Duncan Macgregor, History of the development of the East African Rift System

21 Pages·2014·6.41 MB·English

by  Duncan Macgregor

JournalofAfricanEarthSciences101(2015)232–252
ContentslistsavailableatScienceDirect
Journal of African Earth Sciences
journal homepage: www.elsevier.com/locate/jafrearsci
History of the development of the East African Rift System: A series
of interpreted maps through time
⇑
Duncan Macgregor
MacGeologyLtd.,26GingellsFarmRoad,Charvil,BerkshireRG109DJ,UK
a r t i c l e i n f o a b s t r a c t
Articlehistory: Thisreviewpaperpresentsaseriesoftimereconstructionmapsofthe‘EastAfricanRiftSystem’(‘EARS’),
Received6May2014 illustratingtheprogressivedevelopmentoffaulttrends,subsidence,volcanismandtopography.These
Receivedinrevisedform25July2014 mapsbuildonpreviousbasinspecificinterpretationsandintegratereleaseddatafromrecentpetroleum
Accepted17September2014
drilling. N–S trending EARS rifting commenced in the petroliferous South Lokichar Basin of northern
Availableonline2October2014
KenyaintheLateEocenetoOligocene,thoughthereseemtobefewfurtherdeepriftsofthisageother
thanthoseimmediatelyadjoiningit.AtvarioustimesduringtheMid-LateMiocene,aseriesofsmallrifts
Keywords:
anddepressionsformedbetweenEthiopiaandMalawi,heraldingthemainregionalriftsubsidencephase
EastAfricanRiftSystem
andfurtherriftpropagationinthePlio-Pleistocene.Awidevariationisthusseenintheagesofinitiation
History
ofEARSbasins,thoughthemajorityoffaultactivity,structuralgrowth,subsidence,andassociateduplift
Neogene
Timereconstructionmaps ofEastAfricaseemtohaveoccurredinthelast5–9Ma,andparticularlyinthelast1–2Ma.Thesepercep-
tionsarekeytoourunderstandingoftheinfluenceofthediversetectonichistoriesonthepetroleum
prospectivityofundrilledbasins.
(cid:2)2014ElsevierLtd.Allrightsreserved.
Contents
1. Introduction......................................................................................................... 232
2. Pre-EARSrifts........................................................................................................ 234
3. ConstructionoftimereconstructionmapsforEARS1andEARS2Rifts ......................................................... 235
4. EARS1Rifts(?Eo-OligocenetoMidMiocene),Figs.5–8...................................................................... 239
5. EARS2Rifts(Mid-LateMiocenetoRecentandongoing) ..................................................................... 243
5.1. EasternBranch.................................................................................................. 243
5.2. WesternBranch................................................................................................. 247
5.2.1. AlbertineBasin.......................................................................................... 247
5.2.2. LakeEdwardtoLakeTanganyika............................................................................ 248
5.2.3. RukwatoMozambique ................................................................................... 249
6. Offshorebranch...................................................................................................... 250
7. Petroleumimplications................................................................................................ 250
8. Conclusions.......................................................................................................... 250
Acknowledgements................................................................................................... 250
References .......................................................................................................... 251
1.Introduction poorlyunderstood.Theobjectiveofthisreviewpaperistherefore
tointegratethevoluminousliteratureontheEARSwithpartially
TheEastAfricanRiftSystem(‘EARS’)representstheworld’sbest releasedsubsurfacewelldata,inordertocompileaseriesofmaps
exampleofanactiveriftsystem,althoughitsdeeporiginsremain showingtheprogressivedevelopmentofthesystemthroughtime.
Thesemapswillremaindynamicasmoredataisreleasedondril-
lingintheregionandfurtherstudiesareundertaken.
⇑ There is widespread confusion in the application of the term
Tel.:[phone].
‘East AfricanRift System’(‘EARS’). Multipleages of rifting in East
E-mailaddress:[email protected]
http://dx.doi.org/10.1016/j.jafrearsci.2014.09.016
1464-343X/(cid:2)2014ElsevierLtd.Allrightsreserved.
