THE GEOLOGY OE TÎI5 HASAMT AREA^ ^ ™VALLEY, SOUTH TURKAI'IA, KE2TÏA AMO r t f B H f^ m y c F r if à ^ PL\ccE?Ht= vcicftroic a c e k :- 3 Tliesis presented for the Degree of Doctor of Philosophy in the University of London \ \ Stephen Donald Weaver ProQuest Number: 10098242 All rights reserved INFORMATION 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 10098242 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.O. Box 1346 Ann Arbor, Ml 48106-1346 11 ABSTRACT The geology of a previously unmapped 170 square miles ( 440 sq. kms. ) area of South Turkana, lying within the Kenya rift, is described. A contoured ard coloured geological map is presented. The stratigraphical succession established which totals about 6000 feet ( 18 OO m. ) in thickness and consists largely of volcanic rocks, ranges in age from about 25 to 4 million years b.p. New potassium/argon age determinations on some of the lavas are presented and reinforce the established stratigraphy • Particular attention is paid to the Pliocene succession which consists of a thick, widespread basalt formation overlain by a number of trachyte volcanoes. The morphology of the multicentred trach^rte volcanoes suggests that they are a type not described before,. The petrography of the lavas of the Nasaken area is described in detail. Chemical analyses of rocks and minerals are presented and the petrogenesis of the Pliocene lavas is discussed with Ill particular reference to the relationship between the basalts and the voluminous trachytes and the origin of the chemical variation within the trachytes. The trace-element geochemistry of the Pliocene lavas is compared with the geochemistry of lavas from other areas within the East African rift system • IV CONTENTS page CHAPTER 1 : INTRODUCTION a) Scope of work 2 b) Situation and. access 2 c ) Physiography 3 d.) Drainage and water resources 5 e) Climate 6 f ) Population 9 g) Flora and fauna 9 h) Conduct of field work 10 i) Previous geological work 10 j) Geological setting 11 k) Note on time-scale employed 14 l) Note on systems of measurement 15 CHAPTER 2 : STRATIGRAPHY 1) Basement System 17 2) a Turkana basalt formation 21 b Correlations and age 26 3 ) a Kowun volcano 27 b Correlations and age 33 4 ) a Kasorogol melanephelinite 35 b Nakasuw and Kanitiriam trachyphonolites 35 0 Kanitiriam microfoyaite 36 d Correlations and age 37 e History of the area west of the watershed 38 5) Tirioko basalt formation a Introduction 39 b Kagumnyikal basalts 41 page 5 ) 0 Napeitom mugearites 44 d Napeitom basalts 45 e Lotokongolea basalts 46 f Napeitom limestone 48 g Correlations and age 50 6) Pliocene trachyte volcanoes a Introduction 51 b Trachyte intrusions of the Kasamanang valley 53 c Kafkandal volcano i Introduction 56 ii Ngapawoi unit 58 iii %ong unit 59 iv Moru Angitak unit 60 d Nasaken trachyte complex i Introduction 61 ii Nasaken area 63 iii Balakaicha area 6? iv Nakwamoroi centre 70 V Tirioko basalt inliers 71 vi Emus and the cyclic nature of activity 73 vii Edilteniro area 76 viii Karimorega centre 77 ix Katatarope valley 79 X Morphology of trachyte flows 81 e Kanatim volcano 82 f South Turkana group of volcanoes 82 g Geochronology of the South Turkana group 84 7 ) Recent deposits 86 VI page CHAPTER 3 : STRUCTURE 1 ) Regional structure 89 2) Structure of the Nasaken area 90 3) Form and structure of the major volcanic units 94 CHAPTER 4 : PETROGRAPHY 1 ) Classification 99 2 ) Mineralogical components a Plagioclase feldspars 102 h Alkali feldspars 102 c Olivines 105 d pyroxenes 109 e Amphiboles 111 f Aenigmatite 113 g Micas 113 h Magnetite 114 i Quartz 114 j Feldspathoids and analcite 115 k Apatite 115 1 Secondary minerals 115 3) Textures a Basalts and intermediate lavas 117 b Trachytic lavas 118 4) Petrography of the lavas a Introduction 120 b Turkana basalt formation 120 c Kowun trachytes 121 d Kasorogol melanephelinite 123 e Kanitiriam microfoyaite 123 f Kanitiriam and Nakasuw trachyphonolites 124 vil page 4 ) g Tirioko basait formation 124 h Nasaken volcanics - basic and intermediate lavas 128 i Kafkandal and Nasaken volcanics - trachytic lavas 129 j Pulaskite syenites 132 k Pétrographie features of welded tuffs 134 CH/iPTER 5 : GEOCHEMISTRY and PETROLOGY 1 ) Introduction 163 2 ) Conpositions of the Miocene lavas 166 3) Geochemistry of the Pliocene lavas a Introduction 171 b Mineral chemistry 172 c Chemistry of the lavas 177 4 ) Petrogenesis of the Pliocene volcanic rocks a Relationship between basalt and trachyte 