Pleistocene calcretes from eastern Tunisia: The stratigraphy, the microstructure and the environmental significance Wissem Gallala a,c, * , Mohamed Essghaïer Gaied b,c , Elhoucine Essefi d,e , Mabrouk Montacer b,c a Département des Sciences de la Terre, Faculté des Sciences de Gabès, Cité Riadh, Zirig, 6072 Gabès, Tunisia b Faculté des Sciences de Sfax (FSS), Route de Soukra, Km 3.5, BP 802, 3018 Sfax, Tunisie Département des Sciences de la Terre, Tunisia c UR: GEOGLOB (Code 03/UR/10-02), Tunisia d École Nationale des Ingénieurs de Sfax (ENIS), Tunisia e UR: Dynamique Sédimentaire et Environnement (UDSE) (Code 03/UR/10-03), Tunisia article info Article history: Received 26 February 2009 Received in revised form 21 April 2010 Accepted 23 April 2010 Available online 4 May 2010 Keywords: Pleistocene Calcrete Geochemistry Mineralogy Genesis Eastern Tunisia abstract This paper is meant to study the stratigraphy, the mineralogy, the microstructure and the geochemistry of Pleistocene calcretes from eastern Tunisia in order to infer the environmental factors intervening in their formation. Samples of eight profiles of Pleistocene calcretes from eastern Tunisia were examined on the basis of a variety of techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-ray Dif- fraction (XRD), chemical analysis and Atomic Absorption Spectrophotometer (AAS) techniques. Then, the obtained data underwent a statistical analysis on the basis of Factor Analysis (FA) and Principal Compo- nent Analysis (PCA). On the basis of field missions, five different horizons have been differentiated from bottom to top of all profiles: nodular, powdery, massive Brecciated and laminar horizon. The mineralogical study shows two minerals categories inversely proportional: calcite and (quartz and the clay). It shows also shows that Palygorskite is the dominant clay mineral. The escarpment edge is capped by a limestone containing fibrous palygorskite. Finally, superficial calcrete are described: a brecciated horizon which occurs in pockets on the plateau surface. This study about eastern Tunisia revealed the occurrence of successive cycles of calcretisation. Pedogenesis, water table oscillation, sedimentogenesis and stromatogenesis are the intervening factors in the calcretisation process. During the Pleistocene, they interfered with each other according to the climatic pulsations. From the studied case, it may be noticed that the formation of each calcrete horizon is the result of a dominating process that takes place during a distinguishable stage. In the first stage, the pedogenic process is developed by palygorskite formation including authi- genic replacement or formation from a precursor mineral, neoformation from the breakdown products of such minerals or neoformation from suitable solutions. In the second stage, the powdery horizon is formed in the slope of the distal zone which presents a drained environment. In the third stage, several diagenetic processes (cementing, compaction, dissolution...) contribute to the formation of the laminar and massive horizon. Since it is exposed to dryness for a long period, the massive horizon is harder and more compact. In the fourth stage, the banding of light–dark in the laminar horizons reflects a dry-wet season alternation seasons. Dark beds are formed by the stromatolitic cover were developed dur- ing the wet season, whereas light beds were developed in an extremely arid climate argued by the pres- ence of the detrital grains. In the fifth stage, the brecciated horizon, which occupies the channels, is formed by well rolled concretions, which present a dismantling material of Early and Middle Pleistocene calcretes after the Post-Villafranchian compressive phase. Thus, calcretisation seems to have been con- trolled by periods of uplift and stability of the slope, given that calcrete formation might be inhibited by the activation of the sedimentation of colluvial materials as a consequence of the tectonic activity. We also suggest that groundwater and biological activity may play a significant role in the development of pedogenic, sedimentological and polygenetic calcrete cycles within the same sedimentary basin. The alternation of dry and wet climatic periods may be responsible for the calcrete genesis. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction From a climatic point of view, calcretes are typical surface for- mations of the semi-arid and subhumid landscapes (Estrela and 1464-343X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2010.04.009 * Corresponding author at: Faculté des Sciences de Sfax (FSS), Route de Soukra, Km 3.5, BP 802, 3018 Sfax, Tunisie Département des Sciences de la Terre, Tunisia. Tel.: +21622874565; fax: +21674274437. E-mail addresses: gallala_wissem@yahoo.fr (W. Gallala), moh_gaied@yahoo.fr (M.E. Gaied). Journal of African Earth Sciences 58 (2010) 445–456 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci