Rudite

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Rudite is a general name used for a sedimentary rocks that are composed of rounded or angular detrital grains, i.e. granules, pebbles, cobbles, and boulders, which are coarser than sand in size. Rudites include sedimentary rocks composed of both siliciclastic, i.e. conglomerate and breccia, and carbonate grains, i.e. calcirudite and rudstone. This term is equivalent to the Greek-derived term, psephite. Rudite was initially proposed by Grabau[1] as "rudyte." It is derived from the Latin word, "rudus," for "crushed stone," "rubbish," "debris," and "rubble."[2][3]

Rudites can be deposited in a variety of nonmarine and marine environments. In nonmarine settings, gravels, which later were lithified to become rudites, accumulated within fluvial channels, within alluvial fans, and as glacial deposits. In marine environments, rudites were deposited along shorelines as part of beaches, as basal conglomerates during marine transgressions, and in the deep sea by slumps and turbidites. Rudites composed largely of rounded siliciclastic gravel are conglomerates and others composed of angular siliciclastic gravel are breccias.

Pettijohn [4] gives the following descriptive terms based on grain size, avoiding the use of terms such as clay or argillaceous which carry an implication of chemical composition. The Greek terms are more commonly used for metamorphosed rocks, and the Latin for unmetamorphosed:

Descriptive size terms
Texture Common Greek Latin
Coarse gravel(ly) psephite (psephitic) rudite (rudaceous)
Medium sand(y) psammite (psammitic) arenite (arenaceous)
Fine clay(ey) pelite (pelitic) lutite (lutaceous)

References[edit]

  1. ^ Grabau, A.W. (1904) On the classification of sedimentary rocks. American Geologist. vol. 33, pp. 228-247.
  2. ^ U.S. Bureau of Mines Staff (1996) Dictionary of Mining, Mineral, & Related Terms. Report SP-96-1, U.S. Department of Interior, U.S. Bureau of Mines, Washington, D.C.
  3. ^ Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4
  4. ^ Pettijohn F. J. (1975), Sedimentary Rocks, Harper & Row, ISBN 0-06-045191-2