is an arenaceous sedimentary rock composed mainly of feldspar
and quartz and varies in colour (in a similar way to sand), through
grey, yellow, red, and white. Sandstones are often relatively
soft and easy to work which therefore make them a common building
and paving material.
are clastic in origin (as opposed to organic, like chalk or coal).
They are formed from the cemented grains that may be fragments
of a pre-existing rock, or else just mono-minerallic crystals.
The cements binding these grains together are typically calcite,
clays and silica. Grain sizes in sands are in the range of 0.1mm
to 2mm. (Rocks with smaller grainsizes include silts and clays
and are typically called argillaceous sediments. Rocks with larger
grainsizes include both breccias and conglomerates and are termed
principle mechanism for the formation of sandstone is by the sedimentation
of grains out of a fluid, such as a river, lake or sea. The environment
of deposition is crucial in determining the characteristics of
the resulting sandstone, which on a finer scale include its grainsize,
sorting, composition and on a larger scale include the rock geometry.
Principal environments of deposition may be split between terrestrial
and marine, as illustrated by the following broad groupings:
Rivers (levees, point bars, channel sands) Lakes
1.Shoreface sands, Deltas, Turbidites (submarine channels)
Types of sandstone
the geological characteristics of a sandstone have been established,
it can then be broadly divided between three groups:
1. arkosic sandstones, which have a high (>25%) feldspar content
SEDIMENTARY ROCKS GENERAL
2. quartzose sandstones, such as quartzite, which have a high
(>90%) quartz content.
3. argillaceous sandstones, such as greywacke, which have a significant
rock is formed from the weathered remains of other rocks. Sedimentary
rocks are formed from overburden pressure as particles of sediment
are deposited out of air, ice, or water flows carrying the particles
in suspension. As sediment deposition builds up, the overburden
(or lithostatic) pressure squeezes the sediment into layered solids
in a process known as lithification ("rock formation")
and the original connate fluids are expelled.
rocks are composed largely of silica (i.e. quartz), with other
common minerals including feldspars, amphiboles, clay minerals
and sometimes more exotic igneous minerals. Sedimentary rocks
are classified as clastic, that is, they are composed of discrete
clasts of material (rather than being composed of organic material
as is the case for a limestone).
minerals precipitating out of the ocean cover the ocean floor
with layers of calcite which can later form limestone.
rocks are economically important in that they can be used as construction
material. In addition, sedimentary rocks often form porous and
permeable reservoirs in sedimentary basins in which petroleum
and other hydrocarbons can be found.
is believed that the relatively low levels of carbon dioxide in
the Earth's atmosphere, in comparison to that of Venus, is due
to large amounts of carbon being trapped in limestone and dolomite
sedimentary layers. The flux of carbon from eroded sediments to
marine deposits is known as the carbon-cycle.
shape of the particles in sedimentary rocks has an important effect
on the ability of micro-organisms to colonize them. This interaction
is studied in the science of geomicrobiology. One measure of the
shape of these particles is the roundness factor, also known as
the Krumbein number after the geologist W. C. Krumbein.
rock is one of the three main rock groups. See also igneous and
rock is formed from the weathered remains of other rocks.
CLASSIFICATION OF ROCKS