Home » Rock Directory

Greywacke

A sedimentary rock consisting of angular fragments of quartz, feldspar, and other minerals set in a muddy base. Its origin is problematic, since, according to the normal laws of sedimentation, sand and mud should not be laid down together, and some geologists attribute its formation to submarine avalanches or turbidity currents. Deposits of greywacke are found on the edges of the continental shelves.

Greywacke (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly-sorted, angular grains of quartz, feldspar, and small rock fragments set in a compact, clay-fine matrix. It is a texturally-immature sedimentary rock generally found in Palaeozoic strata. The larger grains can be sand-to-gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume.

An old rock name that has been variously defined but is now generally applied to a dark gray, firmly indurated, coarse-grained sandstone that consists of poorly sorted, angular to subangular grains of quartz and feldspar, with a variety of dark rock and mineral fragments embedded in a compact clayey matrix having the general composition of slate and containing an abundance of very fine-grained illite, sericite, and chloritic minerals.

Graywacke is abundant within the sedimentary section, esp. in the older strata, usually occurring as thick, extensive bodies with sole marks of various kinds and exhibiting massive or obscure stratification in the thicker units but marked graded bedding in the thinner layers. It generally reflects an environment in which erosion, transportation, deposition, and burial were so rapid that complete chemical weathering did not occur, as in an orogenic belt where sediments derived from recently elevated source areas were poured into a geosyncline.

Graywackes are typically interbedded with marine shales or slates, and associated with submarine lava flows and bedded cherts; they are generally of marine origin and are believed to have been deposited by submarine turbidity currents. CF: arkose; subgraywacke.

See also: Graywacke; Greywacke Illustration

[ Close ]