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Six of the best known Precambrian iron-formation basins of North America display consistent patterns of sedimentation. In three of the basins, the iron formations are almost exclusively lutaceous, contain few sedimentary structures other than lamination, and include representatives of all four of James' sedimentary facies. In the other three basins, the iron formations are predominantly arenitic and contain a much wider variety of sedimentary structures. All four of James' facies are represented among their lutaceous members, but the arenitic portions belong only to the oxide and silicate facies. The predominantly arenitic iron formations are underlain by coarsening-upward, quartzose shelf sandstones, whereas the predominantly lutaceous iron formations are underlain by sl te-turbidite sequences and/or pillow lavas. Five of the iron formations are overlain by slate-turbidite sequences, and the sixth is truncated unconformably. Several conclusions can be made. (1) The iron formations vary considerably in their internal sedimentary character and lie conformably between siliciclastic rocks deposited in a variety of marine environments. Hypotheses that restrict the deposition of iron formation to a nonmarine or to a specific marine environment are not likely to have broad applicability. (2) The sedimentary structures and the stratigraphic settings show that the lutaceous iron formations are relatively deep-water sediments; yet some belong to James' oxide facies. The dominance of ferric iron is not sufficient to prove a shallow-water depositional environment. ( ) The similarities between the character of a given iron formation and that of the siliciclastic unit beneath it suggest a close environmental relation between the two.
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