Tectonophysics
-
191
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
SEQUENCE AND GEOCHRONOLOGY
OF THE KENYA
RIFT VOLCANICS
B.H. BAKER ’ , L.A.J. WILLIAMS ‘, J.A. MILLER 2 and F.J. FITCH 3
i Department
2 Department
3 Department
of Geology, University of Nairobi, Nairobi (Kenya)
of Geodesy and Geophysics, University of Cambridge, Cambridge (Great Britain)
of Geology, Birbeck College, University of London, London (Great Britain)
(Received June 10, 1970)
ABSTRACT
Baker, B.H., Williams, L.A.J., Miller, J.A. and Fitch, F.J., 1971. Sequence and geochronology
Kenya rift volcanics. Tectonophysics, 11: 191-215.
of the
The Upper Cenozoic volcanic formations associated with the Kenya rift valley are divided into seven
main stratigraphic groups representing widespread ‘plateau’ formations, and a number of central volcano groups of more local&d occurrence. 75 K/Ar isotopic age determinations including 30 unpublished dates are listed and the age-range of most of the volcanic groups is given. A tabulation of the
sequence of volcanic groups is provided and their distribution is illustrated.
The Kenya rift volcanics were erupted nearly continuously from Early Miocene to Holocene times.
Miocene volcanism was mostly of nephelinites, alkali basalts and phonolites. Pliocene activity was
trachytic, phonolitic and nephehnitic in central and southern parts of the rift valley, while contemporaneous basaltic volcanism took place along its whole length. The only extensive rhyolitic activity
occurred in northwest Kenya in Pliocene times. Late Pliocene and Quaternary volcanism was mainly
trachytic in the rift valley floor and basaltic to the east.
INTRODUCTION
Systematic mapping of the Kenya rift valley and surrounding areas by the Geological
Survey of Kenya now covers more than three-quarters of the outcrop of Cenozoic volcanics. The purpose of this contribution is to provide a brief summary of the main
stratigraphic groups that can be established from the literature and from first-hand knowledge of much of the ground. There is a need for a provisional stratigraphic framework
as a basis for future work.
The Cenozoic volcanic formations are of two kinds: extensive formations showing
little lateral variation that were erupted from fissures or numerous small centres, which
form the main areas of volcanic plateaux and plains; secondly, more localised formations
erupted from central volcanoes, which are often intercalated within or rest upon the
plateau volcanics. Many of the larger central volcanoes are characterised by well differentiated volcanic series, whereas the plateau formations are generally more homogeneous
in composition(Williams,
1969a, 1970). The distribution of the main volcanic groups
is shown on the geological map (Fig. l), and place names are shown on Fig. 2.
uTlTll
H
Quoternary
bosolts
Quoternory
cent
volcanoes
Pliocene
basal&
und trwhytes
++
.rl+
+
. _ .,_..___
r
&ocene
rh”olltCri
+ and nuqeor1tes
Mmcene
Fig, 1. Geological
Miocene cent,CJ, “clICQ”oe5
and alkahne complexes
bosoits
map of the Kenya
rift volcanics
(locality
names shown on Fig. 2).
:cP .’
.’
;<
-40
\
5.
!
I
i
2
1.
CENTRAL
,
Aberdare
range
VOLCANOES
%/r;\ 44
I3 Kwngwl
3 Eburru
15 Ngong
4
2 Elgon
Borrer
16 01
Domyo Sambu
14
Menengol
5 Emuruangogalok
17 01 Esokut
6 Lenderut
18 Olorgesollle
7 Lodwor
8 Londaam
9 Lorqonot
IO Mt Kenya
I I K~hmonloro
I2 Kdombe
Fig. 2. Locality map.
‘A
\AMBOSELl
8
‘\
“(,
9
t
@‘I
J4Qo
O+
01 Esoyelt
19 Pokka
20
Shombole
‘1.
ii.11. BAIil
I 04
K, L.:\.J. \VILLIAMS.
J.A. MILLL’K .AhL, I .J. I I’l( II
The opportunity has been taken to list nearly ail the isotopic ages on the Kenya
Cenozoic volcanics that are available to the ;iuthors. The few published dates that arc
omitted from the tables arc those that arc inconsistent with observed stratigraphical successions. Anomalous results are the consequence of sample contamination
or result from
the presence of inherited argon, or from subsequent argon losses. The majority of the isotopic ages were obtained by the conventional K/Ar method (Miller, 1967), and three
ages were obtained by the 40Ar/‘YAr method (Fitch et al.. 1969; Fitch and Miller, 1970);
many of them have not been previously published. A number of these have been provided
by the courtesy of the ~onlrnissioner of Mines and Geology. Nairobi; Prof. H. Patterson
of the Harvard Museum of Comparative Zoology. and Dr. G.11. Curtis. Dept. of Geology
and Geophysics. University of California, Berkeley.
The provisional time-scale employed in this contribution
is that most generally used in
East Africa, the beginning of the Miocene, Pliocene and Pleistocene Epochs being placed
at 25, I? and 2.5 n1.y. (million years) respectively. rather than the scales suggested by
Funnell ( 1964) and Berggren ( 1969).
THE PLATEAU
I~O~MATIO~S
Miocene basalts
The oldest volcanic rocks of central and northern Kenya are the Miocene basal& which
rest locally on fossiliferous Lower Miocene sediments or on Precambrian rocks.
Basal& correlated with those of the Samburu Series of the Nanyuki-Mar&I
area
(Shackleton, 1946) have been traced northwards nearly to the south end of Lake Rudolf
(Baker, 1963a; Dodson, 1963). EquivaIent basalts have been found at the rift valley margins and in its floor around Lake Hannington (McCall, 1967). These basalts have been
correlated by some authors with the Simbara basalts which form the basis of-the Aberdare
Range (Shackleton, 1945;McCall, 1%7), but stratigraphic and isotopic age evidence now
suggests that the latter are considerably younger.
The extensive basalts of Turkana have been correlated with the Samburu basalt type
area (Joubert, 1966; Walsh and Dodson, 1969; Dodson, in press), a correlation confirmed
by isotopic age determinations,
but because of lack of physical continuity across the
floor of the rift valley they may be distinguished by naming them the Turkana basal&
Equivalents of the Samburu basalts west of the central part of the rift valley (the
Elgeyo basalts) rest on fossiliferous Lower Miocene Tambach sediments and crop out
beneath the well dated Upper Miocene phonolites which cap the Elgeyo escarpment
(Shackleton, I951 ; Bishop et al.. 1969; Walsh. 1969).
Nine of the ten isotopic ages fail into the range 14-23 m.y., and the single remaining
age (32 m.y.) is of doubtful reliability. The younger limit of this range is consistent with
the maximum age of the overiying plateau phonolite group of 14 m.y. (Table I).
The Turkana (Miocene) basalts cover much of the floor of the Turkana depression in
northwestern Kenya, and also probably extend to the east of Lake Rudolf. Further south
the Samburu basalts outcrop is limited to a narrow zone on each side of the Lakes Hannington and Baring0 region. They are not exposed south of the latitude of Nakuru, and were
erupted from many small centres in a pre-rift depression in the northern half of Kenya.
