Geology Reference
In-Depth Information
coal is anthracite , a metamorphic type of coal (see Chapter 7),
with up to 98% carbon.
are sandstone, mudstone (or shale), and limestone facies,
respectively.
Many sedimentary rocks in the interiors of continents
show clear evidence of deposition in marine environments.
The rock layers in Figure 6.22 (left), for example, consist of
a sandstone facies that was deposited in a nearshore marine
environment overlain by shale and limestone facies depos-
ited in offshore environments. Geologists explain this verti-
cal sequence of facies by deposition occurring during a time
when sea level rose with respect to the continents. As sea level
rises, the shoreline moves inland, giving rise to a marine
t ransgression (Figure 6.22), and the depositional environ-
ments parallel to the shoreline migrate landward. As a result,
offshore facies are superimposed over nearshore facies,
thus accounting for the vertical succession of sedimentary
facies. Even though the nearshore environment is long and
narrow at any particular time, deposition takes place con-
tinuously as the environment migrates landward. The sand
deposit may be tens to hundreds of meters thick, but have
SEDIMENTARY FACIES
Long ago, geologists realized that if they traced a layer
of sediment or sedimentary rock laterally, it generally
changed in composition, texture, or both. These changes
resulted from the simultaneous operation of different
processes in adjacent depositional environments. For ex-
ample, sand may be deposited in a high-energy nearshore
marine environment, whereas mud and carbonate sedi-
ments accumulate simultaneously in the laterally adjacent
low-energy offshore environments (
Figure 6.22). Depo-
sition in each environment produces sedimentary facies ,
bodies of sediment each possessing distinctive physical,
chemical, and biological attributes. Figure 6.22 illustrates
three sedimentary facies: a sand facies, a mud facies, and a
carbonate facies. If these sediments become lithifi ed, they
Three Stages of Marine Transgression
Three Stages of Marine Regression
Offshore
Low-energy
Near shore
High-energy
Land
surface
Limestone
facies
Shale
facies
Sandstone
facies
Time
line
Time
lines
Time
lines
Old land surface
Old land
surface
Figure 6.22 Marine Transgressions and Regressions
 
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