The Brereton Limestone is a shallow-water, open-marine carbonate deposited over peat or delta-plain muds after delta abandonment and a marine transgression. Six distinct biofacies are recognized, utilizing quantitative analysis of abundance data on 32 fossil types obtained from detailed petrographic examination of 141 samples. The biofacies partly overlap and probably represent coexisting paleocommunities. Data on autecology, lithology, insoluble residue content, and thickness were used to interpret the habitats of each biofacies.

Biofacies V, a low-diversity biofacies dominated by brachiopods and ostracods, occupied turbid-water, mud- or shelly mud-bottom areas during influxes of detrital clays late in the abandonment of the Herrin delta and, also, early in the construction of the Jamestown delta.

Low-relief carbonate mud mounds accumulated within and around baffles provided by thickets of phylloid algae, crinoids, fenestrate bryozoans, or productid brachiopods, and are separated by narrow to broad intermound areas. Shallow-water mud mounds, containing Biofacies I, which is dominated by calcareous phylloid algae and foraminifers, are capped locally by Biofacies VI, a low-diversity biofacies dominated by ostracods. Biofacies VI, occupied the high subtidal to supratidal crests of algal mud mounds which had a stressed (possibly hypersaline) environment. Deeper water mud mounds were occupied by either Biofacies III, a crinoid-mixed fossil biofacies, or by Biofacies IV, which is dominated by fusulinids, strophomenids, and trilobites.

Biofacies II, dominated by sponges, mollusks, and impunctate brachiopods, generally occurred on the flanks of the shallow-water mounds. Biofacies I, III, and IV also occurred in broad, muddy intermound areas and Biofacies III in narrow, winnowed intermound areas.

Spatial distribution of biofacies and inferred habitats is characterized by irregular and local changes, and does not conform to regular, predictable changes perpendicular to a paleoshoreline.

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