AGE RELATIONS OF THE KENYA RIFT VOLCANICS
195
TABLE I
Isotopic ages of the Miocene bawlts
Sample
number
Rock/mineral
analysed
Locality
Formation
Age (m.y.)
26011
basalt 1$2
Lokitaung,
north Turkana
Turkana basalts
(Walsh and Dodson, 1969)
32.2 * 0.5
31.5 f 0.7
264/S
basalt lp2
Lokitaung
north Turkana
Turkana basalts
(Walsh and Dodson, 1969)
23.0 + 0.7
20.7 + 0.4
29314
basalt ‘,’
Nathira,
north Turkana
Upper Turkana basalts
(Walsh and Dodson, 1969)
23.5 + 1.1
23.3 + 0.2
LS-1-64K
basalt 1$4
Loperot,
Turkana
Turkana basalts
(Joubert, 1966)
15.8 & 1.2
LS-2-64K
basalt “4
Loperot,
Turkana
Turkana basalts
(Joubert, 1966)
17.5 f 0.9
LS-2-65K
basalt 1,4
Loperot,
Turkana
Turkana basalts
(Joubert, 1966)
16.7 2 0.8
LS-7-67K
basalt r*’
Lothagam,
Turkana
Turkana basalts
(Patterson et al., .1970)
16.8 f 0.5
IO-7
basalt “4
Lothidok,
Turkana
Upper Turkana basal&
(Walsh and Dodson, 1969)
14.0 * 1.2
27131
basalt 1,3
north of Maralal,
Samburu
Samburu basalts
(Shackleton, 1946)
23.0 + 3.2
19.8 + 3.1
18.5 f 2.9
34/1043
basalt lp3
Elgeyo
Elgeyo basalts
(Walsh, 1969)
15.1 + 3.2
15.6 +_3.2
scarp
r Whole rock K/Ar isotopic age determination.
’ Grasty (Reilly et al., 1966).
3 F.M. Consultants for Geol. Surv. Kenya (unpub~shed}.
4 Geochron Lab. for Prof. B. Patterson (unpubfished).
5 Geochron Lab., Patterson et al. (1970).
Plateau phonolites
Extensive phonohte plateaux are found on both sides of the rift valley in central Kenya,
their outcrop being centred on the Nakuru area. They commonly overlie the Miocene
basalts and overlap them in central Kenya, but did not extend far northwards into Turkana. In southern Kenya they are the oldest Tertiary volcanic rocks.
Shackleton (1946) distinguished three types of phonolites in the areas north and south
of Maralal: the Rumuruti, Losiolo and Thomson’s Falls phonolites, which were separated
on their petrological characteristics. The youngest division - the Thomson’s Falls phonolites - is now tentatively regarded as Pliocene in age on the evidence of a single isotopic
date.
The Losiolo phonolites extend north to the Baragoi area (Baker, 1963a) and southwards
7’ecronophysics, 11 (1971) 191-215
196
I).fi. UAKIIR, L.A.J. WILLIAMS,
J.A. MILLER AN11 !:.J. I-I l(‘li
to the area east of Nakuru (McCall, 1967). West of the rift valley similar phormlrtes cap
the Elgeyo escarpment (Walsh, 1969) and form the Uasin Gishu plateau (Sanders, 1963;
Jennings, 1964). An isolated outcrop area of phonolite around Kisumu in the Kavirondo
Rift (Saggerson, 1952) has isotopic ages which place it in this group. To the south of the
Kavirondo Rift the Kericho and Mau phonolites (Binge, 1962; Williams, in press) crop out
on the western slopes of the Mau range, and extend further south to the Mara and Narok
areas (Williams, 1964a; Wright, 1967). The plateau phonolites north and south of the
Kavirondo Rift may be referred to as the Uasin Gishu and Mau phonolites respectively.
Several isotopic ages show that the Kapiti and Yatta phonolites (Fairburn, 1963; Walsh.
1963; Matheson, 1966) of the southeastern part of the volcanic field belong to the
plateau phonolite group (Table Ii).
The thirteen isotopic ages of plateau phonofites fall into the narrow range 11.0 - 13.5
m.y., indicating that the group was erupted during a short period of time at the end of
Miocene and in earliest Pliocene times. The group is thus a useful widespread and welldated stratigraphical marker.
did-PIi~ce~e p~onolites and rrachytes
The group includes two main formations - the Kabarnet trachytes (circa 7 m.y.) and
the Thomson’s Fails phonolites (circa 6.5 m-y.). The former occurs on the upper part of
the Kamasia Range (Walsh, 1969) and the latter occurs in the Bahati area and in southern
Laikipia (Shackleton, 1946; McCall, 1967).
The lower part of the volcanic succession of the Nairobi area (Nairobi and Kandizi
phonolites, Mbagathi phonolitic trachyte) is tentatively included in this group (Williams,
1967), for it rests upon the Upper Miocene Kapiti phonolite and is overlain by local representatives of the P~io-Pleistocene trachytic group.
The full extent of representatives of the group in the floor of the rift valley is obscured
by younger volcanics. A further complication is the fact that the Pliocene basal& of the
southern part of the rift valley floor (see below) and the Mid-Pliocene phonolites and
trachytes were approximately contemporaneous
(Table III).
The three isotopic ages available from this group suggest that the phonolites and
trachytes in the Thomson’s Falls and Kabarnet regions were erupted before the similar
flows that reached the Nairobi area from the west. The three sub-groups are not obser
in contact and their extent and stratigraphical relations are unknown.
Pliocene rhyolites and mugearites
The Miocene Turkana basaits of northwestern Kenya are overlain by thick sheets of
rhyolites and ignimbrites with intercalated mugearites and trachytes. These reach their
thickest development on the Murua~~r range, but extend as discontinuous occurrences
over much of northern Turkana (Walsh and Dodson, 1969; Fairbum and Matheson, 1970).
No isotopic ages are available, but the group isundoubtedly
post-Miocene. The degree
of dissection of the rhyohtic ranges suggests a Lower Pliocene age.
AGE Rl!LATIONS
OF THE KENYA
TABLI:
II
Isotopic
ages of the plateau
19:
RIFT VOLCANICS
phonolites
Locality
Formation
Age (m.y.)
“’
Tambach,
I$eyo
scarp
Lower Uasin Gishu phonolites
(Walsh, 1969)
13.6 f 0.6
13.5 + 0.3
phonolite
“’
Chepkorio,
Elgeyo scarp
Upper Uasin Gishu phonolites
(Walsh, 1969)
12.0 + 0.3
12.0 f 0.3
MB/II
phonolite
“’
northwest
Kericho
of
Kericho phonolites
(Binge, 1962)
12.4 f 0.2
12.3 * 0.2
MB/12
phonolite
“’
northwest
Kericho
of
Kericho phonolites
(Binge, 1962)
12.1 f 0.2
13.2 ? 0.3
13.4 * 0.4
MB/16
phonolite
I”
Fort Tcrnan,
northeast
of Kericho
Kcricho phonolites
(Binge, 1962)
11.8 _+0.2
11.8+-0.3
MB/1613
phonolite
I”’
northwest
of Kcricho
(same as MH/IZ)
Kcricho phonolites
(Binge, 1962)
12.6 + 0.7
12.5 ? 0.4
KA1749
phonolitc
“7
Ombo. near
Kisumu
Kisumu phonolites
(Saggerson,
1952)
13.1 20.6
MB/IO
phonolite
‘ls
Maseno. north
of Kisumu
Lower Kisumu phonolites
(Saggerson,
1952)
12.3 2 0.2
12.3 _+0.2
LS
phonolitc
“’
Kirimun, south
of Maralal
Lower Rumuruti
phonolitcs
(Shackleton,
1946)
12.0 + 0.6
phonolite
‘&
Kirimun, south
of Maralal
Upper Rumuruti
phonolitcs
(Shackleton,
1946)
11.0+0.6
KA651
phonolite
14
Stony Athi,
southeast of Nairobi
Kapiti phonolitc
(Sikes, 1939:
Saggerson, in press)
13.4
MB/28
phonolite
XG
Stony Athi,
southeast of Nairobi
Kapiti phonolite
(Sikes, 1939;
Saggerson. in press)
12.9 + 0.7
13.1 + 0.5
KA 1054
phonolitc
”
Yatta plateau.
