Anatomy and Weather Induced Internal Heterogeneities of a Pleistocene Carbonate Coastal Dune (Rejiche Formation, Southeast Tunisia)

Hasler, Claude-Alain¹, Gregory Frébourg¹, Eric Davaud¹ (1) University of Geneva, Geneva, Switzerland

Carbonate wind deposits (eolianites) form large porous bodies, which are of significant economic interest as they may constitute aquifer or hydrocarbon reservoirs. The Rejiche formation is characterized by shallow-water to eolian, ooid-rich carbonate deposits. In the el Bibane lagoon area (southeast Tunisia), this formation builds a coastal belt over 40 km long and several hundred meters wide, and restricts access to the open sea from the shallow-water lagoon. These Pleistocene deposits (istotopic stage 5e) have not been submerged by the sea and have been cemented early under vadose conditions. The Rejiche formation has been used for building stones since the Roman times.

The unused quarry of Slob ech-Chergui has ideal outcrop exposures in the eolian facies that reveal numerous vertical (both perpendicular and parallel to the coastal belt) and horizontal sections. They allowed the precise mapping of the microfacies that comprise these fossil dunes. Microfacies were defined according to depositional processes such as grainflow, grainfall or wind ripples, which relative abundance depends on weather conditions. These Facies have their own diagenetic evolution as well as specific ranges of porosity (primary or secondary) and permeability.

Using geographic information system software, a 3D distribution of microfacies was obtained, which enabled us 1) to reconstruct the dunes' evolution through time in order to use it as a paleoclimatic record (wind directions, rain occurrences, vegetal cover) 2) to quantify the microfacies relative abundance and the internal petrophysic heterogeneities for this kind of deposit, 3) to predict the internal heterogeneity variations due to weather changes.