SEDIMENTARY ROCKS/Chert 57 Figure Photomicrographs of chert replaced and cemented sedimentary rocks in thin section (A) Silicified oolite in PPL (plane polarized light) (B) Same view as (A) in XPL (cross polarized light) showing a variety of silica cement and replacement fabrics after the original carbonate sediment Trenton Series, Ordovician, Centre County, Pennsylvania, USA (C) Silicified bioclastic sandstone with abundant sponge spicule tetraxons, viewed in PPL (D) Same view as (C) in XPL Both the matrix and sponge spicules are replaced primarily by microcrystalline quartz Upper Greensand, Cretaceous, Ventnor, Isle of Wight, UK Scale bars 500 mm related to redox boundaries in the original sediment (Figure 8) The spacing of chert bands reflects abrupt, stepwise rises of the redox boundary related to pulses in sedimentation and hiatuses The geometry of the redox boundary (governed by permeability and porosity contrasts in the sediment) generally determines the chert morphology, accounting for the spectrum of burrow-form and tabular cherts commonly observed in the field The replacement by chert can be remarkably selective, with preferential replacement of limestone matrix, or of biogenic debris of a particular original composition, such as high-Mg calcite Very often the earliest stages of silicification in carbonate sequences occur in shell material in which localized silica precipitation/carbonate dissolution is promoted by the bacterial breakdown of organic matter, particularly conchiolin within the shell matrix Thus, as well as the nodular form of chert, selective silicification may result in scattered silicified fossils within limestone In the Cretaceous Chalk of Europe and the USA, flint nodules of irregular form occur at specific widespread stratigraphical horizons, but also in sheets and pipes that cross-cut bedding Flint is generally dark grey, and contains carbonate inclusions, particularly of bivalves and echinoderms Flint nodules have a thin white crust, or patina Many echinoids from the Upper Chalk are filled with flint, the silica having nucleated within the urchin, but without replacing the shell Sponges and burrows are also selectively silicified, with the shape of many flints in the chalk reflecting the morphology of Thalassinoides burrows in which they nucleated (Figure 9) Siliceous sponge spicules probably provided much of the biogenic silica for the formation of flint There are also nodular chert-bearing beds within the Portlandian (Late Jurassic) limestones of southern England (Figures and 8), where bioturbation textures and diagenetic redox boundaries controlled silica precipitation and replacement Siliceous sponges were the main biogenic silica source