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Sandstone Petrology & Sedimentary Rock Information

Sandstone 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.

Sandstones 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 rudaceous sediments.).

The 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:

Terrestrial environments

1. Rivers (levees, point bars, channel sands) Lakes

Marine environments

1.Shoreface sands, Deltas, Turbidites (submarine channels)

Types of sandstone

Once 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
2. quartzose sandstones, such as quartzite, which have a high (>90%) quartz content.
3. argillaceous sandstones, such as greywacke, which have a significant fine-grained element

SEDIMENTARY ROCKS GENERAL

Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in three main ways-by the deposition of the weathered remains of other rocks (known as clastic sedimentary rocks); by the deposition of the results of biogenic activity; and by precipitation from solution. Sedimentary rocks include common types such as chalk, limestone, sandstone, and shale.

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.

Sedimentary 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).

Carbonate minerals precipitating out of the ocean cover the ocean floor with layers of calcite which can later form limestone.

Sedimentary 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.

It 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.

The 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.

Sedimentary rock is one of the three main rock groups. See also igneous and metamorphic.

Sedimentary rock is formed from the weathered remains of other rocks.

CLASSIFICATION OF ROCKS

Rocks Classification Image


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