Paleoenvironmental significance of the monospecific biostromes in the Campanian-Maastrichtian Duwi Formation (Eastern Desert, Egypt)

The Campanian-Maastrichtian Duwi Formation in the Eastern Desert of Egypt is an economic-rich part of the Middle East phosphorite belt, which has been formed during an extensive high-productivity and anoxic episode in the southern Tethys. To understand the consequences of these environmental conditions on the paleo-ecosystems, the sediments and the benthic macrofauna was analyzed. Although the macrobenthic elements are poorly represented throughout the studied sections, four monospecific biostromes were recognized. They included two Ambigostrea oyster biostromes, which have developed under high-energy and fully aerated environments above the storm wave-base. In contrast, the heterodont bivalves Integricardium and Aphrodina biostromes were deposited under mesotrophic, low-energy, and dysoxic regimes. Shell orientation, shell size-frequency, and multivariate analysis of other taphonomic features indicated that one of two Ambigostra biostromes were partially generated by waves and/or storms (i.e. parabiostromes), while the other one is mostly in situ (i.e. autoparabiostromes). In contrast, the heterodont biostromes are intrinsic autochthonous concentrations (autobiostromes). The link between occurrences of biostromes and cyclicity of deposits suggests a clear association to sea-level fluctuations. The autobiostromes dominating the maximum-flooding and early highstand intervals and were differentiated from the parabiostromes, which formed by physical instead of biologic agents during initial transgressions or late highstand. The palaeo-ecosystem was reconstructed and the environmental factors behind the biostromes were identified; the monospecific nature of the biostromes is interpreted to be a consequence of high-stress environment (e.g., high productivity and associated anoxia). The global climate during the Campanian-Maastrichtian, sea level fluctuations, and physiography influenced salinity and oxygen levels and played the pivotal role in the formation of the monospecific biostromes of the Duwi Formation.