Depositional sequences of the Duwi, Sibâîya and Phosphate Formations, Egypt: phosphogenesis and glauconitization in a Late Cretaceous epeiric sea

Abstract

Sedimentary facies, depositional environments and stratigraphic relationships of the Upper Cretaceous Duwi, Sib~iya and Phosphate Formations in Egypt are summarized in order to shed light on the origins and interrelationships between phosphorites and their commonly associated facies. The distribution of phosphorites, glauconitic sands, organic carbon-rich shales, cherts and porcelanites, and bioclastic and fine-grained limestones are correlated in both space and time across the central phosphorite provenance of the country. The correlations are linked by means of sequence stratigraphic analysis and are tied to global eustatic sea-level curves.Two main depositional realms are inferred from the Campanian lithofacies assemblages: (1) a deeper-water hemipelagic environment accompanying maximum transgression dominated by deposition of phosphorites, organic carbon-rich shales and biosiliceous sediments, which, after maximum flooding, shoals upwards into (2) a progradational stage accompanying sea-level fail, during which oyster banks with brackish back-reef sediments (Eastern Desert) and deltaic sediments (upper Quseir Variegated Shale, Nile Valley) dominated eastern portions of the phosphorite belt, while greensands were reworked seaward from earlier inner shelf sediments in the west (Western Desert). These depositional realms encompass (1) transgressive to lower highstand systems tracts and (2) upper highstand to lowstand progradational wedge system tracts, respectively. The reoccurrence of lower Maastrichtian phosphatic shale, marl and associated phosphorite lithofacies at the top of the sequence indicates repetition of the sedimentary cycle.Both phosphorites and glauconitic greensands appear to be the result of current winnowing and reworking of authigenic grains from previously deposited sediments. The resulting deposits range in scale from thin lag layers, to amalgamated beds, to giant sand waves up to 10 m thick. In the Egyptian setting, the common factors linking massive phosphorite and glauconite occurrences may have been the co-occurrence of humid climate, deep lateritic weathering in hinterlands, anoxic bottom waters on the inner shelf, and high fluvial discharge rates of phosphorus and iron. Superimposed on these factors were influences of cross-shelf currents necessary to concentrate authigenic precipitates, and a sea level change, which acted to markedly reduce the diluting effects of siliciclastic detritus during rapid sea level rise, as well as vertically stack 'authigenic facies' during sea level fall.

Origin and Significance of Late Pliocene Phosphatic Hardgrounds on the Queensland Plateau, Northeastern Australian Margin

“…The carbonate fluorapatites of the QLP hardgrounds show a complete absence of cathodoluminescence, as well as very low levels of MnO, as detected by electron microprobe, and this is consistent with other studies of marine carbonate fluorapatites (e.g., Glenn and Arthur, 1988;Glenn, 1990b). We think that this is not because of the availability of dissolved Mn in pore waters, but, rather, because CFA does not normally accommodate Mn 2+ in its structure.…”

“…In oxygenated waters, FeOOH compounds sorbed phosphate, but upon encountering reducing conditions, this sorbed phosphate is released (Froelich et al, 1979;Krom andBerner, 1980, 1981;de Lange, 1986;Schaffer, 1986) and is available for precipitation as a primary authigenic phase (e.g., O'Brien et al, 1988;Froelich et al, 1988;Schuffert, 1988;Glenn and Arthur, 1990;Glenn, 1990aGlenn, , 1990b. The iron reduced in this process, perhaps by microbial mediation, becomes available for incorporation in other authigenic phases (the dolomites of the QLP hardgrounds) or may be fluxed by chemical gradient back to the relatively more oxygenated seafloor, where the Fe 2+ is re-oxidized and re-precipitated (the goethite of the QLP hardgrounds).…”

Calcareous nannofossils as paleoproductivity indicators in Upper Cretaceous organic-rich sequences in Israel

“…Paleogeographic reconstructions suggest that the shoreline during this period was about 100 km to the southeast during the Campanian, and even further to the east in the Maastrichtian (Bender, 1974;Germann et al, 1987;Glenn, 1990). The accumulation of organic-rich carbonates, cherts and phosphorites, as well as the microfossil assemblage composition, indicates highly productive paleoenvironments, associated with a large-scale upwelling system that existed along the southern margins of the Tethys (Reiss, 1988;Almogi-Labin et al, 1993;Eshet et al, 1994).…”

High-Resolution Sequence Stratigraphy, Condensed Sections, and Flooding Events off the Great Barrier Reef: 0-1.5 Ma

“…Since the 1930s and 1940s, such intervals have been recognized as characterized by the following features: (1) enrichment of well-preserved fossils and fossil fragments, (2) faunal mixing: fossils from different paleontological zones that were mixed up together within a bed, and (3) widespread distribution of sediments having negligible thicknesses (Heim, 1934;Rod, 1946). Since then, researchers have increasingly recognized that intervals of sequence condensation also frequently were marked by the occurrence and, in some instances, a high (even minable) abundance of authigenic minerals (e.g., Glenn, 1990). Of continuing debate, however, is how much sediment reworking and winnowing contributes to the condensation process (e.g., Rod, 1946;Jenkyns, 1971;Kennedy and Garrison, 1975;Krajewski, 1984;Kidwell, 1991;Burnett et al, 1988;Glenn and Arthur, 1990;Föllmi, 1990;Snyder et al, 1990).…”