east of Thika
Yatta phonolite
@‘airburn. 1963
13.2
Sample
number
Rock/mineral
analysed
MB/8
phonolite
MB/9
l-63K
LS-2
63K
I Whole rock
’
3
4
’
6
7
*
K/Ar isotopic age determination.
Feldspar determination.
Nepheline determination.
Evernden and Curtis (1965).
Miller (Bishop et al., 1969).
Geochron
Lab for Prof. 13. Patterson (unpublished).
Curtis (Bishop et al., 1969).
First determination
by the total degassing 4o Ar/3gAr method;
40Ar/39Ar
isochron age determination
(Bishop et al., 1969).
Tectonophysics,
11
( 197 1) 19 1 2 15
sxond
the average
plateau
age of a
Isotopic ages of the Mid-Pliocene phonolites and trachytes
Sample
number
-
~ock/nlineral
Locality
analysed
_--.-___----.-
Formation
Age tm.y.i
JM.734
trachyte Iv3
Lelian,
Kabarnct
Kabarnet trachytes
(Walsh, 1969)
7.1 20.3
7.3 + 0.3
7.3 + 0.4
w.227
phonohte
Thomson’s
Falls
Thomson’s Falls phonolites
(Mc<‘alI, 1967)
6.7 _+0.5
6.2 + 0.5
6.4 S 0.5
KA663
phonolite ”
Beacon Ranch
Athi River
Nairobi phonolite
(Williams, 1967)
5.2
IS3
_---._
’ Whole rock K/Ar isotopic age determination.
’ feldspar determination.
3 P.M. Consultants for Geol. Sure. Kenya (unpublished).
4 Evernden and Curtis (1965).
Pliocene basalts
Basalts outcrop on marginal step-fault platforms in the southern part of the rift valley
and they also occur beneath a cover of younger volcanics along parts of the floor. The
Kirikiti basalts west of Magadi (Baker, 1958, 1963b), the Singaraini and 01 Keju Nero
basalts east and northeast of Magadi (Baker, 1958; Matheson, 1966), and the basaltic
flows in the Narok area (Wright, 1967) are ali overlain by plateau trachytes representing
the upper part of the Plio-Pleistocene trachytic group described below. There is little
direct evidence of the lower age limits for these episodes of basaltic volcanism, but a
Pliocene age for the Kirikiti basaits is confirmed by isotopic dates of about 5 m.y. Moreover, the Singaraini basalts rest on, or interdigitate with, volcanics of the Mid-Pliocene
01 Esayeiti-01 Esakut centres, and the 01 Keju Nero basalts rest locally on flows from the
Olorgesailie volcano of presumed Pliocene age. The basalts of the Narok area are evidently
intercalated in a Pliocene pyrocIastic sequence which includes ignimbrites (Williams, 1964b).
In the central part of the rift the extensive Kaparaina basaits overlie the Mid-Pliocene
Kabarnet trachytes of the Kamasia Range and occur below fossiliferous Pho-Pleistocene
sediments (Martyn, 1967; Martyn et al., 1968). The Kwaibus basalt north of Nakuru is
overlain by the Plio-Pleistocene Lake Hannington phonolites and rests on Pliocene tuffs
and sediments (McCall, 1967; Walsh, 1969).
The precise northward extension of Phocene basalts is at present uncertain but isotopic
dates ranging from 8.3 to 2.5 m.y. show that representatives of the group occur at Lothagam and Kanapoi in south Turkana (Patterson, 1966; Patterson et al., 1970). Some of the
basaltic formations of the nearby Loriu plateau may also belong to this group (Rhemtulla,
1970) and it is likely that part of the little-known basaltic region east of Lake Rudolf is
of the same age. Basalts capping Upper Tertiary sediments in the Laisamis-Merti region
east of the rift valley have been tentatively assigned to the Pliocene (W~li~s,
1966)
(Table IV).
AGE RELATIONS
OF THE KENYA
TABLE
IV
Isotopic
ages of the Pliocene
RIFT VOLCANICS
199
basalts
Sample
number
Rock/mineral
analysed
Locality
Formation
Age (m.y.)
MB/38
basalt
1’2
Oloibortoto
R.,
\;,est of Magadi
Kirikiti
(Baker,
basalts
1958)
5.13 + 0.10
MB/39
basalt
1,2
Oloibortoto
R.,
west of Magadi
Kirikiti
(Baker,
basalts
1958)
5.03 f 0.20
5.07 f 0.10
MCZ
S 65-K
basalt
1,3
Kanapoi,
south Turkana
Kanapoi basalt
(Patterson,
1966)
2.9
2.5
KA226 1
basalt
1,4
Kanapoi,
south Turkana
Kanapoi basalt
(Patterson,
1966)
2.71 + 0.26
KA2262
basalt
1*4
Lothagam,
south Turkana
Sill in Lothagam group
(Patterson et al., 1970)
3.71 + 0.23
KA2294
basalt
iy4
Lo thagam ,
south Turkana
Pliocene basalts
(Patterson
et al., 1970)
8.31 + 0.25
’
2
3
4
Whole rock K/Ar isotopic age determination.
F.J. Fitch and J.A. Miller (unpublished).
Geochron
Lab (Patterson,
1966; Patterson and Howells,
Gilbert (Patterson
et al., 1970).
1967).
The Pliocene basalt group is loosely defined and contains
spanning much of Pliocene time. Presently available evidence
tensive basalt formations of the central rift are Mid-Pliocene
panied by eruptions of central volcanoes such as 01 Esayeiti
which are also Mid-Pliocene (see Table VIII).
