Structural and stratigraphic controls on diagenesis within carbonate platforms in rift basins

The depositional architecture of carbonate platforms on the crest of rotated footwall fault blocks has been well studied, but with little attention to the controls on the structural and diagenetic processes that affect these platforms. This is critical, since porosity and permeability can be modified significantly by diagenesis from marine, meteoric fluids and hydrothermal fluids circulating within interparticle pore systems and along faults throughout platform growth, burial and uplift.

In the Suez rift, an aborted arm of the Late Oligocene to Miocene Red Sea Rift, pre- and syn-rift carbonate platforms are exposed within tilted and rotated fault blocks. Dolomitized areas are focused along major block-bounding faults, particularly in areas of significant structural complexity, such as rift-scale accommodation zones. Detailed characterisation of one of these platforms, the Hammam Fauran Fault (HFF) Block, western Sinai, shows that dolomitisation of the pre-rift Eocene Thebes Formation occurs in two principal forms:

• stratabound dolomite, that preferentially replace remobilised facies (debris flows and grainstone turbidites) and extend discontinuously away from the HFF for up to 2km. These bodies are offset by the Gebel Fault that formed during the earliest syn-rift, implying dolomitisation at rift initiation. 87Sr/86Sr isotope data support this, with a late Oligocene age.
• massive, non-facies-selective dolomitised pods, up to 500m wide, confined to the fault damage zone and interpreted to have formed at the rift climax. This is supported 87Sr/86Sr isotope dates equivalent to Miocene seawater.

Stable isotope and trace element data indicate dolomitisation from contemporaneous seawater at temperatures up to 120˚C. Fluid flow models show that the proto-HFF and early-rift intra-block faults could have provided up-fault conduits for fluids convecting within the Nubian Sandstone aquifer at rift initiation. Since the HFF tipped out within the sediment pile, the permeability architecture favoured discharge of fluids via the Thebes Formation to form stratabound dolomite bodies. At rift climax, the HFF breached the surface, and seawater convection along the fault plane and within the damage zone formed the massive dolomite pods.

The results of this study indicate dolomitisation can occur on carbonate platforms in rift basins at zones of structural complexity. In particular, the massive dolomite bodies are located within the linkage zone of two segments of the HFF against the Zaafarana regional transfer zone, which enhances structural complexity and permits vertical fluid flow. The shape, size and distribution of the geobodies were therefore strongly influenced by the timing of fluid flux in relation to faulting along basement-connected lineaments. The dolomite and limestone underwent further diagenetic modification by marine and meteoric processes during uplift and footwall rotation. The result is complex, multi-scale, fracture-connected pore networks, influenced by the extent of dolomitisation.