Anatomy and origin of a Cretaceous phosphorite‐greensand giant, Egypt

Abstract

Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark‐coloured shales, glauconitic sandstones and bioclastic and fine‐grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon‐rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea‐level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution‐collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron‐oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters. This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.

Tethyan Phosphates and Bioproductites

“…In Egypt (Garrison et at., 1979;Glenn and Mansour, 1979;Germann et at., 1987;Issawi, 1989;Glenn and Arthur, 1990), phosphate deposits are distributed over a vast zone between the Red Sea and the oases of Dakhla and Kharga in the western desert. After the deposition of the Nubian Sandstone, a fluvio-marine formation covering the entire area of northeastern Africa until the Cenomanian, marine influences developed during the Campanian, resulting in the deposition of sandy shales and variegated shales immediately below the phosphatic sediments.…”

Color, Mineralogy and Composition of Upper Jurassic West Siberian Glauconite: Useful Indicators of Paleoenvironment

“…Such heterogeneity certainly influences the color of glaucony grains, and is related to a complex pattern of sedimentological and chemical evolution before burial. For example, brown grains or brown rims on green grains are commonly found within marine greensands and associated facies, such as phosphorites (Glenn & Arthur 1990, Zamirailova et al 1999, Martín-Algarra & Sánchez-Navas 1995, 2000. Commonly, such brown grains are synsedimentarily replaced by phosphate and ferruginous materials.…”

Geochemistry, mineralogy and depositional environment of black shales of the Duwi Formation, Qusseir area, Red Sea coast, Egypt

“…Ross and Bustin 2009). The enrichment of organic matter reveals a period of high primary productivity in a shallow environment conducive to the preservation of organic matter (Glenn and Arthur 1990). Ni can be incorporated into organic matter, and so may be used as a marker for the original presence of organic matter (cf.…”