Some important sedimentary rocks

Some important sedimentary rocks are as under;

(1). Conglomerate

  • Nature: Consolidated gravels. Colour variable.
  • Mineral Composition: Rounded pebbles are set in a fine-grained matrix. The matrix commonly consists of sand and silt and it is cemented by silica, calcium carbonate, or iron oxide.
  • The individual pebbles may be entirely composed of quartz or maybe rock fragments that have not been decomposed.
  • Texture: Very Coarse-grained
  • Varieties: Fine conglomerates grade into coarse sandstones. If the rock contains angular or subangular fragments, it is called “breccia”. The angularity of rock fragments in breccia suggests that this material could not have traveled very far from its source.

(2). Sandstone:

  • Nature: Arenaceous. Colour variable according to the type of cementing material. Rocks having silica or calcite as their cementing material are light in colour, those that contain iron oxide are red to reddish-brown.
  • Mineral Composition: Quartz is the chief mineral constituent. Small amounts of felspar, mica, garnet, etc. may also occur. Cementing material may be silica, calcite, iron oxide, clay, or chlorite.
  • Texture: Sandstones are composed almost entirely of well-sorted, subangular to rounded sand grains. The texture of sandstone is (i) coarse-grained when the size of grains is between 2-0.5 mm, (ii) medium-grained when the size of the grains is 0.5 – 0.25mm, and (iii) fine-grained when the size of the grains is 0.25 – 0.10 mm.
  • Structure: The common structure seen in the sandstones are stratification, current bedding, ripple makes, and rain prints.
some important sedimentary rock
Sand Stone
  • Varieties:
    • Orthoquartzite: White Siliceous sandstones in which most of the grains as well as cement consist of quartz are called orthoquartzite.
    • Grit: It is sandstone containing sharply angular grains.
    • Arkose: A coarse-grained sandstone containing notable amount of felspar is called arkose.
    • Graywackes: It is grey-coloured rock containing poorly sorted angular fragments of quartz and basic igneous rocks, and fine-grained chlorite or clay material. Graywackes may contain as much as 30% fine-grained clay or chlorite or both. The finer-grained graywackes grade into the shales.
    • Glauconite sandstone: It is a green-coloured sandstone containing a mineral called glauconite.

(3). Shale:

  • Nature: Argillaceous. Colour variable. Sales are often soft and can be scratched by a knife.
  • Mineral Composition: Shales are composed mainly of clay minerals like kaolinite, montmorillonite, and illite. Small amounts of other minerals such as quartz, mica, and chlorite are also present.
  • Texture: Very fine-grained with grains with grain size less than 0.01 mm.
  • Structure: Lamination, ripple marks, and some organic structures may be present.
  • Varieties: 
    • Calcareous shale: When a considerable amount of calcium carbonate is present.
    • Ferruginous Shale: When a considerable amount of iron oxide is present.
    • Carbonaceous Shale: When a considerable amount of carbonaceous (organic) matter is present.
    • Siltstone: It is a rock containing compact silt (grain size; 0.01 – 0.1)
    • Mudstone: it is a structureless rock containing compacted mud.
Shale; an example of sedimentary rock
Shale Rock

(4). Limestone:

  • Nature: Calcareous rock. Formed chemically or organically. Commonly white, grey, or cream-coloured. Often contains fossils. Limestones are identified by their softness, fossil content, and effervescence in dilute hydrochloric acid.
  • Mineral Composition: Calcium carbonate is the chief constituent. Magnesium carbonate is also present in variable amounts. Chalcedony, silt, and clays are present as impurities. Some limestone may also contain calcareous shells of marine animals.
  • Texture: Limestone is a fine-grained rock. It is commonly compact and massive. Some limestones may have politic structures. Organic structures are also common.
  • Varieties: The important varieties of limestones are as follows;
    • Chalk: The porous fine-grained and generally friable limestone composed mainly of foraminifera shells, is known as chalk.
    • Oolitic limestone: This limestone is mainly composed of rounded grains resembling fish roe. It is believed to have been formed by chemical precipitation. Under the microscope, each grain (oolith) is seen to be made up of concentric layers of CaCO3, often with a bit of wheel at the center.
    • Marl: Impure limestones in which the percentage of clay and calcium carbonate is almost equal, are known as “marl”.

(5). Dolomite:

  • Nature: The Dolomites resemble limestones.
  • Mineral Composition: The chief constituents of dolomite rock is dolomite minerals [CaMg(CO3)2]. It may also contain some calcite. The Dolomites are generally not formed by original chemical precipitation. They are formed when the calcium carbonate of the limestone is replaced by dolomite. This process is called “dolomitization”. Depending upon the relative proportion of calcite and dolomite present, the limestones and dolomites are classified as follows.
    1. Limestones: Those rocks which contain more than 90% calcite and less than 10% dolomite.
    2. Dolomite limestone: Those rocks that contain 90-50 % calcite, and 10-50 % dolomite.
    3. Calcitic Dolomite: The rocks which contain less than 10-50 % calcite, and 50-90 % dolomite.
    4. Dolomites: Those rocks which contain less than 10% calcite and more than 90% dolomite.
  • Texture: Dolomite is a fine-grained rock. It is compact and massive.
dolomite is an important sedimentary rock
Dolomite: a sedimentary rock

(6). Iron Formation:

  • Nature: Iron-formation is a banded iron rock. It is formed due to the chemical precipitation of iron oxide and chert.
  • Mineral Composition: The iron-formation consists mainly of chert-magnetite, chert-hematite, and chert-hematite magnetite. Other minerals that are commonly present are siderite, ankerite, and charmostie.
  • Texture and structure: In these rocks, the sedimentary banding is generally well preserved.

(7). Laterite:

  • Nature: Colour si often red, brown, or yellow. Laterite is a residual product of weathering in a hot humid climate. It occurs as a mantle over bed rocks.
  • Mineral Composition: Laterites are essentially clays rich in aluminium and iron hydroxides with minor amounts of silica.
  • Texture: Porous and concretionary.
  • Varieties: Laterites rich in aluminium hydroxides are called “bauxites”. Bauxites commonly show “pisolitic” structures.
Laterite; an important sedimentary rock
Laterite: an important sedimentary rock


Related Material:

  1. Sedimentary Rocks and their formation
  2. Classification of Sediments
  3. The texture of the Sedimentary Rocks
  4. Structural features of the sedimentary rocks
  5. Composition of Sedimentary Rocks


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