Banded iron formation, Karijini National Park, Western Australia. 2.1 billion year old rock showing banded iron formation. Banded iron formations (also known as banded ironstone formations or BIF s) are distinctive units of sedimentary rock that are almost always of Precambrian age.anded iron beds are an important commercial source of iron ore, such as the Pilbara region of Western Australia and the Animikie Group in Minnesota.