A Middle–Late Pleistocene palustrine–lacustrine–travertine system (Borj Edouane Unit, NW Tunisia): sedimentology, stable isotopes, and palaeohydrological implications

The carbonate Borj Edouane Unit represents the northernmost documented Middle–Late Pleistocene continental carbonate deposits in Tunisia. The unit consists of two main facies associations, asymmetrically distributed: (1) calcrete–palustrine–laminar crust; (2) microbialites. The first accumulated carbonate formed in the distal zone of alluvial fans (calcretes) and in shallow lakes subjected to periodic exposure (palustrine facies) that occupied the westernmost strip of the basin. The second association formed in permanent lakes (stromatolites: oncoids and planar stromatolites) located in the central areas of the basin, fed by springs associated with a concentration of vertical faults in the eastern area of the basin (travertine facies). At time of deposition, these facies communicated with the lake via a transitional lacustrine–travertine environment (transitional travertine–stromatolite facies). All facies show a restricted range of δ 18 O values around − 5.8 ± 0.5‰, suggesting formation from paleoprecipitation largely of Atlantic derivation, brought about by westerly winds. As for the δ 13 C values, these show a wide range (− 6.4 to 11.7‰), overall relatively high values for all facies, with the lower values corresponding to the calcrete–palustrine association and the higher ones linked to the microbialite facies. The trend in travertines probably mimics an increasing influence of carbon derived from dissolution of carbonate bedrock of the Cretaceous substrate, while that of palustrine samples records increasing respired-C contribution. The sedimentological and isotopic characteristics of the Borj Edouane Unit reflect wetter conditions at the time of deposition, and point to a humid period at the Middle–Late Pleistocene transition.