relative ages of loess and till in two quaternary palaeosols in gorges valley, mount kenya, east...

JOURNAL OF QUATERNARY SCIENCE (1997) 12 (1) 61–72 CCC 0267-8179/97/010061–12 $17.50 1997 by John Wiley & Sons, Ltd.

Relative ages of loess and till in twoQuaternary palaeosols in Gorges Valley,Mount Kenya, East AfricaWILLIAM C. MAHANEY Geomorphology and Pedology Laboratory, Atkinson College, York University, 4700 Keele Street,North York, Ontario M3J 1P3, CanadaRENE W. BARENDREGT Department of Geography, University of Lethbridge, Lethbridge, Alberta T1K 3M4, CanadaWALTER VORTISCH Prospektion und Angewandte Sedimentologie, Institut fur Geowissenschaften, MontanuniversitatLeoben, A-8700 Leoben, Austria

Mahaney, W. C., Barendregt, R. W. and Vortisch, W. 1997. Relative ages of loess and till in two Quaternary palaeosols in Gorges Valley, Mount Kenya, EastAfrica. Journal of Quaternary Science, Vol. 12, 61–72. ISSN 0267-8179

Received 7 April 1996; accepted 4 October 1996

ABSTRACT: Two compound palaeosol profiles formed in loess and till were studied in theoldest early Quaternary end moraine (Gorges age) system on Mount Kenya. Although both tillsappear to have similar weathering histories, the overlying loessic sediments, serving as secondaryparent materials for the palaeosols, are considered to have different ages on the basis of colour,presence or absence of clay films, and field texture (especially percentage of silt). Palaeomagneticdeterminations showed both tills to have reversed remanent magnetism, suggesting that theywere emplaced during the Matuyama Chron. The overlying loessic sediments in profiles GOR55and GOR58 show normal magnetism and grade upward into the lower zone of bioturbation inthe A horizon complex. However, in GOR58 the surface loessic sediments have normal remanentmagnetism with considerable secular variations, suggesting it was emplaced over a longer timeframe during the Brunhes Chron (i.e. , 0.78 Ma). Profile GOR55 contains larger amounts ofgibbsite, indicating more aggressive leaching over time, and an absence of secular magneticvariations suggests deposition over a shorter time interval.

The study of volcanic feldspars and ash, using scanning electron microscopy, in the twoprofiles, shows that sand grain (63–250 mm) weathering is slightly different in the two tills;relative weathering effects (e.g. corrosion of grain surfaces and neoformation of clay minerals)are greater in profile GOR55 than in profile GOR58. Overall the data indicate that some sitesin the lower tropical Afroalpine timberline environment may have aeolian covers that areconsiderably younger than the deposits they overlie. 1997 by John Wiley & Sons, Ltd.

KEYWORDS: palaeosols; Quaternary; loess; till; Mount Kenya.

enia forest, it is very likely that the tussock grass area hasIntroductionexisted since the Last Glacial Maximum (LGM) or longer.Field observations of texture and relative degree of weather-

Early Quaternary glacial deposits were first mapped and ing suggest that GOR58 has a younger loess cover depositedanalysed in Gorges Valley (Fig.1) by Mahaney and Barend- some considerable time after glaciation. Site GOR55 appearsregt in 1984 and 1986 (Mahaney et al., 1989). Two palaeo- to have an older loess that may have been emplaced shortlysols, in the outer moraine crest, show important palaeo- after the Lake Ellis Glaciation (Fig.2) during the early Brunhesmagnetic and weathering records that appear to be related Chron. Because these palaeosols are similar to other earlyto their relative ages, topographic positions, and vegetation and middle Quaternary palaeosols in Gorges Valley (Mahaneycovers. Both palaeosol sites are situated on the outer et al., 1989), we attempted to establish firmer age controlsmoraine; profile GOR55 is under Hagenia forest, whereas using palaeomagnetism and weathering relationshipsGOR58 is under tussock grass in a cleared area. Because (including neoformation of clay minerals studied using SEM).the timberline zone consists of discontinuous stands of Hag- We also sought to determine whether or not we could

detect differences in state of weathering and rates of loessdeposition between the two palaeosols using palaeomagnet-

*Correspondence to: W. C. Mahaney. ism.Contract grant sponsor: NSERC (Canada)Contract grant sponsor: Deutsche Forschungsgemeinschaft

62 JOURNAL OF QUATERNARY SCIENCE

Figure 1 Location of sites GOR55 and GOR58 in lower Gorges Valley, Mount Kenya. Elevations in metres.

known with precision, but present mean annual temperatureis probably similar to a meteorological station at ca. 3000 ma.s.l. on the western flank of the mountain (i.e. approximately10.0°C; Coetzee, 1967). Present mean annual precipitationis approximately 1000 mm (Coetzee, 1967).

