■6 Londiani caldera seen from across Kilombe caldera 11 THE GEOLOGY OF THE LONDIANI AREA OF THE KENYA RIFT VALLEY Thesis presented for the Degree of Doctor of Philosophy in the University of London William Barry Jones ProQuest Number: 10107352 All rights reserved INF0RMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed a note will indicate the deletion. uest. ProQuest 10107352 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.Q. Box 1346 Ann Arbor, MI 48106-1346 Ill ABSTRACT 2 An area of about 900 square miles (2300 km ) at the junction of the Kenya and Kavirondo Rift Valleys was studied and a map of it on a scale of 1 : 50,000 is presented. The formations present are alkaline lavas and tuffs ranging in age from 12 m.y. b.p. to recent and can be divided into a basanite to phonolite series older than 7 m.y. and a basalt to trachyte series younger than 7 m.y. The formations are grouped into four assemblages, each consisting of rocks derived from sources in about the same area. A series of trachytic ash flows about 4 m#y. old, the Eldama Ravine Tuff, and two trachyte volcanoes, Londiani of 3 m.y. b.p. and Kilombe of 2 m.y. b.p., together with their associated syenite bombs are described in detail. The structure of the area is dominated by the Equator and Mau Monoclines which form the western margin of the Kenya Rift Valley. Faults are relatively unimportant but show three distinct trends which can be related to structures in the basement. Chemical analysis was carried out on about 200 rocks, particularly concentrating on the Eldama Ravine Tuff and the Londiani and Kilombe Trachytes. This, with the petrography, showed that the rocks within the basanite to phonolite series and the basalt to trachyte series are related in general but not in detail by fractional crystallisation- It is also showi that in the trachytic rocks Na, Pe, Y and the Lanthanides are very mobile. IV, ACKNOWLEDGEMENTS I acknowledge with gratitude the receipt of a Natural Environment Research Council grant during the first three years of this project. I would like to thank Professor B.C. King, director of the East African Geological Research Unit, for the opportunity to work in this very interesting area and for criticism of my work. I am indebted to several colleagues at Bedford College for useful discussion of geological problems, especially Professor W.W. Bishop, Dr. I.L. Gibson, Mr. M. Golden, Mr. P.S. Griffiths, Mr. P. Truckle and Dr. S. Weaver and to Dr. and Mrs. L.A.J. Williams for their support in the field. Thanks are also due to Mrs. M. Brook for K/Ar age determinations at the Institute of Geological Sciences and to Mr. H, Lloyd for wet chemical analysis. I am especially grateful to Miss Elizabeth Burton who gave freely of her own time to prepare diagrams and colour maps. Lastly I would like to mention the late Ngua Muli whose efficient running of my camp enabled me to get on with the job. CONTENTS Page No. Chapter I INTRODUCTION Situation and Access 1 Topography 2 Drainage 4 Climate, Flora and Fauna 5 Population 7 Previous Geological Work 9 Chapter II REGIONAL GEOLOGICAL SETTING Introduction 12 Pre-Plateau Phonolite Formations 12 The Plateau Phonolites 15 The Timboroa Assemblage 15 The 7 m.y. Faulting Episode 17 The Kapkut Assemblage 18 The Londiani Assemblage 19 The Menengai Assemblage 21 Summary 21 Chapter III STRATIGRAPHY Introduction 23 Plateau Phonolite 24 Kericho Phonolite 24 Timboroa Assemblage 24 Luiribwa Phonolitic Nephelinite 25 Timboroa Phonolite 26 Makutano Tuff 27 Kapkut Assemblage 32 Hawaiites 32 Lembus Mugearite 33 VI Kapkut Trachyte 35 Londiani Assemblage 38 Trachytic Ash Flow Tuffs 38 A) Mau Tuff 38 B) Eldama Ravine Tuff 40 Trachyphonolites 51 Saos Mugearite 52 Trachytes 54 A) Londiani Trachyte 57 B) Kilombe Trachyte 63 C) Minor Trachyte Centres 75 Londiani Plain Basanite 76 Menengai Assemblage 78 Menengai Trachyte 78 Lower Menengai Tuff 79 Rosoga Tuff 87 Esageri Beds 87 Upper Menengai Tuff 91 Gogar Tuff 96 Chapter IV STRUCTURE Introduction 98 Faulting 98 Faulting of the Timboroa Assemblage 98 Faulting of the Kapkut Assemblage 103 Faulting of the Londiani Assemblage 105 Faulting of the Menengai Assemblage 109 Age of the Faulting 110 Correlation of the Faulting with Adjacent Areas 111 Monoclines 112 The Equator Monocline 112 vil Monoclining of the Mau Highlands 121 Discussion of