Classification of the sediments (that form the sedimentary rocks) is made on the basis of the sources of the sediments. Some of the rocks are formed from the sediments obtained from pre-existing rocks, while some are formed from the sediments sourced from the minerals precipitated from water. On the basis of the above mentioned two sources of the sediments, they are classified into two types:
(1). Clastic Sediments
The broken fragments of pre-existing rocks ranging in size from minute clay particles to very large boulders. Clastic rocks are formed by the mechanical accumulation of grains of clastic sediments. Depending upon the size of constituent grains the clastic rocks are classified into three groups: (i) rudaceous rocks, (ii) arenaceous rocks, and (iii) argillaceous rocks.
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These rocks are formed by the accumulation of bigger rocks fragments such as gravels, pebbles, and boulders. If the grains are rounded, the rock is called “conglomerate” and if they are angular, the rock is termed as “breccia”.
These rocks are composed almost entirely of sand grains. When individual grains are rounded, the rock is called “sandstone” or “grit” if the grains are angular.
These rocks are made up of very fine-grained sediments. “shale and “mudstones” are typical argillaceous rocks, which are composed of clay-sized sediments.
There are some clastic rocks, which do not fit into the above-said classification. They require consideration of mineral composition also. For example, when appreciable quantities of felspars are present in sandstone, the rocks is called “arkose”. When the sandstone contains an appreciable quantity of clay as well as angular quartz grains, the rock is called “graywacke”. In addition, there are many clastic rocks, which contain grains of more than one size. For example a rock containing a mixture of sand and silt may be classified as “sandy silt stone” or “silty sandstone” depending on which particle size dominates.
(2). Nonclastic Sediments
Nonclastic rocks include those sedimentary rocks which are formed by the chemical precipitation of minerals from the water or by the accumulation of remains of animals and plants. They are classified into two groups: (i) Chemically formed rocks, and (ii) Organically formed rocks.
Chemically Formed Rocks
These rocks are formed when the mineral matter in the solution is precipitated from water, usually because of changes in water temperature or in the chemical content of water. Such chemical sediments are derived from the dissolution of materials from older rocks and subsequent transportation of dissolved chemical substances into a sea or lake. On the basis of composition, the chemically formed rocks are classified as follows.
(1). Carbonate Rocks: Limestones and dolomite are the most abundant carbonate rocks. They are formed by the chemical precipitation of calcium carbonate from seawater.
(ii). Salt Rocks: Evaporation is the major process involved in the deposition of chemical precipitates. The salt deposits formed by the evaporation of saline lakes are called the “evaporites”. The principal minerals of these deposits are chlorides and sulfates of Na, K, Mg, and Ca. Rock-salt, gypsum, and anhydrite are by far the most abundant minals of evaporites. They commonly form massive beds.
(iii). Ferruginous Rocks: This group includes those rocks which are formed by the chemical precipitation of iron oxides. Such rocks contain a high proportion of iron-bearing minerals such as siderite, hermatite, chamosite, and pyrite. “Iron-stone” is an example of “ferruginous rocks”.
(iv). Siliceous Deposits: Siliceous rocks are formed when silica is precipitated from water. Examples of such deposits are flint, chert, jasper, and agate.
Organically Formed Rocks:
These rocks are composed mainly of the remains of animals and plants. Organically formed rocks are subdivided into two groups; (i) biochemical rocks, and (ii) organic rocks
(i). Biochemical Rocks: The biochemical sediment is produced when plants and animals living under the water, extract from it dissolved minerals matter, usually calcite, to form shells or other hard parts. These shells accumulate on the ocean floor in great quantities to form sedimentary rocks. An example of a biochemical rock is a “shell-limestone”.
(ii). Organic Rocks: Rocks containing organic matter belong to this group. An example of such rocks is “coal’. These are also called the “carbonaceous rocks”.