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Summary Article: Greywacke
from Guide to Minerals, Rocks and Fossils

Colour Grey to black, sometimes greenish, usually dark in colour. Texture Typically contains sharply angular grains (up to 2 mm) in a finer-grained matrix. Structure Frequently massive; graded bedding is typical. Individual graded beds are coarse-grained (sandy, perhaps with a few small pebbles) at the base, and pass upwards into silt or clay at the top. Fossils rare. Slump structures common. Mineralogy Coarser grains consist of quartz, feldspar and rock fragments; matrix too fine-grained to be distinguished by the naked eye. The green colour is due to the presence of chlorite. Field relations Greywackes are deposited rapidly in deep marine basins, by turbidity currents; these are masses of sediment-charged water which flow down slopes on the sea bed, and deposit their load of sediment in deep water. Individual units are probably deposited very rapidly, hence their ill-sorted nature.

© 2003 Philip's

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