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Eiszetta/ter Gegentuarl114- 14'
Hannouer 19927 f i g , 4 : : 6 .
Stratigraphy, Geochemistry, and Paleomagnetism
of Late Quaternary Bedrock and Paleosols, Karisoke Area,
Virunga Mountains, Northwestern Rwanda
Wnlrau C. N1rnexev. Roxai-o G. V. HrNcocr & fuxe \}7. Brnt :oHrct ' )
Quaternarv , s t ra t ig raphv , geochemis t rv , pa laeomagnet ic research .
Afr ica, Northwestern Rwanda, Virunga lvlountains
Abstract: A surface soil and rn'o buried paleosols, units
I i - l l i - iV (upper) and IV ( lower)-V, provide important infor-
nrat ion on paleocl imare/drainage in the Virunga N{ountains
i rom dr ier ro lve t rer paleocl imat ic episodes of the last g lacia-
r ior . r and middle postglacia l per iods. In general , the strat igra-
phv indicates a per iod of bedrock weather ing, fo l lowed bv
eprsodic emplacement of ruf faceous sedimenrs accompanied
bv rvearherrng in i t ia l lv in a dr ier and perhaps colder paleo-
envirooment. Later , dur ing rhe last g laciatron ( interstadia l ) ,
increased stream acr iv i tv emplaced ai luv ium, covered wi th a
th in la l 'e r of tut f . a l l of which is weathere d more v igorouslv
rhan rhe under lv ing sediments. The surface soi l (ground soi i )
shows comparative lv less we athe nng and move ment ,tf
mobi le e lements indicat ing the lace middle to late Holocene
paleoenvironment was somewhat drier and perhaps cooler
con.rpared with the early Holocene. The geochemistry of
these three soi ls shows considerable de ple t ion of Na, Ca, and
K in the middle paleosol re lat ive to the older bur ied paleosol
and to the surface hor izons. i ron is an importanr indicator of
paleocl imate. suggest ing that the middle l I and I I I uni ts of
the upper paleosoL.fbrmed under a paleocl imate wetter chan
todav. The radiocarbon-dated middle paleosol indicates i t
could have been exposed to subaer ia l weather ing f rom the
Kalambo Inrerstadia l ( - l5.ooo vr BP) to the mrddle Holo-
cene. Tht tota l rveather ing in the middle uni t is too ad-
vanced to have occurred dur ing the Flo locene. The re lat iveiy
high conccntrat ions oi Th rn local bedrock, tuf fs, a l luv ium
end s lope wash indicate rhar i t is the source of radioact iv i tv
reporred bv other workers in The Virunga N{ountains.
' ) Adresses o i thc au thors : Pro f . Dr . \ { ' t t - t - t ' l r t C . M, tH, t -
r i , \ . Gcont r r rpho logv and Pedo loqv Labora t t l rv . Depar t r ren t
, r t ' G e o q r e p h r , . \ t k i n s o n C . r l l c g e . \ b r k U n i v e r s i t v N o r t h
\ , . , r k . O n t e r i , r . C ; r . n a c l a \ ' 1 j - l I P I D r . R . G . V . H . t l t t - , t x .
SL( ) \ \ 'POKE I tc rc to r Fec i l i t v . rnd Dcpar tmct r t , . r i Chcn l ice l
l i r r g r r r c c r r n g . r n d A p p i i e d C h c r n i s r r r ' . L , ' n i v e r s i n o f I o r r ) l l t t ) .
I \ ) f r ) n r ( ) . O n r l i r r , ' . ( l . i n ' . r r l r r \ t t S L \ t . P r , r f . R i , r . t ' \ \ ' . B . ' , l i 1 , \ '
l ) i i t , , l . [ ) ( f ) i i r u t t c r ] i , r t ( : t ' o { r r t p h r . [ - c t h b r i . - i g c U r l i v c r s i t v .
l - c r h b r r . . i s c . . \ l b c r t r r . C . L r r r r t l r l ' 1 K . . \ l ' r
IStrat igraphie. Geochemie und PalAomagnetik
spdtquartdrer Gesteine und Paliobdden
im Karisoke-Gebiet der Virunga-Berge. Nordrvest-Rwarrdal
Kurz fhssung: E in re ze n te r Boden und zwe i begrabene
Pal ioboden. E inhe i ren l l - l l l - lV loberer ) und IV (unre re r ) -V ,
l ie fe rn w ich t ige ln fo rmat ionen uber Pa ldok l ima und Abf lu f l -
verhd l tn isse in den V i runga-Bergen \ ron t rockeneren zu
leuchreren pa ldok l imat ischen Phasen des Spdtg iaz ia ls b is zu
n i i t t le ren pos tg laz ia len Zet ren . Zusammeniassend be t rach-
re t . we is t d ie s t ra t ig raph ische Abtb ige zun lchs t au i e i r re Vcr -
w i r re rungsper iode des Fes tges te ins h in . der e ine Phase fb lg t .
