Upper Cretaceous facies development and depositional environments, Gebel Somar area, north central Sinai, Egypt

Facies analysis of the exposed Upper Cretaceous sequence at Gebel Somar area revealed that the Cenomanian — Turonian carbonates are related either to platform — epicontinental sea or to deeper shelf- margin carbonates. Their microfacies types are equivalent to the Standard Microfacies (SMF) types, namely the bioclastic wackestone or bioclastic micrite (SMF-9) of shallow open marine platform facies (FZ. 7), rudstone with reef debris (SMF-6) accumulated in fore-reef slope (FZ. 4) and hemipelagic wackestone (SMF-3) deposited under deep shelf marginal conditions (FZ. 3). The Coniacian-Santonian rocks show rapid facies change and manifest oscillating depositional environments from restricted marine shoals to shelf facies with open circulation. The SMF types of bioclastic wackestone (SMF-9), grainstone with coated bioclasts (SMF-11), grainstone with ooids (SMF-15), grainstone with pellets (SMF-16), grainstone or pelsparite with aggregate grains (SMF-17) and pelleted lime mudstone-wackestone (SMF-19) are discernible in the Coniacian-Santonian carbonates. These SMF types are platform facies belts either winnowed (FZ. 6), open (FZ. 7) or restricted (FZ. 8) platforms. The rapid facies change of the Coniacian-Santonian may reflect mild tectonic instability other than sea-level fluctuations. This tectonic instability may be the first imprint of Syrian are deformation, evidenced at the studied area by complete absence of Campanian strata. The Maastrichtian rocks are formed of pelagic sediments deposited under deep shelf margin conditions (SMF-3) in a wide basin or depocenter generated as a result of the Syrian arching system. The facies characteristics and the diagenetic sequence as well as the geologic history of the study area during the Late Cretaceous time are also discussed and interpreted.