subgraywacke

subgraywacke, dark-coloured sedimentary rock that contains from 65 to 95 percent free quartz, in grains 0.06 to 2 mm in diameter, held together by a matrix with a low mud content and often a high carbonate content. Some geologists favour a definition of graywacke (q.v.) that permits no more than 75 percent free quartz in the rock, and they thus would class as subgraywackes those rocks with (1) over 25 percent unstable materials (i.e., feldspar and rock fragments), (2) more rock fragments than feldspar, and (3) more voids (open pore spaces) or mineral (often carbonate) cement than clay or mud matrix.

Subgraywackes of the first type occur in thick deposits in great subsiding troughs, where they either dominate the rock sequence or occur as a minor component with less-siliceous graywackes. These rocks are derived from areas with many quartz-rich rocks or areas in which intense chemical weathering eliminates the less-stable materials before the sediment is deposited.

Subgraywackes of the second type constitute more than one-third of the sandstones in the geologic column, occurring in rock sequences of all ages. They commonly consist of rounded and well-sorted grains in layers often inclined to the general plane of bedding. They probably were deposited by density currents (highly turbid, bottom currents) and are derived from areas that contain mainly sedimentary rocks and low-grade metamorphic rocks (altered by low heat and pressure). Because they occur in coal deposits, they may have a partly nonmarine origin; some are thought to have accumulated on coastal floodplains and deltas or similar environments. Mudstones and shales often occur interbedded with these rocks.