Encyclopedia of Geology || SEDIMENTARY ROCKS | Ironstones

Variations in the chemistry of primary fluid inclu-sions from ancient halite deposits are significant.They also imply that seawater chemistry has changedsignificantly. Variations are in phase with inferredseafloor spreading rates, global changes in sea-level,and the primary mineralogies of ancient marine car-bonates and evaporites. Of particular significance isthe fact that inclusions in halites of the same age fromdifferent geological basins exhibit similar compos-itions. This suggests that the association with dolomi-tization (proposed by Holland et al.) is incorrect: moreinterbasin variation in the amount of dolomitizationwould be expected, resulting in a greater variation inthe chemistry of fluid inclusions than that observed. Itis surprising, however, that the question of whether ornot variations in sodium : potassium ratios matchmodel predictions was not addressed. More recent,unpublished, work suggests that Cretaceous and Per-mian seawaters were enriched in potassium and rela-tively depleted in sodium, as would be expected fromthe Hardie hypothesis.

See Also

Minerals: Sulphates. Sedimentary Environments: LakeProcesses and Deposits. Sedimentary Rocks: Mineral-

ogy and Classification; Dolomites. Tectonics: Hydrother-

mal Activity; Hydrothermal Vents At Mid-Ocean Ridges;

Rift Valleys.

Further Reading

Hanor JS (1996) Variations in chloride as a driving force insiliciclastic diagenesis. In: Crossey LJ, Loucks R, andTotten MW (eds.) Siliciclastic Diagenesis and FluidFlow: Concepts and Applications, pp. 3–12. SpecialPublication 55. Tulsa: Society for Sedimentary Geology.

Hardie LA (1990) The roles of rifting and hydrothermalCaCl2 brines in the origin of potash evaporites: a hypoth-esis. American Journal of Science 290: 43–106.

Hardie LA and Spencer RJ (1990) Control of seawatercomposition by mixing of river waters and mid-oceanridge hydrothermal brines. In: Spencer RJ and ChouI-M (eds.) Fluid–Mineral Interactions: A tribute to H-PEugster, pp. 409–419. Special Publication 2. SanAntonio: Geochemical Society.

Hite RJ (1983) The sulphate problem in marine evaporites.In: Schreiber BC (ed.) Proceedings of the 6th Inter-national Salt Symposium, Toronto, pp. 217–230.Alexandria, VA: Salt Institute.

Kendall AC (1989) Brine mixing in the Middle Devonian ofwestern Canada and its possible significance to regionaldolomitization. Sedimentary Geology 64: 271–285.

Lowenstein TK and Spencer RJ (1990) Syndepositionalorigin of potash evaporites: petrographic and fluid inclu-sion evidence. American Journal of Science 290: 1–42.

SEDIMENTARY ROCKS/Ironstones 97

Ironstones

W E G Taylor, University of Lancaster, Lancaster, UK

� 2005, Elsevier Ltd. All Rights Reserved.

p0020

Introduction

Ironstones have been critical to industry and indus-trial revolutions. They have been essential raw mater-ials since the dawn of the Iron Age (about 700 bc).Without iron-rich deposits many of the manmadestructures and utensils that we take for grantedtoday – tall urban buildings, power pylons, bridges,ships, cutlery, hammers, saws, and the seeminglyindispensable motor car – could not exist.

The point at which an ironstone deposit is con-sidered to be an ore has changed considerably overthe years and depends upon the particular economic,technological, social, and political circumstances atthe time. Nowadays deposits need to have an ironcontent in excess of 60% by weight to be worked,whilst in the mid-twentieth century ironstones with28% iron by weight were regularly extracted as ores.

The quality of the potential ore, and in particular theproportion of phosphatic material, is an importantfactor that has to be considered in the mining of iron.

Initially, the availability of water power and theproximity of coal were the factors controlling pro-duction. The middle of the nineteenth century saw achange from coal-fired furnaces producing cast ironto the Bessemer process, which produced steel. Laterin the same century, the open-hearth process and var-ious refinements were developed. Each of these newproduction processes demanded ores of a particularquality.

