GREYWACKE
A texturally immature sedimentary rock containing larger grains in a fine-grained matrix of clay-and silt-sized particles. The larger grains may range from sand- to gravel-sized particles and are composed of quartz, rock fragments and feldspar. In a greywacke, the matrix materials should constitute more than 15% by volume.These rocks are marine sediments. They may form from a slurry of sediment deposited in deep ocean environments from fast-moving currents. When this is the case, the rocks may exhibit a variety of sedimentary features.
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