Page 19 - Sedimentology and Stratigraphy

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Stratigraphy
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Stratigraphy
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6 Terrigenous Clastic Sediments: Gravel, Sand and Mud 26.2.2009 8:13pm Compositor Name: ARaju

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Fig. 2.1 A classification scheme for sediments and sedimentary rocks.
as molluscs. Limestones constitute 10–15% of the which are lithified sediment: lithification is
sedimentary rocks in the stratigraphic record. the process of ‘turning into rock’ (18.2). Mud,
silt and sand are all loose aggregates; the
Evaporites These are deposits formed by the precipi-
addition of the suffix ‘-stone’ (mudstone, siltstone,
tation of salts out of water due to evaporation.
sandstone) indicates that the material has been
Volcaniclastic sediments These are the products of lithified and is now a solid rock. Coarser, loose gravel
volcanic eruptions or the result of the breakdown of material is named according to its size as granule,
volcanic rocks. pebble, cobble and boulder aggregates, which
become lithified into conglomerate (sometimes
Others Other sediments and sedimentary rocks are
sedimentary ironstone, phosphate sediments, organic with the size range added as a prefix, e.g. ‘pebble
deposits (coals and oil shales) and cherts (siliceous conglomerate’).
sedimentary rocks). These are volumetrically less A threefold division on the basis of grain size is
common than the above, making up about 5% of used as the starting point to classify and name terri-
the stratigraphic record, but some are of considerable genous clastic sediments and sedimentary rocks:
gravel and conglomerate consist of clasts greater
economic importance.
than 2 mm in diameter; sand-sized grains are between
In this chapter terrigenous clastic deposits are consid-
2 mm and 1/16 mm (63 microns) across; mud
ered: the other types of sediment and sedimentary
(including clay and silt) is made up of particles less
rock are covered in Chapter 3.
than 63 mm in diameter. There are variants on this
2.1.1 Terrigenous clastic sediments scheme and there are a number of ways of providing
and sedimentary rocks subdivisions within these categories, but sedimen-
tologists generally use the Wentworth Scale
A distinction can be drawn between sediments (Fig. 2.2) to define and name terrigenous clastic
(generally loose material) and sedimentary rocks deposits.