Geology Reference
In-Depth Information
Texas coast has extensive such fl ats. These microbial
mats are useful to interpret the stratigraphic record
because they are extensive, easily recognized, and have
a fairly high preservation potential because they pro-
vide cohesion to the substrate, especially in carbonate
successions (e.g. sabkhas) where cementation facili-
tates preservation. Additionally, they are they are
restricted to the intertidal and supratidal zone where
erosion is relatively uncommon permitting them to be
buried by washover sediment that is carried across
barrier islands.
3.2.1.2
Three-Dimensional Structures
Three-dimensional cyanobacterial structures, typically
called stromatolites or algal stromatolites, are also
important surface structures in the intertidal zone. For
many years these features were exclusively associated
with the intertidal zone with the most spectacular
examples being in Shark Bay in Western Australia
(Fig.
3.3
). The combination of the intertidal environ-
ment with carbonate sediment and tropical climate
provides for rapid lithifi cation and high preservation
potential. These structures are present in the rock
record from the Precambrian to the Holocene (Fig.
3.4
).
They are now known to develop subtidally also, but in
locations where tidal currents are swift (Dill et al.
1986
) so that it is appropriate to consider them as
tidalites. Stromatolites are typically carbonate features
but have been reported from siliciclastic sediments as
well (Davis
1966
) .
Many organisms are infaunal and produce distinct
structures, typically in the form of burrows. These
Fig. 3.1
Examples of intertidal tracks (bear) (
a
) and burrow
spoil (
b
) that are common on intertidal fl ats but that have very
little preservation potential
Fig. 3.2
Microbial mats
(
arrows
) on the surface and in
the stratigraphy of a
wind-tidal fl at on Padre
Island, Texas, USA