Methods

Palaeosol profiles were described following the nomencla-ture of the Soil Survey Staff (1951, 1975) and Birkeland(1984). Particle size classifications follow the Wentworthscale of Folk (1968) for sand/silt (63 mm) and Soil Survey Staff(1975) for silt/clay (2 mm). Particle sizes were determinedby dry sieving the coarse particles (2000–63 mm) and bysedimentation for fines (, 63–0 mm); procedures follow thoseoutlined by Day (1965) and Bouyoucos (1962). Coloursfollow the system of Oyama and Takehara (1970).Figure 2 Stratigraphy for older tills on Mount Kenya showing

Samples of the , 2 mm fraction were studied by X-rayrelative time of emplacement of till of Gorges age (after Mahaneyet al. (1989) and from Mahaney (1990)). The ages follow Cande diffractometry (oriented powder mounts), and sands (63–and Kent (1995). 2000 mm) by scanning electron microscopy (SEM with

energy dispersive X-ray analysis of the sand fractions). TheXRD results are given as peak heights (Whittig, 1965; Mah-aney, 1981). Palaeomagnetic samples were collected in plas-Field areatic cubes (2.4 cm sides) and analysed on a Schoenstedt(SSM-1) spinner magnetometer following procedures outlinedin Barendregt (1984, 1995), and Barendregt and MahaneyThe two palaeosols are located in the outer Gorges end

moraine at 3000 m a.s.l. (Fig. 1) within the Hagenia wood- (1988). Azimuthal orientations were measured in the fieldusing a magnetic compass. As the method of sampling wasland dominated by Hagenia abyssinica and tussock grass

(mainly Festuca pilgeri). Site GOR55, situated under a Hag- restricted to the use of separately oriented cubes, only softsediments were sampled. Sediments influenced substantiallyenia canopy, is shaded and probably has a wetter microcli-

mate. Site GOR58, located under tussock grass is open to by root systems or containing pebbles, krotovinas, and/ormodern roots were avoided.wind action and is probably drier. Temperature data are not

63QUATERNARY LOESS AND TILL AGES, MOUNT KENYA

Particle sizeResults and discussion

Particle size trends (Fig. 3) in the two palaeosols were studiedin order to determine differences between the tills andPalaeosol stratigraphyoverlying aeolian sediments. Both profiles have somewhatdifferent distributions of clay and silt with depth. Both pro-The two palaeosols (Figs 3 and 4) are Alfisols (Tropudalfs,files have similar amounts of clay in their till horizons andSoil Survey Staff, 1975). Both profiles have similar thick-evidence (ca. 20% clay in both B horizons) for translocationnesses of aeolian sediment, either bona fide loess or sandyof clay from the A to the B horizon complexes within thewind-blown sediment typical of mountain environmentsoverlying loessic materials. Profile GOR55 has pronounced(Owen et al., 1992), as in the Ah1 horizon of GOR55 andclay films in the B horizons. The larger amount of clay inthe Ah2/Bt1 horizons of GOR58, over till; however, profilethe Ah1 horizon of profile GOR58 may result from differentGOR55 has a slightly greater silt content in the Bt1 horizon,vegetation covers at the two sites (tussock grass at GOR58which could result from aeolian deposition over a longerand trees at GOR55). The Ah2/Bt1 horizon complex oftime (see Birkeland (1984), Mahaney (1978) and Nelsonprofile GOR55 contains more silt than the same horizons(1954) for discussion of loess amount/thickness versus age),in GOR58. This suggests that it was deposited some con-from the influence of forest in trapping aeolian slit and/orsiderable time after the Gorges Glaciation (middle Matuyamafrom silt movement downward in the profile. In general, theChron), and was followed by a long period of subaerialchanges in field texture—silty loam in GOR55 to loam inweathering. Greater sand percentages in the upper horizonsGOR58—without any important change in percentage clay,of both profiles are considered to be of aeolian origin,show more silt in the former. The reddest colours in theprobably derived locally from younger morainic material.two profiles, although variable, tend to reach 5YR 4/4 and

