Stable isotopic composition and its relation to origin and diagenesis of some Upper Cretaceous dolomites and dolomitic limestones from Egypt

The Upper Cretaceous sedimentary sequence in northern Egypt consists of a variety of platform carbonate units in which originally calcitic components have been totally or partially replaced by dolomite. Replacement is partial in the Abu Roash sequence whereas it is complete in the Bahariya Oasis and Gebel Ataqa sequences. Detailed examination of sedimentary structures, lithologies, fossil contents, and fabric relationships demonstrates that these shallow marine facies comprise a shoaling-upward sequence bounded above by a major pre-Middle Eocene unconformity overlain by a Middle Eocene carbonate sequence. Petrographic, cathodoluminescence, trace element, and isotope techniques were used to investigate the genesis and textural relationships among two generations of dolomite and late void-filling calcite in this sequence. Early diagenetic dolomite records limestone replacement in the presence of marine-derived fluids, whereas the later stage records dolomitization and cementation in the presence of mixed meteoric-marine water. However, microcrystalline dolomite of the Red Bed Series shows trace elemental and isotopic signatures of hypersaline dolomitization. Cathodoluminescence reveals that replacive matrix dolomite rhombs exhibit dully luminescent cores with dull to bright rims, whereas dolomite cement exhibits finely-zoned bright luminescence. However, dolomite cement and stabilization of previously formed dolomite in the Bahariya Oasis sequence are represented by dolomite that shows a unique bright creamy CL color. The excess radiogenic Sr of this dolomite reflects an increased supply of meteoric-water which intensified weathering of overlying more radiogenic clastic sources. Finally, the late void-filling calcite spar, a distinctive CL-zoned phase, occurs throughout the stratigraphic sequence. Examination of this cement within the Maastrichtian chalks which regionally overlie the dolomite sequence has demonstrated that this calcite is related to infiltration of meteoric water during the regional erosional exposure. Petrological and geochemical examination of the Maastrichtian chalks from Bahariya Oasis and Abu Roash provides insight into factors that control porosity development in fine-grained calcitic carbonates. High porosity chalks at Bahariya Oasis have depleted isotopic values and low concentrations of minor elements. In contrast, the low porosity chalks at Abu Roash have more enriched isotopic values and elevated concentration of Sr, Na, Mg, Fe, and Zn. (Abstract shortened with permission of author.