Records of global production are scarce beforethe latter half of the twentieth century, and certainlyin Europe much of the exploitation predates thatcentury. In Great Britain the maximum annual outputwas in the order of 18 Mt (Milliontonnes) and oc-curred during two main periods – 1870–1890 and1940–1945. In the former period the main type ofore extracted was from the blackband and claystoneironstones (see below) of the Carboniferous rocksof various coalfields, whilst in the latter period the

Figure 1 Ordovician ironstones, Betws Garmon, North Wales, UK (inset showing the steeply inclined stoping method of under-

ground mining for the deposit and a residual pillar of magnetite-rich material).

98 SEDIMENTARY ROCKS/Ironstones

ooidal ironstones of Northamptonshire and Lincoln-shire were the dominant ores. Although Ordovicianooidal ironstones from North Wales were extracteduntil early part of the twentieth century (Figure 1).

The terms used to describe both the processes ofironstone formation and the ironstones themselveshave been many and varied. Attempts to simplifyand standardize the terminology have recently metwith some success, mainly through the InternationalGeological Correlation Programme (IGCP). Forexample, the terms ‘Clinton’ (from the Silurian Clin-ton Group of New York State, USA) and ‘Minette’(from the Jurassic Minette oolitic ironstones of north-eastern France and adjacent areas) as descriptions ofironstone types have proved to be unsatisfactory andhave now fallen into disuse.

Definition

Largely because iron may invade and impregnate awide range of rocks, defining what constitutes an ir-onstone has proved difficult. Exhortations to mergethe nomenclatures of the various iron-rich deposits,such as the banded iron formations and ironstones,have been resisted on the basis that the mineralogy,petrology, and genesis of these deposits are distinctand separate. A precise definition of ‘ironstone’ wasagreed only in the last decade of the twentieth centuryand stems from the work of the IGCP 277 (Phaner-ozoic Ooidal Ironstones). Ironstones are sedimentaryrocks consisting of at least 15% iron by weight,which may be quoted as 19% FeO or 21% Fe2O3

or an equivalent admixture in a chemical analysis.They occur almost exclusively in the PhanerozoicEra and are distinguished from the mainly Precam-brian banded iron formations (see SedimentaryRocks: Banded Iron Formations) by their lack ofboth regular banding and chert and by their age:banded iron formations were produced when therewas a deficiency of oxygen in the Earth’s atmosphere.The ferruginization (iron enrichment) may be theresult of either direct deposition or subsequentchemical changes.

Ironstone Mineralogy

The iron-ore minerals may be oxides (includinggoethite, haematite, and magnetite), carbonates (usu-ally siderite), or silicates (normally berthierine or cha-mosite). They may be associated with other carbonateminerals, sulphides and/or phosphatic minerals.

Goethite – FeO(OH) – is commonly formed byoxidation during weathering. Also, in many ooidalironstones, it can result from the oxidation of berthier-ine; the two minerals may be found intermixed, oftenin alternate concentric layers, in ooids. Limonite wasformerly thought to be a distinct mineral with thecomposition 2Fe2O3 3H2O but is now consideredto have a variable composition (and properties) andto consist of several iron hydroxides (commonlygoethite) or a mixture of iron minerals. Generally, itoccurs as a secondary alteration product. Haematite –Fe2O3 – can be an important mineral in some iron-stones, where it is usually formed as a late-stage

Figure 2 Typical photomicrographs of the three categories of

ironstone. (A) Blackband ironstone. Note the dark organic-rich

lamination in a mainly sideritic matrix. Plane-polarized light;

horizontal dimension is 1.3mm. (B) Claystone ironstone. Note

the lack of organic material in the mainly sideritic matrix.


diagenetic product of the alteration of goethite. Ex-perimental synthesis indicates that this transform-ation occurs at a temperature above 80�C and at adepth of about 2 km. Magnetite – Fe3O4 – normallyforms during the low-grade metamorphism of iron-stones, although the mineral has been reported fromunmetamorphosed deposits in Libya.

Siderite – (Fe,Mg,Ca,Mn)CO3 – is a very importantmineral in ironstones. It is the only iron-bearing min-eral in many claystone and blackband ironstones.Substitution of magnesium, calcium, or manganesefor iron in the structure of siderite has been hypothe-sized to be related to the environment of formation.Substitution appears to have been greatest in marinesediments and in those ironstones formed during thelater diagenetic stages of non-marine sediments.

