wacke, sedimentary rock composed of sand-sized grains (0.063–2 mm [0.0025–0.078 inch]) with a fine-grained clay matrix. The sand-sized grains are frequently composed of rock fragments of wide-ranging mineralogies (e.g., those consisting of pyroxenes, amphiboles, feldspars, and quartz). The grains are angular and poorly sorted with many minerals retaining growth forms that resulted from low abrasion. The matrix, which contains appreciable amounts of clay minerals, may constitute up to 50 percent of the volume. Of the clay minerals, chlorite and biotite are more abundant than muscovite and illite; kaolinite is absent. The abundant matrix tends to bind the grains strongly and form a relatively hard rock.
Common structural features of wackes include repeated graded bedding and such deformational features as folds and deformed bedding, which appear to have formed soon after deposition; cross bedding is absent. Cyclic sequences of deposition are common within wackes, with basal sandstone followed by laminated sandstone and shale at the top.
The characteristics of wackes all point to rapid deposition in turbidity currents (density currents resulting from an increase in sediment concentration) in a tectonically active region. Wacke sequences may be several thousand metres thick, strongly suggesting rapid subsidence in geosynclinal regions. They occur in almost all fold mountain belts except those dominated by limestones, such as the Canadian Rockies.