197 b Origin of the syenites 201 c Differentiation within the salic lavas 202 d Chemical variation within a single trachyte flow 2O4 e Rare-earth element distributions in feldspars 2O5 f Residual trace-element ratios and magma-types 206 CONCLUSIONS 208 REFERENCES 211 APPENDICES 1) Analytical methods 226 2 ) C.I.P.W. norm 228 3 ) Specimen localities 229 viii LIST OP FIGURES after page 1. Location of area 8 3 2 . Relief map 3 3. Physiographic divisions 4 4. Basement drainage pattern 5 5. V/ater supplies 6 6 . E.A.G.R.U. areas 11 7. East African rift system 11 8 . Basement outcrop in South Turkana 17 9. Basement map for area 8 20 10. Successions within the Turkana basalt formation 21 11. Turkana basalt lava succession 23 12. Extent of Kowun volcano 28 13. Cross-section through tholoid and basalt cone 31 14. Kowun fault-strip succession 31 15. Geological evolution of area west of the watershed 38 l6. Serial E-W sections along Kasamanang valley 40 17. Members of the Tirioko basalt formation - 40 18 . N-S longitudinal section, Cheror to Napeitom 42 19. Radiating basalt cooling columns 48 20. Napeitom gorge succession 48 21. Structural contour map for base of trachyte volcanoes 52 22. Form of trachyte intrusion along fault plane 55 23. Map of Kafkandal volcano 56 24. Section through Epong 59 25. Section through Moru Angitak 59 26. Moru Angitak from Plate 24 60 27. Nasaken volcano, source and flank zones 62 28 . Rose diagram of dyke orientations 64 IX after page 29. Succession at Nasaken 66 30. Sketch of hill at 683678 69 31. Sketch of hill at 705772 69 32. Succession at Nakv/amoroi 71 33. Succession at Emus 73 34. Formation of ash and ash-flov; eruptions after Rittmann 74 35. V/elded tuff specimen 8/ll4a 76 36. Successions in Edilteniro river 76 37 . Flow directions in Katirr basalt 77 3 8 . Succession in Karimorega area 78 39. Section through a trachyte flow 81 40. Kanatim succession in Edilte river 82 41. South Turkana group of volcanoes 82 42. Geochronology of South Turkana group 86 43. Regional structure 89 44. Cross-section from the Kerio to the Suguta 89 45. Modal classification of volcanic rocks 99 46. Normative classification of alkali rhyolites 99 47 . Graph of v femics for Menengai and Nasaken trachytes 100 48 . 2V determinations of alkali feldspars IO3 49. Alteration of olivine 108 50. Graph of modal quartz v dark minerals, Nasaken trachytes 129 51. Crystallization sequences and rection series 134 52. Alkali / silica plot, rift volcanics 163 53. Compositions of olivines and pyroxenes 172 54. Compositions of feldspars 175 55. Normative feldspars 176 56» Alkali / silica plot. Pliocene rocks of Nasaken area I86 57 . A-F-M diagram, Pliocene lavas I86 58 . Major-oxides plotted against Solidification Index 187 X after page 59» Alkali / silica plot, Nasaken and Ribkwo areas 189 60. Graph of 2r against Solidification Index 194 61. Si02 3 CaO against Zr 194 62. AI2O3 , MgO against Zr 194 63. Total iron-oxide, total alkalis against Zr 194 64. Peralkalinity against Zr 200 65. Si02 - AI2O2 - total alkalis diagram 201 66. R.E.E. patterns in feldspars 201 67 . R.E.E. patterns in lavas 202 68. Residua system 2O3 69. Alumina - alkali ratio diagram 2O3 70 . Chemical variation through a single flow 204 71* Zr/Nb against Zr for northern rift volcanics 207 72 . Zr/Nb against Zr for East African volcanics 207 XI LIST OP PLATES after page 1. Photomicrograph, garnet-plagioclase symplectite 19 2. Photomicrograph, layered plagioclase-amphiholite 19 3. Nakasuw trachyphonolite hills 21 4. Agglomerate, Turkana basalts 23 5. Botryoidal oalcite, Turkana basalts 23 6, Agglomerate ridge, Kowun 28 7. Agglomerate centre, Kowun 32 8. Panorama west to north from Kowun 36 9. Kanitiriam hill 36 10. Kov/un trachyte dipping beneath Lotokongolea basalt 41 11. Kagumnyikal hill 41 12. Pahoehoe basalt 47 13. Basalt dyke cutting Kowun trachytes 47 14. Radiating basalt cooling columns 48 15. Basalts in Napeitom gorge 48 16, Earth pillars in Napeitom gorge 49 17. Napeitem limestone 49 18 . Lodukai dyke 54 19. Lodukai dyke 55 20. Cone sheets near Cheror 55 21. Trachyte dyke, Kasamanang valley 56 22. Panorama, Ngapawoi lavas 58 23. Centre in Ngapawoi unit, Kafkandal 59 24. Moru Angitak centre 60 25. ditto 60 26. ditto 61 27. Explosion breccia, Nasaken 64 28 . ditto 64 Xll after page 29. Exfoliation weathering in basalts at Nasaken 65 30. Bedded pumice tuffs, Nasaken 65 31. Bills west of Nasaken 66 32. ditto 66 33. Balakaicha area 67 34. Trachyte plugs 68 35.