Berthierine – (Fe2þ,Fe3þ,Al,Mg,Mn)2(Si,Al)2O5

(OH)4 – is a 0.7 nm repeat serpentine. Reportedvariations in the chemical composition may reflectthe analytical difficulties of dealing with very fine-grained samples. The formation of berthierine inironstones is the subject of some debate and will beconsidered later. Chamosite – (Fe2þ

5 ,Al)(Si3Al)O10

(OH)8 – is a 1.4 nm repeat chlorite with a very similarchemical composition to berthierine. Experimentalwork has shown that berthierine may be transformedinto chamosite at a temperature of about 150�C and adepth of about 3 km. The phyllosilicate glauconite –(K,Na)(Al,Fe3þ,Mg)2(Al,Si)4O10(OH)2 – is generallythought to be restricted to marine environments andoccurs in some ironstones.

Other carbonate minerals, such as calcite, arago-nite, dolomite, and ankerite – Ca(Mg,Fe)(CO3)2 –may be particularly common in ironstones both as aconstituent of the cement and as discrete bioclasts.Phosphate minerals, such as francolite (carbonatefluorapatite) and vivianite – Fe3(PO4)2 8H2O – canbe major components of ooidal ironstones. They canbe a detrimental contributory factor to the viability ofa deposit as an ore, particularly since, in most cases,the mineral grains are very small and difficult toseparate.

Plane-polarized light; horizontal dimension is 1.3mm. (C) Ooidal

ironstone. Ooids, showing selective replacement by siderite and

phosphatic minerals, are set in a berthierine-rich mudstone

matrix. Plane-polarized light; horizontal dimension is 5.2mm

(reproduced with kind permission of Kluwer Academic Publish-

ers from Young TP (1993) Sedimentary iron ores. In: Pattrick RAD

and Polya DA (eds.) Mineralization in the British Isles, pp. 446–489.

London: Chapman & Hall, plates 14b, 14a and 14e).

Types of Ironstones

Extensive use of high-precision microscopy and ana-lytical techniques has allowed detailed insights intothe composition and formation of ironstones. Aformal classification of ironstones has not yet beenuniversally accepted, but three distinctive categorieshave been recognized (Figure 2).

Blackband Ironstones

Blackband ironstones are, typically, fossiliferoussapropel-rich (usually with an organic content in

excess of 10%) finely laminated sideritic ironstones.Although non-laminated types are known, more fre-quently they are formed of alternating siderite- andorganic-rich laminae. They are found almost exclu-sively above coal seams in a lacustrine parasequence


with mudstone and seat earth deposits (Figure 3).Palaeontological and mineralogical evidence indicatesthat these ironstones were formed during freshwaterinundation. Unlike non-marine clayband ironstones,there is an absence of early diagenetic pyrite, and theoccurrence of coal balls (calcite–pyrite concretions) isan indication of marine incursion. The ironstones

Figure 3 Idealized stratigraphical column for blackband iron-

stones showing relative water depths of sedimentation, not to

scale.

Figure 4 Idealized stratigraphical column for claystone ironstone

typically form thin (less than 10 cm) sheets of lessthan 10 km2 extent and often change laterally intolimestones with similar textural characteristics. Bogiron ores, which occur as lenses of ferruginous con-cretions within peat deposits, are thought to be themodern analogues of blackband ironstones.

Claystone Ironstones

Claystone or clayband ironstones have been the basisof the steel industry in many industrialized countries,largely because of their association with coalfields.Essentially, they are accumulations of iron carbonates(usually siderite) that have replaced the non-marineshales of coal-measure cyclothems (parasequences)and occur as either thin sheets or, more commonly,layers of concretions (Figure 4). Occasionally thesesheets may extend over several hundred square kilo-metres. Normally, each concretion is unlaminatedand does not contain high amounts of organic mater-ial, and the siderite grains are usually microscopicor sub-microscopic in size (less than 10 mm). Marineclaystone ironstones are predominately rich in an-kerite with pyrite, and production of siderite issuppressed. Irregularly shaped sphaerosiderites (ballironstones), which usually occur at the base of palaeo-sols, are composed of siderite cement in the form ofdistributed spherulites (0.5–1 mm in diameter).

