Forms and deposits of the continental quaternary of the Saharan margin of Eastern Maghreb (tentative synthesis) Jean-Louis Ballais, Hédi Ben Ouezdou To cite this version: Jean-Louis Ballais, Hédi Ben Ouezdou. Forms and deposits of the continental quaternary of the Saharan margin of Eastern Maghreb (tentative synthesis). Journal of African Earth Sciences, Elsevier, 1991, 12 (1/2), pp.209 - 216. 10.1016/0899-5362(91)90070-F. hal-01562556 HAL Id: hal-01562556 https://hal-amu.archives-ouvertes.fr/hal-01562556 Submitted on 15 Jul 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Journal of African Earth Sciences, VoL 12, No. 1/2, pp. 209-216, 1991. 0899-5362/91 $3.00+ 0.00 Printed in Thailand @ 1991 PergamonPress pie Forms and deposits of the Continental Quaternary of the Saharan margin of Eastern Maghreb (Tentative Synthesis) J. L. BALLAIS*and H. BEN OUEZDOU** *Facult6 des Sciences Humaines et Sociales, 94, Boulevard 9 Avril 1938, 1007 Tunis, Tunisie **Centre des Sciences de la Terre, LN.R.S.T., B.P. 95, 2050 Hammam-Lif, Tunisie Abstract- The Saharan margin of Eastern Maghreb extends from Auras in the West to the Gulf of Gab~s in the East. The Plio-Quaternary boundary is very complex in the West where it is characterized by the persistence of Tertiary sedimentation with the development of ablation forms, presence of crusts and the formation of a lacustrine system. This is followed by the development of accumulation forms during the Middle Pleistocene and mostly during the Upper Pleistocene to Holocene with contemporaneous diminution in the gra~ulometry of the deposits. The evolution of gyperetes at the expense of calcretes crust suffered a partial reverse during the Middle Pleistocene. At this time a lacustrine phase settled down in the Algero-Tunisian Chotts (sebkhas). The end of the Middle-Pleistocene corresim~ded with a major climatic break with the disappearance of the calcrctes. By the end of the Upper Pleistocene, the gylx~etes attained their maximum development and latter disappeared during the Holocene. R~sum~ - Ce brian provisoire s'appuie sur les connaissa~ces acquises depuis 25 ans dans les Auras, les Nemencha, la r6gion des grands Chotts alg6ro-tunisiens, le Dahar et la plaine c&ih'e de Gab~s. I1 permet de mettre en 6vidence roriginalit6 de la charni~re Plioc~ne-Quatemaire et le rSle des variations climatiques dans revolution quatemaire. La eharni~re Plioc~ne-Quatemaire monlre,d'une part, lapersistance de certaines earact~ristiques tertiaires (continuit~ de la s&iimentation : d6tritique darts la plaine c.&i/~re, laeustre darts la r6gion des grands Chotts) et, d'autre part, 1'apparition de nouveiles caract6ristiques (formation de crofites gypseuses dans lar6gion des grands ehotts et formation de crofites calcaires ailleurs). Les formes sont plus vari6es h rOuest (pseudo-rafia, glacis, coul6es boueuses) qu'~ rEst (p6diment ou glacis). Une r6duction du nombre des niveaux s'observe 6galement de rOuest (3 niveaux) ;~ rEst (1 seul niveau). Au Pl~istoe~e moyen, les formes d'accumulation (glacis d'accumulation dans les Aur/~s, terrassedans le Dahar et dans laplaine c6ti~:re) commencent ~se substituer aux formes d'ablation (glacis d'ablationcouvert). AuPl6istochae sup6rieur- Holoc~e, c.eRe tendance se g6n6ralise (terrasse, c6ne alluvial et 6pandage) ~ rexception de la r6gion des grands Chotts tunisiens. De lam~me facron, la granulom6trie des d6p6ts, grossih'c au Pl6istoc~neinf6rieur et moyen, devient fine au Pl6istoc~ne sup6fieur (avec un gradient W-E) et ~ dominante fine h l'Holoc~ne. Enfm, r6volution g6ochimique montre le d6veloppement progressif, au Pl6istoc.~ne moyen et surtout au Pl6istoc~e sup~eur, des crofites gypsenses 0imit6es aux conf'ms des grands Chotts au Pl6istoc~ne inf6rieur) aux ddpens des crofites calcaires; en effet, si les deux types de crofites altement encore au Pl~istoc~e moyen, les crofites gypsenses dominent totaleanent an Pl6istoc~ne sup6rieur (sauf dam les Matmata) avant de disparaitre au cours de rHoloc~ne. Cette 6volution met en 6vidence rexistence d'une coupure climatique fondamentale/, la fm du Pl6istoc~ne moyen et d'autres moins importantes t la fin du Pl6istoc~e inf6rieur et ~ la fin du Pl6istoc~ne sup6rieur. L'absence d'une phase lacustre au P16istoc~e moyen posait donc un probl~me majeur et se trouvait en contradiction avec les earact6ristiques elimatiques de cette p&iode. Des datations radiom6u'iques (T/U) ont pennis r6cemment de conf'Lrmerrexistence d'une telle phase. INTRODUCTION zones that are probably due to Quaternary climatic variations as well as differences in other regional Since the last 25 years, knowledge of the parameters. continental Quaternary of the Saharan margin of