5/4 in the B horizon complexes (somewhat redder inGOR55). The cemented subsoil in profile GOR58 may berelated to different microclimates, or to differential releaseof Si upon weathering and its reprecipitation as a cementing Mineralogyagent. The relatively smaller amount of gibbsite in profileGOR58 tends to support the notion of different degrees of The mineralogy of the , 2 mm fraction was analysed in

order to determine if distributions of clay and primary min-Si removal and leaching, as discussed by Vortisch et al.(1987) and Mahaney (1990). erals might provide useful lithostratigraphic or weathering

Figure 3 Silt and clay distributions with depth in profiles GOR55 and GOR58; from Mahaney (1990). Aeolian/till boundaries (1,2) weredrawn on presence/absence of silt caps in the upper parts of profiles 55 and 58. Unit 1 is left off the profile diagrams by convention (SoilSurvey Staff, 1975).


Table 1 Mineral distribution in profiles GOR55 and 58

Site Soil Mineralsa

horizonH I G Q F M

GOR55 Ah1 — — — xx x xAh2 — — — xx x trBt1 — — x xx x trBt2 — — x xx x x2Cox1 — — x x x x2Cox2 — tr x tr xx x2C3 — — tr x xx x

GOR58 Ah1 — — — xxx x trAh2 — — — xx x xBt1 — — — xx x xBt2 — — x x tr tr2Cm1 — — x x xx x2Cm2 tr — — x xx x2Cu tr — — x xx tr

aH, metahalloysite; I illite; G, gibbsite; Q, quartz; F, feldspar;M, magnetite. Relative amounts were determined from peakheights; nil (—), trace (tr), small amount (x), moderateamount (xx), and high amount (xxx).

Palaeomagnetism

Palaeomagnetism is valuable for differentiating sedimentsdeposited during the early Pleistocene from those depositedduring the middle and late Pleistocene. The boundarybetween the Brunhes Normal Chron and the MatuyamaFigure 4 Stratigraphic and palaeomagnetic profiles for siteReserved Chron is 0.78 Ma (Cande and Kent, 1995).GOR55 (A) and Site GOR58 (B), lower Gorges Valley, MountAlthough polarity subchrons lasting from 10 to 200 ka occurKenya from Mahaney (1990). GOR55 is located under Hageniawithin the Brunhes and Matuyama, most sequences of glacialforest; GOR58 under tussock grass. Both sites are on the outerdeposits on Mount Kenya reveal stratigraphic evidence ofGorges moraine crest. Unit 1 is an aeolian unit; unit 2 is till.multiple glacial and interglacial events spanning long periodsof time. On the basis of this strategraphical evidence(Mahaney et al., 1989) as well as soil weathering histories(this paper), materials sampled at GOR55 and GOR58 rep-resent time frames that far exceed the estimated temporalvariables. Among the primary minerals, in both profiles,

quartz is more abundant in the palaeosol sola (A/B horizon extent of known polarity subchrons. Palaeomagnetic charac-teristics of the two sections were studied in order to deter-complex) and lower in the tills (C horizons) (Table 1). As

an aeolian indicator it was first observed on Mount Kenya mine if the loesses were deposited soon after the till or atdifferent intervals of time. We also wished to establish theby Mahaney (1982), and in subsequent studies (Mahaney,

1989, 1990; Vortisch et al., 1987) it has proved particularly relative age of the tills (e.g. Brunhes versus Matuyama) andto determine whether or not polarity subchrons were present.useful in distinguishing aeolian from glacial sediments.

Feldspar (mainly anorthoclase) is more abundant in the Remanence measurements and stepwise alternating field (AF)demagnetization of selected pilot specimens (Figs 5 and 6)tills than in aeolian cover sediments (Table 1). Magnetite is

distributed rather uniformly in the Cu horizons, but is present from each lithological unit (GOR55 and GOR58) werecarried out at the levels indicated in Figs 5 and 6. Qualitativein trace amounts only in the palaeosols, suggesting a higher

degree of weathering and diagenesis of the Fe2O4. assessment of changes in both direction and magnitude ofthe magnetic vectors measured led to the selection of aAmong the weathering products in the , 2 mm fractions,

metahalloysite is found only in the lower parts of the GOR58 20 mT cleaning field for the remaining samples.On the basis of the low median destructive field of theseprofile. Its absence in the GOR55 profile may be related to

