Geology Reference
In-Depth Information
focused on muddy environments and on ichnology
(see Chap. 4). Many would credit George deVries
Klein with the fi rst detailed work on tidal sedimentol-
ogy in North America during his doctoral research on
the Bay of Fundy while at Yale University under the
direction of John Sanders (Klein 1963 ) .
Although rudimentary, he showed how basic tidal
processes produced recognizable sedimentary struc-
tures that could be present in the stratigraphic record.
The term tidalite had is origin as the result of this work
(Klein 1971, 1972 ). It is now applied to all sediments
and sedimentary structures that have accumulated
under the infl uence of tides. The following discussion
will address how tidal signatures can be recognized in
stratigraphic successions and how they can be distin-
guished from similar features that are deposited under
non-tidal conditions. Some structures and sequences
can form in multiple depositional environments, e.g.
fl uvial, deltaic, shallow marine or lacustrine. Some
situations exist where sediments also accumulate under
tidal conditions but there are no tidal signatures pre-
served, e.g. estuaries, tidal fl ats, or tidal inlet systems.
The lack of tidal signatures may be due to energy con-
ditions with little or no sediment transport, or may be
due to the infl uence of waves that mask or rework any
evidence of tidal activity. Because of these circum-
stances, it is pretty certain that the strata that are inter-
preted as containing tidalites represent only a portion
of those sediments that have actually accumulated
under tidal conditions.
As discussed in the Chap. 1, tides occur in cycles of
various durations. These cycles are an important key to
the production of tidal signatures. The most important
and most easily recognizable of these cycles in the
stratigraphic record are those of daily and monthly
duration, but longer cycles can also be recognized. The
daily or diurnal cycles may represent one or two tidal
cycles depending upon the location; diurnal or semi-
diurnal tides. The lunar monthly cycles represent
2-week or fortnightly periods associated with phases
of the moon. A gradual change occurs from relatively
high energy to relatively low and back to high energy
again in both cycles. These energy cycles are refl ected
in a combination of grain size and bed thickness
changes. In the absence of erosion and the presence of
suffi cient sediment supply, the packages of sediment
that accumulate will display easily recognizable tidal
signatures. Longer cycles also exist that can be recog-
nized; monthly, seasonal and longer (see Chap. 1).
Although tides and related processes produce various
signatures that permit us to recognize tidal infl uence in
various environments, there are other features that are
associated with various tidal environments, especially
tidal fl ats. Some are biogenic, some are chemical and
others are physical.
3.2
Sedimentary Structures
Various types of features are known that form on the
sediment surface that are typical products of tidally
infl uenced phenomena; a few that are indicative of
only tidal conditions. These features include biogenic
structures and physically produced structures. Because
of their development on the sediment-fl uid interface,
they may be destroyed during burial and even if pre-
served, they may not be visible because most of them
require exposure of the bed surface to be recognized.
3.2.1
Biogenic Structures
3.2.1.1 Surface Structures
Benthic organisms occupy several tidally-infl uenced
environments; on both intertidal and subtidal surfaces.
Generally these are somewhat low physical energy
environments because regular and intense sediment
mobility precludes colonization by epifaunal or infau-
nal organisms. Less dynamic substrates will permit
organisms to be present on a regular basis. Those ani-
mals that are exclusively epifaunal leave tracks, trails,
resting places, and surface expressions of burrows that
have a low preservation potential (Fig. 3.1 ). The only
circumstances whereby such features might become
preserved in the stratigraphic record would be during a
fairly deep and rapid burial. Recognition generally
requires exposure of a bedding surface.
The one biogenic surface feature that is likely to be
preserved depending on sediment composition, and
displays environmental signifi cance is microbial mats
that are composed of blue-green algae, now more com-
monly termed cyanobacteria. These mats typically are
associated with the spring high tide to supratidal por-
tion of tidal fl ats. They are especially common along
wind tidal fl ats (Fig. 3.2 ), such as along the coast
of Texas (Miller 1975 ) . This environment typically
does not experience regular lunar tides but is subjected
to water level change produced by wind. The south
 
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