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Phanerozoic Ironstones: an Introduction and Review

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Phanerozoic Ironstones: an Introduction and Review Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Phanerozoic ironstones: an introduction and review T.P. Young S U M M A R Y: 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. This introduction to the papers presented at the Developments in the study of Geological Society International Symposium on ironstones 'Phanerozoic Ironstones', held at the University of Sheffield in April 1987 owes much to informal Recent increased interest in the genesis of oolitic discussion with colleagues and friends during the ironstone-formations is largely due to advances meeting and field excursions. in the technology available for their examination Studies of ancient ironstones are dominantly and in the framework of sedimentology and of those with an ooidal texture, deposited with diagenesis within which they can be interpreted. syndepositional iron enrichment in marine en- In most ironstones the oxide, hydroxide and vironments. Ironstones with texture and silicate phases are very fine-grained; classical mineralogy similar to those of the marine optical petrographic technique has not been able examples are also found in non-marine contexts, to resolve microstructure, and has, therefore, and examples are described in the recent litera- fallen short of providing adequate basic des- ture (e.g., Siehl & Thein 1989; Kearsley 1989). cription of the rocks. The major advances of These non-marine examples have much to offer recent years in techniques of high-resolution in the search for the conditions giving rise to imaging and chemical analysis by electron beam iron-rich sediments. instruments have provided a suite of important Ironstones without ooidal textures (e.g., tools for petrographic and mineralogical in- blackband ironstones, claystone ironstones and vestigation; high-resolution scanning electron sphaerosiderites) are currently rather better microscopy, backscattered electron imagery, understood than ooidal examples (Spears 1989). analytical transmission microscopy and electron It is the advance of the study of marine ooidal probe micronanalysis have allowed a more ironstones with which most workers in the field detailed analysis of the textures and chemical are primarily involved, and with which 12 of the composition of the mineral phases present in 13 case studies in this volume are concerned. ironstones. The extremely fine-grained nature of This review will therefore concentrate on the the phases is highlighted by Hughes (1989), who research into marine examples. was unable to obtain single-crystal energy From YOUNG, T. P. & TAYLOR, W. E. G. (eds), 1989, Phanerozoic Ironstones ix Geological Society Special Publication No. 46, pp. ix-xxv Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 x T.P. Young dispersive spectroscopic chemical analysis for temporal distribution of oolitic ironstone- berthierine, despite using the technique (analy- formations have been analysed and show tical transmission electron microscopy) which cyclicity on several scales (Van Houten & Arthur provides an analysis from the smallest volume 1989). Two major peaks of ironstone production currently possible. Clearly there is much more to in the Jurassic and Ordovician have been be obtained through these areas of research. The interpreted by Van Houten & Purucker (1984) as small particle size of the phases in ironstones has being associated with high global stands of sea similarly so far limited the investigation of their level and a high degree of continental dispersion, isotopic compositions, but the imminent with large areas of peneplained terrain. These capability for ultra-fine-scale measurement of authors indicate a temporal relationship between the stable isotopes ratios of carbon, oxygen and the major periods of deposition of oolitic sulphur will be of great significance for the study ironstone-formations and periods of repeated of ironstones. transgressions. Van Houten (1985) modified his The rather unusual sedimentological con- view and indicated that 'neither dispersed ditions during the formation of most oolitic continents nor major highstand of sea level was ironstone-formations have, in the past, given rise a necessary factor'. These conditions are clearly to great problems in interpretation. The in- favourable, nevertheless, for sea-level stand and creased understanding of event stratification, sea-level change control the ability of the cyclic sequences and episodic sedimentation has continental areas to supply large amounts of iron provided a new framework within which the to the marine environment during reworking of conditions of deposition of these ironstones can terrestrial weathering products. be investigated. Within these major peaks of ironstone de- A further major influence on the investigation position, periods of a few to a few tens of of ironstones is the great advance in the study of millions of years were conducive to enduring or diagenetic processes. In the last twenty years the recurring conditions of ironstone formation. broad framework for the understanding of the This intermediate scale of temporal distribution chemical processes involved in early diagenesis has been attributed to hypothetical 32 Ma has been established. Although the study of the polytaxic oligotaxic cycles (Van Houten & authigenesis of silicate minerals is not so Arthur 1989), but the great variation in the advanced as that of carbonate or sulphide length of these events probably indicates that phases, much can now be said about the variations in local tectonic conditions, sea level chemical conditions required for the formation and sediment supply may also be very important. of the mineral phases found in ironstones The requirements for ironstone deposition may (Spears 1989). well locally be satisfied by such processes as a 'clastic trap', but only when many other parameters are also satisfied on a regional or The genesis of ooidal ironstones global level. The problems associated with the origin of Small-scale patterns, which may show a finer ooidal ironstones are threefold: why are these cyclicity on a scale of a few hundreds of rocks iron-rich, why do they have the mineralogy thousands of years, have been tentatively attri- that they do (particularly why do they have this buted to 400 000 year Milankovitch cycles (Van specific silicate mineralogy?) and why do they Houten 1986; Van Houten & Arthur 1989) or to have an ooidal texture? These problems are by local tectonic controls. The low deposition rates no means solved, but advances have been made required for the production of ironstones would in our understanding of many areas. Any model make them very sensitive to minor environ- for the origin of ironstones must simultaneously mental changes (Van Houten 1986). address the problems of the deposition of the The palaeogeographical distribution of iron- ironstone-bearing units, of mineral genesis, and stone deposits has been suggested as evidence for of the origin of the texture. Each of these areas climatic control on their formation. The asso- will be discussed before the presentation of a ciation of oolitic ironstone-formations with well working model. vegetated land masses at moderately low latitudes, which may have had lateritic soils, has been claimed for the Jurassic (Taylor 1951; The origin of oolitic ironstone-formations Bubinicek 1961; Hallam 1975; Hallam & Much can be learnt about the controls on Bradshaw 1979; Van Houten 1985), but some ironstone deposition from the study
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