SEDIMENTARY ROCKS/Banded Iron Formations 39 Member, it has been identified in a number of other well-preserved BIF units elsewhere The characteristic mesobanding of BIF is not present in GIF GIF also has alternations of iron-rich and iron-poor material, but these are typically coarser and much less regular The coarsely crystalline chert bands are commonly wavy or lenticular Both ironrich and silica-rich bands may be granular, more particularly the latter The iron-rich bands of GIF, as the name implies, often consist of a close-packed and lithified mass of granules or ooliths, averaging about a millimetre in diameter They are made up of iron oxides, with or without quartz, and the intergranular material consists mainly of the same minerals, but usually with a lower iron content Continuity of Banding A remarkable stratigraphic feature of Hamersley Basin BIF is its exceptional lateral continuity Thus the Dales Gorge Member, used above as a model for the description of banding, and the other main BIF units, are easily identifiable over the entire basin area BIF macrobands within the Dales Gorge Member are similarly recognisable through their constant relative thicknesses, and within BIF macrobands both individual centimetre-scale mesobands, as well as sub-millimetre microbands within them, have been correlated over 300 km This degree of fine-scale lateral stratigraphic continuity is reminiscent of evaporites, particularly in the Permian of Europe and the United States Metamorphic and Tectonic Modification Because of their exceptional freedom from postdepositional metamorphism and tectonic deformation, the BIFs of the Hamersley Basin have been used as ‘type examples’ for the introductory descriptions above; the BIFs of the Transvaal and correlative Griqualand West basins of South Africa, which are of similar age, also have the same excellent preservation But in this respect they are atypical of the majority of BIF occurrences Many Early Precambrian examples have suffered significant metamorphic modification, which begins with coarse-grained recystallisation (annealing) of both the initially microcystalline chert and fine-grained iron oxides, and continues with the growth of iron-rich silicates (e.g., grunerite, ferrohypersthene, fayalite) Even in early metamorphic stages the delicate fine textures within the banding tend to become blurred, but the essential iron-rich/ iron-poor alternation of the mesobanding shows a robust resistance to complete obliteration Most metamorphism of BIF appears to be isochemical, but there is some evidence of chemical modification at higher grades Older Precambrian BIFs also tend to have undergone significant tectonism, particularly those of Archaean greenstone belts of all continents, where the BIFs often form curvilinear steeply dipping units which are useful in deciphering complex structures It is characteristic of deformed BIF in these belts that it appears to have reacted sensitively to tectonic stress, forming complex internal flowage folds defined by the banding Distribution Over the Earth BIFs are widely distributed throughout the Precambrian areas (cratons and shields) of all continents except Antarctica, where only one occurrence is so far known BIFs of the older cratons include the oldest known example, at Isua, in Greenland, aged about 3.8 Ga They are consistently present in the greenstone belt sequences of all the main old cratons Examples include the Abitibi Belt of the Superior Province of Canada, the greenstone belts of the Yilgarn and Pilbara Cratons of Australia, and the those of the Baltic Shield, the North China Craton, the Amazon Craton of Brazil, the Kaapvaal and West African Cratons, and the BIFs of the Ukraine Craton, notably at Krivoi Rog; most of these are relatively thin, tectonised, and metamorphosed Most of these greenstone-associated BIFs have ages between 2.8 Ga and 2.5 Ga A different style of BIF occurrence is present in four of the Gondwana continents (South America, southern Africa, India, and Australia) In this type the BIFs occur as well preserved, gently dipping supracrustal sequences, which may form conspicuous topographic plateaus The Caraja´ s Formation of the Amazon Craton, the Caueˆ Itabirite of the Sa˜ o Francisco Craton (both in Brazil), the Kuruman Iron Formation and Penge Iron Formation of South Africa, and the Mulaingiri Formation of the Indian Karnataka Craton all belong to this group, which includes the BIFs of the Australian Hamersley Basin, already mentioned above; these occurrences have been called the Great Gondwana BIFs, and are mostly younger than those present in greenstone belt occurrences Further mention of a final distinct category of BIF occurrences, which includes Rapitan in the Yukon, and those of the Damara Belt, in Namibia, is made below the next heading Distribution in Time Mention has already been made not only of the general restriction of BIFs to the Precambrian, but also to