in der tu f f ige Sch ich ten abge lager t wurden. D ie Ze i t war von
einer Verwitterung unter trockeneren und viel leicht kdlteren
Bed ingungen beg le i te t . Spdter , wdhrend des le tz ten Gla-
zials. kam es bei verstd.rkter Aktivi tdt der Fl irsse zur Sedi-
menta t ion von Schwemmlandab lagerungen, d ie von e iner
d i innen Tuf f -Sch ichr r iberdeckr wurden. D iese Sedrmenrc
s ind in tens iver verw i t te r t a ls d ie l iegenden Sch ich ten . Der
rezenre Boden ze ig t e ine verg le ichsweise ger inge Verwi t te -
rung mob i le r E lemente . was darauf h indeute t , da l l d ie
K l imaverhd l tn rsse im spdten NI i t te l -Ho lozdn und Spdt -Ho lo-
zdn erwas r rockener und v ie l le ich t k i rh le r waren a ls im Fruh-
Ho lozdn. D ie Geochemie der c i re i Boden ze ig t e ine bedeu-
tende Abnahme von Na. Ca und K i rn mi t t le ren Pa ldobode n
im Verg le ich zum i l te ren Pa l ioboden und zu den rezentcn
Boden. E isen a ls e in rv rchr iger ind ika tor fu r das Pa l lok l ima
de u te t darau f h in . da t l d ie mi t t le ren E inhe i ren I I und I I I des
oberen Pa lcobodens un ter f -e uch teren K l imabed ingungen a ls
heure en ts t r tnden s ind . D ie Rad iokarbon-Dat ic rungen dcs
mi r t le rcn Pa l r iobodens ze iqen. da l l d icscr Boden c i t rc r s r . rb -
lc r i sche n Veru i t te r r . rnq vom Ka i ln rbo- ln te rs tad ia l ( : l i ( )00
Jehre vor he u tc ) b is z t r rn NI i t t c l -Ho loz in r tusgescrz t u 'a r . l ) i c
g c s a n r r c V c r n ' i r t e r l i n q i n r n r i t t l c r e n A L l s c h n i t t i s t z t t * c i t
t i r r tge-schr i t t cn un t . l rv r j .h rcnd r ' l cs Ho loz l ins c r l t s t r l r t t l c t l . Drc
r c l e r i r ' [ r o h c K t . r n z c n t r ' r t i o n v o n T [ r i r t d c n l t t s t c l t c t t d c t t I : c s t -
g c s r c i r t c n . f L r i f i ' r r . S r h s ' c r t t t t t i r r t c l s e d i r n c r t r c t t t t t t d F l r L r t g -
r u t s c h r n a s s c n z c i g t . , l e t i l ' 1 r d i c Q u c l l c r i c r R : r t l i , r e k t i , , i t : i r i : t .
\ ' { )n ( ]c r : r r r r ie rc . " \L t t , r rc t t i rL ls r ' l c t r V i rL r r ig r - [ ] c r {c r t l - t c r t , . l t te I l
Stratigraphy, Geochemistry, and Paleomagnetism of Late Quaternary Bedrock
1 Introduction
The surficial geology and geochemistry of the Virunga
Mountains in northwestern Rwanda are very poory
understood (ANrnuN and MacNEs 1983; HRRRIS
1982). Of the f ive Rwandan volcanoes, Karisimbi
(4510m) is the highest and contains a complex series
of lava flows which cover the largest area. Over 110
parasit ic cones circle the summit area and presumably
are the source of most pyroclastic sediments and rocks
fbund around the mountain. One young lava f low
(precise location unrecorded) from Karisimbi has been
dated 16,000 yr by K/Ar (H,tnnts 1982). The follow-
ing discussion concerns the stratigraphy, geochemistry
and paleomagnetism of a section on the northeastern
flank of Karisimbi (Figure 1) at 3100m (in the val ley
area between Karisimbi and Visoke).
2 Field Area
Thc Virunga Volcanoes in northwestern Rwanda (on
the Zaire-Rwanda-Uganda border Figure 1), consist
mainlv of rocks of basaltic composition (MEnctrn
19U4). In some cases, thick cover over rocks of tufface-
ous origin overl ie the basalt; al l are usually weathercd
to depths of t .o to 3.0m depending on age . Surf icial
deposits of loess, colluvium and alluvium are often
weathered in to th ick (= 2 .0m) brown (10YR 4/4)
colored paleosols sometimes forming compound units
( e . g . s i m i l a r t o I I A 1 1 b a n d I I A l 2 b h o r i z o n s i n
Figure 2) that appear to have formed during the last
glaciation when the timberline was depressed several
hundred meters.
The site is located in the upper port ion of the Hagenia
woodland dominated by Hagenia abyssinica (Cou,
1967; HqoBERG 1964). The presence of a dark moor-
land-like paleosol, at or near the surface, that can be
traced to elevations as low x 2110m indicates a sub-
stantial vegetation depression. Because the thick
moorland-like paleosol is similar to one observed on
Mount Kenya (900km to the east) (MAHANEY 1990),
it is considered an important paleoclimatic indicator.
The present-day climate is wet with mean annual pre-
cipitat ion at approximately 2000mm (FosseY 1982).
Evapotranspiration is unknown. The temperature in
the upper Hagenia forest may approximate the mean
annual remperature of t0 t 6'C at a similar alt i tude
on Mount Kenva (MnHnNEv & BovEtt 1986).
RUHENGERI
;;
AA
II
,LAKE (
LUH0ilooLUH0iloo
INT€RNATIONAL BOUNMRY
ROADS
AIRSTRIP
PEAKS (el€val,ons In m)
CONTOIJRS
FOOTPATHS
CRAT€RS
CAMPING S ITES. HUTS
HEATH COVIREO LAVA
MIX€D COLONIZING WOOOI AND
BAMAOO
HAGTNIA HYPERICUM FORTST
GIANT STNECIO & LOEIL IA
Frg . 1 : Locat ion o f sampl ing s i te in the V i runga Mounta ins , nor thwesrern Rwanda
l 1 6 Wrr.Lr lu C. Mrrr l l r . i - , RoNt l r ; G. V. Harc_(x.K & Rr,xr , $I . B.rHr, : , tHrt ; r
3 Methods
't 'he KAR t sectior-r was described using the nomen-
c la ture o f the Sorr - SuRvsy St 'A l , r - (19t1 , 1975) andBtnxt ' t- , tNo (1984). Soi l and sedimenr colors werecstimated usirrg the color chips of Oy,qun & T,rrtl l , \R,{ (1970). Samples were col lected and dried afterre nroving the granule and pebble size material(> 2 mm). Par t ic le s ize determinat ions fo l low the\(entworrh scale (For.r 1968)for coarse material (sand
and si l t) and the Sctrr. Sunvey S't 'AFf (197t) for f inepart iclcs (clav). Part icle sizes were determined by drys iev ing for coarse mater ia l (2mm-63prm)(Dnv t96t )and by sedimentation for f ines ( < 63 ptm) (Bouvou-(tos 1962). Each sample was X-rayed using Ni-f i l teredCu Kcr radiat ion to determine rhe mineral content ofthe ( 2ptm fract ion fol lowing procedures outl ined by\X/ r rn" r ' rc ; (196t ) and M,cunNEy (1981) . Sa l ts wereanalyzed fol lowing procedures of Bowln & \fncox(196t). The pH was measured by electrode using aso i l -wa te r r a t i o o f 1 :1 . The ( 2mm samp les we resubsampled and approximately I to 2g was placed insmall f- l ip-rop polyvials for neutron acrivarion analysisin the SLO\f lPOKE nuclear reacror at the Universitvof
'l 'oronro (H,rNc.ctc-r< 1978, l9B4).
I 'o determinc the concenrrar ions o f U, Dy, Ba, T i ,Mg, Na, V, A l , Mn, Ca and K, which produce shor t -l ivcd radioisotopes, the samples were irradiared se-r ia l ly f r ; r l minute ar a neurron f lux o f 1 .0 x 1011r) 'cm l. s I in the SLO\fPOKE nuclear reacror arthc University of Toronto. After a delay t ime of t9minutes (which al lows the very short- l ived 28Al todecay to acceptable levels), each sample was assayedusing 5 minute counrs with on-site gamma-ray spec-tromcrers, as described by HaNc.ocK (1984). Appro-priatc gamma-ray peak areas were measured and rhechcrnical concentrarions calculated using the compara-tor nrethod.
The samplcs were then batch irradiated for 16 hours ata neu t ron f l ux o f 2 . , x l gn . cm ' -2 . s l , and a f t e r await ing t ime of approximately 7 days the concenrra-t ions o f Sm, U, Yb, La, As, Sb, Br , Sc, Fe and Nawere similarly determined, with the Na giving a cross-check with the f irst analysis. After a two-week delay,the samples were recounted and the concentrat ionsof Nd, Ce , Lu, Ba, Th, Cr , Hf , Sr , Cs, Ni , Th, Sc,Rb, Fe, Co, Ta and Eu were measured. Once again,the Sc and Fe were used to cross-check the secondphasc of the analvs is .
4 Results and Discussion
4.1 Strat igraphy
A sequencc of slope wash and al luvium, interbeddedtuff i , and basalt that is presumably younger rhan
= 10 ,000 v r (H , rnn rs 1982) p rov idcd : i n oppor run i r v
to s tudy wea the r ing and pa leoso l genes is i n a humid
tropical serr ing over a short span of t ime . ' l 'he
wcath-
ered basal t and tuf f (uni ts V arrd IV) shown in Figure
2 are exposed in a gul ly a long the lowcr f lanks of Kar i -
s imbi Volcano. Because the two lower uni ts were less
highly weathered (based mainly on color d i f l l ' rcnccs
observed in thc f ie ld) , i t was inferred that they had
formed ovcr a s l - ror t span of t ime fo l lowed by a longer
per iod of weather ing which produced hucs as browrr
a s 1 0 Y R 6 / 1 . U n i t I I I a p p e a r e d r o b c m o r c h i g h l y
a l te red w i th co lo rs o f t oYR4/6 ind ica t ing tha r Fc -
hydroxides and oxides might make up the bulk of rhc
weather ing products. Because the lower iwo srrar igrr-
phic uni ts consisr of pyroclast ic /volcanic rock rn" , . i , r1,
that were at least a l tered to a modcrate degree on a
f low surface dated at = 10,000 yr , i t is bel ieved thcv
migh t have wea the red du r ing thc d r ie r pa leoc l i rna rc
which accompanied the ear ly stadia l of the last g lacia-
t i o n = ) 0 , 0 0 0 - l ) , 0 0 0 v r B P ) ( M , r n r r r , r , , 1 9 9 0 ) .
IAt2b
mBtb
III82bIII82b
MCloxb
MC2oxbMC2oxb
UAb
M82bM82b
VCb
SLOPE WASH
WATERLAID TUFF 8 A'HALLUVIUM
WEATHERED TUFF B LOESSWEATHERED EASALT
ROOTS
UNWEATHEREDEASALT
Strat igrapl ' rv of the KAR I sccr ion,
V i runga Moun ta ins .
