Geology Reference
In-Depth Information
and also as the result of suspended sediment delivered
during times of storm surge.
In low latitude areas, mangroves occupy this same
general environment. These trees extend from the shal-
low subtidal environment to the supratidal level. They
occupy water bodies that range from normal marine
salinities to fresh water but they do require tidal fl ux
(Dawes
1998
). Like marsh grasses, mangroves pro-
duce easily recognizable peat deposits that represent
the intertidal zone and therefore provide a good indica-
tor of sea level at the time of accumulations.
Caution should be exercised with the interpretation
of peats in the stratigraphic record. Some peats are
deposited
in situ
and some are not. Transported plant
material that produces peats is most commonly com-
posed of sea grass debris. Recognition of any
in situ
root material, especially in marsh and mangrove peat
is the best indicator of the intertidal zone and sea level
position.
Another type of tidal cyclicity has been recognized
in the shells of estuarine mollusks (Murakoshi et al.
1995
). Detailed examination of the growth lines on the
shells of the bivalve,
Potamucorbula
, show cyclic pat-
terns of thick and thin layers representing spring-neap
cycles. These patterns appear as miniature tidal bun-
dles (Fig.
3.25
). Such a bivalve can be transported but
if found
in situ
it would be considered as a tidalite.
Fig. 3.23
Photo of a tidal bundle sequence showing spring and
neap packages from the margin of a tidal channel in Martens
Plate, German Wadden Sea
Fig. 3.24
Photo of stacked
sequences of tidal bundles, all
oriented in the same direction
from the Cretaceous in the
San Juan Basin of New
Mexico
