GRAINS: Non-skeletal Grains: Intraclasts and Extraclasts
2003. "GRAINS: Non-skeletal Grains: Intraclasts and Extraclasts", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Intraclast - A fragment of penecontemporaneous, commonly weakly consolidated, carbonate sediment that has been eroded and redeposited, generally nearby, within the same depositional sequence in which it formed (Folk, 1959 and 1962).
Lumps - In modern sediments, irregular composite aggregates of silt- or sand-sized carbonate particles that are cemented together at points of contact: in ancient carbonates, similar-appearing lobate grains that are composed of carbonate mud (micrite). After Illing (1954); no longer widely used.
Grapestone - Sometimes used to describe aggregates of silt-sized carbonate crystals (or grains), but more properly applied to grape-like clusters of such aggregates bound together by cements or organic encrustations.
Extraclast - A detrital grain of lithified carbonate sediment (lithoclast) derived from outside the depositional area of current sedimentation (Folk, 1959).
Calclithite - A rock formed chiefly of carbonate clasts (extraclasts) derived from older, lithified limestone, generally external to the contemporaneous depositional system. Commonly located in arid settings, along downthrown sides of fault scarps. Term coined by Folk (1959).
Intraclasts and extraclasts are found in deposits of any age from Archean to Recent. Intraclasts are especially common in Precambrian to Mid. Ordovician strata, where they form widespread flat-pebble conglomerates. Such deposits probably reflect the abundance of microbial deposits and the scarcity or absence of macrofaunal grazers and burrowers during that time period (e.g., Garrett, 1970).
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A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks