<P> Superior type are the second and larger form of BIFs . They primarily formed during the Paleoproterozoic era, occurring on continental shelves and can be found around the world . Superior types were formed by chemical precipitation in shallow waters, primarly due to the low atmospheric and ocean oxygen levels, resulting in high iron levels in the oceans . Under calm shallow conditions, oxygen released during photosynthesis by blue - green algae, would combine with the iron creating magnetite, which would then sink and deposit on the floor . </P> <P> Until 1992 it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally . Iron - rich waters would then form in isolation and subsequently come into contact with oxygenated water . The Snowball Earth hypothesis provided an alternative explanation for these younger deposits . In a Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 million years ago (mya) that nearly or totally depleted free oxygen . Dissolved iron then accumulated in the oxygen - poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron . </P> <P> An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal - rich brines in the vicinity of hydrothermally active rift zones . Alternatively, some geochemists suggest that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs . </P> <P> Banded iron formations in Northern Minnesota were found directly underneath a thick layer of ejecta from the Sudbury Basin impact . At the time of formation Earth had a single supercontinent called Columbia with substantial continental shelves . An asteroid (estimated at 10 km across) slammed into waters about 1,000 m deep some 1.85 billion years ago . Computer models suggest that the tsunami would have been at least 1,000 metres high at the centre, and 100 metres high about 3,000 kilometres away . Those immense waves and large underwater landslides triggered by the impact stirred the ocean, bringing oxygenated waters from the surface down to the ocean floor . </P>

Banded iron formations in marine sediments provide evidence of