Geology Reference
In-Depth Information
Fig. 3.22
Diagram showing the spring-neap cycles in tidal bundles (From Visser
1980
)
Cross-Strata Structures
The cross-stratifi cation produced by migrating bed-
forms occurs in scales that range from a few centime-
ters to tens of centimeters. Migrating ripples that
develop when small bedforms develop may display
climbing ripple cross-strata when an abundant sedi-
ment supply is present (i.e. sediment fallout is much
greater than bedform migration). These distinctive
structures are typically present in multiple depositional
environments; especially fl uvial and those with tidal
infl uence. Unless a cyclic reversal of the directional
orientation of the cross-strata occurs, there is no reason
to interpret them as tidalites. Such a bimodal organiza-
tion of migrating ripples is rare.
A more important cross-strata structure that quali-
fi es as a tidalite is
tidal bundles
(Fig.
3.22
). Tidal bun-
dles are special cross sets that are generally at least
10 cm thick and are commonly near 50 cm. Each bun-
dle is a couplet of typically heterolithic cross-strata
that develop from the migration of small dunes (mega-
ripples) but they may also be monolithic. As these bed-
forms migrate with the tidal currents there is variation
in the velocity over the spring-neap cycle that produces
differences in bundle thickness due to changes in dis-
tances of bedform migration. Under the infl uence of a
strong predominant current, either fl ood or ebb, and a
very weak subordinate current there is a succession of
sandy cross-strata separated by thin mud drapes from
slack tide conditions (Fig.
3.22
).
The tidal bundles accumulate in a sequence of
trough cross-strata that shows rhythmic changes in
individual bed thickness from spring to neap and back
as each tidal cycle takes place (Visser
1980
) . Individual
bundles may only be a centimeter or two thick during
spring conditions and a few millimeters during neap
conditions (Fig.
3.23
). These bundles are excellent
examples of tidalites and show a high preservation
potential because they develop on channel margins and
fl oors where burial can be rapid. They are almost
always oriented in a single direction in a sequence of
several stacked cross-sets (Fig.
3.24
).
Because tidal bundles occur in medium scale cross-
strata that were formed by similar size bedforms, their
development is the result of relatively strong currents.
As a result there is a high potential for reactivation sur-
faces to be present within the bundle sequence, espe-
cially near spring tide conditions. This limits their
environment of formation to tidal channels, channel
margins or intertidal fl ats where mesoscale bedforms
(< 2 m wavelength) are present.
Miscellaneous Tidal Infl uence Indicators
There are other sedimentary phenomena that may be
considered as tidalites. These may be located within
the intertidal zone but do not show any evidence of
tidal processes. Examples include the plants that grow
only in this specifi c environment. Salt marsh grasses
are excellent indicators of the position of sea level and
are associated with tidal processes. In general, marsh
grasses are restricted to the upper part of the intertidal
zone; typically between neap and spring high tide
(Frey and Basan
1985
) .
Spartina
is typically lower in
elevation than
Juncus
. These two taxa are the most
common marsh species at present and can be recog-
nized easily in peats. The surfaces upon which these
plants are located receive sediment by tidal processes
