Clastic Rock


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Related to Clastic Rock: clast, Carbonate rock

Clastic Rock

 

(fragmentai rock), sedimentary rock consisting wholly or chiefly of fragments of various rocks (magmatic, metamorphic, or sedimentary) and minerals (quartz, feldspar, mica, and sometimes glauconite and volcanic glass). Clastic rocks are subdivided into cemented and uncemented, or friable, rock. In cemented clastic rock, the bonding agents include carbonates (calcite and dolomite), silica oxides (opal, chalcedony, and quartz), iron oxides (limonite, goethite), and argillaceous minerals. Clastic rock often contains such organic remains as the shells of mollusks and tree trunks and branches.

The classification of clastic rock is based on the size of the fragments. There are coarse fragmental rocks, or psephites, whose fragments exceed 1 mm (uncemented psephites include blocks, boulders, cobbles, rock debris, grus, and gravel; among cemented psephites are conglomerates, breccia, and grit); sandy rocks, or psammites, with particles of 1–0.05 mm (sand and sandstone); silty rocks, or aleurites, with particles of 0.05–0.005 mm (aleurite and aleurolite); and clayey rocks, or pelites, with particles of less than 0.005 mm (clay, argillite). Sometimes the boundary between aleurites and pelites is set at a particle size of 0.001 mm. Clayey rock may be either chemical or detrital in origin. There are also mixed clastic rocks composed of fragments of different sizes—sandy, aleuritic, and clayey. Among mixed clastic rocks are various loams and sandy loams, which are widely found, particularly among recent continental deposits. Within the structural subtypes clastic rocks are further subdivided according to the mineral composition of the fragments and other features. Clastic rocks also include the products of volcanic eruptions—volcanic rubble and ash—both friable rocks and their cemented varieties, such as tuff, tuff breccia, and rocks that are intermediate between fragmental and volcanic rocks (tuffite and tufogenic rock).

Coarse fragmental rock is formed where the terrain is rugged and the environment highly dynamic; a level terrain and slow water and air currents are necessary to produce sandy, aleuritic, and clayey rocks. Clay particles precipitate chiefly in quiet water. In the littoral zone of seas and oceans, pebbles and gravel are deposited on the beach and in shallow water, but as one moves toward the depths of the basin, these rocks are replaced by sands, aleurites, and finally clayey silts (at depths below the action of waves and currents). However, gravel beds and sands are encountered at great depths as a result of the action of bottom and turbidity currents.

Clastic rocks are important building materials, and sand is used in the glass and metallurgical industries. River and sea sands contain placers of gold, platinum, precious stones, minerals (titanium, tin, tungsten), and rare and radioactive elements.

REFERENCES

Logvinenko, N. V. Petrografiya osadochnykh porod (S osnovami metodiki issledovaniia), 2nd ed. Moscow, 1974.
Rukhin, L. B. Osnovy litologii, 3rd ed. Leningrad, 1969.

N. V. LOGVINENKO

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