SEDIMENTARY ROCKS/Ironstones 101 wide range of mineralogy, textures, and chemical compositions Because they possess oolites, shell fragments, and mud matrices in various admixtures similar to limestones (see Sedimentary Rocks: Limestones), most researchers in the field use the petrographic terminology advocated by Young to describe and classify ooidal ironstones Most ooidal ironstones are less than a metre in thickness and are laterally persistent over approximately 100 km2 A few deposits are in excess of 15 m thick (e.g the Gara Djebilet Ironstone in Algeria) Although an idealized stratigraphical model for this type of ironstone consists of an upward shoaling sequence from black shales at the base, through progressively coarser deposits, to the ironstone at the top (Figure 5), in practice there are many deviations from this standard Ironstones develop during periods of reduced sediment input (starvation), with abundant bioturbation, and often exhibit signs of storm reworking to form tempestites The earliest-formed minerals are usually iron oxides and silicates Iron-rich carbonates may be generated subsequently, often during early diagenesis Modern Examples of Ironstone Development Bog iron ores are found associated with peat deposits in swampy conditions Typically they contain hydrated ferric-oxide and manganese-oxide cements but, below the water table, they may be cemented by siderite It has been suggested that microbial activity in tropical climates particularly promotes the direct precipitation of siderite A possible present-day analogue of ancient ooidal ironstones appears to be the verdine facies In this facies, iron-rich aluminous green clay minerals replace bioclasts and pellets Ferruginous peloids, in many cases altered faecal pellets, are known to be forming today in sediments deposited in front of equatorial deltas, such as those on the continental shelves off Senegal, Guinea, Nigeria, Gabon, Sarawak, and east Kalimantan Present-day examples of ferruginous ooid accumulation are rare In the interior of Africa, along the southernmost parts of Lake Malawi, amorphous ferric-oxide ooids have been found associated with geothermal springs, and, in the brackish open water of southern Lake Chad, goethitic brown ooids are being formed In the shallow seas of northern Venezuela, berthierine-rich green-brown muddy ooidal sediments with peloids have been discovered Environment of Deposition and Subsequent Alteration during Lithification Very few generalizations can be made about the sedimentary environment of ironstones Ironstones may be deposited in shallow-marine, interdeltaic, nonmarine lacustrine, and alluvial environments and may interfinger or replace sandy and shelly marine deposits laterally They are frequently associated with Figure Idealized stratigraphical column for ooidal ironstones showing relative water depths of sedimentation, not to scale