the presence of gibbsite; this indicates more aggressive leach- samples, it would appear that detrital magnetite is the pre-dominant remanence carrier. It is one of the principaling conditions, which may have removed all or most Si

released on weathering to the pore solutions. Because the components of the tills on Mount Kenya (Vortisch et al.,1987).gibbsite distributions are so different in the two profiles, this

mineral must have a weathering rather than a detrital origin. All palaeomagnetic samples from GOR55 and GOR58 fallinto two groups: those showing normal polarity (loesses) andFor a full discussion of gibbsite–metahalloysite distributions

in other pre-Liki (pre-Wurm, Wisconsinan) profiles on Mount those showing reversed polarity (tills). Transitional polaritieswere not seen.Kenya see Mahaney (1991) and Vortisch et al. (1987). The

presence of illite in the 2Cox2 horizon of profile GOR55 is All specimens are characterised by a single componentremanent magnetization (Figs 5 and 6). A viscous componentprobably the result of earlier aeolian transport followed by

glacial reworking (see Mahaney et al. (1988) for discussion). is removed by the first step of AF demagnetization. No


Figure 5 Magnetiziation behaviour of pilot specimens from normally magnetized loess deposits at GOR55 (A–C) and GOR58 (A–C)following stepwise alternating field (AF) demagnetization; from Mahaney (1990). Natural remanent magnetization is indicated by lettersNRM. Normalized intensities during alternating field (AF) demagnetization (A), and directional changes projected on a lower hemisphereequal area stereographic projection (B) where solid squares (K) represent positive inclination and open squares (l) negative inclination,and orthogonal vector projections (C) where solid circles (P) refer to N–S axis (horizontal plane) and open circles (s) to up/down axis(vertical plane).

overprint histories were recorded in either till or loess from real, may represent hiatuses in deposition and/or episodesof erosion and thus may also indicate the passage of con-the formation of secondary iron oxides. Table 2 contains a

summary of the polarity data for each lithological unit at siderable time.Palaeomagnetic directions for the tills at GOR55 andboth sites. The tills show nearly identical pole data but the

loesses are significantly different (Fig. 4). Both loess deposits GOR58 (Table 2) indicate that these deposits obtained theirprimary magnetic remanence during a period when the fieldare normally magnetized and are assumed to have been

deposited during the Brunhes Normal Chron. The loess cover was reversed. As these sites are near the margin of glaciation,and it is known from other work (Mahaney et al., 1989;at GOR55 reveals a pole very close to a geocentric axial

dipole (GAD) but shows no evidence of secular variation. Mahaney, 1990) that the earlier Quaternary glaciationsreached farthest down the mountain, it can be assumed thatWe can assume that the loess was deposited over a relatively

short period, and based on morphostratigraphic evidence these deposits were laid down during the MatuyamaReversed Chron and are thus older than 0.78 Ma. The(this paper), this was presumably during the early to middle

Brunhes Normal Chron. The loess cover at GOR58 on the absence of secondary magnetizations (overprints) is note-worthy (Fig. 3). This may imply that these sediments wereother hand shows evidence of at least two stages of depo-

sition: an earlier one during which the sediments record a for the most part well drained, and that ground-water move-ments were insufficient to substantially remobilize iron andrelatively high positive inclination for this sampling latitude,

and a later stage when the sediments record a north/ north- produce secondary magnetizations. Because both tills areknown to be younger than the oldest till on Mount Kenya,westerly declination, and show a tenfold increase in mag-

netization intensity. Based on their rather different palaeo- which is Olduvai in age (1.77–1.95 × 106 yr; Cande andKent, 1995), we assume that both GOR55 and GOR58 weremagnetic directions and intensities, these two stages of

deposition probably were separated by a considerable period emplaced during the middle Matuyama Chron (Mahaneyet al., 1989).of time. The upper loess may have been deposited during

the late Brunhes. Palaeomagnetic measurements of the loess covers atGOR55 and GOR58 (Table 1) show significant differencesIn contrast to GOR55 the loess record at GOR58 reveals

some secular variation swings. As these swings appear as in the directions of primary magnetization. The intensity ofmagnetization also differs by a factor of 10. This suggestsincomplete cycles of secular variation, the interruptions, if


Figure 6 Magnetization behaviour of pilot specimens from reversely magnetized till deposits at GOR55 (A) and GOR58 (B) followingstepwise AF demagnetization. Labelling conventions as in Figure 5 (from Mahaney, 1990).