AIr 0
2C2C
3C3Css!!
VUA @
;;oo
33I I A t t . ' R
oo
60
/0
80
90
00
00
r20
g r30
==z 4 0
---= 50II
u 6 0
70
II
nnilil
NNT7T7
4 d/ ) l, l
KARl
2 ta
2 2 0 . ^
z r c l , ' ,
2 5 0 f ' ) .
f : : ' ,260
F ig . 2 :
Srra t rg raphv . Geochcmis t ry , and Pa leomagnet ism o f La tc Quatc rnar ,v Bcdrock I 1 r
' l ' hc ovc r l v i r rg un i t s (1 , l I and I I I ) , cons is t i ng o f s lope
rvash, a l luv iurn and water la id tuf f , are considerably
\ : o l r n g e r j u d g i n g b v a I ' C d a t e o f 6 3 ) 0 t 2 0 0 y r B P
( l l G S - 1 2 1 . 1 ) o n c h a r c o a l i n t h e I I A l l b h o r i z o n .
Bccausc rhc p rec ip i ta t i on i s so h igh , the pa r t i c le s i ze
so coersc (Tablc 1) , and roots prevalent in the upper
(wo un i r s (F igu rc 2 ) , i t i s h igh lv l i ke l y tha t some
riorvnward Inovcment of geobiochemical contami-
nenrs n r igh t h l r , c occu r red . Un i t s I I and I i I appear to
havc forrnccl through the interstadia l of the last g lacia-
r i on (K : r l ambo) , t hc l as t g lac ia l max imum and ea r l y to
nr ic lc l lc Holocct te.
' l ' hc pa r t i c le s i ze da ta i n Tab le 1 show some impor tan t
rrcncls wi t i r rcspect to c lay and s i l t that bear on the
rrorpl ' rogcncsis of the two bur ied paleosols. The lower
praleosol Iuni ts lV ( lower) and V] is considerably
higher in s i l t compared wi th the weathered bedrock
which rn ight bc re lated ei ther to the ai r fa l l in f lux of
t uf l .s arrd / or loess (g lacia l ly-crushed quartz grains
indicatc acol ian t ransport of sediments f rom the
Ruwcnzor i Mountains or some other g laciated area;
M, t t t , t x t ' r ' 1990) f i om the nca rby Ruwenzor i Moun-
tu ins or to reworking of local ly der ived s i l t fo l low-
ing a nrajor vcgctat ion bel t depression dur ing colder
s tac l i a l c l ima te a t o r near the end o f the las t g lac ia t i on .
C la r t rans loca t ion in th i s l ower pa leoso l i s on lv on the
orclcr of ' I % indicat ing a dr ier paleocl imate that was
n() t c()nducivc to the downward movement of f inc-
g ra ined pa r t i c l cs .
' l hc uppc r pa lcoso l un i t s [ ( i l , I I I , and lV (upper ) ] a l so
ton ta in h igh s i l t r c la t i ve to the su r face s lope wash un i t
end the wc: i thcred bedrock (V). As in the lowcr paleo-
sol th is increasc could be re lated to the cmplacement
,r f pvroclast ic mater ia l or to sort ing and reworking of
older scdinrents by streams. The increase of percent
c l a r , i n t h e l l A l i b a n d i l I B 2 b h o r i z o n s i n d i c a t e s a
paleosol wi th considerable power to form clay re lat ive
to thc under l y ing pa lcoso l . Th is i nc rease o f c la1 , i n thc
I I IB2b ho r i zon suppor ts the hypo thes is o f i nc rcased
downward movement re lat ive to the lowcr and upper
uni ts in the sequence. Because the surfacc soi l is
formed in slope wash reworked from rrearby slopes
covered w i th py roc las t i c sed imcn ts i t i s poss ib le on ly to
point out that h igher sand percentages suggest suf f i -
c ient ly h igh surface run of f to movc coarsc-grained
sed imen ts ; i nc reas ing s i l t and c lay w i th dep th i nd i ca tc
the poss ib i l i t y o f some downward movement .
4.2 Minenlogy
in order to test the hypothesis of a voung ( < 50,000
yr) age for the paleosols in the KAR I sequence we
analyzed the mineralogy of the I 2 pm fractron i r r
each hor izon. In the oldest bur icd paleosol only a
t race amoun t o f i l l i t e was de tec ted in the IVB2b .
Quartz is present in a l l hor izons wi th moderatc
a m o u n t s i n t h e V C b a n d I V B 2 b h o r i z o n s d i m i n -
ishing somewhat in the IVAb hor izon. Volcanic fe ld-
spars (mainly anorthoclase) are present in smal l
amounts throughout wi th sontewhat lower ref lect ions
in the IVAb ho r i zon . Sma l lamoun ts o f hemat i te were
also detected.
The mineralogv of the upper paleosol shows a s imi lar
absence of c lay minerals wi th hemat i te appcar ing in
smal l to t race quant i t ies as the chief weather ing pro-
duc t . Quar tz i s h ighes t i n un i t I I and in the IVC2oxb
(moderate amounts) hor izons. Volcanics fe ldspars are
h i g h e s t i n t h e I I A 1 1 b , I I A 1 2 b , a n d I I I B I b h o r i -
zons, decl in ing somewhat in the lower hor izons. The
ground soi l is not iceably d i f ferent in that quartz
(abundan t amoun ts ) and vo lcan ic fe ldspars ( sma l l
amoun ts ) a re h igher than in thc two bu r ied pa leoso ls .
l eb le l : Par t i c le s izc ra t ios fo r hor izons in the KARl pa leoso l scquencc , V i runga Mounta i r ts , Nor thwcstc rn Rwar ld ; r
i l o r r z o n D c p t h
(cnr )
Sand (uzo )
( 2 r n m - 6 3 i i m )
S i l t ( % )
(63-2 pm)
Clai,' ( 7o )
( < 2 p n r )
r \ 1( ,ox
l l A l l L r
t l r \ l l b
I i l L l l b
I t l l l I l r
[ \ ' ( . I ox l t
I V ( . I o x b
IV r\ lr
t V t s 2 b
\'(. t l
0 - 1 3
l - i - ) 4
14- t-0
ttJ- 94
9, i - 10J
1 0 j - 1 I t
l ] i - l 7 i
r 73 r82
n J 2 - 1 9 1
r9t-201
201-241
82.4
6 6 . 8
4 ) . 0
t6 . ' 1
7 1 . 0
29 . )
4 5 . 1
8 2 . I
4 > . I
4 2 . 1( r ) .4
1 1 . 1
2 5 . 1
1 2 . 3
1 6 . 8
2 2 . , 4
59.1
i 7 . 8
r t . 7
48.9
5r .63 2 . 2
6 . tj . r -
t 4 . /
6 . 8
2 . 6
1 l . l
6 . t
2 . 2
) . 4
6 . 11 l
r lrl \Wrr-lrau C. MauaNEy, RoN,rr_u G. V. HaNc_ocK & fu.Nn W. B,rnlxoHlr;.r
' l 'able 2: Dry colors, pH and tota l sal ts in paleosol sequence KARI, Virunga Mountains, Northwestcrn Rwanda
Hor izon Depth
(cm)
f) rrr r
Color
pH( 1 : 1 )
E . C .
n l
Cox
I I A I I b
I I A I 2 b
I I I t s I bI I I B 2 bl V C l o x bl V C 2 o x bI V A bI V t s 2 bVCL)
0 - 1313- t4)4- / t )
70 - 94
94-r03
1 0 3 - 1 3 t
r 3 t - r 7 3r 1 3 - r 8 2r82-19tr9t-205205-241
1 0 Y R 2 t 2 2 t 3r 0 Y R 4 t ) 4 t 21 0 Y R 3 / 310 YR 3 /410 YR t / 31 0 Y R 4 / 61 0 Y R 7 / 3l0 YR 6 /310 YR 3 /410 YR 6 /31 0 Y R 6 t 3 5 t 3
t . 0, . 55 1
> . 4
t . t5 . 5t . 8t . 8t . 8t . 8
. 2 8
. 0 t
.09
. 0 t
. 0 t
.04
.06
.04
.08
.04
. 0 7
" Colors fbllow Oyama and Takehara (1970)
Also, pyroxene which exists in rrace amounrs in rheburied paleosols is higher in the ground soil. Becausequartz is relatively rare in the volcanic rocks of theVirunga Mountains it may signal airfall influx, per-haps, from the nearby Ruwenzori Mountains. Theplagioclase feldspars are noriceably higher in the A Ihorizon when compared with the lower paleosols andwith the Cox horizon indict ing they are being conrri-buted from fresh ourcrops up slope. Although it maybe inappropriate ro compare rhe 1 2 pm fractionmineralogy with the ( 2 mm fraction geochemistry,elevated Na and K concentrarions are also observed inthe A 1 and Cox horizons (see Table 3).
4.3 Soil Chemistry
Soil colors, pH and total salts (Table 2) were studiedto determine whether they could be used to dif feren-t iate rhe three units. The pH trends in the upper rwounits show sl ight increases with depth; the lower unitis uniform. These data indicate sl ight movement ofH * ions in the upper two units and none in the lowerunit. The data for total salts reveal only sl ightly higherconcenrrarions in the A and Ab horizons relat ive tothe other horizons. Based on rhe geochemical datadiscussed later the salts present musr be NO and SOrather than Cl. The soil colors shown in Table 2 are allin thc 10 YR range suggesting younger material that isnot highly altered. These yel lowish red colors are com-patible with the l iberation of moderare amounrs ofgoethite and minimal to rrace amounts of hematite .Thc color trends suggest that the II IB2b horizonmight contain the highesr concenrrat ion of l iberated
oxides, hydroxides and clay minerals. Table j con-firms this prediction with its highly elevated Fe con-tent in the II I B 2 b horizon.
4.4 Geochemistry
The chemical analysis data for the KAR I section,which are l isted in Table j , show considerable variab-i l i ty. Some of the variat ion may be associated withdif ferent l i thological materials or with weathering,paleocl imate and/or drainage. Because Na and K arethe most mobile of the major elements their deplet ionin the tuff /ash, al luvium, and lower tuff / loess hori-zons indicates that weathering effects were especiallyprominent in forming these Inceptisols. This supposi-t ion is born out by a check of the inter-elemenr corre-lat ions using Pearson correlat ion coeff icients, whichshow that Na and K are highlv posit ively corrclated.Both are posit ively correlated with Rb and are nega-t ively correlated with Sc, T, Cr, Ni. Br and Cs.
