ENVIRONMENT AND DIAGENESIS OF AN UPPER CRETACEOUS BIOCLASTIC OYSTER LIMESTONE BED, RED-SEA COAST, EGYPT

An Upper Cretaceous bioclastic oyster limestone bed, exposed at the Re d Sea coastal strip, has been investigated for its depositional enviro nment and the early diagenetic modifications affecting its components. The deposition of this stratum body marks the end of prevailed euxini c conditions and the setting up of an oxic milieu. This dramatic chang e in depositional conditions is related primarily to the change in the position of oxygen minimum zone during sedimentation. The noticeable negative shift in the delta(18)O-values of the studied skeletal parts (-2.7 to -6.4 parts per thousand PDB) are attributed to some habitat-r elated controls and the dilution of marine water with a great fresh wa ter influx. In addition, mild diagenetic alterations that have affecte d some of these skeletals, as indicated by their enriched manganese va lues and orange-colored luminescence, are also- in part- responsible f or the delta(18)O negative shift. Meanwhile, the negative delta(13)C s ignatures (-2.2 to -5.6 parts per thousand PDB) are probably related t o an upward flux of isotopically-light and reduced pore waters to the bottom water where the oysters are thought to have lived and/or to the reducing conditions during which the alteration of these skeletals oc curred. Moldic porosity is quite common although partly or completely occluded by clear equant low-Mg calcite of marine origin. These submar ine mosaics are probably formed under oxidizing conditions in the phre atic zone as indicated from their non-luminescence character and stabl e isotopic values. The wide overlap range between the oxygen and carbo n stable isotopic signatures of these mosaics and the skeletal particl es may indicate they both are formed under the same conditions provide d little effect exerted by the latters. A promising possibility of hyd rocarbon accumulations in the area east of Qusier (off shore) is expec ted.