Table 2 Summary table of stratigraphy and mean remanent magnetization directionsa

Site Thickness Lithology Age Polarity N M D I K a95 Pole(metres)

Latitude Longitude

GOR55 1.00 Loess Brunhes? Normal 13 0.13 357 +2 186 3 86°N 329°GOR55 0.60 Till Matuyama? Reversed 8 0.35 179 −5 112 5 87°S 192°GOR58 0.25 Loess (top) Brunhes? Normal 7 0.06 338 +5 26 12 68°N 315°GOR58 0.45 Loess (bottom) Brunhes? Normal 11 0.004 1 +17 119 4 81°N 46°GOR58 1.00 Till Matuyama? Reversed 26 0.034 180 +11 10 15 85°S 42°

aN, number of specimens; M average intensity of magnetiziation (AM −I ); D, average declination (degrees); I, averageinclination (degrees); K, precision parameter (Fisher, 1953); a95, radius of the cone of 95% confidence about the resultant vector.

that the loesses at the two sites are of different age and of clay minerals (on sand grain surfaces) might prove usefulin differentiating the two profiles.probably also of different provenance. The changes in direc-

tion and intensity within the loess profile at GOR58 suggest In profile GOR55 we scanned every horizon from thesurface loessic material to the tills at depth. The strongestthat these sediments were deposited over a considerable

span of time. Furthermore, the presence of partial records weathering effects were observed in the palaeosol horizonsin the form of solution etching, grains coated with gibbsite,of secular variation suggest that periods of erosion and/or

non-deposition may account for some of this time. decomposed feldspar grains, dissolved microliths in volcanicglass, and weathered glass with Al-rich coatings.

Glacially crushed feldspar grains with pronounced con-choidal fractures and relatively sharp-edged grain relief arefound throughout profiles GOR55 and GOR58, indicatingScanning electron microscopythat some grains which survive glacial transport are oftenreworked by aeolian processes (see Vortisch et al., 1987;The fine and very fine sand fractions (63–250 mm) of the

loess and till deposits were studied by SEM in order to Mahaney et al., 1988) (Fig. 7A). Some grains in the Ah2horizon show minor etching features and are coated withdetermine if relative weathering features and neoformation


Figure 7 (A) Glacially crushed feldspar (arrows = crushing planes) with conchoidal fractures and relatively sharp-edged microrelief inGOR55—Ah2. (B) Alkali feldspar with striations and crushing features (arrows) in the Ah2 horizon with slight degree of etching.(C) Solution etching in the Ah2 horizon on a feldspar grain—solution etching parallels cleavage planes. (D) Gibbsite-coated grain (in Bt2horizon); thicker coatings often appear on older more extensively weathered grains.

Fe (determined by X-ray microanalysis) and others (feldspar crushing features, in the Ah1 horizon points to the recentinflux of airfall sediments. An alkali feldspar grain in thegrains) show solution etching parallel to cleavage planes

(Fig. 7B and C). Some feldspar grains in the Bt1 horizon Ah1 horizon shows pronounced solution etching and a smallhole possibly caused by magmatic fluids (Fig. 10D). Glass inhave high Mn coatings; others have very high Al contents

with almost no Si. In the Bt2 horizon many feldspar grains the Ah2 horizon is often rounded, showing nearly dissolvedmicroliths (Fig. 11A). These weathered ash grains occurhave thick Mn coatings; others have Al-rich coatings (Figs 7D

and 8A); surfaces of other etched feldspar grains have Si:Al together with fresh grains of glacially crushed quartz(Fig. 10C) and rare grains of weathered hornblende (Fig. 11B).ratios of 1:1 (determined by EDS).

In the subsoil horizons (2C group), fresh feldspar grains In the Bt1 horizon the nodular variety of gibbsite was foundon Na-feldspar (Fig. 11C); on other feldspar grains weather-(Fig. 8B) are mixed with weathered grains, many of which

show complex weathering histories (Fig. 8C shows more ing is indicated by dissolution etching parallel to cleavageplanes (Fig. 11D). X-ray microanalysis showed more numer-advanced weathering in the lower left of the enlargement).