Four other correlat ion groups including major elc-ments are a lso apparenr . Magnesium, Ca, Mn, Co, V,Sr, Cl, and Ba are al l highly posit ively correlated andare negarively correlated with As, Sb, Br, the rareearth elements (REEs), Hf, Ta, Th, and U. Alumini-um is posit ively correlated with As, Sb, the REEs, Hf.Ta, Th, and U, and is negarivelv correlated with Cr,Sc, V, T i , Mn, and Co. T i tan ium, Sc, V, Cr , N i . andMn are al l highly posit ively correlated and arc nega-t ively correlated with As, Sb, REEs, Hf, Th, and U.Iron does not correlate with any of the other majorelemenrs, although i t is negatively correlated with Sb,the REEs, Hf, Ta, Th, and U.
Strat igraphy, Geochemistry, and Paleomagnet ism of Late Quaternary Bedrock
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I{ i t is possible to relate these geochemical correlat ionfindings ro rhe specif ic mineralogy of the section, rhentherc are signif icant quantir ies of at least f ive primaryrnincrals combirred to make up rhe sedimenrs in theKAR I secr ion.
l 'he Na and K are depleted in the tuff and ash, in thea l luv iun ' r , in the lower rwo hor izons ( lVAb andIVB2b) in the tu f f and loess, ind icat ing major leach_ing of rhese materials during or after their formation.1'he Mg, Ca, er al.-r ich materials are highest in theslopc wash material, in the lYc2oxb horizon in thetuff and loess, and in the weathered basalt horizon(VCb) . A lumin ium and i ts assoc ia ted e lements tendto increase in concentrat ion downward rhrough thesection, with e xce prional ly high leve ls in the IV C 1 oxbhorizon in the tuff and loess. Tiranium and the asso-ciated transit iorr metals tend to be deplered in thel l lB2b hor izon in the a l luv ium and in the next lowerIVCloxb hor izon in the tu f f and loess, and in theIVAb horizon in the tuff and loess. They thereforetend to be depleted in rhe highest si l t-containingunirs of the section, implying that they may be asso-ciated with material in either rhe sand or rhe clay frac-t ions o f the so i ls .
l 'he distr ibution of Fe wirh depth shows trends thatarc probably related to paleocl imate and r ime of expo_sure ro subaerial wearhering processes. The highest Feconrenr , by far is found in the I I IB2b hor izon in theal luvium. This high Fe conrenr could result from dif-tercnccs ln parenr materials or from relat ive move_nrenr (deple t ion /enrichment) caused by dif ferentpaleocl imates. The Fe levels are nexr highest in thes lope wash mater ia l , and in the IVC2oxb hor izon inthc tuff and loess. I f one compares the two buriedpaleosols [ ( I I , I l I , and IV (upper) )and ( IV ( lower) andV)], i t appcars that the lower one was exposed forc i thcr a shor ter r ime or that i t fo rmed in a pa leo-cl in-rate with lcss avai lable moisrure than the upperone. Ir is l ikely that some of the Fe is organical ly com_plcxcd into a soluble form (see paleosol colors in Table2), which al lowed i t to move downward in the upperburicd paleosol, eventual ly enriching the buried Bhorizon. l f this hypothesis is correct one is deal ingwith a considerably stronger paleocl imate in units I I ,l l l and IV (upper) compared with IV ( lower) and V.l 'he Fe data tend ro suppon the notion that the lowerpaleosol fbrmed during one of the stadial intervalscluring the last glaciat ion, while the upper paleosolt irrmed n-rainly during the Kalambo Interstadial, LastGlacial Maximum, and early postglacial period up roaround 6330 t 200 yr Bp (BGS- r2I4). While paleo_ecological data are scarce from the East Afr ican moun-tains, the period from 8000-4000 yr Bp is consideredto have been wetter and warmer than the early andla ter Holocene (M,ru , lN i jy 1988, 1989) .
Fig. 3: Clay and si l t distr ibutions with dcothin the KARl secr ion.
The high concenrrarions of U and Th through the sur_face soi l and buried paleosols suppon the scanty infor-mation on radioactive basalts discovered during a geo_physical survey in the Virunga Mountains during theear ly 1980 's (ANrux & MAGNTTE 1983) . Leve ls o f Uand Th are highest in the tuff and loess.
A chondrite normalized plot of the rare earth elc_ments for the sample suite shows relat ive unifbrmirvboth in curve shape and in magnitude (see Figurc 4).In this f igure , rhe outermost data points are shownwith sol id black l ines and the profi l is for representa-t ive samples A l , Cox, and IVAb are inc luded fordeta i l .
The relat ive uniformity of the patrern indicates thatthe ( 2 mm soi l (paleosol) fract iorr samples wereformed from material of similar magmatic composi-t ion, probably over a short geological t ime span.
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Stratigraphy, Geochemistry, and Paleomagnetism of Late Quaternary Bedrock
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Fig. ' i: Chondritc-normalized values for rare-earth elements
in selected paleosol samples from the KARl section,
Virunga Mountains.
4.5 Paleomagnetism
lnJuly, 1987, samples for paleomagnetic analysis were
collected between Visoke and Karisimbi volcancoes
(Figure l) . In tot^1,73 polycarbonate cubes (2cm x
2 cm X 2 cm) were inserted into the sediments using
hand pressure, and where necessary through gentle
tapping with a rubber mallet. In addition, several
blocks of unweathered vesicular basalt were marked
and collected in the field, and cored in the laboratory
using a bench drill. Analysis was carried out using a
DSM-1 spinner magnetometer in the paleomagnetic
laboratory at the Pacific Geoscience Center.
