SEDIMENTOLOGICAL AND PALAEONTOLOGICAL STUDIES ON THE THAMAD AREA, EAST CENTRAL SINAI, EGYPT

Abstract

The Cretaceous!Paleogene sequence overlying the sandstone of the Mallia Formation in the Thamed area is differentiated into the Galata, Wata, Thamad, Rajim, Sudr, Esna and Egma Formations.

Mineralogical analyses indicate that quartz is the main component in the sandstones and clay intercalations of the Mallia Formation. Kaolinite predominates the clay fraction of the claystone and sandstone matrix of the formation, although minor amounts of illite and smectite are present. Most of the clay minerals are of detrital origin. Kaolinite also predominates the clay fraction in the Galata, Wata and Thamad Formations but it diminishes in the Rajim Formation and replaced by smectite. Variable amount of kaolinite, smectite and illite is recorded in the Esna Formation while smectite became the main constituent at the Paleocene/Eocene boundary.

Calcite is the main component of limestones and chalk while dolomite and quartz are the main constituents of dolostones, chert, respectively. Many accessory minerals including opal-CT, pyrite, marcasite, machinawite, hematite, goethite, chamosite, apatite (dahllite and fluorapatite), variscite, celestite, albite, orthoclase, evaporites (halite, gypsum, anhydrite), clay minerals (kaolinite, smectite, illite/smectite, illite, glauconite, hyalosite), brucite and clinoptilolite are identified within different formations.

The sandstones of the Mallia Formation are dominantly quartzarenite with subordinates of litharenite, sublitharenite and quartzwackes. They are mostly derived from continental blocks and rarely of recycled orogen. The •sandstone cements are mostly clays, iron oxides, calcite, halite, barite and rarely quartz. The prevailed diagenetic processes are the infiltrated clay cutans, cementation by iron oxides, calcite and rare quartz overgrowth and dissolution of unstable minerals.

Cementation by calcite precludes early compaction of sandstone and led to preservation of considerable porosity. Dissolution of calcite cements and unstable detrital grains such as feldspars and ferromagnesian minerals resulted in great amount of secondary porosity and good sandstone reservoir. Isotopic composition of calcite cements indicates that the early calcite cement was deposited from normal marine water or brine water diluted by meteoric water at temperature 26° C whereas the poikilotopic late stage calcite type was deposited from meteoric water at temperature 36° C at depth about 630 m.

A great variety of carbonate microfacies are recorded and used for interpretation of depositional environments of different formations. The studied rocks had suffered from many diagenetic processes such as micritization, cementation, compaction and stylolization, replacement, silicification, dolomitization and dedolomitization.