D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252 233
Africa are recognised (Fig. 1), including extensive systems of theSemlikiextensionoftheAlbertineBasin(seebelow),although
Permo-Triassic and Cretaceous age, in each case themselves con- determiningwhetherthisinitiationistrulysynchronouswiththe
sistingofseveralstackedriftphases(Kreuser,1995).Theterm‘East riftingshiftintheTurkanaAreaneedsbetterbiostratigraphiccon-
African Rift System’ (‘EARS’) tends to be applied only to rifts of trolthanisavailableatpresent.Thisdivisionisthereforeusefulfor
Cenozoic age (e.g. Chorowitz, 2005), but even here there may be descriptive purposes, but is likely to be refined in the future as
confusion,astherearethreeoverlappingsubsetsofCenozoicrifts moreprecisedatingbecomesavailable.Offshoreriftsactiveduring
(Figs.1and2),namely(a)thoseofEarlyPaleogeneage,whichusu- this timescale are also presented and discussed in this paper,
ally represent a continuation of rifting from the Late Cretaceous thoughtheseareofdoubtfuldirectaffinitytotheonshoreriftsys-
(e.g. AnzaGraben,(Morleyet al., 1999c)),(b) thoseof?Eo-Oligo- temsandlesseasilyfollowtheEARS1/EARS2division.Essentially,
cenetoEarlyMioceneage,and(c)thecurrentlyactivesetofrifts, therefore, this is a paper which assimilates our current state of
dating,accordingtothiswork,backtotheMidMiocene.Thispaper knowledge on Oligocene-Recent rift activity in East Africa. The
is confined to the study of the two youngest of these phases, i.e. namesappliedforeachofthebasinsdescribedareshowninFig.2.
thosewhicharenotcontinuationsoftheNW–SECretaceousrifts, Data and interpretations assimilate previous regional and
thoughitisrealisedthatsomeoftheEARSriftsmaybereactiva- semi-regional evaluations of the EARS (e.g. Chorowitz, 2005;
tionsofearlierrifts(e.g.RukwaBasin,Fig.2).TheEARSriftsshow Morley et al., 1999a), published outcrop-based studies, particu-
a general N–S trend, except over the long-lived Ubendian line of larly of volcanic dating and geometries related to faulting (e.g.
basement weakness through the Rukwa Basin (Klerkx et al., Wichura et al., 2011) and lakebed lithological studies (e.g.
1998),wheretheyswingtofollowNW–SEbasementtrends(Figs.1 Tiercelin et al., 1992). Geophysical contributions include gravity
and2,Delvaux,2001).Thelimitednumberofriftsactivefrom?Eo- modelling to identify deep rifts (e.g. Ebinger et al., 1989), pub-
Oligocene through to the Early-Mid Miocene boundary, of which lished subsurface seismic studies (e.g. Scholz/Project Probe,
the type example is the South Lokichar Basin (Morley et al., 1989) and new seismic mapping by the author of the PROBE
1999b),arereferredtointhispaperlooselyas‘EARS1’rifts,while seismic over Lake Tanganyika, Lake Malawi (including Sure-
the more extensive Late Neogene (Mid Miocene to Recent) rifts, stream Petroleum reprocessed data) and Lake Turkana. Public
typified by the Albertine Basin (Pickford and Senut, 1994), are domain seismic data and mapping by other authors have also
referred to as ‘EARS 2’ rifts. In most cases, in the onshore, these been incorporated from various sources over the Lake Albert,
twophasesseemtobegeographicallydistinct,asisillustratedon Rukwa, South Lokichar, Turkana, Omo, Chew Bahir, Anza and
Fig. 1, and the distinction has been historically emphasised in Kerio basins (as listed in Table 1).
northernKenya,whereVetel(1995)andMorleyetal.(1999b)doc- Available well information, with the fullest datasets from
umentamajorshifteastwardsthroughtimeinthelocationofrifts. pre-2005 wells, is shown in Table 1. There has been a recent
TheMidMiocenedivisionbetweenthetwophasesalsoseemsto spate of drilling of Ugandan, Ethiopian and Kenyan EARS basins,
mark theonset of Neogenerifting in the Western Branch, within though only sparse data is available on post-2005 wells through
Table1
Publishedwellandseismicdata,plussourceofmappedfaults,overtheEastAfricanRiftSystem.