Plio-Pleistocene
f 0.3
f 0.2
a variety of formations
suggests that the more exin age, and were accomand the Aberdare Range,
trachytic group
Trachyte lavas and ignimbrites occupy nearly all of the floor of the central and
southern parts of the rift valley and also occur locally on the marginal plateaux. The
group consists of a lower part of trachytic tuffs with prominent ignimbrite units which is
best exposed on the Kinangop and Bahati platforms at the eastern side of the central rift
(Thompson and Dodson, 1963 ; Thompson, 1964; McCall, 1967). Thinner far-reaching
ash flows of similar composition and compatible stratigraphical level are found east of the
rift on the flanks of the Aberdare volcano (Suguroi tuffs, Shackleton, 1946) in the Nyeri
area (Nyeri tuffs, Shackleton, 1945; Baker, 1967) in the Fort Hall and Thika areas
(Fairburn, 1963, 1966) and around Nairobi (Kerichwa valley tuffs, Williams, 1967;
Saggerson, in press). West of the rift valley similar ignimhrites form most of the Mau Range
(Williams, in press), and extend northward toward Molo and Eldama Ravine (Walsh, 1969;
Jennings, in press). Trachyte and locally some basalt flows occur intercalated in the ignimbrite succession of the rift floor. The precise age relationships of the various isolated
Tectonophysics,
ll(l971)
191-215
tut‘f sub-groups cannot bc established, but there is little &~ubt that similarity crf’stratigraphical level justifies their ccwelation.
The upper part of the group is represented by the Plateau trachyte series of the Magadi
area (Baker, 1958, 1963b) and extends nor~eastwards
on to the eastern shoulder of the
rift valley as the Lirnuru, Tigoni, Karura and Kabete trachytes of the area west and northwest of Nairobi (Saggerson, in press). In this area, on the Kikuyu escarpment and in the
Kijabe area, trachytic flows and pyroclastics are interbedded (Thompson, 1964), and
suggest that the two divisions of the group grade laterally and vertically into one another.
The Lake Hannington phonolites (phonolitlc trachytes) &ver a wide area of the floor
of the rift valley north of Nakuru (McCall, t967), and are included in the group, for
their equivalents southwest of Lake Baringo overlie the Plio-Pleistocene Chemeron sediments (Martyn, 15167; McCall et al., 1967).
In the Gilgil and Nakuru regions the Gilgil and Willan’s Farm trachytes, and the Ronda
and Lake Nakuru phonolites and trachytes rest upon the main tuff-ignimbrite sequence
(McCall, 1967). In the area east of Narok, tuffs and ignimbrites containing intercalated
thin basaft flows are overlain by the Plateau and Angata Naado trachytes (Wright, 1967).
Recent age determinations
on tuffs from an area east of Lake Rudolf indicate a PlioPleistocene age for the Koobi Fora beds (Behrensmeyer, 1970; Fitch and Miller, 1970).
These pyroclastics may represent an isolated occurrence of formations equivalent to the
Plio-PIeistocene trachytic group, but their geological setting is unknown.
The group thus has a complex m~~rostratigraphy and detailed subdivision will only be
accomplished after more detailed mapping, It undoubtedly
represents an important phase
of pyroclastic and effusive trachytic volcanism of Late Pliocene to Mid-Pleistocene age,
originating from fissures OKlow volcanoes in the rift valley floor (Table V).
The isotopic ages quoted indicate a provisional age range of 2.5 - 5 m.y. for the ignimbritic lower part of the group, while the trachytic Iavas of the Magadi-Nairobi region
(Plateau trachytes and Limuru trachytes) have a range from 1.7 to 0.6 m.y. The limited
data suggest that the i~~~nbrit~s are mostly Upper Pliocene, whereas the bulk of the
trachyte flows are Early to Middle Pleistocene in age.
Quaternary basal&
Three main fields of ~~aternary basafts occur I SO-250 km east of the rift va.Ifey each
composed of numerous small basaltic cones with striking linear patterns inditiative of
fissure control. The Chyulu basalts comprise same hundreds of small basaltic cones resting upon an apron of flows, with a conspicuous NW-SE alimment (Saggerson, 1963).
Very similar to the Chyulu basalts are the Nyambeni and Thiba basalts forming a
NE-SW chain ofcones and vents in cen~al’Kenya (Baker, 1967; Rix, 1967), and the
Huni HiIls basalt chain of northern Kenya (D&y, I%%).
The Marsabit basal8 on the other hand were erupted from a Iow basaltic dome surmounted by parasitic vents, cones and maars.
Relatively minor occurrences of Quaternary basalts are found in the floor of the rift
valley - the Elmenteita basafts (Thompson and Godson, 1963; McCall, 1967) and the
Holocene basalts of the idands of Lake Rudolf.
All these basal& are believed to be of Late Pleistocene to Holtocene age because of tie
AGE RELATIONS
OF THE KENYA
TABLE
V
Isotopic
ages of the Plio-Pleistocene
Sample
number
Rock/mineral
analysed
MB/32
ignimbrite
MB/25
trachyte
KA411
KA654
RIFT VOLCANICS
trachytic
201
group
Locality
Formation
Age (m.y.)
Langata,
Nairobi
Kerichwa valley tuff
(Williams, 1967)
4.84 f 0.12
5.67 f 0.10
iy4
Nairobi
city
Nairobi trachyte
(Williams, 1967)
3.45 + 0.16
3.17 + 0.06
trachyte
23
south Kinangop
trachyte in Kinangop tuffs
(Thompson
and Dodson,
1963)
3.34
trachyte
‘3
Limuru
northwest
Limuru trachytes
(Williams, 1967)
1.72
24
of Nairobi
MB/26
trachyte
54
Limuru,
(=KA654)
Limuru trachytes
(Williams, 1967)
1.55 + 0.09
1.59 F 0.09
KA650
trachyte
13
NairobiMagadi road
Plateau
(Baker,
trachytes
1958)
1.74
MB/35
trachyte
1!4
Magadi
Plateau
(Baker,
trachytes
1958)
1.42 + 0.05
1.36 + 0.06
MB/36
trachyte
lT4
west of Magadi
Plateau
(Baker,
trachytes
1958)
0.72 + 0.09
0.91 + 0.06
MB/37
trachyte
q4
southwest
Lengorale trachyte
(Baker, 1963b)
0.63 f 0.09
0.64 + 0.09
IBl
pumice
r*’
Koobi Fora, east
of Lake Rudolf
Koobi Fora beds
(Behrensmeyer,
1970)
3.02 + 1.6
3.45 * 1.2
2.50 f 0.5
1B2
pumice
55
Koobi Fora, east
of Lake Rudolf
Koobi Fora beds
(Behrensmeyer,
1970)
2.37 + 0.3
2.64 f 0.29
’
2
3
4
5
of Magadi
Whole rock K/Ar isotopic age determination.
Feldspar determination.
Evernden and Curtis (1965).
F.J. Fitch and J.A. Miller (unpublished).
Second age (IB 1 and IB2) by total degassing 4o Ar/3gAr age determination;
third determination
(IB 1 only) is plateau age obtained by step-heating
40Ar/3gAr
analysis (Fitch and Miller, 1970).
excellent preservation of their surface features. No isotopic dates are available from them,
but a radiocarbon age determination from below a basalt in the Chyulu range gave
480 f 200 years (Saggerson, 1963, p.31).
CENTRAL
VOLCANOES
Miocene central volcanoes
The nephelinite-phonolite
volcanic centres of western Kenya and eastern Uganda form
Tecronophysics, 11 (1971) 191-215
20
B.li. IIAIWR. L.&J.
WILLIAMS.
J.A. MILLER
Ai’iU k.J. I:ITt’H
a well-defined petrographic sub-province characterized by highly undersaturated volcanics
with abundant pyroclastics, and with alkaline intrusives and carbonatites. The chief examples of nephehnite-phonolite
volcanoes in Kenya are Elgon and Kisingiri; the activity
of Tinderet, and perhaps also of Londiani, extended from Miocene into Pliocene times.
