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The Great Sand Sea in Egypt

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The Great Sand Sea in Egypt Developments in Sedimentology, 59 THE GREAT SAND SEA IN EGYPT FORMATION, DYNAMICS AND ENVIRONMENTAL CHANGE - A SEDIMENT-ANALYTICAL APPROACH by Helga Besler Geographisches Institut, Universitat Koln, Germany with contributions from A. Bolten, O. Bubenzer, A. Hilgers and A.J. Van Loon ELSEVIER Amsterdam • Boston • Heidelberg • London • New York • Oxford Paris • San Diego • San Francisco • Singapore • Sydney • Tokyo TABLE OF CONTENTS List of Figures ix List of Tables xi Preface xiii 1 Introduction to the Great Sand Sea 1 2 Methods of Investigation 5 2.1. The Main Objectives 5 2.2. Selection of Suitable Methods 6 2.3. Brief Description of the Procedures 8 2.3.1. Digital elevation models and watershed analysis on the basis of satellite and radar data (A. Bolten and 0. Bubenzer) 8 2.3.1.1. Introduction 8 2.3.1.2. Digital elevation data 8 2.3.1.2.1. ASTER elevation data 10 2.3.1.2.2. Elevation data from the Shuttle Radar Topography Mission 12 2.3.1.2.3. Quality and accuracy of the digital elevation models used 14 2.3.1.3. Generating a uniform elevation database for the Western Desert of Egypt 17 2.3.2. The topographic surveys 19 2.3.3. Sedimentological analysis 24 2.3.3.1. Granulometry 24 2.3.3.2. Morphoscopic analysis 25 2.3.3.3. Heavy-mineral analysis 26 2.3.3.4. Scanning-electron microscopy 26 2.3.4. Salinity measurements 27 2.3.5. Luminescence dating of sand deposits (A. Hilgers and A.J. van Loon) 27 2.3.6. Meteorological observations 30 3 Topography of the Great Sand Sea 33 3.1. Geological Setting and Bedrock Topography 33 3.1.1. Geological formations and their spatial pattern 33 3.1.2. Bedrock landforms 34 3.1.3. Reconstruction of palaeo-drainage systems by digital elevation data (0. Bubenzer and A. Bolten) 39 3.1.3.1. Introduction 39 3.1.3.2. The potential bedrock relief and the hypothetical palaeo-drainage system 41 3.1.3.3. Conclusions 46 vi Table of Contents 3.2. The Pleistocene Draa 46 3.2.1. The southern longitudinal draa of the trades 49 3.2.2. The northern transverse draa of the westerlies 52 3.3. The Holocene Dunes 58 3.3.1. Dunes on the transverse draa 59 3.3.2. Dunes on the longitudinal draa 60 3.3.3. The southward progradation and the northern extension of dunes 67 3.3.4. Comparison with modern winds 68 3.4. The Sand Budget 71 4 The Granulometric Analysis 73 4.1. Historical Developments 73 4.2. Grain-Size Parameters 75 4.2.1. Mean grain size (Mz) 76 4.2.2. Sorting (So) 79 4.2.3. Skewness (Sk) 80 4.2.4. Kurtosis (X) 81 4.3. The System of Granulometric Sand Types 82 4.3.1. Granulometric sand types of the draa 85 4.3.2. Granulometric sand types of the dunes 88 4.3.3. Granulometric types of the sandstones 94 4.4. The Response Diagram 95 5 Luminescence Dating 99 5.1. The Sampling Sites 99 5.2. Laboratory Procedures and Results (A. Hilgers) 105 5.2.1. Equivalent-dose determination 106 5.2.1.1. Sampling, sample preparation and measurement facilities 106 5.2.1.2. Measurement procedures for De estimation 106 5.2.1.3. De determination for the Great Sand Sea samples 110 5.2.2. Dose-rate determination 112 5.2.2.1. Uncertainty about the state of equilibrium of the radioactive-decay chains 114 5.2.2.2. Impact of varying overburden thickness on the cosmic-dose contribution 115 5.2.2.3. Influence of uncertainty in water-content variations 115 5.3. Interpretation of Dating Results 118 5.3.1. OSL ages of the draa 118 5.3.2. Growth rates of the draa 119 5.3.3. Younger aeolian reworking of draa 121 5.4. Comparative Discussion of Regional Datings: Time Slices in the Great Sand Sea 123 Table of Contents vii 6 Quartz Sand as an Indicator of Humid Periods 129 6.1. The Salinity of Sands / 129 6.1.1. Water in dunes ,' 129 6.1.2. Interpretation and discussion of the salinity 130 6.2. The Rubification of Sands 135 6.2.1. Introductory discussion of red dune sands 135 6.2.2. Reddened sands in the Great Sand Sea 138 6.2.2.1. The western transection 139 6.2.2.2. The middle transection 139 6.2.2.3. The eastern transection 141 6.2.2.4. Colour observations during the topographic surveys 143 7 Heavy-Mineral Analysis 145 7.1. The Total Heavy-Mineral Content 145 7.2. The Heavy-Mineral Assemblages 148 8 Scanning Electron Microscopy 153 8.1. Brief History of the Development 153 8.2. How to Prevent a Biased Interpretation 154 8.3. General Properties of the Quartz Grains 155 8.4. Correlations with Dating 156 8.5. The Most Frequent Microstructures: Controversial Phenomena 160 8.6. Discussion of the Environmental Developments 164 8.6.1. Processes in the Holocene 164 8.6.1.1. The more common features 164 8.6.1.2. The environments at the dating sites 170 8.6.1.2.1. The eastern sites 170 8.6.1.2.2. The sites in the middle 172 8.6.1.2.3. The western sites 174 8.6.1.3. The spatial pattern 175 8.6.2. Pre-Holocene processes in the Great Sand Sea 176 8.6.3. Major sand sources 181 8.6.3.1. Environments outside the Great Sand Sea 181 8.6.3.2. Major differences 186 8.6.3.3. The southwestern sand source 186 8.6.3.4. The eastern sand source 189 8.6.3.5. The western sand source 190 8.6.3.6. Sand sources for the southern middle sand sea 192 8.7. The Question of Earlier Cycles of Draa/Dune Formation 193 9 The History of the Great Sand Sea Compared to that of Other Active Ergs 195 9.1. Sand Sources and Their Influence 195 9.2. Draa Patterns and Draa Formation 197 9.3. Time Slices of Draa Formation and Reworking 202 9.4. Conditions During the Holocene Climatic Optimum 206 9.5. The Final Aridization 208 viii Table of Contents Appendix 1: List of Samples and Their Investigated Parameters 211 Appendix 2: Procedure ofSEM Feature Analysis and Environmental Composition Exemplified for Sample No. 35/96 219 References 225 Index 243.
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