GRAINS: Non-skeletal Grains: Pellets and Peloids
2003. "GRAINS: Non-skeletal Grains: Pellets and Peloids", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Pellets - Small (typically 0.03 to 0.3 mm long), spherical to ovoid or rod- shaped grains composed of carbonate mud (micrite). Most pellets lack internal structure and are uniform in size and shape in any single sample; in the strict sense, pellets are the fecal products of invertebrate organisms (see Folk, 1959).
Peloids - Allochems formed of cryptocrystalline or microcrystalline calcium carbonate with no restrictions on the size or origin of the grains (McKee and Gutschick, 1969). This term allows reference to grains composed of micritic material without the need to imply any particular mode of origin — it is therefore a useful “term of ignorance” covering possible pellets, indistinct intraclasts, micritized ooids or fossil fragments and even some microbial or inorganic precipitates that are not necessarily even “grains” in the sense of primary constituents as opposed to interstitial early diagenetic “cements”.
Pellets and peloids occur in Precambrian through Phanerozoic strata; pellets are important sediment constituents mainly in Phanerozoic strata. Structured crustacean pellets are especially prominent in Jurassic and Cretaceous rocks (although they are known from middle Paleozoic to Recent strata).
Pellets and peloids are composed of aggregated carbonate mud and/or precipitated calcium carbonate. Thus, their original composition is (or was) aragonite or calcite (of any Mg level) or a mixture of both. Pelletal glauconites and phosphorites also are common.
Fecal pellets are produced wherever worms, crustaceans, holothurians and other grazing, burrowing, or swimming invertebrates (or vertebrates) exist, but most pellets are destroyed prior to burial. Rapid cementation, usually bacterially mediated, aids preservation, as does rapid sedimentation in low-energy settings. Thus, lagoons (especially hypersaline ones), low-energy tidal flats, and sheltered or relatively deep-water platforms are common sites of pellet preservation. Fecal pellets of pelagic zooplankton, especially copepods, are common in Cretaceous to Recent deep-sea deposits.
Fecal pellets must be distinguished from microbial peloids or inorganic, peloidal marine cements, especially those composed of high-Mg calcite. Such precipitates are especially common in reef cavities, subtidal to intertidal stromatolites, hot springs or other travertine deposits, and submarine vent areas.
Peloids have varied origins and environmental associations. Algal or fungal boring and micritization of grains are common in a variety of open marine to restricted or coastal settings with relatively slow or intermittent sedimentation rates. In particular, areas subject to occasional storms that move grains from active areas of formation to quiet sites of destruction are especially prone to peloid formation. Such sites include backbarrier or back-bar grass flats, lagoons, and protected deeper shelf settings.
Figures & Tables
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