Forms and deposits of the continental quaternary of the Saharan margin of Eastern (tentative synthesis) Jean-Louis Ballais, Hédi Ben Ouezdou

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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￿

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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 (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 (glacisd'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-

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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 (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). A conglomerate with calcareous cement subdivided into 2) several meters high or stony (capping glacis 3?) truncate the Lower Cretaceous fans which downstream grade into large extensive sandstones of Draa el Ghorifa (Southern bank of and more functional playas. FeJaJ ). Elsewhere, in certain sectors like the southern piedmont of the Meflaoui Chain, five THE REGION OF THE LARGE Quaternary levels can be recognized. (Ben TUNISIAN CHOTT8 Ouezdou et Zargouni, 1988). A low terrace and a (Coque, 1962) glacis, with coarse grained materials without encrusting succeed two Glacis-fans with The Pliocene-Quaternary boundary gypcretes. Finally, a conglomerate with calcareous- The Tertiary ended with the deposition of the sandstone cement with Levallois industry Continental Terminal Complex consisting of seve- constitutes the oldest level. ral hundreds of meters of sandstones, clays and conglomerates. Three regional types can be THE WESTERN MARGIN OF DAHAR distinguished. At the top, gypsiferous deposits (Coque, 1962) containing Cerastoderma glaucum represent- ing a first "lagoonal" phase dated as Lower The Lower Pleistocene Vfllafranchian from mammalian fauna at Ain During the Lower Pleistocene a pediment Brimba. These deposits were often deformed developed on the Senonian limestones with a slight during a Villafranchian tectono-orogenic phase. dip towards the west. A zoned fragmented calcrete (reg) overlies the calcareous substratum. Near the The Middle Pleistocene axes of the main wadis, the Lower Pleistocene It is characterized by the development of glacis 4 consists of a very consolidated conglomerate with and 3. Glacis 4 is limited to a few mounds in front angular calcareous stones (Coque, 1962). of Jebel Tebaga and is covered by gypcrete. The ablation Glacis 3 (0,50 m to 6 m of heterometric The Middle Pleistocene cover consisting of the most rounded and the less The Middle Pleistocene consists of a terrace fiat pebbles in all the 4 glacis), consits mainly of about 5 m high with coarse-grained deposits mounds or interfluves covered by a gypcrete. It containing pebbles 10 to 20 cm long cemented into grades into a terrace near the wadis. The cover a conglomerate. Some of the pebbles are fragments revealed an Acheulian industry. ofcalcrete of Lower Pleistocene age (Coque, 1962). Recent observations (Ballais, 1984: BaUais etal., The Upper Pleistocene 1988) distinguished the following: Glacis 2 is generally the most developed. The - a terrace with a complex composiUon consist- cover (0,50 m to 5 m) consists of fewer rounded and ing of silts sometimes gypseous and rare relatively more fiat stones than the precedent. It conglomeratic lenses with two calcrete separated passes laterally into a terrace. A Mousterian by a gypcrete. This unity is probably Middle industry characterizes the base of the cover while Pleistocene. a bladelet industry is present towards the top. - a terrace, about 4 m high consisting of lenses of A second "lagoonal" phase with Cerastoderma conglomerates or coarse-grained friable g/aucum is contemporaneous with the develop- deposits, alternating with silts and calcareous ment of this glacis. The gypcrete which later seals concretions. This unity is probably Upper the cover, equally fossilizes the dunal accumul- Pleistocene. ations south of the Chotts. - a terrace with thick deposits of about 2 m consist- ing of coarse-grained lenses and sandy with silty Holocene lenses. This unity is probably of Holocene age. Glacis i developed during the Holocene. It is well developed at the foot of the high chains and around THE MATMATA BASINS the base level but is also a simple terrace along the (Coud~-Gaussen et al., 1984) wadis. Its coarse-grained cover (0,50 m to 2 m) does not contain a gypcrete but locally silty Peridesertic loess deposits consisting of very fine gypseous sands of I to 2 m thickness. sands and silts mainly quartzeous are deposited 212 J. L. BALLAISand H. BEN OUEZDOU within the basins on the Matmata Plateau. These