Ooidal Ironstones

Ooidal ironstones are characterized by the presenceof ooids and/or pisoids and are very diverse, with a

s showing relative water depths of sedimentation, not to scale.


wide range of mineralogy, textures, and chemicalcompositions. Because they possess oolites, shellfragments, and mud matrices in various admixturessimilar to limestones (see Sedimentary Rocks: Lime-stones), most researchers in the field use the petro-graphic terminology advocated by Young to describeand classify ooidal ironstones. Most ooidal iron-stones are less than a metre in thickness and arelaterally persistent over approximately 100 km2. Afew deposits are in excess of 15 m thick (e.g. theGara Djebilet Ironstone in Algeria). Although anidealized stratigraphical model for this type of iron-stone consists of an upward shoaling sequence fromblack shales at the base, through progressively coarserdeposits, to the ironstone at the top (Figure 5), inpractice there are many deviations from this standard.Ironstones develop during periods of reduced sedi-ment input (starvation), with abundant bioturbation,and often exhibit signs of storm reworking to formtempestites. The earliest-formed minerals are usuallyiron oxides and silicates. Iron-rich carbonates may begenerated subsequently, often during early diagenesis.

Modern Examples of IronstoneDevelopment

Bog iron ores are found associated with peat depositsin swampy conditions. Typically they contain hy-drated ferric-oxide and manganese-oxide cementsbut, below the water table, they may be cemented bysiderite. It has been suggested that microbial activity

Figure 5 Idealized stratigraphical column for ooidal ironstones s

in tropical climates particularly promotes the directprecipitation of siderite.

A possible present-day analogue of ancient ooidalironstones appears to be the verdine facies. In thisfacies, iron-rich aluminous green clay minerals re-place bioclasts and pellets. Ferruginous peloids, inmany cases altered faecal pellets, are known to beforming today in sediments deposited in front ofequatorial deltas, such as those on the continentalshelves off Senegal, Guinea, Nigeria, Gabon, Sara-wak, and east Kalimantan. Present-day examples offerruginous ooid accumulation are rare. In the inter-ior of Africa, along the southernmost parts of LakeMalawi, amorphous ferric-oxide ooids have beenfound associated with geothermal springs, and, inthe brackish open water of southern Lake Chad,goethitic brown ooids are being formed. In the shal-low seas of northern Venezuela, berthierine-richgreen-brown muddy ooidal sediments with peloidshave been discovered.

Environment of Deposition andSubsequent Alteration duringLithification

Very few generalizations can be made about the sedi-mentary environment of ironstones. Ironstones maybe deposited in shallow-marine, interdeltaic, non-marine lacustrine, and alluvial environments andmay interfinger or replace sandy and shelly marinedeposits laterally. They are frequently associated with

howing relative water depths of sedimentation, not to scale.


phosphates, coals, evaporites or laterites, and mosthave no direct relation to volcanism.

Blackband ironstones have many of the character-istics of bog iron ores, which are developed in situ,soon after deposition, by a reaction between organicmaterial and underground colloidal iron-rich solu-tions under a thick vegetative cover. Progression ofthe process could yield siderite by reduction. Alter-native evidence has been put forward suggestingthat these deposits could form by direct sideritic pre-cipitation from tropical swamp waters that are al-ready rich in iron. Blackband ironstones are alwaysdeveloped in close proximity to coal seams, so eitherprocess could be feasible.

The diagenesis of the fine-grained claystone iron-stones has been studied in great detail (Figure 6).Most became enriched in iron during very early dia-genesis along or near the sediment–water interface.Based upon distinct chemical reactions involving theoxidation of organic matter buried within the sedi-ment, diagenetic zones have been established. Al-though the zones can be considered as due to burial,their development is especially dependent upon theavailability of oxidizing agents and organic matter,the sedimentary environment, the nature and amountof organic material, the composition of the inorganicsediment, the hydrological regime of the sedimentarypile, and the composition of the overlying water. The

Figure 6 Summary of reactions and zonation that may occur d

conditions (after Curtis and Coleman 1986, Spears 1989 and reprod

Young TP (1993) Sedimentary iron ores. In: Pattrick RAD and Poly

Chapman & Hall, Figure 9.5 after Curtis and Coleman 1986 and Spe

reactions below the oxic zone may be complicated bykinetic controls, which could explain the occasionalappearance of residual ferric iron in an anoxicenvironment. Because some siderite concretions aredeveloped early and are associated with many non-sequences, the sedimentation rate must have beenrelatively low (less than 40 m Ma�1). Whilst claystoneironstones are formed during diagenesis by thegrowth of siderite in the pore spaces of argillaceousmaterials, sphaerosiderites form by the direct precipi-tation of siderite from pore fluids, and their size andshape probably reflect a higher growth rate. They canoccur in a variety of environments, including the deepsea, but are usually products of a waterlogged zonebelow a leached soil profile.

