DIAGENETIC IMPLICATIONS AND PROVENANCE OF PRE-CENOMANIAN SANDSTONES, SHEIKH ATTIA AREA, EAST-CENTRAL SINAI, EGYPT  ALAA M. SALEM, ABD ELMONEIM T. ABD ELLIAMEED1, IBRAHIM HASSANEIN2, and AHMED M. FOUDA1 Dept Geology, Fac Education, Kafr ElSheikh, Tanta Univ.; 1Dept. Geology, Fac. Science, Tanta Univ.; 2Petroleum Research Inst., Cairo, Egyptز The pre-Cenomanian (Cambro-Silurian and Early Cretaceous) sequence in the Sheikh Attia area comprises intertidal to shallow marine sandstones in its lower part (~ 60 m), and braided fluvial sandstones in the rest of the sequence (~ 110 m). The detrital assemblage of these sediments was strongly modified by diagenesis, largely due to dissolution and replacement of detrital feldspars and rock fragments. Detailed quantitative petrographic study allowed the reconstruction of the original detrital composition and, hence, aided in provenance interpretation. The major detritus for the sandstones was derived from the Arabian Shield with a possible contribution from recycled older sandstones. Diagenetic changes include, approximately in paragenetic sequence, infiltrated clay coatings, cementation by iron oxides, quartz and calcite dissolution of detrital feldspars and calcite cement, and precipitation of kaolinite and late quartz. Early cementation by calcite prevented strong compaction and preserved considerable amounts of macroporosity in most of the sandstones, whereas the absence of early cements in some samples promoted compaction and porosity destruction. The ð18O values (-2.8‰ to +4.8‰) of calcite cement indicate precipitation at temperatures of ~250C from sea water and evaporitic brines. This early-formed calcite predates authigenic kaolinite as well as kaolinitized and dissolved feldspars.