Well Basin Reference Keypoints
Kingfisher1,2,3 Albertine Loganetal.(2009) ThickPlio-Pleist.LateMioceneoverBasement.Tie
toseismic.Publishedbiostrat
Turacowells Albertine/Semliki AbeinomugishaandKasande 1400mthickMid-Late(and?Early)Miocene
(2012),Lukaye(2009) section
Ngassa2 Albertine SserubiriandScholz(2012) EarlyPliocenesourcerocksdescribedat3000–
3250mclosetoBasementsurface
Waki1 Albertine Karpetal.(2012) UndatedsectionwithsourcerocksoverBasement
Nyuni1 LakeEdward Dominion(2011) LatePlioceneoverBasement
Ruzizi1 Ruzizi MorleyandWescott(1999) Undatedsection
Buringa1 Ruzizi MorleyandWescott(1999) Undatedsection
Ivuna1 Rukwa Morleyetal.(1999d) LatestMiocene-PlioceneovertopOligocene
unconformity
Galela1 Rukwa Morleyetal.(1999d) Asabove
SyracuseBoreholeA LakeMalawi Scholzetal.(2011),Lyonsetal. TDat390matdateof1.2Ma
(2011)
Loperot1 SouthLokichar Talbotetal.(2004),Morley EarlyMiocenereservoirsandsourcerocks,Eo-Early
etal.(1999b) OligclosetomappedBasement
EliyeSprings1 Turkana Morleyetal.(1999b) TieforthickPlio-Pleistoceneandthinner?LateMio
sectioninTurkanarift
Ngamia1 SouthLokichar AfricaOil(2013) EarlyMiocenereservoirsandsourcerocks
Twiga1 SouthLokichar AfricaOil(2013) Asabove
Seismicprogramme Basin Reference Mappingoffaultsinthispaperby
ProjectPROBE LakeTanganyika Rosendhal(1988) Author
ProjectPROBE LakeMalawi Scholz/ProjectProbe(1989) Author
ProjectPROBE LakeTurkana Dunkelmanetal.(1989) Author
AmocoSeismicRukwaBasin Rukwa Morleyetal.(1999d) Morleyetal.(1999d)
AmocoSeismicSouthLokichar, SouthLokichar Morleyetal.(1999b) Morleyetal.(1999b)
KerioandTurkanaBasins
EarlyAcademicSeismicLines Albertine Karpetal.(2012) Karpetal.(2012),AbeinomugishaandKasande
(2012)
RecentCommercialSeismicLines SouthOmo,ChewBahir,Ethiopia: AfricaOil(2013) AfricaOil(2013)
TurkanaandSouthKerio,Kenya
234 D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252
Fig.1. AssignmentofriftsinEastAfricatoPermo-Triassic(‘Karoo’),Cretaceous-Palaeogene,Eo-OligocenetoMidMiocene(EARS1)andMid-LateMiocenetoRecent(EARS2)
phases.Latestriftphaseformsbasisofclassification.RiftsactiveinbothEARS1andEARS2phasesarelocatedintheoffshore,innorthernKenya/southernEthiopiaandinthe
RukwaBasinofTanzaniaandareshownhereintheEARS1phase.R=Neogenereactivationofapre-EARSrift,Cretaceous/PalaeogeneinthecaseoftheAnzarift,Karoointhe
caseofriftsalongtheUbendiantrendfromTanganyikatoMalawi.Forthecolourversionofthisandotherfigures,seetheelectronicversionofthispaper.
conference and investors presentations. However, the maps and orPermo-Triasphase.TheNE–SWtrendingRuhuhuriftofTanzania
chronostratigraphic charts in this paper are consistent, to (Figs.2and3)hasbeensubdividedintothreemainriftphasesin
the author’s knowledge, with all wells drilled as of January the Early Permian, Mid Permian and around the Permo-Triassic
2014. boundary(Kreuser,1995).TheyoungerKarooriftphasesseemto
be less well represented within the NE–SW trending rifts of this
age, such as those exploited by the Western Branch of the EARS.
2.Pre-EARSrifts The‘Karoo’riftsareagainreactivatedineasternandcoastalareas
by Early Jurassic rifts, some of which evolved into the successful
AninterpretationoftherespectiveagesofthevariousPhanero- riftsoftheIndianOcean.
zoicriftsacrossEastAfricaisshowninFig.1,basedoninterpreta- In the earliest Early Cretaceous, a series of rifts developed
tions in the literature, mainly the ages of their thickest across Central Africa, related to plate re-organisations in the
stratigraphicfills.WhilethereissomeevidenceforNeoproterozoic Atlanticocean,generallyreferredtoasthe‘CentralAfricaRiftSys-
riftingalongtheUbendiantrendofTanzania(Klerkxetal.,1998), tem’ or ‘CARS’ (Genik, 1992). NW–SE trending members of this
the oldest regionally extensive rift phase is the so-called ‘Karoo’ population of rifts, including the Anza Basin and Sudan rifts,
D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252 235
Fig.2. Keybasinnamesappliedinthispaper.ThelinesofchronostratigraphyshownasFigs.3and4arelocatedhere.
