Banded Iron Formation: Distribution in Time and Paleoenvironmental Significance

Iron-rich sediments have been deposited intermittently throughout earth history, but virtually all significant deposits of the cherty layered rock referred to as iron formation are of Precambrian age. Present evidence, by no means conclusive, suggests three periods of peak deposition rate: mid-Archean (age 3400–2900 m.y.), early Proterozoic (age 2500–1900 m.y.), and late Proterozoic (age 750–500 m.y.). The deposits of early Proterozoic age far outweigh those of all other ages; in the aggregate they account for 90 percent or more of the estimated total of 1015 tons of originally deposited iron formation.