Magnetization data for the Karisoke sediments fall
into three types: (a) single component magnetization
(specimen b, Figure t); (b) mult icomponent magneti-
zation (specimen c, Figure 1); and (c) incoherent
magnetization (specimens a and d, Figure 1).
Approximately one third of the pilot specimens (Figu-
re 5) collected from the Karisoke sediments can be
characterized as a Type A magnetization, that is, they
are single component (except for a small present
earth's field component) and their magnetization
values after stepwise demagnetization plot as a
straight line that reaches nearly to the origin of an
orthogonal project ion. Another th i rd of the pi lot
specimens can be characterized as a type B magneu.za-
t ion, that is , they are mul t icomponent, and af ter step-
wise demagnet izat ion, one of these componcnts shows
a K A R - l - 9 5 6b K A R - r - 9 2 4c K A R - l - 9 7 7
\ d K A R - | - 9 8 5
t - i
. c - ' . . \
d--- - - -
40 50
ALTERNATING FIELD (mT)
WEST EAST-r -r 00 * 0 . 5
N ( a )
. 0 . 3
SOUTH/DOWN
Fig. 1: Magnet izat ion behavior of p i lot specrnrcns
from normally magnetized unconsolidated sediments
following stepwise alternating field (af) dernagnetization.
Natural remanent magnetization rs indicated by letters
NRM. Normal ized magnet izat ion (M/MO) versus af
demagnetization (a), and direction changes during a
demagnetization projected on a lower hemisphere eeual area
stereographic project ion (b) where open squares [LJr and
sol id squares I l ] represent negat ive and posi t ive inc l inat ion
respectively, and orthogonal vector projections (c),
with respect to present horizontal, where solid circles []refer to N/S axis (horizontal plane ) and open crrcles [()0]
to Up/Down axis (vert ical p lane).
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magnetization values which plot as a straight line that
reaches nearly to the origin of an orthogonal projec-
t ion. The remainder of the pi lot specimens can be
characterized as a type C rnagnetization, that is, they
show incoherent magnetization values with stepwise
denragnetization. The basalts beneath the Quaternarysediments show a single component (Type A)magne-
t izat ion (F igure 6) .
'fvpe C magnetizations are recorded in sediments at
0-50 and 140-180cm below the surface (Figure 7).These are zones containing abundant gri t and pebble
size tuff parr icles. The coarser fract ion of these sedi-
20 40 60 80
A L T E R N A T I N G F I E L D ( m T )
NORTH
WEST/UP EAST,/DOWN
Fig. tr: N{agnetization behavior of a pilot specimen
flom thc norniaily magnetized vesicular basalt found
directly beneath the unconsolidated sediments.
Svmbols as in Figure 1.
ments is either too large for alignment in an ambientmagnetic field and hence will show random magneti-zation directions, or was only partially aligned. Ineither case, the coarser fraction of the sediments may
entirely or partially mask the dipole m^gnetrzation ofthe finer matrix fraction. In these sediments, stepwisedemagnetizatton did not produce a clear directionwith an end point near the origin of the orthogonalp lo t .
As can be seen from Figure 7, the inclination and de-clination profiles reveal a suite of normally magne-
tized sediments. The inclination profile shows a some -
what noisy record with values which in places are con-siderably steeper than would be expected for an equa-torial sampling latitude. Inclination values ar rhe
equator should essentially be zero. Some of the large
swings in declination and inclination are associated
with low K. values (Table 4), where K. is a measure
of the degree of internal consisrency within a sample,and are thus suspect. K- values greater than l0 areconsidered acceptable. The magnerization profile
shows a rather consistent pattern of progressive waxing
and waning of the sample intensity which in all likeli-hood is a true depiction of field behavior. This record
would seem to indicate that the 2.4 meters of uncon-
solidated sediments at Karisoke were deposited over aperiod of several thousand years.
5 Conclusions
Two buried paleosols and one surface soil display
different degrees of development compatible withgenesis in: a) a drier stadial paleoclimate, b) wetter
interstadial and early to middle postglacial paleo-
climate, and c) drier later Holocene climate. The
lithologic sequence of sediments supports the paleocli-
matic interpretation in that alluvium (unit III) was
emplaced between short - lived episodes of pyro-
clastic activity that emplaced tuffs (units IV and II).
I f the iaC date of 6300 + 200 yr BP is correct, i t would
appear the surface slope wash sediments buried the
IIA11b horizon during the middle Holocene when
the paleoclimate is known to have been wetter. How-
ever, in all likelihood the upper paleosol dates from
the Kalambo Interstadial of the last glaciation.
The geochemistry of this sequence supporrs the notion
of more intense weathering and translocarion in the
middle paleosol. In particular lower Na and Ca con-
centrations indicate more inrense drainage and the
data for Fe suggest displacement from the Ab complex
into the B2b horizon in the middle paleosol. The
data for U-Th indicate that radioactivity in the local
bedrock is caused bv both elements.
(c)
Samole KAR- l -O32( v e s r c u l a r b a s a l t )
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Stratigraphy, Geochemistry, and Paleomagnetism of Late Quaternary Bedrock r43
DECLINATION INCLINATION
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MAGNETIZATION( A M - 1 )
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l l '
K+ precision parameter (f isner, 1953)where values greater than 10 areacceptable.