experienced multiple rift phases extending into renewed Late 3.ConstructionoftimereconstructionmapsforEARS1and
CretaceousandEocene–Oligocenephases,withinterveninginver- EARS2Rifts
sions in the case of the Anza Basin (Bosworth, 1992; Bosworth
and Morley, 1994; Morley et al., 1999c), where activity is gener- The key product of this paper is a series of sequential maps
ally younger than in the Sudanese rifts. Apatite Fission Track illustratingadevelopingunderstandingoftheprogressivedevelop-
Analysis from surrounding basement outcrop (Foster and mentoftheEARS(Figs.5–12).Themapsshowriftfaultsevidenced
Gleadow, 1996) and vitrinite reflectance data on the Elgal wells tohavebeactiveduringtheintervalslabelled,anillustrationofrel-
(Bosworth and Morley, 1994) evidence that the Anza Rift shoul- ativepalaeo-topography(unquantified)andactivevolcanism.The
ders experienced severe erosion in the Late Cretaceous-Palaeo- periodscovered by the mapsspan approx. ±10% of the ages con-
gene. Such erosional events are not seen in the basin or cerned to allow for uncertainties in the dating and to enable the
surroundingarea, suggesting that theshoulders weretopograph- illustrationofthefullseriesofevents.Faultlocationsandtimings
ically high, similar to modern EARS rift shoulders. Many EARS shownaretakenfromtheauthor’smappingofthePROBEseismic
rifts of the Western Branch exploit these earlier rifts (such cases inthevariouslakesandfromthemappingofotherauthorsonseis-
are labelled on Fig. 1 and are illustrated on Fig. 3), particularly micdata,listedinTable1.Furtherinterpretationsoffaultlocations
along lines of basement weakness, though this seems rarely to and timing are taken from: Africa Oil, 2013; Bonini et al., 2005⁄;
be the case for the Eastern Branch (Fig. 4). Chorowitz, 2005; Cohen et al., 1993; Dawson, 2008; Delvaux,
236 D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252
Fig.3. ChronostratigraphyoftheWesternBranchoftheEastAfricanRiftSystem.ScaleamendedtoaccommodatetheCretaceousandPermianriftphasesonwhichmanyof
theEARS2riftsarereactivated.OnlytheRukwaBasinshowsevidencefor(mild)EARS1rifting.TheAlbertineBasinisthebestcontrolledoftheEARS2rifts,withpartsofLake
Tanganyikabeingconceivabletimeanalogues:otherbasins,particularlytothesouth,seemtobeslightlyyounger,asindicatedbybiostratigraphicdatadiscussedinthetext
onwellsintheRukwaBasinandinexhumedoutcropadjacenttoLakeMalawi.SeeFig.2forlineofsection.Referencesarelistedinthetext.LineofsectioninFig.2.Wells:
K=Kingfisher,T=Turaco,Ni=Nyuni,R=Ruzizi,I=Ivuna,Sa=SyracuseA.
2001; Ebinger, 1989; Ebinger et al., 2000; Foster et al., 1997; Le a. Biostratigraphicorvolcanicdatingwithinwellpenetrations
Fournier et al., 1985; Lezzar et al., 1996⁄; Morley, 1999⁄; Morley intheriftsconcerned,tiedwhereverpossibletoseismic.
et al., 1992; Pickford and Senut, 1994; Ring and Betzler, 1995; b. Biostratigraphicorvolcanicdatingofexhumedoutcropsec-
Saemundsson, 2010⁄; Vetel, 1995⁄; Woldegabriel et al., 1990. tionsofrifts,usuallyonupliftedflexuralmargins.
Those asterisked above contain time reconstruction maps for c. Datingofvolcanicsin outcrops, combinedwithanalysesof
restrictedregionsoftheEARSthatareintegratedintothispaper. theirthicknesspatternsrelativetorift-boundingfaults.
