Sedimentology Lecture Series - Classification of sedimentary rocks

 Classification of sedimentary rocks

    After transportation and deposition loose sediments are consolidated into sedimentary rocks by a process called diagenesis. The sedimentary rocks thus formed may be broadly divided into two groups:

• Exogenetic and
• Endogenetic

    Exogenetic rocks are the products of fragmentation of the source rock outside the basin of sedimentation. Endogenetic rocks are produced out of precipitation from solution within the basin. The exogenetic and endogenetic rocks can be distinguished by their textures. While the endogenetic rocks show an interlocking or crystalline fabric (‘non-clastic’ texture), the exogenetic rocks show discrete framework grains separated by voids. The latter texture is called ‘clastic’ from the Greek clastos, meaning broken

    Classification of sedimentary rocks into clastic and non-clastic, though popular among sedimentologists, may at times lead to ambiguity. A chemically precipitated sediment for example, may also be transported from its place of origin after fragmentation and deposited elsewhere to form a sedimentary rock. Such rocks, although essentially endogenetic, may show a clastic texture.

Folk (1968)

To avoid confusion in such cases, Folk (1968) introduced the following terms:

• Terrigenous,
• Orthochemical &
• Allochemical

    Terrigenous rocks are produced from sediments derived from land areas located outside the basin. The process of derivation is purely mechanical but not all terrigenous sediments are the products of weathering and erosion. Fragments of volcanic origin and the products of crushing by glaciers also add to the terrigenous sediments.

    The orthochemical or truly chemical rocks are produced by chemical or biochemical precipitation within the basin. Evaporites such as halite, anhydrite and gypsum and non-evaporites such as limestone, dolomite and ironstone are examples of such precipitates.

    Intermediate between the terrigenous and orthochemical rocks are the allochemical rocks (allo means false). Produced initially within a basin by chemical precipitation, these rocks are fragmented, transported and re-deposited elsewhere. Both terrigenous and allochemical rocks show a ‘clastic’ texture. Only the orthochemical rocks show a non-clastic texture.

Other type

    Sediments of chemical and terrigenous origin may combine to form rocks of a hybrid nature. Calcareous and carbonaceous shales are examples of such hybrid rocks. While accepting the rationality of Folk’s scheme of classification, it must be appreciated that the terms clastic and non-clastic have thoroughly pervaded sedimentological literature and cannot be totally ignored at present.

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• References:
• Sengupta S M., Introduction to sedimentology (Second edition)
• Pettijohn, F.J. – 1967 – Sedimentary Rocks, Harpers and Bros
• Gary Nichols, 2009 – Sedimentology and Stratigraphy, John Wiley and Sons
• Sam Boggs, 2006 – Principles of sedimentology and stratigraphy, Pearson Prentice Hall