Ferriferous chert-rich and chert-poor chemical sedimentary rock types are here described and the lithostratigraphic units, iron formations, which they form are classified paleoenvironmentally. Paleoenvironments have been deduced from characteristics of enclosing rocks and from sedimentary features, e.g., textures and sedimentary structures, of the iron-rich rocks. The three major paleoenvironmental types span half or more of earth history, an indication that chronological variation in abundance among types may have been due to evolution of tectonic and magmatic environments rather than biologic and/or atmospheric-hydrospheric evolution. The relative abundance of volcanic environments was greatest in the Early Precambrian, of continental-shelf environments in the Middle Precambrian, and of inland-sea environments in the Phanerozoic. All three had comparable representation in the Late Precambrian.

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