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Important Sedimentary Rocks
What is sedimentary rocks?
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place.
Types of Sedimentary Rocks Breccia Conglomerates Sandstones Shale
It is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix that can be similar to or different from the composition of the fragments. The word “ Breccia ” has its origins in the Italian language, in which it means “rubble”. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia , tectonic breccia , igneous breccia , impact breccia , and hydrothermal breccia . There are 3 types of Breccia .
The name “basal breccia ”, originally introduced for rocks in the Azuara impact structure, refers to the fact that this peculiar breccia is regularly found at the base of the post-impact, tectonically undisturbed Upper Tertiary. This rock is formed by the sea waters advancing over a coastal region covered with fragments of chert and other similar rocks. Advancing waters supply fine mud, which is spread over the rock fragments and acts as a binding material.
Basal suevite breccia - ERNSTSON CLAUDIN IMPACT STRUCTURES - METEORITE CRATERS
This rock is also called “crush- breccia ”, such rocks are so named because they are made up of angular fragments that have been produced during the process of faulting. Fault breccias are tectonites formed primarily by tectonic movement along a localized zone of brittle deformation (a fault zone) in a rock formation or province. The fragments so produced due to crushing effect of the block movements subsequently get embedded in clay and other fine material.
Fault breccia - Wikipedia
Agglomerate from the Latin ” agglomerare ” meaning "to form into a ball" is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. It is a specific type of breccia containing angular and sub-angular fragments derived from volcanic eruptions. It may also contain some fused material that has been cemented together with the solid material broken and thrown out of the craters. Agglomerates are typically found near volcanic vents, where they may be associated with pyroclastic or intrusive volcanic breccias.
Agglomerate High Resolution Stock Photography and Images - Alamy
These are sedimentary rocks of clastic nature and also belongs to rudaceous group. They consist mostly of rounded fragments of various sizes but generally above 2mm. cemented together in mixed matrix. A conglomerate typically contain a matrix of finer grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts . The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay. They are closely related to sandstones and exhibit many of the same types of sedimentary structures.
Basal-Conglomerates Having gravels derived from advancing sea-waves over subsiding land masses. Rest on uncomformable surface and contain fragments (clasts) of underlying rocks. Glacial-Conglomerates In which gravel making the conglomerates are distinctly of glacial origin. Volcanic-Conglomerates In which gravel are of distinct volcanic origin but have subsequently been subjected to lot of transport resulting in their smoothening and polishing by river transport before their deposition and compaction or cementation. a. Oligomictic- simple in composition, these gravels are made up of quartz, chert, and calcite. b. Polymictic- In these conglomerates the constituent gravels are derived from rocks of all sorts: igneous, sedimentary, and metamorphic, all cemented together.
a re also classified by the dominant clast size:
Granule Conglomerate 2-4 mm Pebble Conglomerate 4-64 mm Cobble Conglomerate 64-256 mm Boulder Conglomerate 256 mm
What is Sandstones?
Sandstones are mechanically formed sedimentary rocks of Arenaceous Group. These are mostly composed of sand grade particles that have been compacted and consolidated together in the form of beds in basins of sedimentation. Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black.
Columnar Sandstones Are Very Unique Structure
The color of sandstones depends on its composition, especially nature of the cementing material. For example, presence of iron oxide is responsible for the red, brown, and yellow shades; presence of glauconite gives a greenish shade to the sandstones.
Silicious Sandstones Silica (Si02) is the cementing material in these sandstones. Sometimes the quality of the silicious cement is so dense and uniform that a massive compact and homogenous rock is formed. This is named “quartzite”. Calcareous Sandstones Those varieties of sandstones in which carbonates of calcium and magnesium are the cementing materials. The grains are bound together by calcareous cement, which is composed of calcite.
Types of Sandstones
Argillaceous Sandstones These are among the soft varieties of sandstone because the cementing material is clay that has not inherent strength. These stones are less durable stones. All clay stones belong to this group. Ferruginous Sandstones A sandstone containing iron oxide as the cementing material, as grains, or both. Arkose A variety of sandstone that is exceptionally rich in felspar minerals besides the main constituent quartz. Arkose rocks generally occurs in horizons that can be genetically related to some crystalline massif occuring in close neighbourhood .
Greywacke Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. Greywackes are mostly grey, brown, yellow or black, dull-colored sandy rocks which may occur in thick or thin beds along with shales and limestones . Flagstone A variety of sandstone that is exceptionally rich in mica dispersed in parallel or sub parallel layers. Hence, its use in load bearing situations is not recommended. Freestone It is a massive variety of sandstone that is rich in quartz and does not contain bedding planes or any mica.
Uses of Sandstones
They are most commonly used as materials of construction. Sandstones of hard, massive, and compact character are very useful natural resources. Sandstones
Sandstones are the most abundant sedimentary rocks found in the upper 15 km of the crust and make an estimated 15 percent of total sedimentary rocks of the earth.
Did you know?
Shale is a fine-grained, clastic sedimentary rock, composed of mud that is a mix of flakes of clay minerals and tiny fragments. The term shale is sometimes applied more broadly, as essentially a synonym for mudrock . It is the most common sedimentary rock. Shale usually contains other clay-size mineral particles such as quartz, chert , and feldspar.
►Residual Shales- these are formed from decay and decomposition of pre-existing rocks followed by the compaction and consolidation of the particles in adjoining basins without much mixing. ► Transported Shales- these are the deposits of clatic materials of finer dimensions transported over wide distances before final settlement in basins of deposition.
Types of Shales