Evaporite-Carbonate-Siliciclastic Interactions in Extensional Settings, El Qaa Fault Block, Suez Rift, Egypt

Muravchik, Martin; Gawthorpe, Rob; Sharp, Ian

The occurrence of sub-salt hydrocarbon plays in the South Atlantic has recently raised attention towards a better understanding of mixed carbonate-siliciclastic-evaporite rift to post-rift successions. Establishing detailed stratigraphic and sedimentary models for this type of systems is critical to reducing exploration risk and improving success. The location, geometry and stacking patterns of carbonate, evaporite and coarse-grained deltas and their interrelationships are studied in Miocene exposures of the El Qaa Fault Block, Suez Rift, Egypt.

Conglomerate- and sand- dominated deltaic lobes were sourced from the eastern rift shoulder and prograded across the half-graben in a W to WNW direction. They are up to 100 m thick and can extend for more than 2 km in dip direction. Evaporite and carbonate bodies occupy specific positions within the lobes. The carbonate units are of coral-algal origin, typically of tabular geometry intercalating with conglomerates at the topset of the deltaic lobes. They can reach up to 20-30 m thick and extend laterally for 0.7 to 2 km. Occasionally, the carbonate beds extend into the foreset, defyining complete clinothems. Evaporite bodies are preserved as massive anhydrite tabular beds of 5 to 40 m thick that are several kilometers in lateral extent. They tend to occur at the bottomset of the deltaic succession. However, in a few cases they are observed to cover the topset of a previous deltaic lobe. This general pattern is observed to be modified in proximity to the fault border system, with complex stacking patterns controlled by normal fault growth and hangingwall deformation. Hangingwall syncline formation and limb rotation control the pinch-out location and thickening of the facies.

Results from integrated stratigraphic, sedimentary and structural analysis allow two main working hypothesis to be developed: i) that evaporites constitute lowstand facies developed during topset subaerial exposure and incision, or ii) they interfinger and are genetically related to the mixed carbonate-siliciclastic clinoform facies, reflecting local restricted basinal facies. Evidence supporting the two hypotheses and their significance in terms of relative sea-level fluctuations and normal fault growth will be presented and discussed.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013