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Mixed Carbonate-Clastic-Evaporite Depositional Systems in Rift Basins: Insights from the Suez Rift

Gawthorpe, Robert *1; Sharp, Ian 2; Muravchik, Martin 1
(1) Dept. of Earth Science, University of Bergen, Bergen, Norway.
(2) Research Centre, Statoil, Bergen, Norway.

Tectono-stratigraphic and diagenetic/reservoir models for mixed carbonate-clastic-evaporite systems in rift basins have been the subject of renewed interest in recent years following significant hydrocarbon discoveries within the syn- and immediate post-rift interval of the South Atlantic. Although facies models are advanced, many are not linked to the growth, interaction and linkage of normal fault populations and the associated rift topography and bathymetry. In this contribution we use outcrop data from the Oligo-Miocene and Pleistocene-Recent of the Gulf of Suez region to develop dynamic tectono-stratigraphic models for mixed carbonate-clastic-evaporite systems in rift basins.

Gross depositional environments are dominated by fringing reefs, coastal sabkhas, and point-sourced ephemeral clastics. Clastic supply to the basin is largely sourced from major rift shoulder drainages and is typically related to infrequent flash-flood events and wind. Winds and tidal currents are axial, parallel to the elongation of the rift, and have a major impact on facies asymmetry. Windward margins are reef-dominated, whereas leeward margins are characterised by clastic sediment tails, creating pronounced north-south facies asymmetry. Isolated carbonate platforms occurring on intra-rift fault block crests or salt diapirs. Fault block bathymetry and degree of tilt control platform extent and morphology. Drowned footwall carbonate platforms strongly influence subsequent facies development due to the antecedent topography of their margins. Commonly evaporite and clastic units lap out against antecedent platform margins.

Systematic facies partitioning within different systems tracts is noted with HST being reef-dominated whereas LST are evaporite-rhodolith dominated. However, it should be noted that evaporite-rhodolith-reef facies transitions are also typical of lateral variability within individual half graben due to structural or stratigraphic confinement (i.e. not directly sea-level controlled). In addition to structural controls on depositional facies and stratigraphic evolution, fracture-controlled diagenesis is important and typically focused along crustal-scale normal faults. This has resulted in secondary dolomitization, silicification and porosity generation (fracture and mouldic) in the syn-rift Miocene succession and pre-rift carbonates.


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