Geoscience Reference
In-Depth Information
Figure 12.12 Formation and
character of shale.
(a) Mud
collectively consists of indi-
vidual clay particles, along
with some silts, which settle
out of suspension in places
where water is very calm. If
these sediments remain in
place, they will gradually lithify
to form shale. (b) Close-up of
shale. Note the flaky appear-
ance of this rock type.
(b)
(a)
Clastic Rocks
Clastic rocks can be easily identified based on the size of sedi-
ment grains contained within them. In order to get a better feel for
these rock types, go to the
Geo Media Library
and access
Clastic
Rocks
. This animation is a simple interactivity that allows you to
better visualize the character of clastic rocks. To test your under-
standing, be sure to answer the questions at the end of the exercise.
Chemically Precipitated Sedimentary Rocks
When
environmental conditions are favorable, thick deposits of min-
eral compounds can accumulate at the bottom of the ocean or
inland lakes. In ocean environments, this process is associated
with the precipitation and recrystallization of calcium carbon-
ate from seawater. This deposition can occur for a variety of
reasons, including increase in water temperature, intense evap-
oration, upwelling of water, and bacterial decay, to name a few.
An excellent place to see the formation of chemically
precipitated sedimentary rocks is the Bahama Islands, which
lie off the eastern coast of Florida. Figure 12.13 is a satellite
image that clearly shows the precipitation of calcium carbon-
ate in this island chain. In this image, carbonate appears as
the whitish plumes within the matrix of turquoise and bluish
waters that lie between the islands. The turquoise and bluish
colors appear because the water is shallow in the island com-
plex and the satellite sensor can see the ocean bottom. Away
from the islands, however, the water rapidly deepens, which
is reflected by the dark ocean colors. Once the carbonates
accumulate on the ocean floor, they subsequently lithify to
form rock. The Bahama Islands are made of calcium carbonate
deposited in this manner.
In other situations, organisms rich in calcium carbonate
such as clams, mussels, and oysters accumulate on the ocean
bottom after they die. Subsequently, they become cemented
when calcium carbonate precipitates and settles within the ma-
trix of animal remains. The resulting rocks can contain abundant
fossils (Figure 12.14), which enable geologists to reconstruct
environmental conditions at the time the sediments were ini-
tially deposited. Regardless of the particular depositional pro-
cess, whether it is entirely through carbonate precipitation or
due to the accumulation of marine organisms, the resulting
rocks are typically classified as
limestone
. If the deposits
are rich in magnesium, the resulting carbonate rock is called
Limestone
Sedimentary rock consisting of over 50% calcium
carbonate (CaCO
3
).
