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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.

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