The exact genesis of ooidal ironstones remainscontroversial. Particularly, the origin of the ooids isthe subject of a long-lasting debate. The original con-stituents of ooids and how they vary from deposit todeposit are not known with any certainty. It is debat-able whether the ooids grew from solutes, colloidalparticles in solutions, or gels. The ferrous ion inbicarbonate form survives only in an anoxic or redu-cing environment, so this would place a severe con-straint on its presence in solution. Ferruginous ooidsare commonly built of alternating ferric oxide andberthierine sheaths of submicroscopic thickness.Whether the initial crystalline phase was berthierine

uring the diagenesis of sediments in marine and non-marine

uced with kind permission of Kluwer Academic Publishers from

a DA (eds.) Mineralization in the British Isles, pp. 446–489. London:

ars 1989).

p0125


or a precursor ferric mineral (e.g. odinite) is uncertain.Some feel that it was crystallized at the earliest stage,probably from a gel; others have suggested that itformed during the early stages of burial diagenesis.Alternatively, it could be a product of the transform-ation of either a mixture of kaolinite and hydrousferric oxide or a complex synthesis of silicic, ferric,and aluminous substances. The processes involvingmicro-organisms (such as bacteria) are not under-stood, particularly in terms of how they promote thegrowth of ooids. Reworking of ooidal sediments inshallow-water environments often separates, concen-trates, and highly sorts the ooids, forming lenses,which probably accumulated in shallow depressions.Often zonation of ironstones may be observed whenthe body is less affected by redistribution.

The variable nature of the nuclei of ooids and thetrapping of marine microflora during growth indicatethat ooids are probably generated within the hostsediment. Ferruginous ooids could have grown onthe seafloor, at the water–sediment interface, byeither concentric growth due to precipitation of min-eral matter, frequently around heterogeneous nuclei,or mechanical accretion by rolling (like a snowball).Alternatively, they could have grown inside the sedi-ment at shallow depths below the water–sedimentinterface either as early diagenetic microconcretionsor by replacement or addition of iron to peloids.Fluviatile examples do exist (e.g. the Late Oligocenedeposits of Aral Lake, Russia), in which ooids havebeen developed on land and then moved, but this hasnot been convincingly demonstrated to be of generalapplication.

The IGCP 277 came to the conclusion that ferru-ginous deposition must have been due to the interplayof a number of different processes and hence thatthere is rarely a single genetic explanation. The sal-inity of seawater, the carbon dioxide and oxygencontents of the atmosphere, the action of organisms,the sources and availability of iron compounds,seasonal or long-lasting climatic conditions, specificphysicochemical conditions, the marine water depth,diagenetic processes, and tectonism are all potentialfactors. However, the dominant influences seem to bethe local hydrodynamic conditions and the topo-graphical relief of the land and seafloor, which mayhelp to protect the ooidal deposits from excessivedilution by clastics. Paradoxically, the rates of depos-ition of the stratigraphical equivalents of many ooidalironstones do not always correspond to the periodsof lowest detrital input. Changes during burial arenumerous and complex and include the formation ofphosphatic minerals, iron oxides, siderite, pyrite,and quartz. In most cases, these are followed byalterations due to the effects of meteoric waters.

The Ferruginization Process

Although ironstones are generally considered to bethe products of ferruginization during diagenesis, thephysical sedimentary environment is thought to con-trol the style of diagenesis in ironstones. Blackbandironstones are geochemically and isotopically homo-geneous, suggesting stability of conditions duringgrowth. They were probably formed close to the sedi-ment surface, with precipitation of siderite, and notduring progressive burial (Figure 7). The high man-ganese content of siderite, the relatively low calciumand magnesium contents, and the high carbonate con-tent support this. Studies of carbon isotopes showthat calcareous shells from limestones and ironstoneshave similar d13CPDB values (from þ4% to �6%),indicating that the siderites replaced original calciteor aragonite and precluding the domination of metha-nogenesis. As has been previously noted, very earlysiderite could be precipitated directly from swampwaters, but could precipitation have occurred fromprimitive freshwater too?