the Structures 123 Curved Faults 123 Fault Trends 128 Relationship of Vulcanicity to Faulting 130 Evolution of Kilombe Caldera 131 Rift Valley Tectonics 134 A Model for Rift Valley Structures 136 Chapter V THE LAVAS AND SYENITE BOMBS Form and Structure of the Lava Flows 139 Classification of the Lava Flows 141 Petrography of the Lavas 144 Basalts 144 Basanites 146 Hawaiites 147 Mugearites 148 Nepheline Mugearites and Nepheline Hawaiites 149 Trachytes 151 Phonolites 155 Trachyphonolites 156 Pétrographie Evidence for the Petrogenesis of the Lavas 157 Chemistry of the Lavas 162 Introduction 162 Analytical Techniques 162 Comparison of the Two Series, 1 : Major Elements 163 Comparison of the Two Series, 2 : Trace Elements 172 Relationships among the basic rocks 179 The Basalt to Trachyte Trend 162 Volatile Effects in the Trachytes 164 The Syenite Bombs 161 Vlll Introduction 191 Petrography 191 Chemistry 196 Chapter VI THE TUFFS Vulcanology 205 Petrography of the Welded Tuffs 209 Chemistry of the Tuffs 210 SUGGESTIONS FOR FURTHER WORK 219 BIBLIOGRAPHY 221 TABLE 1 : Potassium-Argon Age Determinations 225 TABLE 2.*: Field Characteristics of Kilombe and Londiani Flows 228 TABLE 3 : Petrography of Analysed Kilombe and Londiani Trachytes 232 TABLE 4 : Analyses and Norms Table 4.1-4.4 Basalts and Basanites 233 Table 4.5-4.7 Hawaiites, Mugearites, Nepheline Hawaiites and Nepheline Mugearites 237 Table 4.8-4.12 Trachytes and Alkali Rhyolites 240 Table 4.13-4.14 Phonolites 245 Table 4.15 Trachyphonolites 247 Table 4.16-4.19 Syenites 248 Table 4.20-4.25 Eldama Ravine Tuff 252 Table 4.26-4.28 Other Trachytic Tuffs 258 Table 4.29 Makutano Tuff 261 Table 4-30 Standard Socks 262 IX LIST OF PLATES Page No, Plate 1. Waterfall south west of Londiani Town 6 Plate 2. Mt. Blackett 30 Plate 3. Thick soil profile on Makutano Tuff 30 Plate 4. Water eroded outcrop of E 5 44 Plate 5. Tumulus on Saos Mugearite 44 Plate 6. Tuff dipping off trachyte dome 59 Plate 7. The Visoi gorge 62 Plate 8. The Visoi gorge from the air 62 Plate 9. Kilombe from the air 65 Plate 10. Kilombe Caldera 66 Plate 11. A block of the trachyte flow K 8 68 Plate 12. An outcrop of K 13 68 Plate 13. A porphyritic mugearite xenolith 71 Plate 14. Syenite blocks on Kilombe 72 Plate 15. Close up of a syenite block 72 Plate 16. Tuffs at the waterfall in Kilombe gorge 74 Plate 17. Matebei 74 Plate 18. Limutet 77 Plate 19. Pumice of the Lower Menengai Tuff 77 Plate 20. Fault scarp north west of Menengai 82 Plate 21. The artifact site at Rongai 84 Plate 22. Some of the artifacts at Rongai 84 Plate 23. Canyons in the Esageri Beds 88 Plate 24. The Esageri Beds between Kilombe and Londiani 88 Plate 25. Section through young tuffs at S.E. end of Londiani Mountain 97 X, LIST OF FIGURES Page No, Fig, 1 Topography of the Kenya Kavirondo Rift Junction 3 Fig. 2 Geology of the Kenya Kavirondo Rift Junction 13 Fig. 3 Stratigraphie Columns for the Kenya Kavirondo Rift Junction 22 Fig. 4 Sections across the north of the Mau Highlands and west of Londiani Town. 28 Fig. 5 Sections across the Perkerra Gorge 34 Fig. 6 Stratigraphy and Zr content of the Eldama Ravine Tuff 41 Fig. 7 Rose diagram of dykes and faults in the Eldama Ravine Tuff 50 Fig. 8 Sections across Kilombe 64 Fig. 9 Distribution map of Lower Menengai Tuff 80 Fig. 10 Section at artifact site near Rongai 83 Fig. 11 Distribution map of Esageri Beds 89 Fig. 12 Distribution map of Upper Menengai Tuff 92 Fig. 13 Distribution map of E 2 114 Fig. 14 Distribution map of E 7 115 Fig. 15 Distribution map of E 9 116 Fig. 16 Distribution map of Londiani and Kilombe Trachytes 117 Fig. 17 Plot of dip against age of formations on the Equator Monocline 118 Fig. 18 Map of Kericho Phonolite - Phonolitic Nephelinite junction 122 Fig. 19 Distribution map of M 4 124 Fig. 20 Distribution map of M 3 125 Fig. 21 Fault pattern of Kilombe 127 Fig. 22 Model of the Evolution of Kilombe 133 Fig; 23 Diagrammatic Rift Valley Structures 138 Fig, 24 Normative Classification of tlie Lavas 142 Fig. 25 NcarmatLve Feldspar Diagram for the Lavas 143 XI Fig. 26 Petrography of the Two Trends 159 Fig. 27 Si02 for the Lavas 164 Fig. 28 FeO e MgO ^ SiOg for the Lavas 165 Fig. 29 AJ^Oje CaO v SiOg for the Lavas 166 Fig. 30 Alkalies/FeO/MgO and NagO/KgO/GaO diagrams for the Lavas 170 Fig. 31 Ne/Ks/8iOg diagram for trachytes and phonolites 171 Fig.