Index Geophysics

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

<div>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 presence</div><div>of 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.</div><div>The rudist shells underwent different types of diagenetic alteration</div><div>(e.g. cementation, neomorphism, partial silicification, and compaction).</div><div>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.</div><div>Silicification of the shell components is the more important diagenetic</div><div>event. 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.</div>

Ahmed Sadek M. Mansour

Rudists

Diagenesis

Silicification

Upper Cretaceous

Egypt

Geology

QE1-996.5

Croatian Geological Survey

2004-06-01T00:00:00Z

article

1330-030X

1333-4875

10.4154/GC.2004.03

https://doaj.org/article/e444722cd26e4dad80e39fc8dca23084

Geologia Croatica, Vol 57, Iss 1, Pp 55-66 (2004)

EN

http://www.geologia-croatica.hr/ojs/index.php/GC/article/view/212

https://doaj.org/toc/1330-030X

https://doaj.org/toc/1333-4875

Journal Licence: CC BY