Carbonate-Siliciclastic Delta Interactions Associated to Normal Fault Growth, El Qaa Fault Block, Suez Rift, Egypt

The relationships between coral-algal carbonate bodies and coarse-grained deltaic lobes are studied from Miocene exposures of the El Qaa Fault Block, Suez Rift, Sinai, Egypt. The succession is composed of a series of conglomerate- and sand- dominated deltaic lobes, intercalated with carbonate bodies of coral and algal origin, with the deltas sourced from the eastern rift shoulder and having an overall W to WNW paleoflow direction. This delta system shows a strong progradation pattern across the El Qaa half-graben, with the youngest units reaching the present western margin of the fault block.

The combination of detailed mapping, logging and digital outcrop scanning (LIDAR) allowed the identification of two basic carbonate morphotypes: Type 1) relatively isolated dome bodies on top of the deltaic lobes, and Type 2) low relief tabular bodies integrating the delta topsets. The first type constitutes massive buildups with irregularly planar bases and convex tops that show an elongated ribbon plan-form, transverse to the deltaic lobe progradation direction. These buildups are made up of coral boundstones at their cores that interbed with grainstones and rudstones towards their flanks. They can reach thicknesses of up to 20-30 m at their crests and thin laterally into thin wedges (< 1.5 m) of bioclastic grainstones and rudstones over distances of 250-300 m. The domes interdigitate with conglomerate facies towards their landward margin whereas they are superposed by fine sandstones and mudstones in a seaward direction. Type 2 carbonates consist of laterally extensive tabular coral boundstones, and rhodolith rich rudstones up to several metres thick that vertically and laterally intercalate with sandstones and conglomerates of the deltaic lobes topsets. These carbonate bodies intercalate with conglomerates and pebbly sandstones in a seaward direction, forming clinoform foresets.

The lower portions of the deltaic succession tend to be dominated by Type 1 carbonates, while the upper section shows Type 2 coral bodies to be dominant. In general, an evolutionary trend can be observed from predominantly siliclastic lobe deposition punctuated by coral colonization and growth, into a more mixed carbonate-siliciclastic depositional environment. The two distinctive coral geometries are observed to be related to changes in progradation and stacking patterns of the deltaic lobes controlled by accommodation space creation caused by hangingwall deformation and subsidence.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California