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Graywacke

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.

Sedimentary rock composed of sand-sized grains in a fine-grained clay matrix. The sand-sized grains frequently include rock fragments of wide-ranging mineralogies (e.g., pyroxenes, amphiboles, feldspars, and quartz). The clay matrix may constitute up to 50% of the volume. Of the clay minerals, chlorite and biotite are most abundant. The matrix tends to bind the grains strongly and form a relatively hard rock.

Graywacke is deposited in deep ocean water near volcanic mountain ranges, where underwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. This type of sandstone contains fewer grains made of quartz and more made of feldspars, volcanic rock fragments, as well as silt and clay than most sandstone. It is therefore also known as “dirty sandstone.” The volcanic rock fragments give graywacke a greenish-gray color.

Graywacke sandstone deposits display flat-lying beds, each composed of sedimentary particles of different sizes. The sandstone beds can be from inches to many feet thick and are often separated by thin, dark shale beds. Each sandstone bed was formed during a single turbidite or submarine landslide event and was deposited over a short period of time from hours to days. The thin shale beds formed between turbidite events, when mud particles slowly settled to the sea floor, and may represent thousands of years. Turbidites display graded bedding, that is, the grain size decreases upwards in the bed. During a turbidite event, the larger and heavier grains settle out first. As the energy in the landslide event decreases, finer and finer particles settle out to the sea floor.

Graywacke sandstone occasionally contains fossil mollusks which sometimes can be used to tell when the rock was deposited. The shale layers between the graywacke beds may contain microfossils that also can be used to date the rocks and to determine the depth of water in which they were deposited.

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