Baker, 1986; Ebinger et al., 1989, 1993; Kampunzu et al., 1998; d. Dating of volcanics that are believed to have compositions
McDougall and Brown, 2009, are the main sources for the age of (e.g.rhyolites)thatcorrelatetoperiodsoflikelyextension.
volcanism. e. Extrapolationofmeasuredmodernsedimentationratestoa
Theinterpretationofthetimingofriftinginthisanalysisusesa mappedbasementhorizon.
numberoftechniques,whichcanbelistedinapproximateorderof f. Jump correlations of intervals of similar seismic character
confidencebelow: betweenbasins.
D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252 237
Fig.4. ChronostratigraphyoftheEasternBranchoftheEastAfricanRiftSystem.TimescalelimitedtoEARS1andEARS2riftsasoldersedimentarysections(e.g.Turkana
Grits)areoflimiteddevelopmentandaffinity.NotedistinctionbetweenEARS1riftsinitiatedinOligocenewithpeakriftinginEarlyMioceneandEARS2riftsinitiatedas
depressionswithminorriftingatvarioustimesinMid-LateMiocenewithpeakriftinginPliocene-PresentDay.Referencesarelistedinthetext.LineofsectioninFig.2.Wells:
ES=EliyeSprings,L=Loperot,N=Ngamia.
g. Labelledseismiccorrelationswithininvestorspresentations appliestotheRukwa-MalawitrendoftheWesternBranchwhere
thatcannotbefullyauditedbytheauthor. thereisone wellfullypenetratingthe riftsection tiedtoseismic
(technique a.) and outcrop control in Malawi of an exhumed rift
Inthecontextoftheaboveandthewidevariationsindatabases, section(techniqueb.).Basedonalackofpublishedwelldataatthis
confidence in the interpretation of the precise timing of rifting stage,confidenceisalsolowintheprecisenatureoftimingofrif-
eventsclearlyvariesregionally.Confidenceishighovertheportion tingoffshore,withtheinterpretationshowninferredfromanum-
oftheEasternBranchinKenyaandTanzania,wheretechniquesa. ber of conference papers and diverse seismic images within
andc.canbeemployed,butisdiminishedsomewhatoverEthiopia, investorspresentations(techniquef.).
wheretherearesomeconflictinginterpretationsfromtechniquec. Theinterpretationofpalaeotopographyisbasedonanextrapo-
andlittlepublisheddrillingcontrol.IntheWesternBranch,confi- lationofcurrenttopographybackwardsintime.Thisextrapolation
dence is high in the analysis of the well-drilled Albertine Basin essentially assumes that (a) maximumtopographicvariation and
butlowinLakeTanganyika,wherelowconfidencetechniquessuch amplitude across the region is achieved at Present Day, and (b)
as technique e. need to be used. A level of moderate confidence the rough level of rift shoulder topography was achieved at the
238 D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252
Fig.5. Timereconstructionat28±3Ma,Rupelian,InitiationofEARS1rifts.Keyeventsinclude:(1)firstN–StrendingCenozoicriftsareinitiatedinNKenya?around35Ma.
(probableOligocenestratainLoperot1well),(2)largevolumeoftrapbasaltseruptedfrom28–25MainEthiopia,associatedreliefofAfarplumearound1km,(3)riftingin
Afararound25Ma(Bosworthetal.,2005),(4)earlierMelutandAnzabasinsremainactive,(5)NsungweFm.ofRukwaBasinindicatesmildreactivationofPermian/
Cretaceousrift(Robertsetal.,2012).TopographypriortoEARSdevelopmentbasedonMooreetal.,2009,insouth,AFTAdataofNoble(1997)andFosterandGleadow(1996)
inTanzania,plusOligocenesedimentationratesoffshoreEastAfrica(Morley,1999,Fig.13ofthispaper).Darkershadesonallmapsindicaterelativelyhighertopography.
Volcanicsinred.
time of maximum rift faulting, as interpreted in the paper. The uplift are not seen in spore assemblages within the Kingfisher
critical interpretation that topography is at a maximum in the Wells in the Albertine Basin until around 1Ma (Shaw et al.,
Plio-Pleistocene is supported by a number of lines of evidence, 2009).VolcanicgeometriesanddatingillustratethattheGregory
including (a) the peak of sedimentation rates in the Plio- rift shoulders (technique c.) also did not rise till 1–2Ma (Baker,
Pleistocene in depocentres charged from rivers which drain the 1986;Dawson,2008).
