The lacustrine carbonates in the El Gara Basin (Mid-Late Pleistocene, NW Tunisia)

Resumen

The lacustrine carbonates of the Mid-Late Pleistocene Borj Edouana unit occupy the central part of the El Gara Basin and lie unconformably on the Cretaceous basement. The carbonates are fringed by distal conglomeratic facies of laterally coalescing alluvial fans that surrounded the basin. The two main carbonate facies associations are: calcrete-palustrine and lacustrine. Calcrete facies are represented by massive micritic calcretes. Palustrine facies show scattered gastropods and ostracods, channel and crack porosity, intraclastic-clotted-peloidal and brecciated textures. Laminar crust is linked to the first association, and is present in the south and north margins of the unit. The lacustrine association occupies the central areas of the basin and is dominated by microbialite facies. Two main facies are recognized: flat laminar structures (stromatolites) and oncolites with dispersed gastropods, ostracods and charophytes. Some stromatolite laminae are formed by cauliflower-like to knobby structures. The oncolites are included in a peloidal-intraclastic (microbialite remains) matrix with smaller oncolites. The rich-oncolite facies gradually passes to an intraclastic (microbialite remains), peloidal facies with dispersed oncolites.