Geology Reference
In-Depth Information
Fig. 3.3
Intertidal algal
stromatolites in Shark Bay,
Western Australia (Photo by
M Marsden)
Fig. 3.4
Well-preserved
algal stromatolites
in the Shakopee Formation
(L. Ord.) in southwestern
Wisconsin, USA
develop in intertidal and subtidal deposits and some
are restricted to specifi c environments. Most of these
burrows are near vertical in orientation cutting across
bedding structures. In actuality, these burrows are typi-
cally found in sediments that are extensively biotur-
bated and have little or no stratifi cation preserved
(Fig.
3.5
). They do tend to be preserved easily because
of their occurrence in the stratigraphy beneath the
surface.
Because some taxa live only in certain environ-
ments infl uenced by tidal processes, the trace fossils
that they produce are used to recognize specifi c depo-
sitional environments (see Chap. 4). These may be
intertidal or subtidal. The burrows or escape structures
incorporated in these sediments cannot be considered
as uniquely tidalites. They may be present, however, in
tidalites. In other words, they do not have a distinctly
tidal signature but they may occur in tidal environ-
ments that may or may not have tidal signatures.
3.2.2
Physical Structures
These structures form as the result of sediment trans-
port and accumulation. Most are three-dimensional
with both surface and stratigraphic expression.
