Distribution and Abundance of Rudist Bivalves in the Cretaceous Platform Sequences in Egypt: Time and Space

Authors: MOHAMED S. ZAKHERA

Abstract: As the rudist bivalves represent important organic buildups in the Cretaceous platform sequences, this study emphasizes vertical and spatial distribution of this group of bivalves in the geographic divisions of Egypt, including Western Desert, Eastern Desert and Sinai. Rudists are encountered in different rock facies ranging from mudstones to carbonates. About sixty eight species belong to twenty one genera are reported from Egypt. They belong to six families: Requieniidae, Monopleuridae, Caprotinidae, Caprinidae, Hippuritidae, and Radiolitidae. The Radiolitidae is the most diverse family, comprising eleven genera and fifty-one species, dominated by species of Radiolites, Eoradiolites and Durania. The elevator morphotype of the Radiolitidae became the dominant species in the Turonian sequences. The diversity (richness) peaks in the Turonian (36 species) Cenomanian (26 species) and Albian (9 species), with few records in Aptian, Coniacian, Campanian and Maastrichtian (totally 5 species). As yet rudists are not recorded from Santonian rocks. Geographically, rudists are highly represented in Sinai (60 species) concentrated in the Cenomanian (23 species) and Turonian (32 species), followed by Western Desert (19 species) and Eastern Desert (8 species). Regarding abundance so far, the relatively highly abundant species in Egypt are Eoradiolites liratus (19 sites), followed by Praeradiolite biskraensis and Eoradiolites sinaiticus (7 sites for each), Praeradiolites ponsianus, Durania humei, Radiolites sauvagesi (6 sites for each), Durania gaensis and Radiolites lusitanicus (5 sites for each). The rare occurrence during the Campanian and Maastrichtian may be attributed to stagnant conditions related to deposition of black shales and phosphatic deposits and the change to deep inner shelf setting respectively. The disappearance of rudists from some segments of the sequence is attributed oceanic anoxia or related to shelf drowning especially at the basal Turonian, which may related to global warming.

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