Mainzer Geographische Studien, Heft 9:

Andres, Wolfgang: Studien zur jungquartären Reliefentwicklung des südlichen Anti-Atlas und seines saharischen Vorlandes (Marokko).


Summary: Upper quaternary morphogenesis compared with bordered regions

A phase of intensive vertical cutting introduced the morphogenesis of the later Quaternary. The huge alluvial deposits previously formed (reg) in the southern foreland of the Anti-Atlas and the in part deep weathering of the Devonian and Carboniferous slate at the sole of this gravel indicate long enduring moist periods in the early Quaternary. In the Anti-Atlas the older morphogenesis is characterised by the formation of various generations of pediment.

The phase of pronounced down cutting is dated to the beginning of the last "Pluvial" but one (Tensiftien). In my opinion it can be equated by all means with the phase of the "érosion ougartienne" of the Wadi Saoura/Wadi Guir systems (CHAVAILLON 1964). Distinct phases of erosion are also ascertainable in the Atakor (ROGNON 1967, P. 501) and the Mauretanian Adrar (DAVEAU 1967, p. 49), and are dated by the said authors unanimously to the period of the Middle Acheulian and Upper Acheulian.

The subsequent morphogenesis is characterised in the mountains in particular by the formation of extensive areas of alluvial deposit and as a result of this differs clearly from the older generations of pediment. In the southern foreland of the Anti-Atlas the fluviatile deposits are less extensive and lesser in amount as well as being mainly cut deeply into the older alluvial deposits and are clearly dependent on the present-day course of the drainage channels.

Of the two generations of areas of alluvial detritus fans the more recent came into being in a transitional-phase of pronounced morphodynamic activity from the last "Pluvial" (Soltanien) to an early Holocene arid period. Preceding this was a long period of morphodynamic stability and soil formation with relatively thick vegetation which led to the formation of red soils on carbonate rock (terra rossa type) and red to chestnut-brown soils on silicate parent rock. These soils were for the most part eroded in the intermediate period. The nature of the erosion causes one to presume that a pronounced accentuation of the climate must have occured. Long term arid periods which led to the thinning out of the vegetation covering alternated with phases of heavy precipitation. The fine material of the soil horizons formed previously is to be found both as a constituent of the alluvial detrius-fans deposit as also in the form of enormous "loam terraces" upstream watergaps of the wadis in the hogback of Jebel Bani on the southern edge of the mountains. A 14C-dating of the calctuff lens not far from the basis of the loam terrace near Foum el Hassane produced an age of 11400 y. b. p..

A similar development, i. e. a period of pronounced morphodynamic stability an soil formation with a main period in the early "Pluvial" and a subsequent phase of denudation is assumed for the last Pluvial but one (Tensiftien) and the transition period to the last Interpluvial (Ouljien). A clearly pronounced older generation of areas of alluvial detritus fans and remains of an older loam terrace permit this conclusion.

In the early Holocene period the loam deposits were at first for the most part dissected. In a period about 7000 to 4500 years ago with increasing moisture an increase in vegetation in the western Anti-Atlas came about and grey and grey-brown soils were formed. A complete decalcification of the substrata and a formation of calcareous crusts no longer occured. This Holocene soil formation phase can be equated with the so-called "Sahara Neolithic Age" (approx. 5500-2500 B. C. according to BUTZER 1971). The end of this soil formation period is characterised by a renewed phase of denudation which leads to the formation of a grey- brown slight covering alluvial detritus layer. The expanse of area of this covering deposit in the western Anti-Atlas causes one to presume that during its formation the over-grazing of the vegetation by the increasing keeping of herds by the neolithic immigrants played a role.

The recent arid phase brought along the wadis on the southern edge of the Anti-Atlas further dessection of the loam terrace right down to its basis. On the other hand in Oued Dra in the southern foreland of the Anti-Atlas subrecently until recently a loam accumulation is coming into being which takes its material from the erosion loam-terraces from the edge of the mountains. The morphogenesis of the later Quaternary in the southern and western Anti-Atlas and its Saharan foreland makes it clear that the terms "Pluvial Age" and "Pluvial" if they are to be understood, as in this work, as a period of time, do not suffice to comprise the periods of prehistoric soil formation, slope denudation and fluviatile accumulation. Soil formation took place in each case in a section of a so-called Pluvial, namely mainly in the early Pluvial and over and above this in a moist section of the Holocene. On the other hand the phases of extreme slope denudation and the accumulation of loam terraces are to be dated to the final phases of the Pluvials and the beginning of the Interpluvials. Finally the signs of heavy fluviatile dissection are to be found in the transition periods from an arid period to a Pluvial with increasing precipitation and increased run-off.

These findings correspond largely with the situation described by CHAVAILLON (1964, p. 302) in the course of the account of a "climatic-sedimentary cyde". But if now neither a phase of increased fluviatile erosion nor a period of soil formation nor a period of slope denudation and fluviatile accumulation may fulfil the term "Pluvial", but in each case only cover as their own periods of time just portions of a Pluvial or lie completely outside the same, then this term is no longer to be used for a longer period of time in the sense it has been up to now (Pluvial=Glacial). Thus ROHDENBURG (1970) recommends with justification the introduction of the terms "morphodynamic periods of activity and stability" in place of Pluvial and Interpluvial periods. At the same time a largely chronological correspondence of morphodynamic activity from Central Europe down into the semi-arid sub-tropics is deduced in that the periods of activity are in each case equated with the peak glacial arid periods, whereas the periods of soil formation are initially allocated only to the Interglacials (ROHDENBURG and SABELBERG 1969 b, p. 40), in a more recent work also to the Interstadials (ROHDENBURG and SABELBERG 1973). Admittedly the special problems of the arid areas are pointed out here (RHODENBERG 1970, p. 94; ROHDENBURG and SABELBERG 1973, p. 144 f).

The results of my own studies as also of the findings of ROGNON (1967) for the Atakor and CHAVAILLON (1964) for the Wadi Saoura/Wadi Guir area sketched by way of comparison, however, permit the conclusion that periods of activity with the approach to the arid areas were in each case delayed to the end of a Pluvial (=Glacial) (phase delay). They then correspond better to the nations of RUELLAN (1969). The chronological sequence of morphogenesis and pedogenesis described schematically in RUELLAN (1969, p. 137) for various regions in Marocco which are incorrectedly described by ROHDENBURG and SABELBERG (1973, p. 147 f) as "pendulation theory" in the sense of BALOUT (1952) comes closer in my opinion to the actual situation on the edge of the Sahara than the so-called "parallelism theory" which presumes synchronous periods of activity in all regions. Here it is not only the differing latitudinal position which influences the chronological sequence of periods of morphodynamic activity and stability. In particular the varying height conditions of the mountain areas in the catchment area of the wadis play a decisive role. The Late Quaternary climatic and relief development of the Anti-Atlas in comparison with the results from the High Atlas and central Saharan Atakor (Hoggar) is a good example of this.