greywacke

Greywacke is a dark, coarse-grained sandstone that contains a substantial clay- and fine-grained matrix in which a mixture of sand-sized grains—primarily quartz, feldspar, and lithic fragments—is embedded. The grains are usually angular to subangular, and the rock is typically poorly sorted, with a significant proportion of matrix relative to the framework grains. This composition gives greywacke a dense texture and a characteristic gray color, though shades can range from light gray to nearly black.

Formation and settings: Greywacke forms in high-energy submarine environments, especially along continental margins where turbidity currents deposit sand and clay rapidly. It commonly occurs in thick, gravity-driven sequences known as turbidites and is a hallmark of ancient accretionary prisms at convergent plate boundaries.

Mineralogy and texture: The mineral content typically includes quartz, feldspar, and various lithic fragments, with the matrix dominated by clay minerals such as illite or chlorite. The rock is often cemented by minor amounts of clay, calcite, or iron oxides. Its hallmark fabric—poor sorting, a clay-rich matrix, and angular grains—reflects rapid, short-distance transport and deposition in submarine environments.

Geological significance and nomenclature: Greywacke has been used as a practical descriptor for sandstone with substantial matrix since the 18th century. Modern usage emphasizes its texture and composition rather than strict purity. It is widespread in both ancient and modern sedimentary basins and serves as an indicator of submarine deposition in tectonically active regions.