broken up towards the coast where it is replaced by loess with a thickness that can attain 20 m, a more recent gypcrete. contain 4 to 5 altered layers. This represents BCa accumulation carbonate horizon with calcareous The Middle Pleistocene concretions of different facies (nodules, friable A high terrace lies within the Lower Pleistocene crust, more or less continuous thin flagstone). glacis and corresponds to a deposit of 5 m An upper Pleistocene age is attributed to this maximum thickness, very heterogenous loess. In fact, the most recent three paleosoils consisting of blocks, pebbles and fine to coarse levels have been dated by 14Cas between 10 000 to sands. Semi-fiat and rounded pebbles are pre- 32000 years B.P. The most ancient two levels are dominant within this deposit originating from situated time-wise above the 14C limit. Towards the generally clacareous and dolomitic terrain the western and eastern borders of the Matmata of the Matmata Plateau. The red and beige Plateau, the loess diminish in thickness and silt lenses with calcareous concretions of vary- become more gypseous. Unconformities re- ing degree of development are intercalated within presented by erosional glacis replace the calcretes the coarser-grained deposits. The degree of and the paleosofls towards the east whereas to the consolidation increases upwards. At the top of west gypcretes progressively appear. the deposits, a calcrete developed in layers or as However, recent studies in and around the slabs or more frequently as conglomerate. In Matmata Plateau (Kammoun et Ben Ouezdou, places, silts also contain a calcrete and system- 1984; Ben Ouezdou et Zouari, 1986; Zouari etal., atically superposed coarse-grained deposits and 1986, Ben Ouezdou, 1988) do not entirely confirm the cover of the Lower Pleistocene glacis. The these stratlgraphic datings particularly as the Middle Pleistocene dating of this complex is due 14C datings were obtained from calcareous to final Acheulian industry and the existence of concretions which are extremely unreliable. Mousterian industry at the base of the middle Indeed, the nodules silts and calcretes are al- terrace (see below). ways superposed at the top of the high terrace without ever fit together with it. This high terrace The Upper Pleistocene and the Holocene dates to final Acheulian (Ben Ouezdou, 1986). A middle terrace or an accumulation glacis The silts and sands were probably deposited be- is enclosed within the two most ancient levels fore the downcutting of the high terrace, to- and consists of deposits of variable thickness (3 wards the end of the Middle Pleistocene. The main to 15 m) made up of silty material and gyps- depositional phase of the silts and sands was eous clays. Coarse-grained lenses (rudites) are probably at the end of the Middle Pleistocene. intercalated within this fine series. Occasional- This does not exclude the possibility of silt depo- ly, an erosional level often marked by gypcrete sition during the Upper Pleistocene (Ben Ouezdou, pebbles helps to distinguish two members with- 1986). in this complex. At the top, a crust with variable thickness develops (ranging from several centi- M GABES COASTAL PLAIN meters to 1,5 m). The visible base of the depo- (Ben Ouezdou, 1984, 1986, 1987) sits dated as 28 000 years B.P. (NC) revealed a Mousterian industry (Gobert,1962; Page, 1983; The Pliocene-Quaternary boundary Fontes et aL, 1983). Towards the top, a lithic The Pliocene-Quaternaryboundary in the G ab~s industry was ascribed to Ib~romaurusian coastal plain does not display any unconformity. (Gobert, 1962). Recently it has been ascribed The continuity of detritic continental sediment- to a southern Capsian facies (Ben Ouezdou et al., ation {clays, sands and conglomerates) results in a 1986). The deposition was completed about 8000 gradual indistinct passage between the Tertiary years B.P. (Lower Holocene) (Fontes et aL, 1983). and the Quaternary. A lower terrace, which has been reduced to a bench upstream, generally increases in import- The Lower Pleistocene ance downstream. The deposits which are about 2 An ablation glacis cut down into gypseous clays, m in thickness become increasingly coarser-grain- sands and conglomerates is probably of Lower ed longitudinally. It consists of pebbles, gravels Pleistocene age. Strongly dissected by the hydro- and occasional blocks at the exit of the mountain graphic network and reduced to scattered mounds system becoming increasingly fine grained (sandy) and interfluvial patches, this glacis is covered by a downstream. The deposition which is dated at the conglomerate with calcareous