Carbonate and siliciclastic sedimentation in an active tectonic setting: Miocene of the north-western Red Sea rift, Egypt

Abstract

This study, based on extensive French and European Community field projects, compares shallow-marine sedimentation on fault blocks from the south-western margin of the Gulf of Suez and the north-western Red Sea. These synsedimentary blocks have been examined on various scales. On a regional scale, the fault blocks are grouped according to their distance from the western periphery, this factor determining not only the importance of siliciclastic facies relative to carbonates but also the structural behaviour of each block. Blocks attached directly to the periphery have thin, fairly variable sedimentary covers. Others separated from the periphery by one or more half-graben have somewhat thicker sequences often with better developed carbonate platforms, while one example (Gebel el Zeit), situated close to the axis of the Gulf of Suez rift, has very limited coarse siliciclastics and thin shallow-marine carbonates due to relatively high rates of subsidence.

Structural parameters have also been the main factor determining local sediment geometries. Virtually all tilt- blocks are characterized by a variable asymmetry expressed by the direction of tilting (towards the axis or periphery of the rift), vertical relief, and orientation of each structural unit with respect to the general north-north-west-south-south-east axis of the rift. These morpho-structural attributes determine not only the relative importance of carbonate and siliciclastic facies but also the occurrence of specific facies, such as reefs.

The complexity of the sedimentary cover on any given block depends mainly on its tectonic stability. All blocks, irrespective of their position in the rift, exhibit the effects of synsedimentary tilting, generally in the form of multiple reef systems and localized depositional sequences. However, these are best developed at Gebel el Zeit where a series of angular unconformities, stratigraphic gaps and synsedimentary erosion of both pre- and syn-rift sediments are clearly visible. Structural instability on many blocks has also resulted in the collapse of platform margins while major north-east- south-west fault systems crossing certain platforms have also contributed to their demise. These fault systems, oblique with respect to the dominant ‘Clysmic’ north- north-west-south-south-east trend, also served as corridors for coarse siliciclastic discharges exported from the periphery towards the axis of the rift.

Although tectonic movements have dominated mid- Miocene sedimentation, they have not been the sole factor. In spite of marked lithostratigraphic variation among all blocks, an overall palaeoenvironmental change occurs from early-mid Miocene open-marine conditions to mid and late Miocene restriction, culminating in evaporite precipitation. This change may be related either to global sea-level movement, or to regional tectonic adjustments which culminated in the separation of the Red Sea and Mediterranean. Climatic conditions relating both to temperature and wind directions appear to have been significant factors influencing the thickness and geometry of carbonate platform sediments. Relatively thin carbonate buildups and modest platform dimensions may have resulted from the relatively high latitudes of the area during the Miocene, while a predominant north-westerly wind direction, parallel to the axis of the narrow basin, may have contributed to the asymmetric development of reefs and marine sands.

The mid Miocene sediments exposed along the north- western margin of the Red Sea rift clearly demonstrate the intimate relationships between tectonics and sediment geometry, both on regional and on local scales. Because these geodynamics concern the relatively early phases of rifting, they illustrate the tectonic, eustatic and climatic factors governing the distribution of carbonate and siliciclastic sediments with respect to the first phases of individual platform development.