._~..." 1.,'- ~ . .'" II I wREPORT No. 2S ;.__..,.__-_’: COLONY AND PROTECfORATEPROTECI‘ORATE OF KENYA GEOLOGICAL SURVEY OF KENYA . GEOLOGY OF THE SOUTH-EAST MACHAKOS AREA DEGREE SHEETSHEET 52, S.E.SE. QUADRANT (with coloured map) I f ' by I R. G. DODSON, M.Sc. , Geologist I I III II ) 19531953 II mPRINTED BY THE coyEkmmqrm,GOVERNMENTPR.INTER,NAIROBI Price: Sh.Sh.6/506f50 ,t III II !~, --.- CONTENTS PAGEI’\GE I.I. Introductionllzll'uduclion . 1 II.I]. PreviousPrenioux‘ GeologicalGavfugical Work..Wurk 1 J III.III. Physical Features.features . 2t IV.IV. Summary ofDI" Geology660.10g} and GeologicalGotf-lne'cnl HistoryHistm'} ofu ' theIhc Area.'\I'c:': 51)I V.\'. DetailsDCIEIIIS ofUI' GeologyGeolog} 7 1. BasementBewanrn SystemScm 7 (1):1 : Meta-sediments\Ii‘iél-\C\III‘I':L‘I‘:E\ 8 (2):1 J Migmatites.\Iiglnuulcs . 9 (3)I31 IntrusivesIrrrusjws .. 13 (4)I43 Mineral\‘Iincral beltshclls 14 (5)I15: Metamorphism\Ietamnrphim: 15 (6)[m Structures.Snaictura . 16 2. Tertiary-Turtmrg.i\Yatta:m:=. PlateauPI.1‘.J:.E.: phonolite.I‘II‘HI‘UIIYC... 17 3.n. PleistocenePICNHL‘L‘HC deposits.LICITUKILEH . 17 (1)IIJSECIRIC‘[11\.Sediments 17 (2)9.2) LavasLu 4% 18 4.-‘r. SuperficialSupctficirsl deposits.dC‘flUMh . 18 VI.\ I. Economic|3cunomic Geologyclugj. 19 1.I. Minerals\1jner;ll< 19 2. Walt“Water.. 19 VII.VI 1, ReferencesRCIL‘TC‘ICCQ 20 LIST OF ILLUSTRATIONS“LIB | R \ I IONS Fig.Hg. 1.I Erosionbrown“. surfaces.we Illgu . 3 Fig.Flg 2.3 StructuresSIE‘LICIUZ‘C“ and mineralI‘HI‘L‘I :l beltsDC :\ 15 MAP'\I ‘-.!’ Geological(jeuioglcal map ofml thetil: south-east\L‘LélII‘Eihl Machakos\Iuclmkc-a area::ucur scale\cruic II::125,000" ‘: [NJ-U . at end Foreword Thel‘he report on the south-east partpazt ofor the Machakos.\'liiCl1iil\US area is the fifthti.th report published since the Geological Survey of Kenya was expanded with the assistance ofot' the Colonial Development and Welfare\Vclfare Vote. The southern part of MachakosMuchukos arealiI'CLt proper,proper. to the west, has also been mapped and a:1 report will be published in the near future,future. while a report on the mapping of the KanzikoKan/iko urea.area, on the east.east, is in preparation.preps-tition. Unfortunately,Unfortunately. the reconnaissance search of the south-easternsoutheastern Machakos\‘lachakos area has not revealed the presence oiof numerous small mineral deposits such as ureare knownknmyn in the country south of MachakosMach-altos town. Itit appears unlikely that the area will prove to have any economic importance in the future.future, so forfar as mineral deposits are concerned. It is.is, however, the site of the MaltueniMakueni land settlement scheme and,(ind. as such,such. is anFm area of the greatest importance in the Colony. Thethe area is traversed by the well-knownwelllnown phonolite-coveredphonoiitc-coyereo Yatta‘r'utta plateau. The lavalaw hashas always been regardedregarded as the remains:‘ezrmins of an1111 extensive flowfirm- that found:‘nund its way down a valley in middle Tertiary times.times, but Mr. Dodson has discovered evidence that suggests a diiTerentdifferent origin. He believes thutthat the lavalax-"i. L“-C:'l1L“.~overlies at: zone(one Oiof faultingiitlting of Tertiary age, and that it is unlikelynnliLcl} that at any partpert of the plateau it flowedflowed for more than a short distance. He also considers thatlhzit flowsflours moulded by whenvalleys are rare.rue. Nairobi,Nairobi. WILLIAM\N'llllA \‘J PULFREY,[’ULIJREY. 