The nephelinite volcanoes contain Miocene fossiliferous sediments near their bases
(Bishop and Whyte, 1962; Bishop, 1967).
The Elgon volcanics consist of nephelinites, melilitites, phonolites and pyroclastics
(Davies, 1952; Searle, 1952). Miocene fossiliferous deposits are found low in the succession on the northeastern side of the volcano (Walker et al., 1969).
The isotopic ages at present available (Table VI) are inadequate to determine the age
range of the EIgon volcanics with certainty, but they suggest that the main phase of activity occurred between 15 and 22 n1.y. ago, and was therefore broadly contemporaneous
with the eruptions at Kisingiri volcano farther south.
The Kisingiri volcanics comprise nephelinites, melilitites and pyroclastics belonging to
a large volcano at the western end of the Kavirondo Gulf, Lake Victoria (McCall, 1958).
Some nepheliniti~ and phonoljti~ volcanics were derived from smaller centres between
Kisingiri and the carbonatite complex at Homa mountain (Saggerson, 1952) but the
precise ages of these volcanoes are unknown.
The middle division of the Kisingiri volcanics contains a variety of pyroclastics and
sediments in which Lower Miocene fossil assemblages have been found on Rusinga Island
(Shackleton, I951 ; Whitworth, 1954; McCall, 1958), Mfanganu Island and at Karungu on
the mainland (McCall, 1958). Most of the isotopic dates were determined to establish the
age of these fossiliferous beds (Evernden and Curtis, 1965; Bishop et al., 1969).
The general succession on Rusinga Island is as follows (Shackleton, 195 1; Van Couver.
ing and Miller, 1969):
Lunene lavas (Upper Kisingiri volcanics) 16.5 m.y.
Kiangata agglomerate
Hiwegi Series
19.6 m.y.
Rusinga agglomerate group
21.7(15.2)m.y.
Kiahera Series
The 15.2 m.y. date from the Kiahera Series (Table VI) is incompatible with the other
ages from Rusinga and there is some uncertainty about the lower limit for the Kisingiri
and Rusinga successions. It seems likely, however, that the main eruptions in this r&on
took place during the age range 16-22 m.y.
The association of phonolites and nephelinites at Lodwar, Turkana (Walsh and Dodson,
1969), suggest the presence of a phonolite-nephelinite
centre of Upper Miocene age
(Table VI).
Major volcanoes of Pliocene age were distributed along the floor and shoulders of the
developing rift. Tinderet and Londiani occur near the intersection of the Kavirondc
trough and the main meridional rift; the Aberdare Range and Mt. Kenya are situated on
its eastern side.
The Tinderet volcanics occur towards the eastern end of the Kavirondo graben, and
AGE RELATIONS
OF THE KENYA
TABLE
VI
Isotopic
ages of the Miocene
central
RIFT VOLCANICS
203
volcanoes
Locality
Formation
Age (m.y.)
ly5
Bukwa,
Uganda
basal Elgon volcanics
(Walker et al., 1969)
22.0 f 0.2
21.9 * 0.2
nephelinite
‘&
Bukwa,
Uganda
basal Elgon volcanics
(Walker et al., 1969)
19.8 f 1.5
17.2 f 0.4
nephelinite
lt7
Bukwa,
Uganda
Lower Elgon volcanics
(Walker et al., 1969)
17.4 f 0.3
near summit,
Mt. Elgon
Upper Elgon volcanics
(Searle, 1952)
15.5 + 0.5
15.1 + 0.5
15.3 f 0.5
Sample
number
Rock/mineral
analysed
BW/ 1
nephelinite
_
_
25/211
phonolite
KA336
waterlaid
tuff qg,io
Ru singa
Island
Kiahera Series
(Shackleton,
195 1)
15.2 f 1.5
15.9 f 1.5
14.6 f 1.4
KA656
tuff %I0
Rusinga
Island
Kiahera Series
(Shackleton,
195 1)
21.7
RK-11N
agglomerate
“i
Rusinga
Island
Rusinga agglomerate
(Shackleton,
195 1)
RK-8
nephelinite
‘,11
Rusinga
Island
Lunene lava
(Shackleton,
195 1)
16.8 +_0.6
16.3 f 0.5
Rusinga
Island
Lunene lava
(Shackleton,
195 1)
16.5 + 0.7
16.6 f 0.7
RK-9
nephelinite
‘,a
i”’
RF.Bl
tuffaceous
sediment ql*
Karungu,
south Nyanza
Upper Series, Karungu
beds (McCall, 1958)
AL218
uncompahgrite s12
Rangwa,
south Nyanza
Rangwa complex
(McCall, 1958)
243150
phonolite
Lodwar
Turkana
Lodwar volcanics
(Walsh and Dodson,
’
2
3
4
5
6
7
’
9
lo
’i
l2
I3
4J 3
rest house,
Whole rock K/Ar isotopic age determination.
Biotite determination.
Nepheline determination.
Feldspar determination.
Miller (Walker et al., 1969).
Armstrong
(Bishop et al., 1969).
Armstrong
(Walker et al., 1969).
F.M. Consultants
for Geol. Surv. Kenya (unpublished).
Evernden and Curtis (Evernden et al., 1964).
Evernden and Curtis (1965).
Miller (Van Couvering and Miller, 1969).
Miller (Bishop et al., 1969).
Grasty (Reilly et al., 1966).
Tecfonophysics,
ll(1971)
191-215
group
lake
19.6 + 0.2
19.5 f 0.2
22.5 + 0.4
19.3
19.2
19.0
19.0
1969)
f
f
f
f
0.3
0.3
0.3
0.3
14.9 + 0.5
12.5 + 0.3
B.H. BAKER. L.A.J. WILLIAMS, J.A. MILLER AND l..l. f:l’I’(‘II
204
are nephelinites, phonolites and basanites with abundant pyroclastics of Miocene age
among the early products. Miocene f~~ssiliferous sediments and limestones known as the
Koru beds are intercalated near the base of the succession on the southern and western
sides of the volcano (Shackleton, 195 I; Binge, 1962). Ages of about 19 -~ 20 m.y. were
recorded from these basal pyroclastics and associated sediments (Table Vii). Later tuffs
and agglomerates near Fort Ternan are intercalated with flows of the Uasin Gishu phonolites and yielded ages of about 14 m.y. (Bishop et al., 1969). The final products from
Tinderet volcano are dated at 5.5 m.y.
TABLE VII
Isotopic ages of Tinderet volcanics (Mi~en~P~o~en~)
-
Sample
num her
Rock/mineral
analysed
Locality
Formation
Age (m.u.1
KA427
tuff a3
Fort Teman,
south of Tinderet
fossiliferous tuffs
(Bishop and Whyte, 1962)
14.0
MB/13
carbonatite
nlff 24
Songhor,
west of Tinderet
Koru beds
(Btnge, 1962)
19.9 f 0.6
19.7 * 0.5
MB/15
tuff h4
Fort Teman,
south of Tindere t
fossiliferous tuffs
(Bishop and Whyte, 1962)
14.7 t 0.7
14.0 + 0.2
MB/‘18
nlff g4
KVrIl,
southwest of Tinderet
Koru beds
(Binge, 1962)
19.6 f 0.3
19.5 f 0.3
421465
basanite tYs
Summit
Tinderet
Upper Tideret
(Binge, 1962)
42/431
nephelinite tp5
Lumbwa,
southeast of Tinderet
Tindere t volcanics
(Binge, 1962)
8.9 + 0.5
9.4 k 0.5
9.2 + 0.5
KA1775
basanite lz6
Songhor,
west of Tinderet
Tinderet volcanics
(Binge, 1962)
9.9 + 0.5
volcanics
5.6 + 1.3
5.5 r 1.3
5.8 + 1.4
t Whole rock K/Ar isotopic age determination.
