Lithofacies, cyclicity, diagenesis and depositional environments of the Upper Cenomanian, El Heiz formation, Bahariya Oasis, Western Desert, Egypt

Abstract

The Upper Cenomanian El Heiz Formation comprises seven lithofacies, four are dolostone lithofacies represented by marly dolostone (MLD), vuggy dolostone (VD), sandy dolostone (SD), and dedolostone (DD). The limestone lithofacies are mostly dolomitic marl (DML) and dolomitic echinoidal foraminiferal wakestone (DEFW). A single claystone lithofacies occurs intercalated with the above lithofacies.

Two coeval inner- and outer-shelf environments are proposed for the El Heiz Formation. The inner-shelf that extends from the central to the southern extremity of the Bahariya Oasis is characterized by cyclic sequence, each cycle consists of claystone at the base and dolostone at the top. These cycles were formed by extrinsic mechanism and described as gradual, non-gradual and truncated cycles. The outer-shelf lithofacies with cyclic limestone and claystone occur in the northeastern parts of the Bahariya Oasis. The cycles are of the gradual type, formed by auto-cyclic mechanism, representing deeper-water environment.

Two phases dolomitization can be recognized in the El Heiz Formation. Early dolomitization produced inclusion-rich dolomite at the end of cycles under intertidal zone. The second phase of dolomitization was formed by mixing marine and meteoric water, producing dolomite overgrowths rimmed on the early inclusion-rich dolomite. Dedolomitization occurs at the tops of cycles especially at the inner marking repeated subaerial exposures.