QUATERNARY EVOLUTION OF A MOUNTAINOUS AREA IN N.W. TUNISIA. A GEOMORPHOLOGICAL AND PEDOLOGICAL ANALYSIS Dr. R. H. G. Bos PUBLICATIES VAN HET FYSISCH-GEOGRA- FISCH EN BODEMKUNDIG LABORATORIUM VAN DE UNIVERSITEIT VAN AMSTERDAM nr. 19 (1971) SOL OFFSETDRUK/AMSTERDAM QUATERNARY EVOLUTION OF A MOUNTAINOUS AREA IN N.W. TUNISIA. A GEOMORPHOLOGICAL AND PEDOLOGICAL ANALYSIS Dr. R. H. G. Bos PUBLICATIES VAN HET FYSISCH-GEOGRA- FISCH EN BODEMKUNDIG LABORATORIUM VAN DE UNIVERSITEIT VAN AMSTERDAM nr. 19 (1971) CONTENTS ABSTRACT page 9 ACKNOWLEDGEMENTS 11 SUMMARY 13 PART ONE : GENERAL ASPECTS OF THE AREA STUDIED AND ITS SURROUNDINGS Chapter I 21 INTRODUCTION 1.1. Material 1.2. Field work Chapter II 25 CLIMATE Chapter in 27 GEOLOGY HI. 1. General HI. 2. Northern Part in. 3. Southern Part Chapter IV 33 HYDROGRAPHY IV. 1. General IV. 2. Subdivision of the River System TV. 3. Development of the River System IV. 4. Conclusions PART TWO : THE VALLEY OF THE OUED SELOUL Chapter V 37 THE GEOLOGICAL SETTING V.l. Numidian Flysch V. 2. Medjanian Flysch V. 3. Triassic Breccia V.4. Relationship between Parent Rock and Slope Chapter VI 41 THE OUED SELOUL VI. 1. Phases of Incision of the Oued Seloul VI. 2. Old Flat Parts of the Terrain on the Marls VI. 3. Flat Parts of the Terrain on the Soft Shales VI. 4. Conclusions Chapter Vu 47 MASS MOVEMENTS Vu. 1. General Classification of the Mass Movements VE. 2. Concentrated Types of Mass Movement on the Soft Shales VH.2.1. Earthflow Vu. 2.2. Debris Avalanche VII. 2. 3. Slump Vu. 2.4. Subsidence VII. 3. Concentrated Types of Mass Movement on the Marls VH. 3.1. Rockslide Vu. 4. Areal Types of Mass Movement Vn.4.1. Humid Solifluction VH.4.2. Mudflow Vu. 4.3. Distribution of the Areal Types over the Slopes Vu. 4.4. Explanation of the Complex of Stabilized Slope Deposits Vu. 4.5. Tor-like Sandstone Outcrops Vu. 4.6. Conclusions Chapter Vm 65 COLLUVIUM Vm. 1. Mass Movement and Mass Transport . 2. Normal Soil Erosion and Accelerated Soil Erosion . 3. Accumulation of Colluvium in the Corrasional Troughs Chapter IX 69 NATURE OF THE MASS MOVEMENTS RELATIVE TO THE CLIMATE IX. 1. Scheme of the Mass Movements in the Valley of the Oued Seloul IX. 2. Changes in the Nature and Extent of the Mass Movements Chapter X 73 SLOPE DEVELOPMENT IN CONNECTION WITH THE PARENT ROCK AND THE NATURE OF THE PREVAILING PROCESSES X. 1. Effect of the Parent Rock on Slope Development X. 2. Effect of the Areal Types of Mass Movement on Slope Development X. 3. Effect of the Concentrated Types of Mass Movement on Slope Development Chapter XI 77 EVOLUTION OF THE SELOUL VALLEY AS EVIDENCED BY GEOMORPHOLOGICAL DATA XI. 1. Table of the Geomorphological Processes XI. 2. Relative Dating XI. 3. Scheme of the Relative Dating XI. 4. Absolute Dating Chapter XII 87 PEDOLOGY XH. 1. Soil Map, Scale 1 : 25,000 Xu. 1.1. Introduction Xu. 1.2. Legend XII. 1.3. Description of the Indivudual Soil Mapping Units shown on the Map (supplemented with Profile Descriptions and Tables) Xu. 1.4. Relationship between Some of the Most Wide- spread Soil Units in the Valley of the Oued Seloul XII. 2. Summary of the Recent Pedogenesis XH. 3. Polygenetic Phenomena Xu. 3.1. Soils of the Triassic Breccia XII. 3.2. Soils of the Transition of the Triassic Breccia to the Oligocène Flysch Deposits XII. 3. 3. Soils of the Oligocène Flysch Deposits XH. 3.4. A Pseudo fossil Phenomenon XH. 4. Interpretation of the Polygenetic Phenomena Relative to the Climate XII. 4.1. Pedological Scheme XII. 4.2. Effect of the Parent Rock and the Geomorpho- logical Development on the Soil Profile PART THREE : THE VALLEY OF BEN METIR Chapter xm 115 SIMILARITIES AND DISSIMILARITIES BETWEEN THE VALLEY OF THE OUED SELOUL AND THE VALLEY OF BEN METIR xm. 1. Geological setting of the Valley of Ben Metir Xm. 2. Geomorphology of the Valley of Ben Metir Xm. 3. Mass Movements in the Valley of Ben Metir Xm. 4. Pedology of the Valley of Ben Metir XIII. 4.1. Soils of the Oligocène Flysch Deposits Xm. 4.2. Soils of the Miocene Lagoonal Deposits and the Mio-Pliocene Continental Deposits xm. 4.3. Recent Pedological Development of the Valley of Ben Metir Xin. 5. Conclusions Xin. 5.1. Conclusions as to the Geomorphology Xm. 5.2. Conclusions as to the Pedology SAMENVATTING 127 APPENDIX 135 INVESTIGATION INTO THE PROBABLE CAUSES FOR THE DIFFERENCES IN STABILITY OBSERVED ON THE SLOPES IN THE TRIASSIC BRECCIA AND ESF THE SHALES OF THE OLIGOCENE FLYSCH 1. Grain-size Distribution 2. Morphoscopic Rounding Index Analysis 3. X-ray Analysis with the Guinier-De Wolff Camera 4. Swelling Capacity 5. Soil Aggregate Stability 6. Plasticity Index 7. Moisture Adsorption in a Climatic Cabinet 8. Determination of the Liquid, Solid and Gaseous Phase 9. Differential Thermal Analysis and the Thermo-gravi- metric Analysis 10. X-ray Diffractometer Analysis 