Wacke

A sandstone with 15% to 75% fine-grained interstitial material.

Also:
i. A dirty sandstone that consists of a mixed variety of angular and unsorted or poorly sorted mineral and rock fragments, and of an abundant matrix of clay and fine silt; specif. an impure sandstone containing more than 10% argillaceous matrix. The term is used for a major category of sandstone, as distinguished from arenite.

Ref: AGI

ii. A clastic sedimentary rock in which the grains are almost evenly distributed among the several size grades; e.g., a sandstone consisting of sediment poured in to a basin of deposition at a comparatively rapid rate without appreciable selection or reworking by currents after deposition, or a mixed sediment of sand, silt, and clay in which no component forms more than 50% of the whole aggregate.

Ref: AGI

iii. A term commonly used as a shortened form of graywacke. This usage is not recommended.

Ref: AGI

iv. Originally, a term applied to a soft earthy variety of basalt, or to the grayish-green to brownish-black claylike residue resulting from the partial chemical decomposition of basalts, basaltic tuffs, and related igneous rocks. Mining term signifying a large stone or stoniness in general.

Synonym of: vake

Ref: AGI