Geology Reference
In-Depth Information
◗
Figure 6.27
Fossils
b
Shells of extinct ocean-dwelling animals known as horn corals.
a
Skull of the dinosaur
Allosaurus
on display at the Natural History
Museum in Vienna, Austria.
Microfossils are particularly useful for environmental
studies because hundreds or even thousands can be recovered
from small rock samples. In oil-drilling operations, small rock
chips known as
well cuttings
are brought to the surface. These
samples may contain numerous microfossils, but rarely have
entire fossils of larger organisms. These fossils are routinely
used to determine depositional environments and to match
up rocks of the same relative age (see Chapter 17).
rocks above and below. A vertical sequence consisting of the
Tapeats Sandstone, Bright Angel Shale, and Muav Limestone
is present in the lower part of the canyon (Figure 6.28b), all
of which contain features, including fossils, clearly indicating
that they were deposited in transitional and marine environ-
ments. As a matter of fact, all three were forming simultane-
ously in different adjacent environments, and during a marine
transgression they were deposited in the vertical sequence
now seen. They conform closely to the sequence shown in
Figure 6.22.
In some of the later chapters on Earth history, we refer
to river deposits, ancient deltas, carbonate shelf deposits, and
transgressive-regressive sequences. We cannot include all of
the supporting evidence for these interpretations, but we can
say that they are based on the kinds of criteria discussed in
this chapter.
of Deposition
Geologists rely on textures, sedimentary structures, and fos-
sils to interpret how a particular sedimentary rock body was
deposited. Furthermore, they compare the features seen in
ancient rocks with those in deposits forming today. But are
we justified in using present-day processes and environ-
ments to make inferences about what happened when no
human observers were present? Perhaps some examples will
help answer this question.
The Navajo Sandstone of the southwestern United States
is an ancient desert dune deposit that formed when the pre-
vailing winds blew from the northeast. What evidence justi-
fi es this conclusion? This 300-m-thick sandstone is made up of
well-sorted, well-rounded sand grains measuring 0.2-0.5 mm
in diameter. Furthermore, it has cross-beds up to 30 m high
(
IN SEDIMENTARY ROCKS
Sand and gravel are essential to the construction industry;
pure clay deposits are used for ceramics, and limestone is
used in the manufacture of cement and in blast furnaces
where iron ore is refi ned to make steel. Evaporites are the
source of table salt, as well as chemical compounds, and
rock gypsum is used to manufacture wallboard. Phosphate-
bearing sedimentary rock is used in fertilizers and animal
feed supplements.
Some valuable sedimentary deposits are found in
streams and on beaches where minerals were concentrated
during transport and deposition. These
placer deposits
are
surface accumulations resulting from the separation and
concentration of materials of greater density from those of
lesser density. Much of the gold recovered during the initial
stages of the California gold rush (1849-1853) was mined
from placer deposits, and placers of other minerals such as
diamonds and tin are important.
◗
Figure 6.28a) and current ripple marks, both typical of
desert dunes. Some of the sand layers have preserved dino-
saur tracks and tracks of other land-dwelling animals, ruling
out the possibility of a marine origin. In short, the Navajo
Sandstone possesses features that point to a desert dune dep-
ositional environment. Finally, the cross-beds are inclined
downward toward the southwest, indicating that the prevail-
ing winds were from the northeast.
In the Grand Canyon of Arizona, several formations are
well exposed; a
formation
is a widespread unit of rock, espe-
cially sedimentary rock, that is recognizably different from the
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