Geology Reference
In-Depth Information
Fig. 3.25
Photo and diagram showing the tidal cyclicity recorded in the growth lines in a bivalve shell from the Pleistocene of
Tokyo Bay, Japan (From Murakoshi et al.
1995
)
3.3
Paleotidal Range
(Fig.
3.26
). In both of the conceptual models the thick-
ness of the complete stratigraphic sequence is equal to
the tidal range. Other similar models have been pro-
posed, however a complete sequence such as included
in these models is almost never preserved as such in
the stratigraphic record. Most commonly the top por-
tion is missing due to erosion, but other components
might also be missing just because of the specifi c cir-
cumstances at the site of accumulation.
Both transgressive and progradational tidalite sequ-
ences occur in the modern and ancient stratigraphic
records. These also are partial sequences and therefore
do not permit accurate paleotidal range for the environ-
ment of deposition. A detailed study of the Wood
Canyon Formation in Nevada permitted Klein (
1972
) to
construct a paleotidal model for the stratigraphy from
the base of the intertidal zone to the supratidal
(Fig.
3.27
). Such theoretical models provide a decent
answer to the question but problems exist. The base is
sometimes diffi cult to determine and, as mentioned
above, the upper part of the sequence is commonly
removed by erosion prior to the overlying accumula-
tion. The base can be diffi cult to recognize because
wave infl uence can destroy any tidal signatures that
might accumulate there. The top is easier to identify if
marsh deposits and/or desiccation features are present.
To date good methods do not exist to determine
paleotidal range other than the presence of a complete
stratigraphic sequence from the base to the top of the
intertidal zone and these are quite rare.
Another aspect of tidalites concerns the tidal range in
which they were deposited. This information is important
in reconstructing the environments of deposition of the
respective tidalites. Because tidalites have been recog-
nized in sediments that were billions of years old it is
likely that tidal range has changed overall. We know that
the earth/moon relationship has changed over that time as
the distance between them has increased (Kvale et al.
1999
; Chap. 1 this topic). This undoubtedly also caused
changes in tidal range, most likely a decrease on a global
scale. A generalization by Shaw (
1964
) stated that ancient
epeiric seas had small tidal ranges. Additionally, Irwin
(
1965
) and later Boggs (
2001
) were of the opinion that
tidal waves could not progress over the shallow environ-
ments of these seas. This has been shown to be untrue by
Klein and Ryer (
1978
) who gave both modern and ancient
examples of epeiric seas and broad shallow shelves where
tide-dominated conditions were common (see Chap. 13)
and their respective tidal ranges were greater than
microtidal (<2 m). There have been efforts in the past to
determine the tidal range under which specifi c tidalite
sequences were deposited (e.g. Klein
1971,
1975
) .
In modern tidalite sequences there is a general ten-
dency to equate a complete sequence from sediments
at the base of the intertidal environment to those at the
upper portion with the tidal range (Evans
1975
; Knight
and Dalrymple
1975
). The stratigraphic models are
hypothetical and include the entire potential sequence
