The Lower Cretaceous sections in northern Sinai are composed of the Risan Aneiza (upper Barremian‐middle Albian) and the Halal (middle Albian‐lower Cenomanian) formations. The facies reflect subtle paleobathymetry from inner to outer ramp facies. The inner ramp facies are peritidal, protected to open marine lagoons, shoals and rudist biostrome facies. The inner ramp facies grade northward into outer ramp deposits. The upper Barremian‐lower Cenomanian succession is subdivided into nine depositional sequences correlated with those recognized in the neighbouring Tethyan areas. These sequences are subdivided into 19 medium‐scale sequences based on the facies evolution, the recorded hardgrounds and flooding surfaces, interpreted as the result of eustatic sea level changes and local tectonic activities of the early Syrian Arc rifting stage. Each sequence contains a lower retrogradational parasequence set that constituted the transgressive systems tracts and an upper progradational parasequence set that formed the highstand systems tracts. Nine rudist levels are recorded in the upper Barremian through lower Cenomanian succession at Gabal Raghawi. At Gabal Yelleg two rudist levels are found in the Albian. The rudist levels are associated with the highstand systems tract deposits because of the suitability of the trophic conditions in the rudist‐dominated ramp.

The stable isotope record and diversity of Cenomanian and Turonian rudists were compared for areas located at low paleolatitudes (10°N -Northern Egypt; 5 samples; 13 analyses) and middle paleolatitudes (35°N -Czech Republic, Bohemian Cretaceous Basin, BCB; 4 samples; 7 analyses). The oxygen isotope data indicate that the δ 18 O value of seawater was not identical for both areas. Moreover, local variations in seawater δ 18 O in the range of at least ±1‰ are probable in the shallow water near-shore environment where the rudists lived. Supposing a value of -1‰ VSMOW for contemporaneous seawater, the paleotemperature can be calculated as being between 24.5 and 31.5°C for the Late Cenomanian and between 32.9 and 34.7°C for the Early Turonian in the BCB. The δ 18 O seawater value for the northern part of Egypt was probably slightly lower. Calculation using a seawater value of -1.5‰ VSMOW gives temperatures for the Egypt in the range of 31.0 to 38.8°C for the Late Cenomanian and 35.5 to 41.2°C for the Turonian. The sclerochronological isotope profiles within one shell show changes of the calcite δ 18 O value, probably reflecting seasonal changes, with the largest observed within-shell temperature shift corresponding to 8°C. The analyzed pilot sample set shows that the BCB and Egyptian rudists contain valuable paleoenvironmental information and that a detailed isotope study is needed. Rudist generic diversity of both areas was also compared. The Cenomanian and Turonian rudist assemblages from Egypt and the BCB show similar generic diversity, and also display a similar marked diversity decrease across the Cenomanian/Turonian boundary interval. It is important to note that both areas have similar diversity on the generic and partially species level during the Cenomanian and Turonian. •

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