* Biotite determination.
3 Evernden and Curtis (1965).
4 Miller (Bishop et al., 1969).
’ FM. Consultants for Geol. Surv. Kenya ~unpublished).
’ Curtis (Bishop et al., 1969).
The Aberdare volcanics consist of the Simbara Series basalts (Shackleton, 1945;
Thompson, 1964; Fairburn, 1966), which are overlain on the higher slopes of the range
by the Sattima Series composed of phonolites, trachytes, mugearites and basal&. The
youngest extrusives are the Laikipian basalts (Shackleton, 1945,1946; Fairburn, 1966;
Baker, 1967) which overlie both the earlier groups on the eastern side of the range. The
Aberdare volcanics overlie Upper Miocene plateau phonolites (Rumumti phcmolite and
Kapiti phonolite) and the Pliocene Thomson’s Falls phonolite (McCall, 1967). Some of
-
AGE RELATIONS
OF THE KENYA
RIFT VOLCANICS
205
the Aberdare lavas are overlain by representatives of the Plio-Pleistocene trachytic group
and by the Mt. Kenya volcanics. Thompson (1964) correlated part of the succession in
the Kijabe area with the Sattima Series, but subsequent work strongly suggests that this
Kijabe sequence belongs to the Plio-Pleistocene trachytic group. The availability of only
a single isotopic date (Table VlII) does little to establish the age of the Aberdare volcanics but the general stratigraphical relationships indicate an age range of approximately
5 - 6.5 m.y.
The Mt. Kenya volcanics are composed mainly of phonolites succeeded by trachytic
lavas derived from parasitic vents and fissures (Baker, 1967). Late basaltic cones on the
flanks of the mount~n are related to extrusion of the Quaterna~ Nyanlbeni and Thiba
basalts. The Mt. Kenya volcanics overlie the Pliocene Laikipian basalts of the Aberdare
centre. The two dates (Table VIII) indicate a Late Pliocene age (2 - 3.5 m.y.)‘for the
main eruptive phase of Mt. Kenya consistent with the stratigraphic relations of these
volcanics.
The earliest flows from Londiani volcano (Jennings, in press), immediately east of
Tinderet, may have been the phonolitic nephelinites and nephelinites which rest on the
plateau phonolites. The later volcanics are phonolites, basalts and abundant trachytes which
rest on, or are intercalated with, members of the Plio-Pleistocene trachytic group. There
are no isotopic ages for the Londiani volcanics but activity was probably confined to the
Pliocene.
Kilombe, a trachytic caldera volcano on the floor of the rift east of Londiani, is
probably also a Late Pliocene centre though no isotopic dates are available to confirm the
age (Jennings, in press).
01 Doinyo Sambu is a Plio-Pleistocene composite volcano in northern Tanzania; a few
flows extend northwards into Kenya (Saggerson, 1966). Tuffs and agglomerates are accompanied by basalts, trachytes, phonolites and tephrites, and the volcanics are overlain
by fossiliferous sediments of Middle Pleistocene age (Isaac, 1967). The volcano is bisected
by a major fault so that its eastern half is buried beneath sediments of the Natron Basin.
Only a single date is available (Table VIII).
The Shombole volcano is a composite nephelinite-phonolite
centre on the rift floor at
the Kenya-Tanzania
border (Baker, 1963b). Its age is not accurately known but is probably Pliocene, for it predates the Pleistocene plateau trachytes. Lenderut, a basanitetephrite-~desite
volcano near Shombole is probably also of Pliocene age, but no isotopic
dates are available (Baker, 1963b).
Olorgesailie, a dissected Pliocene centre rising from the rift floor southwest of Nairobi,
contains a nephelinite-basalt-trachyte-phonolite
association (Baker, 195X). The dated
phonolitic nephelinite (Table VIII, KA653) is tentatively correlated with similar flows at
Olorgesailie (Matheson, 1966).
01 Esayeiti, 01 Esakut and Ngong volcanoes, southwest of Nairobi, were all active
before the eruption of Pleistocene plateau trachytes (Matheson, 1966; Williams, 1967;
Saggerson, in press). The first two have some nephelinites in addition to prominent
basalts, trachytes and phonolites, whereas Ngong volcano contains melanephelinites,
basanites and thephrites. Three dates from 01 Esayeiti (Table VIII) show an age range
from 5.6 to 6.7 m.y.
Tectonophysics, 11 (1971) 191-215
‘(I6
I( II. bAKl~l<. 1 .-\.J. WILLlAXIS..l.,\.
Isotopic ages of Phocenc
-.-.--.__-._-
central volcanoes
_...-.
- .-.....-.
- --
-.
._-..
hllI.LLK
___--..
,\Kl)
t.1.
I llt‘li
_ .___.
Sample
number
Rock/mineral
analysed
L.ocality
I:ormation
Age (m.y.1
W/220
basalt
Sagana K..
south of Nyeri
Simbara Series,
Aberdare volcanics
(Shackleton,
1945;
Baker, 1967)
5.5
KA47s
phonolite
near Embu,
southeast of Mt. Kenya
main eruptive phase,
Mt. Kenya volcanics
(Schoeman,
1951;
Baker. 1967)
3.1
KA657
nephcline
syenite %s
summit
Mt. Kenya
nepheline syenitc
(Baker. 1967)
2.64
KA653
phonolitic
nephelinite
Loitigoshi,
Kajiado area
Olorgesailie volcanics
(Baker, 1958;
Matheson,
1966)
5.8
‘*4
“’
A5
plug
IL I.0
KA662
trachy te ‘*’
Shanamu,
near Olorgesailie
Olorgcsailie volcanics
(baker. 1958)
2.7
KA 1060
phonolite
Magadi road,
south of 01 Esayeiti
01 Esayeiti volcanics
(Matheson,
1966;
Williams, 1967)
6.7
MB/33
tephrite
Magadt road,
south of 01 Esayeiti
01 Esayeiti
5.64 + 0.18
5.85 f 0.19
west of Lake Natron,
northern Tanzania
Sambu lavas
(Saggerson,
1966:
Isaac, 1967)
2.02
Lowoi,
west of Baragoi
Tirr Tirr volcanics
(Baker, 1963a)
3.9
3.8
3.6
271301
’
*
3
4
’
6
7
trachyte
“’
26
I.4
volcanics
(Matheson,
1966:
Williams, 1967)
f 0.4
+ 0.4
+ 0.4
Whole rock K/Ar isotopic age determination.
Feldspar determination.
Nepheline determination.
l:.M. Consultants
for Geol. Surv. Kenya (unpublished).