Glacial-crushing features are more pronounced in the 2C ous Al-rich coatings corresponding to gibbsite on grain sur-faces in the Bt2 horizon than above or below (see Table 1horizons (Fig. 8C); many relatively clean feldspar grains alter-

nate with coated grains of weathered ash (Fig. 9A). Some for XRD data on gibbsite in the Bt2 horizon).The till in GOR58 (2Cox2 and 2C3 horizons) is nearlyweathered glass particles display dissolved microliths of vari-

able composition (Fig. 9B). Even in the 2C3 horizon at the identical to the till in profile GOR55. Glacially crushedgrains alternating from fresh to weathered occur with grainsbase of the palaeosol about 20% of grains still have Al-rich

coatings (Fig. 9C and D). In all, chemical corrosion appears of weathered ash (Fig. 12A). Some plagioclase and K-feldspargrains exhibit variable etching features depending on theirgreater in the palaeosol, with gibbsite coatings on more than

50% of grain surfaces through the entire profile. As in susceptibility to weathering. Overall, weathering is less inten-sive in the till bodies than in the overlying loesses. However,previous studies of older Brunhes and Matuyama age profiles

(Vortisch et al., 1987; Mahaney, et al., 1989; Mahaney, both tills were weathered over long time spans in the Matuy-ama Chron prior to the deposition and subsequent weather-1990), most weathering features occur on volcanic glass par-

ticles. ing of the overlying loesses during different time spans ofthe Brunhes Chron.The GOR58 profile contains larger amounts of quartz in

the sand fractions of the palaeosol sola (A and B horizons)horizons than in the C horizons. In the Ah1 horizon com-pound grains of haematite (Fig. 10A) occur together with Interregional correlationssome opal phytoliths (little knots in the phytolith grain arepores) (Fig. 10B), X-ray microanalysis of which confirmed Long-distance correlations with other tropical alpine areas

are not possible because elsewhere probable middle to earlyhigh Si contents. A fresh quartz grain (Fig. 10C), with glacial


Figure 8 (A) Etched feldspar grain with thick gibbsite coating in GOR55—Bt2 horizon. (B) Analbite grain in 2Cox1 horizon with freshsurface showing effects of glacial crushing (arrows). (C) Relatively fresh feldspar grain with glacial crushing features (arrows) in the 2Cox2horizon.

Quaternary glacial deposits have not been dated. However, GOR55 and GOR58, it is likely they were deposited before(e.g. GOR55) and after (e.g. GOR58) this middle Brunheson Kilimanjaro, deposits of the First Glaciation (Downie and

Wilkinson, 1972) may be younger than the Gorges tills on warming event. This would make the two loesses equivalentto the Naro Moru Glaciation (early Brunhes) (GOR55) andMount Kenya. Deposits equivalent in age to the Gorges tills

are unknown in the Ruwenzori Mountains (Osmaston, 1989), the Teleki Glaciation (middle Brunhes or Oxygen IsotopeStage 6 (GOR58) on Mount Kenya; Mahaney, 1990).on Mount Elgon (Hamilton, 1982) and in the Simen Moun-

tains of Ethiopia (Hurni, 1989). It is possible that the older The tills at sites GOR55 and 58 may correlate with CaCO3

fluctuations in the base of oceanic core 82PCS18 (Jansendiamictons in the highlands of New Guinea are early Quat-ernary in age, but evidence for a glacial origin is lacking et al., 1986), but it is impossible to determine which cold

shift or shifts coincide with the Gorges Glaciation on Mount(Peterson and Hope, 1972). There are no highland tropicalareas with geochronologies supported by palaeomagnetic Kenya. Unfortunately this core record begins at 1.4 Ma, with

earlier turbidite deposits having erased older parts of thedating, despite the fact that Barendregt and Mahaney (1988)showed that palaeomagnetic methods work well in tropical record. It is only with the recovery of further marine cores

that we will be able to make tentative correlations with thetills (presumably a response to their high water content andfluidity during deposition). Similarly, the younger loesses of East African highlands.Brunhes age, described here for the first time, can only becorrelated to other documented sections on Mount Kenya(Barendregt and Mahaney, 1988; Mahaney et al., 1989).

