughs (Fig.
3.10
) as the result of