Evernden and Curtis( 1965).
F.J. Fitch and J.A. Miller (unpubtished).
Curtis (Isaac. 1967).
Quatemary central volcanoes
Most of the major Quaternary central-type eruptions in Kenya occurred along the
central and northern sections of the rift floor. Many of these centres are surmounted by
well preserved calderas, and the bulk of the volcanics are younger than the Plio-Pleistocene
trachytic group.
AGE RELATIONS
OF THE KENYA
RIFT VOLCANICS
201
Three large caldera volcanoes in the central sector of the rift floor show signs of Recent
activity. Trachytic lavas at Menengai (McCall, 1967) are accompanied by ignimbrites/
froth flows, whereas Suswa is dominantly phonolitic (McCall and Bristow, 1965);
Longonot is a basalt-trachyte centre (Thompson and Dodson, 1963).
Eburru is heavily mantled with Quaternary pyroclastics but both phonolites and
trachytes are reported (Thompson and Dodson, 1963).
The Kilimanjaro volcanics form a complex Quaternary volcano having three main
eruptive centres. The flows of the northwestern flank (Williams, 1969b) have been correlated with stratigraphic groups established on the main part of the mountain (Wilcockson
et al., 1965). The earliest volcanics are probably Lower Pleistocene in age and are mainly
olivine basalts, basanites and trachybasalts, but thin nephelinitic flows also occur. Most of
the later lavas are rhomb porphyries, phonolites and trachytes which came from the Kibo
centre where activity continued until Recent times (Downie, 1964). An age of about
1 m.y. (Table IX) for the earliest exposed basalts on the Kenya side of the border confirms suggestions from other studies that the main phase of activity at Kilimanjaro occurred during the Pleistocene.
TABLE
IX
Isotopic ages of the Kilimanjaro volcanics (Quaternary)
Sample
Rock/mineral
analysed
Locality
Formation
Age
number
KA940
basalt
I,2
Mawenzi,
Kilimanjaro
Mawenzi trachybasalt,
Kilimanjaro
volcanics
(Downie et al., 1956)
0.514
KA947
basalt
“*
Kilimanjaro
Lavaturm Series
Kilimanjaro
volcanics
(Downie et al., 1956)
0.463
KA937
obsidian
Kilimanjaro
Rhombporphyry
Series
Kilimanjaro
volcanics
(Downie et al., 1956)
0.365
I,*
(m.y.1
_
basalt
‘*3
Ngong Narok,
Amboseli area
Lower olivine basalts
Kilimanjaro
volcanics
(Williams, 1969b)
0.420
_
basalt
1,3
Naiperra,
Amboseli
Lower olivine basalts
Kilimanjaro
volcanics
(Williams, 1969b)
0.850 f 20%
1.100 + 15%
+ 15%
1 Whole rock K/Ar isotopic age determination.
* Evernden and Curtis (1965).
3 G.H. Curtis (personal communication,
1968).
Teleki’s volcano (the most recently active centre in Kenya; it last erupted at the end
of the last century) and Andrew’s volcano are basaltic cones on the flanks of the
“Barrier”, a Pleistocene basalt-trachyte-phonolite
caldera at the south end of Lake Rudolf
(Dodson, 1963).
NX
it.Fi. HAKI:K.
L..,\.J. WILLIA%fS. J.A. MILLkK
AND I ..I. 1 I I’( Ii
Siiali (McCall. 1968), ~n~uruangogolak (Rhenltuila, 1970) and Pakka (J.S.C’. Steal,
personal comrnunjc~tion,
1969) are impressive basalt-trachytc caldcra volcanoes on the
rift floor between Lake Rudolf and Lake Baringo. Holocene plug-domes, tholords and
restricted flows of’ rhyolitc and comendite occur west of Trike Naivasha (Thompson and
Dodson, 1963). The state of preservation and presence of fresh flows at each ot‘ them
testify to the youthful nature 01‘these centrcs.
SUMMARY
AND CONCLUSIONS
The observed relationships between many volcanic formations (both plateau volcanics
and those derived from major central volcanoes) are shown in Tables X and XJ, and the
main volcanic group names used in this account are also indicated. No attempt is made to
quote complete successions for each area or region, and a number of minor Rows are
omitted. Undated sedimentary intercalations were also ignored in compiling the tables.
It will be seen that with very few exceptions the sequence of eruptive events indicated by
the isotopic ages agrees well with stratigraphic successions established in the field. The
role of palaeontological studies in estimating the ages of some volcanic successions in
East Africa has been reviewed by Bishop et al. ( 1969) in a paper in which the ages inferred from assemblages of mammalian fossils were compared with isotopic dates obtained from associated lavas and tuffs; some problems of potassium-argon dating and
possible sources of error were discussed.
Nearly ail the plateau volcanics and the products of many central volcanoes were
erupted from areas within the rift valley or at its margins, showing that from the Early
Miocene onward there was tectonic control of the location of volcanism. In contrast, the
Quaternary multicentre basalt fields and several of the Iargest central volcanoes were
located outside the rift valley and the element of structural control is less clear. The
Miocene nephelinite-phonoiiteecarbonatite
volcanism of the Kavirondo Gulf is obviously related to the subsequent development of the Kavirondo branch graben (Shackleton,
195 I). The similar volcanoes of eastern Uganda are unrelated to fault lines but occur in a
region of strong contemporaneous
monoclinal flexuring. The Plio-Pleistocene Kilimanjaro
and Mt. Kenya volcanoes are located east of the rift valley on extensions of the Pangani
and Nyambeni volcano-tectonic
lineaments.
Nearly all Pliocene and Early Pleistocene volcanism took place within the rift valley.
On the other hand Late Pleistocene and Holocene volcanic activity was strongly bimodal,
taking the form of donlinarltly trachytic central eruptions in the rift floor, with concurrent
extensive basaltic activity in a zone 1SO--250 km east of the rift valley. The main area of
volcanism tended to shift eastward with time: in the Miocene the activity was within the
rift zone and to the west. in the Pliocene it was almost wholly within the rift and in the
Quaternary the volcanism was in the rift floor and to the east.
The sequence of the volcanic groups and their provisional time ranges are shown in
Fig. 3. It is possible to distinguish eight principal phases of volcanism based upon the
character of the volumetrically predominant rock-types:
( I) A Miocene basaltic and nephehnitic phase characterized by multicentre alkalibasalt eruptions in the proto-rift zone in central and northern Kenya, accompanied by the
building up of the nephelinite-phonofite-carbonatite
volcanoes of eastern Uganda and
western Kenya, from 23 14 m.y., with activity at Tinderet extending to 5 m.y.
1
isotopic dates.
