Sedimentology Lecture Series - Rudaceous

 Rocks of Mechanical Origin – Rudaceous

     Sedimentary particles of mechanical (terrigenous) origin are classified according to their size. Particles larger than 2 mm constitute gravel (Latin-rudite), those between 2 and 1/16 mm constitute sand (Latin-arenite), while those finer than 1/16 mm constitute silt and clay (Latin-lutite). Accordingly, sedimentary rocks of mechanical origin are classified broadly into three groups: Rudaceous, Arenaceous and Lutaceous (aka Argillaceous).

Rudaceous Sedimentary Rocks (Conglomerates and Breccias) Unconsolidated sediments coarser than 2 mm are grouped under the term gravel. Indurated gravels are called conglomerates. While the particles constituting a conglomerate are essentially rounded, a breccia consists of gravel-size clasts which are angular. The term rubble is applied to an assemblage of loose angular fragments of gravel size.

    Conglomerates may be classified either in a purely descriptive way (example: chertconglomerate, limestone conglomerate) or according to the mode of their origin (fluvialconglomerate, glacial conglomerate).In the comprehensive genetic classification proposed by Pettijohn (1975), conglomerates are grouped into four major categories which are geologically meaningful: epiclastic, pyroclastic, cataclastic and meteoric.

     Epiclastic conglomerates are the commonest of all conglomerates. These consist of gravels of terrigenous origin. The conglomerates of fluvial origin as also those generated on beaches by wave action are of this type.

     The Intraformational conglomerates, which are also epiclastic, are composed of clasts derived from within the basin. Shale-pebbles, produced out of mud cracks, are good examples of intraformational conglomerates, but the process may be more complicated.

     Extraformational epiclastic conglomerates include oligomicts, petromicts (polymicts), and diamicts.

• Oligomictic conglomerates consist of pebbles of homogeneous composition,
• Petromictic (polymictic) conglomerates consist of pebbles of varied rock types.
• Diamictic conglomerates are characterized by a high proportion of matrix. These may be of glacial (tills or tillites) or
• Non-glacial (tilloid) origin. Pebbly mudstones (argillites) are also included in this group. Conglomerates found in alluvial fan deposits are good examples of tilloids.
• Other important types includes pyroclastic conglomerates of volcanic origin,
• Cataclastic produced by earth movements and
• Meteoric produced by meteoric impact.

Want to know more check out this video.

• 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