SEQUENCE STRATIGRAPHY OF THE SUBSURFACE UPPER CRETACEOUS TERTIARY SUCCESSIONS NORTHERN WESTERN DESERT, EGYPT. MOHAMED KHALIL and ALAA MOSTAFA  Dept of Geology, University, of Assiut, Assiut 71516, Egypt.; Dept. of Geology, University of Al-Azhar, Branch of Assiut 715124, Assiut, Egypt;  E-mail: mokhalil@acc.aun.eun.eg E-mail: alaamos @ hotmail. com The Cenomanian-Middle Eocene successions in Sifa-1 and Faghur-l wells are studied through a sequence stratigraphic analysis of two composite logs covering four rock units (Bahariya, Abu Roash, Khoman, and Apollonia formations). Four mega-sequences separated by well-defined and pronounced unconformities were recognised. These represent four main depositional regimes, which include 24 depositional sequences, separated by depositional sequence boundaries. These depositional boundaries are believed to represent temporal aerial exposures and/or non-deposition accompanying local tectonics and sea level fluctuations related to the global sea level falls indicated by Haq et al. (1987). Diagenetic features accompanying aerial exposures are documented in the form of meteoric cements, dolomitization, vugs etc. Marine diagenetic features such as cementation, hardgrounds, and micritization are documented in the transgressive and highstand microfacies types. The lowermost major sequence (MS-I) consists of three or four depositional sequences, which are characterised by fluvial to fluvio-marine delta system with gradual transgression, that led to flooding of the former system by mixed siliciclastic-carbonate sequences and then depositing shallow-water carbonate-dominated sequences covering most of the shallow ramp area. The next overlying mega-sequence (MS-2) includes seven depositional sequences, which are represented by shallow-marine carbonate-dominated sequences, that developed during the general gradual regressive phase with intermittent transgressive pulses of the sea during the Turonian-Santonian times. There are several discontinuity surfaces, which indicate gaps in deposition. This unit seems to have been deposited in a shallow regressive phase on the shallow ramp of the Tethyan Sea. The third mega-sequence (MS-3) comprises six (in Faghur-1) and three (in Sifa-l) depositional sequences, which are characterised by shallow-marine facies in the lower part and grades upward to open-marine deeper facies during the Campanian-Maastrichtian times. The uppermost mega-sequence (MS-4) was deposited after a long gap covering most, or all, the Paleocene times, and represented by regional unconformity surface in many parts of Egypt. During the deposition of MS-4 sequences the sea transgressed over the northern Western Desert leading to deposition of regressive shallow carbonate sequences (sub-tidal to shallow-intertidal settings) with many discontinuity surfaces marked by emergence horizons and depositional hiatus with sharp facies changes. Fisher plot curves show three mega-sequences that resulted from third-order sea level cycles of 0.5 to 5.0 m.y duration (10-485 m thick.) and many fourth-order depositional sequences (15 to 125 m thick.). Each depositional sequence consists of a variable number of parasequences of 0.1 to 0.5 m.y. duration that are commonly one metre to 15 m thick. Many of these successions are composed of shallow-water carbonate deposits with common peritidal facies. The increase in thicknesses of the sequences (i.e. increase the accommodation space) in Faghur means increasing the rates of subsidence. Thin sequences in Sifa compared with Faghur mean less subsidence and mostly total uplifting during the Paleocene times.