PETROGRAPHY AND GENESIS OF CALCRETE AND SILCRETE FROM THE EASTERN PARTOF QATRANI AREA, WESTERN DESERT, EGYPT MOHAMED I. EL-SAYED Dep. Geology, Fac. Science, Beni Suef, Cairo Univ., Egypt Post-Oligocene deposits in the eastern part of Qatrani area contain calcrete and silcrete intergrade duricrust. The calcrete is observed as channel-fill and formed from spherical to subspherical or even discoidal nodules. The silcrete is present associated with the calcrete or as small hills scattered on the overbank deposits. Two types of silcrete are identified; quartzitic and opaline. Petrographically, authigenetic calcite is the main diagenetic processes that affect the host sediments of the calcrete and is present as micrite and sparry calcite. Micrite is composed of tiny crystals and usually forms the bulk of the nodules as micrite free from any siliciclastic grains or containing few host sediments. It is also observed as agglomeratic fabric and micrite films coating some of the host grains. Sparry calcite is present as randomly distributed patches surrounding the framework grains or as corona-like structure of crust and isopachous cement. Moreover, some biogenic structures as root moulds and casts are found. In the silcrete, three fabrics are identified: grain- supported, floating and matrix. The calcrete in the study area is non-pedogenic to pedogenic in origin. The potential sources of carbonate are the exposed calcareous rocks in Fayium depression and the surrounding areas that transported dissolved carbonate in water to the site of deposition. Another source is the dust fallout but its contribution is limited. Biological processes have also been shown to be important in pedogenic calcrete development. In case of the silcrete, probable local silica sources include the direct dissolution of quartz grains, Oligo-Miocene basalt, and release of silica during calcrete formation and the atmospheric fallout. A decrease in pH is considered to be a probable silica precipitating agent. The calcareous crusts have developed probably during the late Pliocene to early Holocene, when wide climatic variations and gradual increase in aridity occurred. After the formation of the calcrete, silica was precipitated.