Claystone ironstones usually have lower manga-nese and 13C enrichment than blackband ironstones,which can be related to slightly later ferruginization,which takes place below the oxic zone in non-marinewaters by diagenetic distribution of iron within thesediment (Figure 8). The relationships leading to theprecipitation of iron minerals are complex and aresusceptible to slight shifts in the concentrations andavailability of reactants especially S and organic C.The thermodynamics of the reactions predict theobservation of manganese enrichment within the con-cretion cores. The iron and manganese would havebeen present in the silicate minerals of the sediment.Sulphate reduction would be inhibited, as the sedi-ments were isolated from potential sources of sul-phate (e.g. seawater), and changes in organic matterwould be methanogenic, giving rise to bicarbonatesrich in 13C (d13CPDB values in the order of þ10%).Also precipitation would be enhanced by an increasein alkalinity resulting from the combination ofchanges in organic matter and the reduction of Fe3þ

and Mn4þ. Any growth at deeper levels of burialwould be slow under decarboxylation conditionswith bicarbonate depleted in 13C. In marine claystoneironstones, sulphate and iron reduction would pro-ceed broadly simultaneously, leading to the produc-tion of iron pyrites. Siderite is normally rare inmarine sediments because iron can become incorpor-ated in carbonates only below the zone of sulphatereduction.

Over the past two decades there have been signifi-cant developments in research into the environmentalconditions under which ooidal ironstones are formed.

Figure 8 Model of mineralization for claystone ironstones (d, diameter) (reproduced with kind permission of Kluwer Academic

Publishers from Young TP (1993) Sedimentary iron ores. In: Pattrick RAD and Polya DA (eds.) Mineralization in the British Isles,

pp. 446–489. London: Chapman & Hall, Figure 9.7).

Figure 7 Model of mineralization for blackband ironstones (reproduced with kind permission of Kluwer Academic Publishers from

Young TP (1993) Sedimentary iron ores. In: Pattrick RAD and Polya DA (eds.) Mineralization in the British Isles, pp. 446–489. London:

Chapman & Hall, Figure 9.9).


Sea-level change may be the most significant geneticcontrol since it can generate very low accumula-tion rates within basins with low overall sedimenta-tion rates (Figure 9). Widespread sediment starvation

could be produced by relative sea-level rise in shallowepeiric seas with a topographically low hinterland.There is a dispute as to whether these conditionsappertain to the end of regression or to the beginning

Figure 9 Model of mineralization for ooidal ironstones; (A), earliest phase; (B) and (C), middle phases; (D), latest phase (reproduced

with kind permission of Kluwer Academic Publishers from Young TP (1993) Sedimentary iron ores. In: Pattrick RAD and Polya DA (eds.)

Mineralization in the British Isles, pp. 446–489. London: Chapman & Hall, Figure 9.13).


of transgression. Similarly, the origin of early sideriticunits can be related to a decrease in the role ofsulphate reduction, a low sedimentation rate (lessthan 40 m Ma�1), and oxygenated and carbon-poorsediments (values of d13CPDB for various cementsvary from �3 to �22%).

The source and influx of iron is a subject of muchcontroversy. In 1856, Sorby proposed that extensiveferruginization occurred during later diagenesis, butthis idea is no longer accepted, since most ferrugin-ous ooids were formed within the depositionalenvironment. There are three proposals for the originof iron enrichment:

1. iron-rich exhalative fluids, supplying the sedi-ment–water interface (some examples do seemto be related to the episodic reactivation offaults involving exhalative hydrothermal or seepsources);

2. mechanical accretion of lateritic terrestrial weather-ing products (e.g. kaolinite and iron oxides) or lat-eritic soils to form ooids in a marine environment,with subsequent transformation to berthierine(this does not seem to be generally applicablesince unaltered primary ooids with mixed ironoxide–kaolinite composition have not been foundin marine ooidal ironstones); and


3. leaching from underlying sediments, especiallyorganic-rich shales, during very early marine alter-ation of detrital material (the diagenetic redistri-bution of iron within sediments is difficult todemonstrate in ancient ooidal ironstones sincethe process would probably require considerabletime).

