Graywacke

(petrology) 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.

A well-indurated dark gray sandstone that is characterized by abundant dark-colored detrital rock fragments and more than 15% clay matrix minerals between sand grains. Graywacke sands were deposited chiefly in marine basins near the edge of continental margins where plate subduction was taking place. Subsequent compressional deformation and uplift of rocks in the sedimentary basins results in the occurrence of most graywackes in Alpine-type (compressional) mountain ranges. See also Clay minerals; Continental margin.

Graywackes have a wide range in mineral composition, which reflects the varied source rocks from which the detritus in them was derived. They tend to be quartz-poor (10–50%), to be rich in both feldspar and unstable rock fragments, and to contain several percent of unstable accessory minerals such as micas, pyroxenes, and amphiboles. Feldspathic graywackes (those in which feldspar exceeds rock fragments) are derived chiefly from plutonic cores of denuded island arcs. Lithic graywackes (those in which rock fragments exceed feldspar) are derived either from volcanic island arcs or from sedimentary rocks in adjacent basins that were deformed and uplifted. Volcanic rock fragments characterize the former type of lithic graywackes, whereas sandstone, shale, and their weakly metamorphosed equivalents characterize the latter type.

Most graywackes were deposited in submarine fans and adjacent basin-plain environments by turbidity currents. They commonly display graded bedding, Bouma sequences, and current-formed and biogenic sole marks. The term Bouma sequence refers to five divisions of a single, ideal turbidity current deposit. Graywackes are interbedded with shale beds that were deposited by dilute turbidity currents and other marine processes. Thicknesses of several miles of interbedded turbidite graywacke and shale accumulated in many basins. Burial and subsequent compressional deformation of these sequences resulted in the generation of clay matrix, loss of porosity, and strong induration. The gray color of the sandstone is derived from rock fragments and organic-stained clay minerals. See also Arkose; Sandstone; Sedimentary rocks; Shale; Turbidite; Turbidity current.