Peloids
Peloid is a nongenetic term for carbonate grains that are composed of microcrystalline or cryptocrystalline calcite or aragonite and that do not display distinctive internal structures. [1]
Physical Characteristics
Peloids are smaller than ooids and are generally of silt to fine-sand size (0.03-0.1 mm), although some may be larger. [1]
Fecal Pellets
The most common kind of peloids are fecal pellets, produced by organisms that ingest calcium carbonate muds and extrude undigested mud as pellets. [1] Fecal pellets tend to be small, oval to rounded, and uniform in size. [1] They commonly contain enough fine organic matter to make them appear opaque or dark colored. [1]
Distinguishing Features
Pellets can be differentiated from ooids by their lack of concentric or radial internal structure and from rounded intraclasts by their uniformity of shape, good sorting, and small size. [1] Because they are produced by organisms, their sizes and shapes are not related to current transport, although pellets may be transported by currents and redeposited after initial deposition by organisms. [1]
Other Formation Mechanisms
Peloids may also be produced by other processes, such as micritization of small ooids or rounded skeletal fragments caused by the boring activities of certain organisms, particularly endolithic (boring) algae. [1] These boring activities convert the original grains into a nearly uniform, homogeneous mass of microcrystalline calcite. [1]
Some marine peloids may form by precipitation around active clumps of bacteria. [1] Other peloids may simply be very small, well-rounded intraclasts formed by reworking of semiconsolidated mud or mud aggregates. [1]
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs
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