Did Weathering on Land or Sea Floor Produce Ironstone?

Michael M. Kimberley

Soil formation has been the initial process of iron concentration for both cherty and noncherty iron formations of all ages, except pelletal glauconite beds. Noncherty oolitic iron formations have formed by a sequence that started with extremely iron-rich coastal soil-saprolite, as in modern southern Sierra Leone where soil composition, including phosphorus, approximates noncherty ironstone. A large soil area simultaneously became covered by an organic-rich swamp because a coastal barrier (e.g., sand bar) started to block drainage. Soil iron dissolved and reprecipitated within overlying organic matter as dispersed siderite, pyrite, and berthierine. With continued coastal subsidence, the barrier eroded, and a transgression rapidly converted swamp sediment into a combination of underlying bertheirine (with or without ferric hydroxide) ooids and overlying pyritic-sideritic carbonaceous mud. Most organic matter slowly oxidized, causing partial replacement of underlying ooids by siderite. Cherty iron formations have formed similarly, but did not accumulate directly above the precursor soils. Instead, the ferriferous organic matter was carried by weak longshore drift, in the absence of metazoans, to a subsiding area prior to organic oxidation-fermentation and iron precipitation. Thick cherty iron formations have formed by numerous soil-swamp cycles, each contributing ferriferous organic matter from rising blocks to adjace t sinking blocks. Rising blocks gently sloped seaward and so, during soil formation, rivers rarely invaded adjacent sinking blocks. Relief along the continental margin or volcanic arc was minimal between rising and sinking blocks Minor remnant volcanic relief locally has preserved transported organic matter, as in the Archean Helen and Outerring iron formations.