The role of clay minerals in effecting ferruginiza-tion is unknown, particularly in respect of the trans-formation of non-iron-bearing phyllosilicates intoiron-bearing ones and the role of iron-rich greentrioctohedral clay minerals of warm seas as possibleprecursors of later ooidal minerals.

Stratigraphical Record (TemporalOccurrences) and Tectonic Settings

It has puzzled geologists that some geological periodshave significant numbers of ironstones, whilst otherperiods are devoid of them. Ironstones are almostcompletely restricted to the Phanerozoic. Blackbandand claystone ironstones are particularly prevalent inthe Carboniferous, when the depositional basins oc-cupied near-tropical locations. Ooidal ironstones areparticularly common in the Ordovician, Devonian,Jurassic, and Cretaceous periods. Most ironstoneswere formed in warm climates, although some weredeposited in cooler climates (e.g. the Late Ordovicianand Late Permian ironstones). Palaeolatitude datahas shown that the Ordovician and Devonian iron-stones formed in a zone of the Gondwanan shelfseas ranging from 45�N to 65�S of the palaeoequator.In the Jurassic and Cretaceous, ironstones formedbetween 70�N and 10�S. For this reason climatecannot be the major contributory factor in theirformation.

Ironstones are largely confined to three types ofcratonic setting.

1. Many developed in anorogenic basins dominatedby prolonged stability and sometimes with com-plex extensional faulting that involved the forma-tion of marine basins and swells in areas ofsubdued relief.

2. Some developed along the margins of cratonsduring initial convergence or divergence of plates.

3. Other ironstones accumulated on the inner sides ofmobile belts at times of diminished deformation.

Glossary

Condensed deposit A relatively thin but uninterruptedsedimentary sequence representing a significantperiod of time during which the deposits have

accumulated very slowly. It is generally correlatedwith a thicker time-equivalent succession elsewhere.

Ferruginization A synonym of ferrification andthe preferred term by IGCP 277 to describe theprocesses of iron-enrichment of various Earthmaterials.

Hardground A zone at the seafloor a few centimetresthick, where the sediment is lithified to form ahardened surface, which is often encrusted andbored.

IGCP The International Geological CorrelationProgramme.

Neoformation A synonym of neogenesis, the forma-tion of new minerals.

Ooid A synonym of oolith and the preferred term todescribe a spherical or ellipsoidal accretionarysand-sized (diameter of 0.25–2 mm) particle in asedimentary rock (mainly limestones and iron-stones). Ooids usually consist of successive concen-tric layers (often carbonates) around a centralnucleus.

Pellet A small, usually ellipsoidal, aggregate of accre-tionary material (mainly micrite) that has, in mostcases, formed from the faeces of molluscs andworms.

Peloid An allochem composed of micrite, irrespec-tive of size or origin, without internal structure.Includes both pellets and intraclasts.

Pisoid A synonym of pisolith and the preferred termto describe small round or ellipsoidal particles(diameter of 2–10 mm) in a sedimentary rock(mainly limestones and ironstones). Pisoids arelarger than ooids and usually consist of concentriclayers around a central nucleus.

Stillstand A period of time when an area of land isstable relative to mean sea-level (or some otherglobal measure), leading to a relatively unvaryingbase level of erosion.

Verdine facies Green marine clay characterized bythe authigenesis (neoformation in situ) of iron-richaluminous clay minerals, especially 0.7 nm repeatodinite, but not berthierine or glauconite.

See Also

Economic Geology. Palaeozoic: Carboniferous. Sedi-mentary Environments: Depositional Systems and

Facies. Sedimentary Rocks: Mineralogy and Classifica-

tion; Banded Iron Formations; Clays and Their Diagen-

esis; Limestones.

Further Reading

Boardman EL (1989) Coal measures (Namurian and West-phalian) blackband iron formations: fossil bog iron ores.Sedimentology 36: 621–633.

SEDIMENTARY ROCKS/Limestones 107

Curtis CD and Coleman ML (1986) Controls on theprecipitation of early diagenetic calcite, dolomite andsiderite concretions in complex depositional sequences.In: Gautier DL (ed.) Roles of Organic Matter in Sedi-ment Diagenesis, pp. 23–33. Special Publication 38.Denver: Society of Economic Palaeontologists and Min-eralogists.

