Origin of Late Cretaceous phosphorites in Egypt

Abstract

The Duwi phosphorite deposits in Egypt are a part of the Middle East to North African phosphogenic province of Late Cretaceous–Palaeogene age. Their origin is controversial. Most previous authors have considered them to be in situ, authigenic deposits. Phosphatic grains in the Egyptian phosphorites are well-rounded phosphatic mudclasts (45–65%) and phosphatic bioclasts (35–55%). Phosphatic mudclasts are internally structureless but sometimes contain silt-sized detrital grains such as quartz and bone fragments, suggesting a reworked origin from pre-existing phosphatic mudstone. Analysis by Electron Probe Microanalyzer (EPMA) confirmed that the phosphatic mudclasts are chemically homogeneous, without any concentric structure suggestive of authigenic origin. The presence of bone fragments within some of the phosphatic mudclasts, the occurrence of phosphatic mud filling bone cavities, and the similar appearance of the matrix of phosphatic mudclasts and the phosphatic mud in the bone cavities suggest that the bioclasts are also of reworked origin and washed out from the phosphatic mudstone. Scanning electron microscope observations of the fractured surface of the mudclasts have revealed the capsule-like texture that is characteristic of bacterially mediated authigenic phosphorites formed under upwelling conditions. Campanian authigenic phosphorites with similar texture have been reported from Israel, suggesting that the phosphatic grains were reworked from pre-existing authigenic phosphorites equivalent to those in Israel. Sedimentary facies analysis of the Egyptian phosphatic sequence suggests that the major phosphorite beds represent transgressive lag deposits that accumulated in an oxic, bioturbated, wave-dominated shelf environment. It is likely that offshore authigenic phosphorites, formed in an upwelling-dominated environment during an early Campanian highstand, were reworked during a mid Campanian lowstand, transported landward, and concentrated in a near-shore, oxic environment during subsequent late Campanian transgressions. Another transgressive-regressive cycle is recognized to have occurred during the late Campanian–early Maastrichtian, suggesting that formation, reworking and redeposition of the Duwi phosphorites resulted from sea-level changes during the Campanian and Maastrichtian.