2nd September,September. 1952.I952. Chief Geologist. ABSTRACT The area described is bounded by latitudes 101' Eu30' S..5. and 220 Hit00' S.,5.: and by longrlongi- tudes 37‘370 30"30' E. and 38380 00’00' E. Apart fromfront the ruggedrugged ground alongtilong the western boundary,boundary. the areaurea consists of flflat,at. low-lyinglow—lying country. Remnants of two former plains represented by summit levelsiE\'Cls of hillshlils are.v’e recognizablerecognizttbie and aL‘. Pleistocene to Recent plain appears to be still in ait stage of development.oexelopuiern, A short accounttic-count isis givenpitch ofol' the physio-physio- graphy ofor thethe YattaYetta plateau and :ta tentative hypothesishypothesis of itsits origin,origin. diil‘erentdifferent from the classical theory.theory, is tiller-ed.offered. The rocks described comprise: [it(1) (meissesGneisses and scnistsschists ofot' the Basement System.System, with abundant pegmatitepeginatitu and quartz veins,veins. (2)(2: Ancient basic intrusivesintrusiyes including amphibolites and a;t pyroxenite,pyroxenite. (3)i3] Tertiary phonolite of the YattaYutta plateau,plateau. (4)t4) River conglomerates, soils, gi'zirels.gravels, etc..etc., (5)r53 Pleistocene olhine-basalt.olivine-basalt. """""'"'... GEOLOGY OF THE SOUTH-EASTSOUTH—EAST MACHAKOS AREA I-INTRODUCTIONI—INTRUDL'CTION The area described in this report is the south-eastsouth—east quadrant of.1: degree sheet 525.; (Kenya),(Kenyat. bounded by meridiansmortdians 37°3"" 30'311' E.1-_'. and 38°38 00'[i'tl’ E. and'.I.11.L1 byb1. parallels11.11.111.313 11"° 30'311-" S.'1'. and 2°3 00'110' S.,8.. of01' which the1113 extent is approximately1;;1L1:L1xin1'..tei_\' 1,2501.251.“ square miles. '11::The whole1111013..of".tltc.the area is nativenative reserve,reserve. the portion eastezzst of the AthiA1112. RiverR5137 being' part of thethe.K'KituitL'.i district,L11 st 'iL‘t. and thatthat westnest of the river part of the Machakos.\luc'.1-.t.-..1s district.Li'.striL' Where inhabited thethe;areaL'_ isl"; occupied by Africans ofol' the UkambaL'lx'anim tribe. Cu:tivationC.'L=.'.?.1‘.:111o11 ofL1: crops and.11'.L1 stock-keepings'LoL'k—keepi 111: areL111? practised in the northern,northern. western and,21nd. to a'.1 lesserless-3: extent,extent. in the easterni.’.t-Jl'l sections.sections DueD' 1L: to the erratic rainfall,1'Ltinr'all.gei1eralgeneral povertypovert} of the soil,soi|.:11dand theth-3 primitive111' 1111:11'1- agriculturalL 1'.|'. 11.".12 methods111311.oLi.' used,ttscd. crop yields)fcids are low.10“. Recently attention has.'L'1s been'ITCCE': focused:L1L'.1scL'. on.. ".the south-eastern" portion where, under the control of.11' the African.-'\l'1"1'C.&L1’1 LandLune SettlementSedicnmnt Board,8-.1.-.'... the Makueni settlementsettfesnent scheme is being developed. The object of01' the scheme is to10.3resettle'3!t" L. members"l"; CTT-“CI: of0. the UkambaCkamba tribe,ts'ibc. hitherto livingliving inin the congested localities':oc:1‘1i'.ies inin the western'LL3s‘I'11 partpas”. of(11' the1:1.- area.'.1.t Bush-clearingBllsh-Clearing and cultivation schemes have been put into practicepl'L-‘LL‘. cc underL111'L1e1' the supervisionopen-131011 of01 EuropeanFur ope-.Ln officersofficers to prevent11.-event the deteriorationdo: erio: 11111 of0." soil$1.11: as.1s had.:tLI. happenedher-hence in‘11". neighbouringIlClLIn'l'K'Jlll'lllLf cultivatedi.' l'\' {[L‘Ll areas.‘.'..t3L15. It["1 appearsL.p'L"Ll1'>1 that131.1: the.3 L’ energeticCllt‘lgt. Il.C efforts:s ofo‘ those1.3 responsible will make1111156 possiblepossible an increasing amount ofot' settlementsettle-rcntiin1 areas.."3:-.s uninhabited.211111'11 Sibltft'. tilltill now. The area was geologically surveyedSt1r\'c}etl betweenbetween. March.\‘11L1'L‘h. and11nd September,S-cgétcrtt'ncr. 1950,L1'5t.‘1.\1.-';thwith the object ofL1I' preparingpreparing. a reconnaissance map and in :111an attempt to discoverdisL‘Ln-L‘r mineral111't-L‘I'al deposits,deposits. or geologicalgeoiogical conditions favourable:‘ox-onrehic to1L1 mineral occurrences.QCCLI‘JI'CIlCLb. Unfortunately,t..".1;'.11't.11‘.;-tL"1}.thethe workworl' that it was. 5 possible to carry out in the time availableanoilable did not revealtor-3:1: any..:1§.' deposits of111' economic value.\Ltltie. Mapping was carried out on11:1 tracingsti'LLL‘inL'rs ofot' aerial'.'1L‘:'i'.1l photographsnt‘.‘tL'1jI"'-'.‘ and'-..'1.;" on0:1 fieldfield sheetss (scale[scale approximatelyLLDDI'OXll'ITctTtil} 1/64,000)1 (14.0001 based onL111. the 1/250,0001. 25LJ.LI'['Jt.‘I military map. _. Machakos\lzchakos E.A.F.E No. 16371113? (1945).{1945). Later the informationintorination obtained was replotted1e111otILLL'1 on the four-1ti: 1/50,0001'5. preliminaryprcl1'111ir1:L1'_\-' plotsplots ofL11" sheet SA37/H-IV.SAB? l—l—l\". With‘.\"it11 a:1 singleszngc exception,.3\L'L[1‘- .L111. namely11:'11..3‘._. in1:1 the1113 caseuse of{11' the YattaYetta phonolite,phono'ite thethe accurateacccratc demarcation of junctionsj'.1nL':io11s and' contacts..mtacts provedprotec- impossible. The reasonreason forfor thisthis is duect1L firstlyfirsti'y to an.".n insufficientirisut‘rici I:1t number1'... " ofL'.I'. exposures,L-.\p-.'.IsI.1rL'Is. and:111L5 secondlysecondh. because of the gradation of.1: the various rockroe}. typesttgrt's of the Basement System53'ste'1' o."of which the area is largely[E1 rg'ely composed. Acknowledgments:.«t('1'.1.1.11x'1‘1311gr111r31zrs: The'Ihe writer wishes'.1-1511:3111 to express his thanks tor-.1 the:‘131L1ltiLc1'sofficers ofoi thet".1L MakueniMel-{tieni settlement scheme for whose kindness.ndncss and21nd assistanceas ..;1'.1L'c hehe is greatly:13' indebted,indetd. and to thethe District CommissionerC'onnnissioner ofoz' KituiKEtL1i forto: helphelp and'.=. :111 hospitality.'r'os} i. '.-.211'-.. II---=.pREVIOUSll—PRI \ IOL 5 GEOLOGICALGE'.0]. {'.GI( IC. \l. WORK Little geologicalgeologicn‘a work has been carried out in the areastreet previous|1:'I:\'io.1s to the presentpresen'. survey. J..1. W. GregoryGregor} (1921,1.192], p. 186)*1861"" gave anEll] account ofot' the:he Yatta1 ' Plateau ascribing' ' its origin to aat volcanic flowflow which followed the course of0" a:2 valley,‘ concluding, "It is therefore clear that either the existing bandhand of(11' lava231:1. isi'.' the1313 remnantr31: of a wider sheet oro." it must havehave been confinedconfined like a'.1 stream..m betweenhet'fi'een the'.lic bankshair-R's of.'1!' a valley". An officialofi'icial reconnaissancereconnaissance of the Makueni.\l.11tt!eni districtdistriCI.