Banded iron formations in marine sediments provide evidence of

Banded iron formations provide evidence of oxygen in the atmosphere. Banded iron formations are sedimentary rocks composed of alternating thin layers formed of iron oxide minerals, such as magnetite and hematite and silica rich minerals such as, chert and iron silicates. Most geologists agree that banded iron formations were precipitated from sea water when oxygen, produced by cyanobacteria, reacted with iron dissolved in sea water.