الفهرس 
Central nile valleyOnly 14 pages are availabe for public view
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Abstract
The origin of chert in the sedimentary succession of Egypt is less certain and much debatable. This study attempts to shed some light on the origin of bedded and nodular chert, the source of silica and the depositional environment of chert and carbonate host rocks. Mineralogical, petrographical and chemical examinations were carried out on some selected samples from the Lower Eocene Thebes Formation at three different localities; southwestern Sinai, Quseir-Safaga district and the Central Nile Valley. Field observations have shown that the degree of chertification is variable in the three studied areas. Generally, chertification effect is highly pronounced in the carbonate rocks of Gebel Duwi compared with the other two areas. However, chert occurs in various forms; nodular chert, bedded chert and quartz geodes. Both the nodular and bedded cherts are common in Gebel Duwi section whereas the latter is abundant in southwestern Sinai area. Meanwhile, the abundance of chert in the Central Nile Valley sections is less than both the Duwi and southwestern Sinai areas. Both the nodular and bedded cherts are present, but chert nodules are abundant at Gebel Shaghab whereas bedded cherts are common at Gebel Sheikh Zsa. Mineralogical studies have shown that chert is composed essentially of quartz, calcite and dolomite together with subordinate amount of impurities. The average content of authigenic quartz in the nodular chert, to some extent, is more than in the bedded chert. Authigenic silica occurs mainly in the form of equigranular and fibrous quartz but rarely as opal-CT. Variation in the silica fabric observed at Gebel Duwi section is more than that recorded at southwestern Sinai and the Central Nile Valley areas. Opal-CT is mainly detected in southwestern Sinai area at Gebel Qabeliat and north Abu Zenima sections. Chemical analysis of some major and trace elements (" ~ "a2+" ~ ", " ~ "2+, " ~ "e2+" ~ ", n2+N,a + and K+) has shown that the concentration of these elements is largely affected by post-depositional changes. The marked DROP in the CalMg ratio between carbonate host rocks.