Geology Reference
In-Depth Information
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horizontal dimensions of length and width measured in
hundreds of kilometers.
The opposite of a marine transgression is a
marine
regression
(Figure 6.22). If sea level falls with respect to a
continent, the shoreline and environments that parallel the
shoreline move seaward. The vertical sequence produced by
a marine regression has facies of the nearshore environment
superposed over facies of offshore environments.
What Would You Do
You live in the continental interior where fl at-lying sedimentary
rock layers are well exposed. Some local residents tell you of
a location nearby where sandstone and mudstone with dino-
saur fossils are overlain fi rst by a seashell-bearing sandstone,
followed upward by shale and fi nally limestone containing the
remains of clams, oysters, and corals. How would you explain
the presence of fossils, especially marine fossils so far from
the sea, and how this vertical sequence of rocks came to be
deposited?
SEDIMENTARY ROCKS
No one was present when ancient sediments were depos-
ited, so geologists must evaluate those aspects of sedimen-
tary rocks that allow them to make inferences about the
original depositional environment. And making such deter-
minations is of more than academic interest. For instance,
barrier island sand deposits make good reservoirs for hy-
drocarbons, so knowing the environment of deposition and
the geometry of these deposits is helpful in exploration for
resources.
Sedimentary textures such as sorting and rounding can
give clues to depositional processes. Windblown dune sands
tend to be well sorted and well rounded, but poor sorting is
typical of glacial deposits. The geometry or three-dimensional
shape is another important aspect of sedimentary rock bod-
ies. Marine transgressions and regressions yield sediment
bodies with a blanket or sheetlike geometry, but sand depos-
its in stream channels are long and narrow and are described
as having a shoestring geometry. Sedimentary textures and
geometry alone are usually insufficient to determine depo-
sitional environment, but when considered with other sedi-
mentary rock properties, especially
sedimentary structures
and
fossils
, they enable geologists to reliably determine the history
of a deposit.
downward in the same direction that the current flowed.
Thus, ancient deposits with cross-beds inclined down toward
the south, for example, indicate that the currents responsible
for them fl owed from north to south.
Some individual sedimentary rock layers show an
upward decrease in grain size, termed
graded bedding
,
mostly formed by turbidity current deposition. A
turbidity
current
is an underwater flow of sediment and water with
a greater density than sediment-free water. Because of its
greater density, a turbidity current fl ows downslope until it
reaches the relatively fl at seafl oor, or lakefl oor, where it slows
and begins depositing large particles followed by progres-
sively smaller ones (
◗
Figure 6.24).
The surfaces that separate layers in sand deposits com-
monly have
ripple marks
, small ridges with intervening
troughs, giving them a somewhat corrugated appearance.
Some ripple marks are asymmetrical in cross section, with a
gentle slope on one side and a steep slope on the other. Cur-
rents that fl ow in one direction, as in stream channels, gen-
erate these so-called
current ripple marks
(
Figure 6.25a, b).
And because the steep slope of these ripples is on the down-
stream side, they are good indications of ancient current di-
rections. In contrast,
wave-formed ripple marks
tend to be
symmetrical in cross section and, as their name implies, are
generated by the to-and-fro motion of waves (Figure 6.25c, d).
When clay-rich sediment dries, it shrinks and develops
intersecting fractures called
mud cracks
(
◗
Physical and biological processes operating in depositional
environments are responsible for a variety of features known
as
sedimentary structures
. One of the most common is dis-
tinct layers known as
strata
or
beds
(
Figure 6.26). Mud
cracks in ancient sedimentary rocks indicate that the sediment
was deposited in an environment where periodic drying took
place, such as on a river fl oodplain, near a lakeshore, or where
muddy deposits are exposed along seacoasts at low tide.
◗
◗
Figure 6.23a), with
individual layers of less than a millimeter up to many meters
thick. These strata or beds are separated from one another
by surfaces above and below in which the rocks differ in
composition, texture, color, or a combination of features.
Layering of some kind is present in almost all sedimentary
rocks; however, a few, such as limestone that formed in coral
reefs, lack this feature.
Many sedimentary rocks are characterized by
cross-
bedding
, in which layers are arranged at an angle to the
surface on which they are deposited (Figure 6.23b, c). Cross-
beds are found in many depositional environments such
as sand dunes in deserts and along shorelines, as well as in
stream-channel deposits and shallow marine sediments. In-
variably, cross-beds result from transport and deposition
by wind or water currents, and the cross-beds are inclined
of Ancient Life
Fossils,
the remains or traces of ancient organisms, are inter-
esting as evidence of prehistoric life (
Figure 6.27), and are
also important for determining depositional environments.
Most people are familiar with fossils of dinosaurs and some
other land-dwelling animals, but are unaware that fossils of
invertebrates, animals lacking a segmented vertebral column,
such as corals, clams, oysters, and a variety of microorgan-
isms, are much more useful because they are so common.
◗
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