Geology Reference
In-Depth Information
entific term for the tiny grain size. You can easily distinguish shale from siltstone be-
cause shale splits into very thin layers (called
laminations
) and you cannot see the indi-
vidual sediment grains without a microscope.
For particles this tiny to settle out of water and be compacted, the water
must not be moving. Therefore, shale and siltstone are often the result of sedi-
ments deposited in still water, such as a lake bed or deep ocean.
Sandstones
are categorized according to their mineral content. Three major types exist:
Quartz sandstones:
These are the most common sandstones and are composed of
well-sorted quartz grains. They are commonly white or tan colored.
Arkose sandstones:
These contain high amounts of feldspar (eroded from granite).
They are often poorly sorted with angular pink or reddish grains.
Greywacke sandstones:
These are composed of sediments eroded from volcanic
rocks (such as basalts). They have some quartz and feldspar but are poorly sor-
ted, angular, and generally dark in color.
The largest sediment grains create conglomerates and breccias (see Table 7-2). Almost
always, these rocks are poorly sorted, with the larger particles held together by a ce-
mented mix of smaller particles. If the sediments are rounded, the rock is a
conglomer-
ate.
If the sediments are angular, the rock is a
breccia.
No grains at all: Chemical sedimentary rocks
Chemical
sedimentary rocks are made not of rock particles but of chemical particles.
They are created when minerals are dissolved from existing rock into water and carried
to the ocean (or a lake). Once in a body of water, the minerals re-form into solids, creat-
ing particles that settle to the bottom and
lithify,
becoming a sedimentary rock. Chemic-
al sedimentary rock particles may be organic or inorganic:
Organic:
Organic sediment grains come from the shells created by an organism
during its lifetime using elements dissolved in the seawater. When the organism
dies, the shell remains and becomes sediment particles on the ocean floor — sub-
jected to the same changes in size, shape, and sorting that I describe for sediment
grains in the previous section. These types of sedimentary rocks may also be
called
biogenic
sedimentary rocks.
