The feeding activities and excretory products of the marine decapod Callianassa major Say and the marine annelid Onuphis microcephala Hartman have been studied in shallow marine environments of the southern Atlantic and eastern Gulf of Mexico coasts of the United States. These filter-feeding organisms produce depositionally significant quantities of argillaceous fecal pellets that are transported and deposited as granular clay with hydraulically equivalent quartz sand grains. At average population densities observed in shallow marine environments, these organisms are calculated to be capable of removing and pelletizing approximately 12 metric tons of suspended materials per square kilometre per year and of depositing fecal pellet mud as thick as 4.5 mm yearly. Recent deposits of Onuphis pellets up to 30 cm thick and Callianassa pellets as thick as 60 cm have been observed.

The complex, species-specific, sand-sized fecal pellets are composed of 80 to 90 percent clay-mineral particles and 5 to 10 percent undigested organic particles and small amounts of quartz sand and silt grains. The digestive systems of the organisms significantly alter the clay mineralogy of the sediments extracted from suspension, and Callianassa major produces fecal pellets of different clay-mineral compositions from fecal pellets produced by Onuphis microcephala. The digestive processes wholly or partly destroy chlorite. Mixed-layer clay minerals are partly destroyed, and kaolinite and illite are in part disordered. Fecal pellets, rich in organic matter, enter the coprophagic cycle where further clay-mineral alteration takes place. Decay of the organic matter creates microreducing conditions within the pellets, thus promoting the processes of glauconitization.

Whereas flocculation is the important process in the deposition of argillaceous sediments in deltaic environments, biogenic pelletization may be the most important process in depositing argillaceous sediments in shallow marine interdeltaic environments.

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