11. Conclusions REFERENCES 161 INLAYS fig. 2, 5 and 21. ABSTRACT An anticlinal valley developed in Oligocène flysch deposits (soft shale, marl, sandstone), including a diapyric injection of Triassic breccia, was subjected to a detailed geomorphological study. It was established that periods of areal mass movements of the flow- type (including humid solifluction) have been active on all parent mat- erials of the flysch area. At present mass movements are operative in the flysch deposits by means of linearly concentrated slide movements limited to one of the parent materials: the soft shales. Neither now nor in the past have mass movements affected the Triassic breccia, as is evidenced by the fossil and polygenetic character of the soils in these breccia. Three types of slopes were distinguished dependent on the rock; the slope development proved to be highly related to the parent rock and the nature of the mass movements. Due to the periodically unstable conditions in the flysch area the soils encountered date nearly all from the Holocene. The geomorphological phenomena closely match the pedological data and may truthfully project the evolution of the landscape in this part of NW Tunisia. It was established that the inter-pluvials were the stable, soil forming periods in this morphogenetic environment. ACKNOWLEDGEMENTS The author wishes to record his sincere appreciation of the assistance and encouragement rendered him during the preparation of this thesis by Prof. Dr. P.D. Jungerius and Prof. Dr. Ir. A. P.A. Vink. Special thanks are also due to the following persons: Dr. Ir. J. van Schuylenborgh, Dr. J. Rupke, Mr. E.A. Kummer, Mr. P.A. Riezebos and Dr. H.R. Volk for kindly reading through part of the manuscript. Mr. D. F. Karel for interpretation of X-ray analyses. Mr. P.C. Beukenkamp for interpretation of Differential Thermal and Thermo-gravimetric Analyses. Mr. E. A. Kummer for interpretation of X-ray diffractograms. Mr. O.K. Hulshof for counting and interpretation of pollen samples. Mr. C. J. Snabilié for drawings and assistance in printing. Miss P. de With for drawings. Mr. A. Eikeboom for preparation of the photographs. Mr. D. Linthout for the translation. Further to Dr. D. G. Jongmans for his hospitality and assistance in Tunisia, and to Dr. S. Segal for discussions and advice. 11 SUMMARY All geomorphological and pedological data discussed in this summary have been compiled in figure 26, the scheme of the landscape and pedogenetic evolution in the area studied. The purpose of this study has been to reconstruct the evolution of the landscape in a mountainous part of the Kroumirie (NW Tunisia) on the basis of geomorphological and pedological observations. Two valleys were studied, the narrow anticlinal valley of the Oued Seloul and the wide synclinal valley of Ben Metir. THE VALLEY OF THE OUED SELOUL Geological Structure The steep anticlinal valley of the Oued Seloul was given special atten- tion to in the course of this study. The interfluves consist of sandstone, and lower in the valley soft shales and marls occur. Together these rocks constitute the Oligocène flysch deposits. Folding of these sedi- ments took place during the lower Miocene and is directed NNE-SSW. The folding is on the whole of a rather simple anticlinal-synclinal nature, locally with injections of Triassic material ('Triassic' diapyr). The eastern valley side of the Oued Seloul locally consists of this Triassic breccia (fig. 6). Mass movements The geomorphological forms in the valley of the Oued Seloul are de- termined especially by processes of mass movement. Results of the mass movements still visible in the landscape indicate that nature and extent of these processes were dependent on the rock on which they 13 were active and on the fluctuating climatic conditions during the Quater- nary. Three criteria were used to arrive at a first subdivision of the mass movements. Apart from water content of the masses and the nature of the movement (flow or slide), two criteria already used by Sharpe (1960), the lithology of the material served to subdivide the mass movements (fig. 18). Present shape and position of the deposits made it possible to desig- nate the processes as active or no longer active, which proved to be an important factor in the reconstruction of the landscape development in the Seloul valley (fig. 10). This permitted a second subdivision into active and fossil (stabilized) mass movements (fig. 18). With regard to the relations rock/mass movement and geological time/mass movements the following has been observed: On the Triassic breccia no traces have been found indicative of recent or past activity of mass movements. Furthermore, a steep valley-side still bears witness of very old vertical incisions of the Oued Seloul in the Triassic deposits.