cement of about base as 5195 + 105 years B.P. (Steinman und 0.5 m thickness. A salmon calcrete overlies the Bartels, 1982), continues during the historic conglomerate. It is relatively well preserved in period as evident by the discovery of fragments of the piedmont zone but becomes more and more pottery at the top of the terrace. Forms and depositson the ContinentalQuaternary of the Saharan margins (TentativeSynthesis) 213 PROBABLE CORRELATIONS Aur6s. On the contrary, at the same period, the calcretes covered all the other surfaces studied. General Characteristics (see Table} Finally, the types and number of forms become The pliocene-Quatemary boundary is character- progressively simple from west to east (see Table}. Izcd by itscomplexity {Riser, 1979}. It stillpresents It is strongly probable that this simplification is the characteristic of the Tertiary era particularly related to regional features due to tectonism, as with regard to the continuation of the sediment- well as to altitude and energy of relief. ation at the Continental Terminal summit. Two The Middle Pleistocene is generally characterized facies can be distlnguishcd: a fluviatile detrltic by the development of accumulation forms and facies in the Gab6s coastal plain and the Aur6s- gypcretes. The accumulation glacis, "6pandages", Nemencha piedmont; as well as a lacustrian facies fans {Aur6s, Nemencha) and terraces (Dahar, Gab6s in the centre of the basin of large Algcro-Tunlsian coastal plain} gradually replaced ablation forms saline lakes. On the other hand, characteristics (covered ablation glacis} which are located in the typical of the Quatemary era are present, region of the great Tunisian saline lakes. The particularly with regards to thc crusts which gypcretes spread globally at the expense of the correspond to a characteristicregional repartition calcretes in a rather complicated manner. This is or distribution. The gypcrete at the top of the obvious toward the west where there is a tendency Continental Tcrminal is centered around the to replace calcretes on the south Aurasian pied- Tunisian salt lakes, Nemencha {Cabot, 1971}, and mont. On the contrary, on the west-ern flank of the eastern part of the( southc)-n picdmont of Dahar, a gypcrete is intercalated between two

Synthetic table of continental Quaternary forms and deposits of the Saharan margin of eastern Maghreb. Tableau synth6tique des formes et d6pfts du Quatemalre continental du Maghreb oriental.

Periods and Age Piedmont south Piedmont south Region of the great Dahar Ma~nata basins Gulf of Gab~s in years BP of Attr~s of Nemencha TunisianChotts Coastal Plain

Holocene Terrace alluvial fan, coarse Glacis 1, lower Terrace, fine terrace, alluvial deposits with Terrace, coarse to fine deposits and fine "6pandage" 10000 fan coarse beds. Gypcrete, terrace 30 0007 G y p crete Gypcrete accumulation Lacustrine h a s e Terrace, coarse Silts with Cover of ablation calcareous glacis, gypseous Upper concretions fine deposit with Terrace, f'me and coarse deposits glacis, coarse to fine deposits Pleistocene deposit with sandy- intercalated gypseous matrix gypcretes 90000 Gypcrete Gypcreteand calcrete Terrace, coarse LACUSTR IN E PHASE Cover 150 000 : of ablation glacis, Cover of ablation Cover of ablation Terrace, coarse glacis, coarse glacis coarse conglomerates and fine deposits Silts with deposits with calcareous deposits cement Middle with intercalated deposits with FLEXU R A T I O N calcareous Calcrctc Gypcrete cover of Pleistocene gypcretes and calcrete and gypcrcte, Gypcrete, cover ablation glacis concretions accumulation of ablation glacis glacis,coarse coarse deposits. calcretes 700 000? deposits. Major tectonic Rejuvenatedfaults Calcrete, Gypcrete, phase north cover of ablation cover of ablation of the Chotts glacis coarse glacis, coarse Calcrete, cover Lower deposits with deposits with Calcrete, sandy-calcareous calcareous- malrix. gypseous matrix. Conglomerates, of ablation glacis, pediment and Calerete, cover silts with conglomerate Gypcrete, cover of ablation glacis, conglomeratic of ablation glacis, coarse, deposits intercalated coarse deposits with sandy- terrace Gypseous clays, with calcareous- Pleistocene calcareous matrix. gypcretes gypseous matrix Pseudo-rafia conglomerate