Fig. 7: Stratigraphic and paleomagnetic profiles for Site KAR1, Virunga Mountains, Rwanda.
Magnetization values for all samples are at 10 militesla (mT) demagnetization.
Uni ts of inc l inat ion and decl inat ion are in degrees; uni ts of magnet izat ion are in amperes per meter A'M-r .
4 . 8 0 . 4
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37283870t50785 32 l--t 466t5t lt 6t 53044'l'l
t 230306 l4268642a2532532599r q
99II
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\Wrllran C. ManaNEy, RoN, r-o G. V. HnNcocK & fuNE W. BaRExonEc'r
Table 4: Mean Remanent Magnetization Directions
Si te
Karisoke I
Karisokc I
Thickness Lithology
210 t m vo lcan ic
tuff & ash
210cm * vesicular
Ag. Polarity
Brunhes N
Brunhes N
D I
3 ' - 0 . t
360 -23
Pole
Lat. Long
8 l ' N 3 0 3 "
80 'N 206"
KK
1 1
N M
t 3 . 0 3
a91
ll
55
N : number o f specrmens
M = average in tens i ty o f magnet iza t ion (AM- l )
D = average decl ination (degrees)
I : average incl ination (degrees)
K = prec is ion parameter (F tsuan, 1953)
tr 91 = radius of the cone of 91 o/o confidence about the resultanr vecror
The Karisoke sediments are for the most part stably
magnetized and record a normal dipole field. The
zones of coarse sediment generally reveal incoherent
magnetization directions which are nevertheless all
normal. Mean remanenr magnetization directions
indicate a local field with an inclination of -0.1
degrees and a declination of 31 degrees indicating thar
a normal dipole field existed at the time of deposition
of these sediments. The underlying vesicular basalt is
l ikewise normally magnetized.
6 Acknowledgements
The authors thank the Natural Sciences and Engineer-
ing Research Council of Canada for financial support
to the SLO\7POKE Reactor Facility and to \fCM
(grant A9021) and RWB (Grant A0t81). They also
thank the Digit Fund and D. P. \7arrs, former Direc-
tor of the Karisoke Research Center for use of facili-
ties, accommodation, and for assistance in working
around mountain gorilla groups in the Karisoke Re-
search Area. Field work was authorized by I'office
Rwandaise du Tourisme et des Parcs Nationaux. Labo-
ratory analyses were completed at the SLOWPOKE
Reactor Facility, University of Toronto, at the Geo-
morphology and Pedology Laboratory, Geography
Department, Atkinson College, York University, and
at the Pacific Geoscience Centre , Energy Mines and
Resoutces, Vancouver, B. C.
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Ftsurn, R A. (1953): Dispersion on sphere . - Proc. of the
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Fox, R. L. (1968): Petro logy of Sedimentary Rocks.
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FossEv , D . ( t9SZ) , Gor i l l as i n the M is t . - 326p . , Hough-
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HaNcocr, R. G. V. (1978): Some aspects of the analyses of
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3 p . ; O t t a w a , O n t . , 2 1 - 2 7 .
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Hanrus, N. (1982): The Virunga Volcanoes. - unpubl ished
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Mountains.
HEosEnc, O.Ogeq, Features of Afroalp ine Plant Ecology.- Almquist and Wiksel ls , 144 p. ;Uppsala.
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paleosols: evidence from East Afrrca and the Rocky
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- (1988): Holocene glaciat ions and paleocl imate on
Mount Kenya, East Africa: Quaternary Science Reviews,
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stratigraphy, Geochemistry, and paleomagnetism 'f Latc euatcrnar' Bcdr.ck f , i i
lv{ . r t t , rsr ' r ' , \7. C. ( l9S9b): euaternary geology of MountKcnya . - i n MIHANEy, W. C . . ed . , eua te rna ry andenvlronmental research on East African mounrains. _,I 2 l - 140; Balkema, Rotterdam.
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Mt Hc t t ' t t , A . (19S4) : V is i r6 de la va l l c6 i f b r rd p la rcau cn t r rle Bisoke ct lc Kar is imbi . - - Unpubl ishcd f ic lc l norcs
Oy , \N , r ' \ , M . & T , rn l r iAR; \ , H . (1970) : R . . i scc l S ra r rc la rd S . i lColor Charts. - -Japan Rcsearctr Counci l f i r r Agr i iu l turc.Fo res r ry a r rd F i shc r i cs .
So l . Sunv ty S lan , (1911) : So i l Su rvev Manua i . Wash ing -ton , D . C . , U . S . De pa r tmen t o f Agr i cu l tu rc l l and roook18 , U . S . Government p r i r r t i ng Of f i cc , 103 p .
- (1971) : So i l ' l ' axonomv:
Wash ing ron , U . S . ( i ovcn r_ment Pr inr ing Off icc, 7 i , i p.
\WHrn tc ; , L . D . (1961) : X - ra1 r d i f l i a r r i on t c r im iq r_ rc - , f o rmineral idcnt i f icat i .n and rn incrakrgical c, i ' rprs i t i r r r .- rn BL , rc . x , C . A . . cd . , f u l c thods o f So i l Ana l r . , s i s . _Amer . S , t c . Ag ron . . p ( r7 l -69 ( r ; l l l ad i s , ,n , \X , ' i r . .
Manuscr ipt acccptcd i l . Novcmbcr 1g90