Eastern Maghreb has increased considerably. FROM THE SOUTHERN PIEDMONT It is now possible to attempt a review of the OF THE ~ TO THE GULF OF GABES: Quaternary geology of the region extending from REGIONAL PRESENTATION OF THE Aures in the west to the Gulf of Gab6s in the east CONTINENTAL QUATERNARY (see Fig. 1). The area can be subdivided into six distinct morphotectonic zones as follows: Aur6s, The Southern Piedmont of the Aur~ (Ballais, Nemencha, Tunisian saline lakes or Chotts, 1984} Dahar, Matmata basins and the coastal plain of The Pliocene-Quaternary boundary - The end Gab6s. This study, reveals the similarities in these of the Pliocene sedimentation ( 1000 m total thick- 209 210 J.L. BALL~Sand H. Bss O~u .." . , J ,.. I /" c ' : ,-'" ,' 'k---~~.,a ~ ..... ' ~ !~ ~ ' '. ."'i ,, .." .." .." "~ ) .... i , ~. "s,,f,-,, L/ Ii </¢"o,, 7,-,..~ ~",.. ...... / ~,, ..... ~ .-.,~-- ~ ,j,-" I ~ '~a (_, <"-,x~~, . .o.~ ~ . Fig. I. I~catlon map. I - main reliefs, 2 - limit of morphotectonic zones. Fig. I. Carte de IocalisaUon. 1 - reliefs principaux; 2 - limite de zone morphotectonique. ness) is characterized by 200 to 300 m of well represented. It is generally an ablation glacis- conglomerates (Guiraud, 1973) probably fan covered by blocks and pebbles sometimes contemporaneous with the first lacustrine strongly consolidated by a calcareous cement. A phase in the Melrhir and Merouane Chotts. As second generation of mud flows is emplaced with well as the preceeding deposits, they are affected the same characteristics as the first. by the tectono-orogenic phase of the Lower The Middle Pleistocene - It is during this period Quaternary. that the glacis attained their maximum extension. A convex-shaped pseudo-rafia was locally The mud flows dissapear as the terraces develop deposited later with a length of 1 km, width of and the eolian sediments accumulate in the west. 20 m at the upstream end and a few meters at the The glacis IV is well developed and is often an downstream end. It consists of blocks that can accumulation glacis-fan (of about I0 m of large attain several cubic meters, often round, loose, stones, bedded, alternating with silt, sands and covered by a calcrete. By a dip of 6% it truncates gravels). It grades laterally into "6pandages" or the Cenomanian limestone which have been cover ablation glacis. It is replaced by the last vertically redressed, over 300 m above the beds of series of mud flows still several kilometers in the present day wadis. length but only 10 to 20 m thick with no terminal The Lower Pleistocene - The Lower Pleistocene crust. is characterized by alternation of glacis and mud The pre Middle Paleolithic Glacis {III) is less flows. The oldest Glacis (Glacis, VI) younger than developed. It is an accumulation glacis-fan or a the LowerAcheulian (Ballais, 1984), is represented glacis - terrace and occasionally a covered ablation by some scattered mounds with several meters of glacis-fan. It is only covered by a gypcrete in the cover consisting of large blocks close to the relief. west and passes laterally into alluvial fans or a very This covered ablation glacis-fan changes eastward restricted terrace. Finally, a lacustrlan phase could into accumulation glacis or fan, The characteristic be produced in the Melrhir and Merouane Chotts calcacrete at the top in the west is .llaitially covered {Ballais et a£, 1985}. by gypsiferous encrusting and then by a gypcrete The Upper PleiStocene - This was a period of which remains exclusive south of Nemencha. Mud limited deposition and geomorphologic forms. flows of several kilometers long and several tens of Glacis II, the last glacis-terrace of same age, often meters thick containing large blocks of several constitutes the principal terrace. It generally cubic meters and with a calcrete at the top are sub- possesses a gypcrete. Downstream it grades into a sequently emplaced. The second glacis (Level V) is much finner fan. A second lacustrian phase is thus Forms and deposits on the Continen~l Quaternary of the Saharan margins (Tentative Synthesis) 211 developed in the Melrhir and Merouane Chotts Cartographic studies and recent geomorphologic (Coque et Gachelin, 1975). observations has led to new data on this region. In The Holocene - This is a period of complex fact around the piedmont south of Jebel Chebket deposition (Ballais, 1986) characterized by the Bouloufa (North chain of the Chotts), the cover of setting up of new eolian accumulations through- the highest ablation glacis (Glacis 3) is characteriz- out the Lower Sahara (Ballais et BenA~,ouz, 1987). ed by a calcareous cement with a calcrete (Abbes et It is followed by the accumulation of a terrace (often a/., 1986).