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Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
Phosphate nodules, including common mineralized internal moulds of macrofauna, are found only at certain levels in the chalky, locally bituminous Ghareb Formation (uppermost Campanian-Maastrichtian) of southern Israel. The microstructure of these nodules suggests that they consist of marine sediment cemented by a microbially-mediated apatite, precipitated locally in protected microenvironments, e.g., within partly closed burrows and shells, or within the upper sediment layer. The globular microstructures commonly recognized in the phosphate cements of the pebbly phosphates are regarded as mineralized cells of endobenthic micro-organisms. The global occurrence of pebbly phosphate in diverse lithofacies throughout the geological column indicates the tolerance of these globular micro-organisms to a wide range of ecological conditions.
The grains in the Upper Cretaceous phosphorites of the Negev (southern Israel) consist of bone fragments, phosphatized faecal pellets and ovular grains of different types. The commonest variety of ovular grain consists of microcrystalline phosphate (microphosphate) containing relics of globular microstructures, either forming the whole grain, coating a foraminifera or bone fragment, or cementing small clasts. The latter types resemble the pebbly phosphates and can be regarded as micronodules. The matrix of these granular phosphorites is mainly micritic, and in places argillaceous. In some Negev phosphorites the ovular grains and their matrix consist mainly of loosely packed tubules. These tubular microstructures resemble mineralized filaments of microphytes which form microbial mats on modern shallow seafloors lying within the photic zone. The origin of these granular phosphorites is discussed, and a comparison with the pebbly phosphates indicates that different ecological controls and microbial mediators resulted in the different kinds of phosphate bodies.
Since most of the phosphate matter can be related to microbial activity, it is doubtful whether direct replacement of solid carbonate by phosphate contributes significantly to phosphogenesis in sediments. It is suggested, however, that in many deposits phosphate-precipitating micro-organisms have penetrated and mineralized the borings of algae and other endolithic groups, or have filled dissolution cavities within lithified carbonates.
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