EARS rift shoulders (Fig. 13), (b) the immaturity of river profiles Thetopographyaroundriftsisassumedtohavebeenreducedin
acrosstheregion(Pauletal.,2014;Ganietal.,2007)and(c)inter- magnitudeforperiodspriortomaximumrifting,followingamodel
pretationsoftheageofthemainthrowsontheriftboundingfaults, of rifts developing initiallyas gentle depressions with low throw
as is discussed below for the basins concerned. For example, the bounding faults apparent on Present Day analogues (Modisi
rise of the 5km high Rwenzori mountains, which represent the et al., 2000) and developed on seismic over Lake Tanganyika by
highest non-volcanic, non-orogenic topography in the world, is Tiercelin and Mondeguer (1991) (Fig. 14). Many, though not all,
dated as Plio-Pleistocene by folding of Pliocene strata in outcrop rifts investigated in this paper show increased structural growth
(PickfordandSenut,1994).Highaltitudetaxaassociatedwiththat intoboundingfaultsthroughtime,i.e.greatestisochronvariations
D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252 239
Fig.6. Timereconstructionat20±2Ma,Aquitanian,AcmeofEARS1rifting.Keyeventsinclude:(1)deeplacustrineshale(sourcerock)developmentinSouthLokicharBasin,
(2)riftinginBroadlyRiftedZoneofEthiopiacommencingbetween25Maand21Ma(Levitteetal.,1974;Boninietal.,2005),(3)someactivityinSugutaBasin(Morley,1999),
whereEarlyMiocenesectionpresent,(4)EthiopiandomedevelopedalongwithpeakofRedSeariftingandshoulderuplift(Bosworthetal.,2005),(5)offshoreriftingactive
fromlatestOlig-EarlyMiocene(Danforthetal.,2012;Jeansetal.,2012;Parsonsetal.,2012).
in Recent times. Prior to rifting, interpretations are made of the through time are similar to those independently derived by Paul
development of broad domes with reliefs as suggested by etal.(2014)onthebasisofriverprofilestudies,thoughdifferfrom
Wichuraetal.(2011)fortheKenyadome,basedontheobserved the assumption used in that paper that African topography was
geometriesoflavaflows.Theinterpretationiseventuallytiedfor essentiallyflatatthestartoftheOligocene.
the Oligocene to a broad interpretation of pre-EARS topography
basedonApatiteFissionTrackAnalysis,drainageanalysisandoff-
shoresedimentationratesanalysis.Thisessentiallyassumessome 4.EARS1Rifts(?Eo-OligocenetoMidMiocene),Figs.5–8
residualpalaeotopographyaroundtheAnzaRiftshoulder(asdis-
cussed earlier) and Tanzanian Craton in a region of Late Creta- A new set of rifts were created in the Palaeogene in northern
ceous-PalaeogeneApatiteFissionTrackagesandhightopography Kenyathatdonotexploitearliertrendsandshow,forthefirsttime
in the northern part of the South African plateau as proposed by in Africa, a distinct N–S orientation, thus marking a significant
Mooreetal., 2009:suchtopographyisalso requiredto explaina change in stress direction (Fig. 5). There are many basins in this
peak in sedimentationrates and turbiditesand supplyin e.g. the region and most are undrilled, but the only basin at this time
Rovuma Delta (Fig. 13). The palaeotopographical patterns shown established from drilling and seismic interpretation to contain a
240 D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252
Fig.7. Timereconstructionat15±1Ma,Langhian,latemilderstageofEARS1riftinginEasternBranchandpossibleearliestEARS2riftsinWesternBranch.Keyevents
include:(1)volcanismfillsuprifttopographyinnorthernKenya,(2)riftsactiveinsouthernMER(Boninietal.,2005),(3)SemlikiSub-BasinofAlbertineBasinprobably
initiatedclosetothistime,basedonstratigraphyofTuracowell,(4)assumedcontinuationofriftingoffshorebasedonsparsepublishedseismiccoverage.
thick(6–7km)stratigraphicsectionofthisageistheSouthLoki- Earlyto earliestMid-Mioceneage (‘Auwerwer Formation’)before
charBasin. thebasinwasupliftedintheLateMiocene-Plioceneontheshoul-
TheSouthLokicharBasinhasnowbeendrilledbyalargenum- derofthelaterEARS2Turkanarift(McDougallandBrown,2009).
berofrecentwells(AfricaOil,2013).ResultsoftheearlierLoperot Vetel(1995)proposesthattheinitiationofthebasinwasaround
1wellhavebeenpublishedindetail(Figs.4and14,Morleyetal., 35Ma,whichisconsistentwiththeavailabledata.
