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.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.