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An argillaceous sandstone characterized by an abundance of unstable mineral and rock fragments and a fine-grained clay matrix binding the larger, sand-size detrital fragments.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a rock of sedimentary origin consisting of small fragments (grains) of sedimentary and chiefly metamorphic and igneous rock with an admixture of fragments of basic volcanic rock and tufogenic material. A significant quantity of cementing material is characteristic. The color is usually dark gray, black, or dark green (owing to the chlorite in the cement). The fragmentary grains in the graywacke are angular or slightly rounded and are poorly sorted. Graywackes are typical for geosynclinal areas where they form thick strata originating during periods of the rapid accumulation of disintegration products from internal uplifts composed of more ancient rock.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Pyrargyrite probably does not occur in the Llallagua stock, but it was an ore mineral with galena in a peripheral vein in graywackes at Uncia, 3 km south of the intrusion.
This fault dips approximately 35 degrees to the northeast, and brings the Mesozoic sediments and Tertiary volcanics into contact with older, underlying shales, graywackes and mafic volcanic rocks.
1) could be summarized as a basement of Upper Precambrian age which crops out in the center of the Olivenza-Monesterio anticlinorium, formed by successive levels of schists, graywackes, shales and graphitic quartzites affected by a low-grade regional metamorphism (Benito et al., 1977).
The rocks exposed at the Hale Creek mine are typical of the Franciscan-Knoxville group of late Jurassic age and consist of sandstones, shales, cherts, greenstones and graywackes. Quartz is common in this diverse assemblage of rocks.
To the south of this lie Silurian graywackes and Devonian sandstones; to the north are Lower Carboniferous dolomite and limestones, often muddy and with shales and cherty horizons.