Diagenesis of Upper Cretaceous Rudist Bivalves, Abu Roash Area, Egypt: A Petrographic Study

Abstract

Upper Cretaceous rudist buildup from the Abu Roash area, Egypt, is characterised by floatstone to rudstone textures. The low diversity of rudist types, the abundant fine-grained carbonate matrix, and the presenceof some coral heads suggest that the investigated buildup was developed under warm, relatively quiet, somewhat protected water conditions, in a shallow shelf area or open lagoon.The rudist shells underwent different types of diagenetic alteration(e.g. cementation, neomorphism, partial silicification, and compaction).Different types of calcite cements are recorded: (a) early marine micritic and bladed cement, (b) early meteoric isopachous and equant calcite, and (c) late meteoric vug-filling calcite of coarse crystalline texture after partial dissolution of the shells. Stabilization of the bimineralic shells took place within a meteoric phreatic milieu, where the aragonitic shell layers were dissolved or recrystallized to more stable diagenetic calcite and the calcitic layers also suffered some recrystallization.Silicification of the shell components is the more important diageneticevent. It postdates the early meteoric diagenetic equant calcite cement and occurred before the precipitation of the late meteoric diagenetic vug-filling cement. Megaquartz and fibrous chalcedony partially replaced the shells. The silica was probably derived from different sources, pressure-solution of quartz and transformation of clay minerals during later compaction of underlying siliciclastic rocks and/or dissolution of silica-producing organisms such as silica sponge spicules and radiolaria. Silicification possibly occurred within a mixing zone environment where meteoric water, which is the carrier of silica, mixed with marine water producing a solution supersaturated with quartz and undersaturated with respect to CaCO3 minerals.