-
Ploteau
phonolites
Pliocene
basalts;
Mid-Pliocene
phonolites and
trachytes;
Pliocene
ten tral
volcanoes
PlioPleistocene
trachytic
group
Kirikiti
basalts
(5.0-5.1)
Olorgesailie
volcanics
(2.7-5.8)
01 Euyeiti
volcanics
(5.6-6.7)
Kabete, Karma trachytes
trachyte (3.2-3.5)
-
Precambrian
gneisses
Kapiti phonohte (12.9-13.4)
-__--------
Simbara Series (5.5) -
--------
phonolitie trachyte
Athi tuffs and lake beds
-Mbagathi
Nairobi phonolite (5.2)
Ngong volcanics
-Nairobi
Kerichwa Valley tuffs (4.8-5.7)
-Tigoni,
Limuru trachytes (1.5-1.7)
East of rift
Kajiado-Nahobi-Thika- Fort Hall areas
(Fairburn, 1963, 1966; Matheson, 1966;
Williams, 1967; Saggerson, in press)
denotes observed contact; names in italics indicate formations of central volcanoes; and numbers in parentheses refer to ranges of
Precambrian
gneisses
Mau phonolite
e--_-.-_-_---_----_________--____~
Singaraini
basahs
_-- ___--__-_-_________-_-__------___----------_-_-
Plateau trachyte series (0.6-1.7)
Rift floor
Magadi area
(Baker, 1958)
----------_----~--_-_---------------
Mau tuffs
Mau ashes
Mau area
{Williams, 1970)
Westof rift
Observed relationships between some volcanic formations in southern Kenya I
TABLE X
x
PSYOPiWstocene
tnachytic
g7acP
(with
intercalated
basalts and
phonolites)
Quaternary
hasalts
@4aternary
central
volcanoes
I
--_-.--_
I
Tuffs of Mau and
Mt. Londiani
trachytes and
Londiani and
Kilombe
Menengai pumice
~auaahes ____
West of rift
Kericho-Molo areas
(Binge, 1962; Bishop et al., 1969;
Jennings, in press)
areas
-
Bahati tuffs
Lake Hannington phonolites
Kinangop tuffs
basal& Mbaruk
phonolites, Ronda
trachytes, Giigil etc.
hienengai pumice
-_---_-_---__
._ -.
..--.--------.
Nyeri tuff
.
Kenya atite (2.6- 3. I)
Laikipiarl hasalts -
-Mt
Kinangop tuffs
Thiba _
basalts
.__- -__.---____._-
Nyambeni
volcanic series
_~
East of rift
Nyeri-Mt Kenya-Fort Hall areas
Shackleton, 1945: Fairburn,
1966; Baker, 1967)
_._. _
pyroclastics and sediments
with intercaiated trachytes
(3.34) and basalts
rhyolites
.-._---___---
trachytes, basalis,
phonolites and
I_.
Upper Menengai Series
Naivasha area
(Thompson and
Dodson, 1963)
older basalts, Elmenteita
Recent basalts, Elmenteita
.~.
Rift floor
Nakuru-Hannmgton
(McCall, 1967)
Observed relationships between some volcanic formations in central Kenya ’
TABLE Xl
2
-:
7
‘:
7
L
zc
E
i-
3
5
E
1
isotopic dates.
Miocene
ten tral
volcanoes;
Miocene
basalts
Plateau
phonolites
Pliocene
basalts;
Mid-Pliocene
phonolites and
trachytes
phonolitic
nephelinites
areas,
’
Simbara Series
------_-
Thomson’s Falls
-phonolite
(6.2-6.7)
Sattima Series
Kwaibus basalt -
Rift floor
Nakuru-Harmington
(McCall, 1967) -
------
Samburu Series (18.5-23.0;
ages from area farther north
where basalts rest on Precambrian
gneisses)
Precambrian gneisses
Kapiti phonolite (12.9- 13.4)
m-_----
Thomson’s Falls
_
phonohte (6.2-6.7)
Simbara Series (5.5)
-----
Sattima Series
East of rift
Nyeri-Mt Kenya-Fort Hall areas
(Shackleton, 1945; Fairnurn,
1966; Baker, 1967)
denotes observed contact; names in italics indicate formations of central volcanoes; and numbers in parentheses refer to ranges of
Precambrian gneisses
Koru beds (19.5-19.9)
Lower Tinderet
volcanics
tuffs and
sediments, Fort
Ternan (14,0- 14.7)
-------
Naivasha area
(Thompson and
Dodson, 1963)
Kericho phonolite (11.8-13.4)
Rumuruti phonolite (1 l.O- 12.0)
____________-_-------__-----_-~
I
Upper Tindere t
volcanics (5..59.9)
--
West of rift
Kericho-Mole areas
(Binge, 1962; Bishop et al., 1969;
Jennings, in press)
TABLE XI (continued
lS.11. IsAKI<K.
T
E WESTERN
ET
KAVIRONDO
0
I
AND
SUSWf I.LO?~GOI
ETC \ ‘OLCANl(
Kit’ JMBF
2
PLATEAU
RIFT
LONC IIAN
MAIN
AND
L.A.J.
\VILLIAMS.
RIFT
VALLEY
TURKANA
J.A. MILLER
ANI)
EASTERN
PLATEAU
I II I I I Im
-=si --=
&JATERNARY
I,.J. I 1’1( I!
BASALTS
A\5
r
m
vo
VOi
3
MT
4
RDARE
KENYA
VOLCANIC!
>
VDLCANICS
5
6
7
0
_x.u~_~,~
<._.__r
____5
‘PLIOCENE
NGONG
OLORGESAILIE
OL CSAYETI-VOCCANICS
PHON,JLITES
8 TRACHYTES
9
IC
II
12
13
14
15
16
17
16
I9
!C
21
!2
!3
Fig. 3. Sequence and age ranges of the volcanic groups.
(2) A Late Miocene phase of fissure phonolite eruptions centred in the NakuruBaring0 region, from 13.5 to I 1 m.y.
(3) An Early (?) Pliocene phase of rhyolites, ignimbrites and mugearites confined to
northwestern Kenya.
(4) A Mid-Pliocene phase (S--7 m.y.) of trachytic and phonolitic activity centred on
the Naivasha-Nakuru-Baringo
region:
(5) A Pliocene basaltic phase during which basalts flooded much of the rift valley floor
and the predominantly
basaltic Aberdare Range was built, with a probable eruptive
maximum around 5-6.5 m.y.;
(6) A Late Pliocene phase of massive trachytic ignimbrite eruptions centred on the
Nakuru-Naivasha region (S--3 n1.y.) with scattered trachytic lava eruptions at 3.5 - 3 m.y.
AGE RELATIONS OF THE KENYA RIFT VOLCANICS
213
This was followed by extensive flood trachyte volcanism in the floor of the central and
southern rift valley (2-0.6 m.y.).
(7) A Late Quaternary phase of mainly trachytic caldera volcano eruptions along the
floor of the rift valley,
(8) A Late Quaternary phase of multi-centre basalt volcanism in three main areas
150-2.50 km east of the rift valley.
The Kenya rift valley traverses one of the world’s largest alkali-igneous provinces,
which contains at least 100,000 km3 of volcanics, of which nearly half is of phonolitic,
trachytic and rhyolitic composition and poses some formidable petrogenetic problems
(Willianls, 1970b). It is notable that the Late Miocene and Late Pliocene to MidPleistocene phases of dominantly phonolitic and trachytic activity coincided with
periods of strong uplift of the Kenya swell and with periods of rift-faulting (Saggerson
and Baker, 1965). On the other hand the Miocene and Pliocene basaltic volcanism occurred at times of comparative crustal stability. These features indicate a degree of interdependence of tectonism and volcanism that strengthen the view that both processes are
the surface expression of fundamental changes of state in the upper mantle beneath the
rift zone.
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