publisher = {Im Selbstverl. des Instit. für Physische Geographie der Freien Universität Berlin, Berlin},

abstract = {Research results are presented from two areas: the Tibesti Mountains (Central Sahara) and Tunisia. In the first study area geomorphological and sedimentological methods were used to reconstruct the origin of two depressions. The valley and 510pe sediments were classified and investigated for Climatic evidence. The reconstructed formation process of these two depressions can be compared with geomorphological development in other parts of the Tibesti. In addition, comparison may be possible with results by P. ROGNON (1967) in the Atakor borderland in the Hoggar Mountains. The upper terrace accumulation phase („Oberterrassen-Akkmnulation“) in the Tibesti probably corresponds to the sedimentation of the „terrasse graveleuse" in the Atakor borderland. Further possibilities of comparison are indicated: both for the older formation process preceding the upper terrace accumulation (e. g. for the period of origin of river systems and erosion surfaces at the time of basin formation, or for periods of lacustrine sedimentation) and for the younger formation process following the upper terrace accumulation (e. g. for the following periods: the accumulation of early Holocene lacustrine deposits, the development of soils formed on top of these sediments‚ the formation of a younger fine material terrace and of the recent valley sediments). Climatic influences seem to have had a similar effect on formation processes in both mountain ranges. The second part of the study deals with research results from various areas in Tunisia concerning the different phases of erosion and accumulation and of soil formation during the upper Pleistocene and Holocene, the results were compared with finds from other parts of Tunisia and the northern borderland of the Sahara. Development seems to have been parallel during some periods, e. g. the period of relatively intense morphodynamic activity with the deposition of debris and gravel during the upper Pleistocene. the soil formation phase about 6000—4700 B. P.‚ and the accumulation period about 2000—1000 B. P. with fine material sediments along the recent valleys. Here too, climatic influences may have been responsible for parallel processes of formation. A comparison of the morphodynamics of the two study areas in the upper Pleistocene and Holocene suggests six formation periods: During the first period (between 30 000 B. P. and approx. 14 000 B. P.) the Tunisian study area underwent an intensified morphodynamic activity (solifluction, sedimentation of debris and gravel, glacis formation); there is also evidence of processes of soil formation about 28 000 to 21 000 B. P. Evidence of phases of relatively intense morphodynamic activity (debris and gravel deposits, glacis formation) about this time is also found in the Tibesti study area. It has not yet been possible to fix the exact dates of the different phases in the first period. The second period, approx. 14 000 to 7400 B. P.‚ is characterized in the Tibesti by phases of lacustrine sedimentation. In Central Tunisia two dates of calcrete crusts, probably due to a soil formation phase. belong to the older phase of this period. In Southern Tunisia an accumulation of mainly fine material occurred in the younger phase of this period. Climatic conditions in both study areas during the second period were probably at times more humid than today. During a short third period after 7400 B. P. the formation of alluvial fan sediments in the Tibesti study area indicates a relatively dry Climate. A similar climatic development seems to have occurred in regions south of the Tibesti. In Southern Tunisia fine material accumulation, probably indicating wetter climatic conditions, ended around 7000 B. P. The fourth period. about 6000 to 4700 B. P.‚ features in Tunisia the formation of dark humous soils. Results from other Maghreb countries Show that during this period a relatively humid climate probably prevailed up to the northern borderland of the Sahara. There a traces in the Tibesti of the formation of dark soils on top of the lacustrine sediments of the second period; these soils have not yet been dated. In the fifth period, about 4700 to 2000 B. P.‚ morphodynamic processes (deposition of gravel) increased in intensity again in the Tunisian study area. In the Tibesti at this time coarse material was probably displaced in the valley and slope areas. It was not possible to date the different phases of this period. During the sixth period, about 2000 to 1000 B. P. fine material was deposited along the recent valleys in Central Tunisia. Comparable sediments are also known in Marocco, Libya and the Hoggar at this time. Similar but as yet undated sediments occur in the Tibesti. Their formation may be due to climatic or anthropogenic influences. Further research is necessary in order to establish an exact chronology and climatic interpretation of these formation phases. Available results do not seem to contradict the hypothesis of a synchronous course of climate fluctuations in the upper Pleistocene and the Holocene in the study areas.},
note = { \url {http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/7400}},
}