The oldest genetic hypothesis for oolitic iron formations based upon thin-section study, that of Henry Clifton Sorby (1856), is defended and amplified. Sorby (1856) proposed that oolitic iron formations originated as beds of calcareous oolite, which were covered by ordinary mud rich in organic matter. Iron was leached from the mud by pore water bearing organic-decay products, resulting in ferruginization of the calcareous sediment. All characteristics of oolitic iron formations and partially ferruginized limestone appear to support this, as does quantitative modeling of present processes to explain the youngest oolitic iron formation, a voluminous deposit less than 5 m. yr. old.

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