1999b;Talbotetal.,2004).Thewellpenetratedadeeplacustrine Two adjoining rifts in the Turkana complex, the South Kerio
shale (the ‘Loperot Shale’), dated as ‘Early Oligocene to possible (Fig.14)andNorthLokicharrifts,haveestablishedEARS2sections
Eocene’,some300msTWTabovethepredictedbasementsurface observable onseismicandtiedtooutcroppingarkosesbelowthe
at a central point within the rift (Fig. 14). The earliest rift units regional mid Miocene volcanics (Morley et al., 1999b; Vetel,
showaplanargeometryonseismic,withoutsignificantthickening 1995). Their sections seem to be thinner than in South Lokichar,
totheboundingfault.Peakriftsubsidencewasprobablyattained atupto2kminNorthLokicharandperhapsupto5–6kminSouth
at the time of deposition of a younger deep lacustrine shale unit Keriowhereseismicdefinitionishoweverpoor.Vetel(1995)spec-
termedthe ‘Lokone Shale’,which showsa distinct wedgeshaped ulatedthese basinsmay havebeeninitiatedlater,perhapsin the
geometryonseismic,andisdatedinrecentwellsasEarlyMiocene Early Miocene, partly due to their thinner nature and partly to
(Africa Oil, 2013). This is overlain by a sandy rift infilling unit of follow a trend of younging volcanism to the east. Drilling is
D.Macgregor/JournalofAfricanEarthSciences101(2015)232–252 241
Fig.8. Timereconstructionat12±1Ma,Serravallian,transitionalphasebetweenEARS1andinitiationofearlysagphasesofEARS2rifts,withmigrationofrifts.Keyevents
include:(1)jumpofriftactivityfromSouthtoNorthLokicharBasinandtoTurkanaBasinaroundendmidMiocene(Morleyetal.,1999b),(2)lackofrifting(Elgayo
escarpmentexcepted)butexistenceofcirca1400mhighdomeevidencedbyvolcanicgeometriesofKenyaphonolitesat13.5–11Ma.(Wichuraetal.,2011),(3)continued
riftingofSemlikiportionofAlbertineBasin(e.g.PickfordandSenut,1994)andcommencementofsubsidencebelowLakeAlbert,(4)abestcaseassessmentofLake
Tanganyikariftssuggestsinthemostlikelycasethattheyhavenotbeeninitiatedbythistime,thoughthiscannotbeexcluded,(5)poorlycontrolledperiodofrelative
quiescenceoffshorebetweenriftphases(e.g.Jeansetal.,2012).
plannedlaterthisyear(2014)toanalysewhetherthecriticalEarly aroundandeastofLakeTurkana(Vetel,1995),whichsuggeststhat
Miocene source rock and reservoir section seen in the South riftingwas concentratedaround South Lokichar.Theexistence of
Lokicharwellsispresentandshouldresolvethisissue. EARS1riftsectionsbelowthemidMiocenevolcanicsbelowLake
ElsewhereinnorthernKenya,sparseevidencesuggeststheexis- Turkana,andthereforebeyondseismicdefinition,cannothowever
tenceofonlythinsedimentarysectionsofEarlyMioceneage. An beruledout.
Early Miocene sandy section is uplifted along the Elgayo Escarp- InthemainpartoftheGregoryRift,Mid-Miocenevolcanicflow
ment in the northern Gregory rift (Bosworth and Maurin, 1993) geometriesaregenerallyunsupportiveofthepresenceofanypre-
whilethinshallowlacustrinedepositshavebeendatedataround existingrifttopography(Wichuraetal.,2011),suggestingtheEARS
20Ma by interbedded tuffs east of Lake Turkana (Ebinger et al., 1 rifts did not propagate any further to the south. Rifting also
2000). These sections, as shown by the maps of Morley et al. seemstohavealsocontinuedfromPalaeocenetimesintheearlier
(1999b),areindicativeoffluvialandshallowlacustrinedeposition, Anza Graben, extending up to an unconformity in the Hothori 1
perhaps in localised small and slowly subsiding rifts. It can be well,roughlydatedasintra-EarlyMiocene(Morleyetal.,1999c).
noted that pre mid-Miocene volcanism is sparse in the region Itisalsopossiblethatthegravity-definedGatomeandLotikipirifts