Conclusions

The two palaeosol profiles show overall trends in particleCorrelations with the marine recordsize distribution, clay and sand mineralogy, and magneticremanence that indicate different ages for the two loessCores from the Canary Basin (30°–33°N) yielded foramin-

iferal assemblages including Globoratalia menardii/tumida) covers. The loess in profile GOR55 is presumed to be lowerBrunhes in age; weathering features on sand grains and aand Pulleniatina, ranging from 3.1 to 3.7 × 105 yr BP (Fig. 13)

indicating a warmer climate during the middle Brunhes higher content of gibbsite suggest greater chemical corrosionand more aggressive leaching than in profile GOR58. Nor-Chron (Jansen et al., 1986). Enhanced meridional circulation

during glacial stages brings the southward movement of cool mal magnetism throughout the loess cover in profile GOR58indicates a Brunhes age, and based on a greater proportionnorthern hemispheric water and a corresponding decrease

in G. menardii, which signals a glacial period. Because of of fresh grains and less neoformation of clay (supported alsoby field observations of clay films), possibly a youngerthe different degrees of weathering in the two loesses at


Figure 9 (A) Feldspar grain (centre) with solution etching in the GOR55—2Cox2 horizon—other grains are probably weathered ashparticles. (B) Dissolved microliths of variable composition on volcanic glass in 2C3 horizon. (C) Gibbsite coated feldspar grains in the 2C3horizon. (D) Weathered ash with Al-rich coatings in 2C3 horizon; Box in centre is area enlarged for more detailed analysis.

Figure 10 (A) Compound Fe-rich grain (probably haematite) in GOR58—Ah1. (B) Soil phytolith with high Si content confirmed by X-raymicroanalysis. (C) Fresh quartz grain with glacial crushing features (arrows). (D) Alkali feldspar in the Ah1 horizon with some solutionetching and small hole possibly caused by magmatic corrosion.


Figure 11 (A) Volcanic glass grains in the GOR58—Ah2 horizon—with dissolved microliths. (B) Weathered hornblende grain in the Bt1horizon with etching parallel to cleavage planes. (C) Nodular aggregates of gibbsite on Na-feldspar. (D) Weathering along parallel cleavageplanes on a feldspar grain.

Figure 12 (A) Glacially crushed (arrows) feldspar grain (centre) in the GOR58—2Cox2 horizon—surrounded by grains of ash (altered glass).(B) Plagioclase grain (Pl) with pronounced etching features above a K-feldspar grain (KP) less sensitive to weathering in the 2C3 horizon.

Brunhes age. As a result of the long hiatus in age between possible for the loess units on Mount Kenya. The tills,however, are older than the hiatuses known to exist in thesethe till and loess, it appears loess sheets were stripped

periodically and replaced with fresh sediment, especially in lower core sections.areas lacking forest cover. Firm long-distance correlations

Acknowledgements We thank the Institute of Geology and Palae-with other areas are not possible at this time; however, theontology, University of Marburg, Germany and the late Klaus Fechertwo loesses may have been deposited before (GOR55) andfor use of and assistance with the scanning electron microscope.after (GOR58) a major mid-Brunhes warm cycle of ca. 4.0Research was supported in part by Natural Sciences and Engineering× 105 yr ago. Tills deposited by an expanded mountain iceResearch Council of Canada (NSERC) grants to WCM and RWBsheet in the early Quaternary are probably older than mostand a Deutsche Forschungsgemeinschaft (DFG) grant to WV and

pre-Wurm deposits described on other East African moun- joint DFG/NSERC grant to WCM. David Hinbest and G. Mahaneytains, but because there are no palaeomagnetically dated assisted with various analyses in the Geomorphology and Pedologysections on other tropical mountains, it is impossible to Laboratory at York University. Palaeomagnetic samples were ana-correlate them with any degree of exactitude. Approximate lysed at the Pacific Geoscience Centre, Sidney, B. C. Janet Allin

drafted the illustrations. Photomicrographs were prepared by Johncorrelations with cores retrieved from the Canary Basin are


Figure 13 Fluctuation of CaCO3 and species of Foraminifera in core 82PCS18 (Jansen et al., 1986) signalling colder/warmer intervals inthe Canary Basin, Atlantic Ocean (from Mahaney, 1990). The relative time of emplacement of the GOR55 and GOR58 loess deposits isshown relative to the mid-Brunhes warming event (stippled area) of Jansen et al. (1986). Dashed lines indicate dating uncertainty. Singlearrow indicates direction of shift in age of Gorges till. Double arrows indicate that palaeosols in loess remained open to the action ofsubaerial weathering.

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relative ages of loess and till in two quaternary palaeosols in gorges valley, mount kenya, east...

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