Clastic Rock

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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.


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


References in periodicals archive ?
The basement of the Golpazari field is composed of Jurassic limestones and Upper Cretaceous clastic rocks.
A limestone quarry in one of the hogbacks, on the northeastern side of Port-Daniel, exposes a dense network of NNE-striking brittle fault planes with minor N-striking fault planes affecting both the limestone of the La Vieille Formation and the red clastic rocks within the karsts (Fig.
The Black Cape exposures, where abundant slickensided fault planes abruptly end at the contact between Early Devonian basalt and underlying clastic rocks (Fig.
However, based on the presence of similar clastic rocks in both formations, the hiatus is not considered to be regionally significant (Johnson 2001b).
The lithological contrasts, the fact that the Mosquito Lake Road Formation apparently evolved in an arc-type tectonic setting (Johnson and McLeod 1996), and the realization that the clastic rocks of the Hanford Brook Formation were deposited on a continental platform (e.
Recent studies of Carlin Trend gold deposits have repeatedly shown that the areal extent and number of ounces of gold contained in near-surface "leakage zones", coupled with the presence of structurally-controlled, high-grade gold "feeder zones" in the Upper Plate clastic rocks, provides an approximate measure as to the size and grade of major gold deposits which are found in the more favourable, deeper Lower Plate carbonate rocks.
The evaluation of cuttings, logs and sidewall cores demonstrated the presence of oil over a 200 feet section of low permeability and low porosity clastic rocks in the Miocene.
The property is underlain by a thick sequence of Pennsylvanian and Mississippian carbonate and clastic rocks, of which the Mississippian Rundle Group shows the greatest exposure.
Gold mineralization occurs in jasperoid and in silicified Mississippian clastic rocks.