Major tectonic phase and sand Pliocene L a c u s t r i a n p h a s e 214 J. L. B~s and H. BEN Otw.zr~ou calcretes and on the southern piedmont of the units. The erosional forms followed major tectonic FeJaJ saline lake, a calcrete, on the other hand, phases in the piedmont south of Auras and consolidates the cover of the glacis 3. However, in Nemencha as well as in the region of the great the Gab~s coastal plain and the Matmata troughs, Tunisian Chotts. The overlap in time between the there is also the formation of calcretes. Dahar-basins of Matmata-eoastal plain of Gab~s The Uppermost Pleistocene-Holocene is on the one hand and the piedmont south of Auras characterized by the predominance of and Nemencha as well as the region of the great accumulation forms (terraces, alluvial fans, Tunisian Chotts on the other hand, may be "~pandages', etc..) throughout the flank of the explained by the age of different geological struc- Sahara with the exception of the region around the tures. As the ftrst group Is relatively old, the second large Tunisian saline lakes. One form only can be group is relatively young. Within the second group, distinguished during the Upper Pleistocene in a the overlap in the break between the great general sense. This is a terrace. Similarly, the Tunisian Chotts and the piedmont south of Auras gypcretes totally dominate during the Upper and Nemencha may be explained by the overlap in Pleistocene with the probable exception of the the beginning of the major tectonic phase (see Matmata troughs which are characterized by silts Table}. with calcareous concretions. These gypcretes From the sedimentologic point of view, the disappeared during the Lower Holocene. The deposits are generally coarse-grained (rudltes) granulometry of the Upper Pleistocene deposits during the Middle Pleistocene. On the contrary, becomes finer following a west-east gradient and they become dominantly fine (lutites or arenites persists during the Holocene with the exception of < 500 ]am) during the Upper Pleistocene and the terrace I ofAuras and glacis I of the large Tunisian Holocene with the exception of the great Tunisian saline lakes. Finally, a lacustrian phase that can Chotts. This also constitutes an additional be attributed to the Upper Pleistocene develops evidence in support of the originality of the within the basin of the Algero-Tunisian large saline region of great Tunisian Chotts. It also confirms lakes. the existence of a major climatic break at the end of the Middle Pleistocene. The Middle DISCUSSION Pleistocene is characterized by a climate which is more humid and aggressive than that of During the Quaternary, the passage is generally the Upper Pleistocene. The Middle Pleistocene from levelling forms to accumulation forms. In- was a lacustrian phase which prevailed deed, covered ablation glacis or less frequently, throughout the Algero-Tunisian Chotts system pediments are replaced by terraces, alluvial fans (Ballais et al., 1985). T/U datings (Fontes et al., and "6pandages". 1987) confirms this and permit the identification of A prlor~ this type of substitution could only be a lacustrian phase during the Middle Pleistocene the effect of shortening of the evolution time after ( 150000 - 90000 years B.P.) as in Wadi Ash-shati the Lower Pleistocene. This does not mean much (Gaven, 1982) and as in the west ofAlgeria (Causse as it is not possible to conclude that the genesis of et al., 1988). the accumulation forms is more rapid than that of From the geochemical point of view, the time the erosional forms. In addition, it is not also range of the distribution of the two basic types of possible to conclude that this genesis is a neces- crusts and encrustings reveal a high degree of sary stage in the development of the erosional complexity and evidence of the importance of forms. regional parameters. Analysis of the distribution However, even if this substitution is general, it is in time and space of these two types of crusts lead not concomitant everywhere. This morphologic to the following conclusions: breake overlaps in time into three phases in the six - the Matmata basins experienced only one type of regions. In effect, for the entire Gab~s coastal crust {calcretes) particularly during the Middle plain-basins of Matmata-Dahar, the break is Pleistocene with the possibility of movements of situated between the Lower Pleistocene and the carbonates during Upper Pleistocene. This Middle Pleistocene. On the other hand, for the region constitutes a special case on the scale of southern piedmont of Auras and Nemencha, the the studied zone because of the absence of passage from erosional to accumulation forms gypcretes. occurred at the end of the Middle Pleistocene - - In the Gabes coastal plain, there was a definite whereas in the region of the great Tunisian Chotts, break between the Middle Pleistocene with this took place at the end of the Upper Pleistocene. calcretes and the Upper Pleistocene - Lower This highlights the originality of the great Tunislan Holocene with gypcretes. This change may be Chotts. This difference could be explained by a related to a climatic change with a tendency difference in the tectonic evolution of the three towards aridity. This is because of the necessity Forms and deposits on the Continental Quaternary of the Saharan margins (Tentative Synthesis) 215 for less water in the mobilization of calcium (1969) in the Algerian Sahara, Couvreur (1978) in sulfate with reference to calcium carbonate. limestones of the Central High Arias and Riser - Dahar and the Auras piedmont experienced only (1978) in the Jebel Sahrho. It certainly confirms one type of crust during the Lower Pleistocene. the conclusions of the studies in the eastern This is the calcrete. During the Middle Maghreb. During the Lower Pleistocene, the Pleistocene, the two types of crusts alternated mountain sectors provided the rafias or the major without any visible break. solifluxion flows as well as the glacis or terraces - Both the great Tunisian Chotts and the piedmont with calcretes, whilst a lacustrian phase with south of Nemencha are characterized by the Cerastoderma g/aucumwas taking place in Touat development of gypcretes from the Lower and Ahnet. The Middle Pleistocene also witnessed Pl~istocene to the Upper Pleistocene with rare the development of accumulation forms such as appariUon of calcretes. the conglomeratic terrace of northern Sahara fans - The alternation of both types of crusts during the and mountain terraces with calcretes. From the Middle Pleistocene probably confirm the Upper Pleistocene, the fans and terraces are pre- complexity of the climatic evolution. If this dominantly detriUc, terrlginous, fluviatile and evolution is characterized by a general tendency fluvio-eolian deposits silt-grained and clayey towards aridity during the Quaternary without (< 60 pm). any regional change in the lithology (Coque, It is also necessary to stress the importance of 1962; Ballais, 1984; Ben Ouezdou, 1986, 1987), the Middle Pleistocene period and the passage to the situation is different during the Middle Upper Pleistocene in the Saharan flank of east- Pleistocene when at least one humid fluctuation em Maghreb. In general, the end of the Middle occasionally reversed this tendency. Pleistocene is marked by fundamental breaks in The crust studied in this paper cannot be morphology, sedimentology and geochemistry. regarded as of purely pedogenetic origin and Morphologic break is witnessed by the disappear- therefore intimately related to the characteristics ance of the big ablation glacis, which were of the substratum. On a regional scale, it has been principally succeeded by terraces. The sedimento- well established in the past that most of the logic break is marked with the end of the exclusive gypcretes, particularly those which directly overlie predominance of coarse-grained deposits a non-gypseous subtratum (often calcareous) are (rudites). Finally, the geochemical break is due to fixation by vegetation of dusts taken by the witnessed by the almost complete disappearance wind from the Sebkhas (great Chotts) (Coque, ofcalcretes and the development ofgypcretes. The 1962). During the Pleistocene, the development of Middle Pleistocene is also characterized by the the gypcretes at the expense of the calcretes, on the complexity of the observed tendencies. It is now piedmont south of Aur~s-Nemencha from east to established that a lacustrian phase reappeared at west, with no modification of the substratum and the end of the period. This phase was traditionally the hydrographic network, is a proof that there is attributed to the Upper Pleistocene (Coque, 1962), increase in aridity (BaUais, 1985), because and seems to be quite generalized throughout gypcretes advanced further and further from the northern Sahara from Libya to Erg Chech. The great Chotts. The contemporaneous appearance of tendency of continuous extention of the gypcretes the first Quaternary wind deposits in the Ziban, from Lower Pleistocene was disturbed by the Just to the west, confirms this interpretation (Ballais development of new calcretes. etal., 1979). On a local scale (wadi Ahmadi on the western flank of Dahar), a gypcrete intercalates REFERENCES between two calcretes in the Middle Pleistocene Abbes, C. et AbdelJaouad, S. avec la collaboration terrace at a time when the basin is only calcareous. de Ben Ouzdou, H. 1986. Carte g~ologique d' El This crust which results from the accumulation of Hamma au 1/ 100 000 (sud tunisten). 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