Curtis CD and Spears DA (1968) The formation ofsedimentary iron minerals. Economic Geology 63:257–270.

Kearsley AT (1989) Iron-rich ooids, their mineralogy andmicrofabric: clues to their origin and evolution. In: YoungTP and Taylor WEG (eds.) Phanerozoic Ironstones, pp.141–164. Special Publication 46. London: GeologicalSociety of London.

Kimberley MM (1994) Debate about ironstone: has solutesupply been surficial weathering, hydrothermal con-vection, or exhalation of deep fluids? Terra Nova 8:116–132.

Odin GS (ed.) (1988) Green Marine Clays, Oolitic Iron-stone Facies, Verdine Facies, Glaucony Facies andCeladonite-Bearing Facies – A Comparative Study. Devel-opments in Sedimentology 45. Amsterdam: Elsevier.

Limestones

R C Selley, Imperial College London, London, UK

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Introduction

Limestones are one of the most important of allthe sedimentary rocks introduced in (see SedimentaryRocks: Mineralogy and Classification). Limestonesare composed largely of calcium carbonate (CaCO3)in the mineral form calcite, but there are several otherimportant carbonate minerals with which limestonesare associated. This article opens by discussing im-portant differences between limestones and sand-stones, and continues by outlining the mineralogy,classification, and rock names of limestones. This isfollowed by a brief account of limestone depositionalenvironments, and, logically, by their postdeposi-tional diagenesis. The article concludes with a descrip-tion of the economic importance of limestones, whichis considerable, and a selected reading list.

Differences between Limestonesand Sandstones

Limestones and sandstones are the two most import-ant groups of sedimentary rocks. However, lime-stones pose a completely different set of problems to

Petranek J and Van Houten F (1997) Phanerozoic OoidalIronstones. Special Papers 7. Prague: Czech GeologicalSurvey.

Spears DA (1989) Aspects of iron incorporation into sedi-ments with special reference to the Yorkshire Ironstones.In: Young TP and Taylor WEG (eds.) PhanerozoicIronstones, pp. 19–30. Special Publication 46. London:Geological Society of London.

Taylor JH (1949) The Mesozoic Ironstones of Britain:Petrology of the Northampton Sand Ironstone. Memoirof the Geological Survey of Great Britain. London: Geo-logical Survey of Great Britain.

Van Houten FB and Arthur MA (1989) Temporal patternsamong Phanerozoic oolitic ironstones and oceanicanoxia. In: Young TP and Taylor WEG (eds.) Phanero-zoic Ironstones, pp. 33–49. Special Publication 46.London: Geological Society of London.

Young TP (1993) Sedimentary iron ores. In: Pattrick RADand Polya DA (eds.) Mineralization in the British Isles,pp. 446–489. London: Chapman & Hall.

Young TP and Taylor WEG (eds.) (1989) PhanerozoicIronstones. Special Publication 46. London: GeologicalSociety of London.

those of sandstones, the solutions of which requirethe application of different concepts and techniques.

First, limestones, unlike sandstones, are intrabas-inal in origin. That is to say they form in the envir-onment in which they are deposited. The sourcematerial of sandstones, by contrast, has beenweathered, eroded, transported, and may finally bedeposited hundreds of kilometres from its point oforigin. Sandstones (or siliciclastic rocks) thereforeoften contain many different minerals. Limestones,by contrast, have a much simpler mineralogy, gener-ally consisting of only calcite and two or three others(which will be mentioned shortly). Siliciclastic sandgrains may hold clues to their source, but tell little oftheir depositional environment. Limestone grains, bycontrast, although largely monomineralic, occur in awide range of sizes and shapes, reflecting their mul-tiple origins. These grains form in specific environ-ments from which they are seldom transported.Limestone grains thus give important clues abouttheir environment of deposition.

When studying sandstones, vertical profiles ofgrain size and analysis of sedimentary structures arethe keys to environmental diagnosis. With limestones,however, it is the analysis of grain type and texturethat aids environmental diagnosis.

The second large difference between sandstonesand limestones lies in their chemistry. Sandstones are

Encyclopedia of Geology || SEDIMENTARY ROCKS | Ironstones

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