The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen being released by photosynthetic cyanobacteria , combining with dissolved iron in Earth 's oceans to form insoluble iron oxides , which precipitated out , forming a thin layer on the substrate , which may have been anoxic mud ( forming shale and chert ) .
The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria .