Benthic foraminiferal biofacies distribution in the Middle Jurassic Safa Formation, Gebel Al-Maghara, Northern Sinai, Egypt: Paleoenvironmental implications

摘要

The Middle Jurassic Safa Formation of northern Sinai, Egypt is subdivided into two members; the sand-rich lower Safa Member and the sand-lean upper Safa Member. They contain abundant and highly diversified benthic foraminiferal assemblages that correlate strongly with the associated depositional environments. Cluster and factor analyses have led to the identification of four biofacies: the Lenticulina and the Eoguttulina-Ophthalmidium biofacies, which are dominated by calcareous assemblages and remains of other invertebrates, characterize normal marine shelf settings; the Ammobaculites and the Ammodiscus-Glomospirella biofacies, which are overwhelmingly dominated by agglutinated assemblages and are completely devoid of calcareous taxa characterizing low salinity marginal marine settings. The vertical distribution of the foraminiferal biofacies along with the corresponding lithofacies characteristics facilitated environmental interpretations and enabled to subdivide the Safa Formation into 14 intervals displaying either an upward shallowing or a deepening tendency. The faunal distribution pattern and the dominant biofacies indicate the strong influence of salinity changes, sedimentation rates, water depth, turbulent bottom and dilution of CaCO3 and to a lesser extent, bottom oxygenation levels as the major controls on benthic foraminiferal distribution. The distribution of agglutinated assemblages in many intervals can not be solely explained in light of the organic matter concentrations and the concomitant low oxygenation levels as previously proposed.