According to Encyclopedia Britannica, banded iron formations are “chemically precipitated sediment, typically thin bedded or laminated, consisting of 15 percent or more iron of sedimentary origin and layers of chert, chalcedony, jasper, or quartz” (Britannica 2010). Many of the formations formed during the second half of the Precambrian period - known as the Proterozoic Era (2.3 billion years ago) - when microorganisms created iron band formations by either chemolitho-autotrophic iron oxidation or oxidation by photosynthetic oxigen production (Konhauser et al. 2002). The iron bands produced during the first half of the Precambrian era - known as the Archean Era (roughly 4 billion years ago) - are not as thick or laterally as long as their Proterozoic counterparts. Scientists believe that these iron bands formed when early microorganisms interacted with the ferrous iron in the water. Because relatively large quantities of free oxygen in the air is needed to produce these iron formations, they are importnat indicators of when oxygen was accumulating in the atmosphere.

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Klein, C.. 2001. Precambrian banded iron formations (BIFS) worldwide: Their geological setting, mineraalogy, metamorphism, and origin. Abstract, Geological Soceity of America Annual Meeting. Paper No. 81-0. Accessed January 17, 2010.

Konhauser, K.A., T. Hamade, R. Raiswell, R.C. Morris, F. G. Ferris, G. Southam and D. E. Canfield. 2002. Could bacteria have formed the Precambrian banded iron formations? Geology 30: 1079-1082. Accessed January 17, 2010.