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Department of Geology, UNCC, PO Box 914, Cardiff, CF1 3YE, UK
Ironstones, particularly ooidal ironstones, have long fascinated sedimentary geologists and have generated an enormous variety of interpretations, but have remained poorly understood. A recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences.
The first part of this introduction examines recent advances, and attempts to produce a synthesis of them, with particular reference to the genesis of marine ooidal ironstones. Current models for the formation of marine oolitic ironstone-formations are discussed, and a working model based on various lines of recent research is proposed. This model invokes the intrasedimentary formation of berthierine ooids in marine environments during post-oxic diagenesis. The ooids may be mechanically modified and mineralogically transformed by reworking, as well as by later diagenesis. Goethite ooids may be oxidized berthierine ooids, reworked lateritic ooids or primary grains. The formation of oolitic ironstone-formations is favoured by a break in clastic sediment supply, continued supply of iron and physical reworking. Many sedimentary environments may supply these parameters, but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply.
In the second part of this introduction the terminology and classification are discussed, and conventions proposed. The term berthieroid is introduced as a non-specific term for material of undetermined berthierine or chamosite composition. The term ironstone is proposed as a petrological term and as an informal lithostratigraphic term. A deposit composed of ironstones may be termed an ironstone-formation. It is recommended that a ‘Dunham’ style of classification is employed petrographically, with -ironstone replacing the -stone of the original carbonate classification. The nomenclature of allochems in ironstones is also discussed. A glossary of ironstone terminology is provided.
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  P. L. Younger The importance of pyritic roof strata in aquatic pollutant release from abandoned mines in a major, oolitic, berthierine-chamosite-siderite iron ore field, Cleveland, UK Geological Society, London, Special Publications, 2002; 198: 251 - 266. [Abstract] [PDF]
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