The Middle-Upper Eocene in the Northern Plateau of the Bahariya Depression, Western Desert (Egypt) and correlation with other Tethyan sectors

Abstract

This study describes the paleoenvironmental evolution of the Middle-Upper Eocene succession exposed along the northern plateau of the Bahariya Depression (Western Desert, Egypt). Based on detailed field observations and facies analysis, twenty-one facies were recognized and grouped into five facies associations: tidal flat to restricted marine (FA1), lagoonal (FA2), ramp crest (FA3), middle ramp (FA4), and siliciclastic-dominated foreshore–shoreface (FA5). These facies associations were developed in a gently dipping mixed siliciclastic-carbonate ramp which was connected with siliciclastic coastal-shelf depositional system. The biogenic assemblages described in the Middle-Upper Eocene deposits show a mixing of photozoan and heterozoan associations, indicating euphotic to mesophotic inner to mid-ramp settings under mesotrophic-oligotrophic conditions. The succession shows a transition from photozoan to heterozoan carbonates and a shift towards inner ramp settings. The formation and the evolution of the northern Bahariya platform reflect the complex interplay of synsedimentary tilting due to post-Middle Eocene inversion phase, high-frequency sea-level changes, transition from warm to humid climate and paleoecological parameters. The sedimentation pattern including distinct lateral changes in facies and thicknesses are largely controlled by tectonics. Long-term sea level changes were responsible for the unconformities associated with major regressions. The Middle-Upper Eocene strata display a characteristic up-section increase in clastic input from the hinterlands through tectonic uplift and/or a change to a cooling climate. The shift to a colder climate, which was a result of a global temperature decrease, led to the maximum northward retreat of the Neo-Tethys Ocean. During the Eocene epoch, carbonates dominated by Larger Benthic Foraminifera (LBF) were prevalent along the entire Tethyan margins. This was linked to a decline in coral reef formations. Comparisons with other southern and western-central Tethyan sectors showed that small coral reef build-ups in widely scattered localities could exist in broad LBF-rich ramps. Consequently, zooxanthellate corals were only able to flourish locally in marginal environments under specific ecological conditions.