glacially-crushed quartz grains in late quaternary deposits in the virunga mountains, rwanda...

Glacially-crushed quartz grains in late Quaternary deposits in the Virunga Mountains, Rwanda - indicators of wind transport from the north? WILLIAM C. MAHANEY

Mahaney, William C. 19900301: Glacially-crushed quanzgrains in late Quaternary deposits in the Virunga Mountains, Rwanda- indicatorsof wind transport from the north? Boreas, Vol. 19, pp. 81-89. Oslo. ISSN

Quartz sand grains in late Quaternary deposits of the Virunga Mountains, northwestern Rwanda, were found to contain glacial-crushing microtextures. Glacially-crushed subangular grains generally lack sharp edges, and carry mainly low frequency, linear crushing features. Because there are no known tills in the Virunga Volcanoes (and because quartz is rare in volcanic terrain), it is likely the grains were delivered by aeolian transport from the Ruwenzori Massif, -150 km to the north. All the grains studied are consistent with crushing under low shear stress, and over short transport distances, in alpine glaciers. An increase in the percentage of glacially-crushed quartz from Holocene to Pleistocene beds likely reflects on the increased strength of the paleowind system (trade winds) during the last glacial maximum. Alternatively, it could reflect on the increased source area for glacially-crushed grains resulting from an increase in the extent of the Afroalpine belt during the last glaciation.

William C. Mahaney, Geomorphology and Pedology Laboratory, Geography Department, Atkinson College, York University, 4700 Keele Street, North York, Ontario, Canada, M3J IP3; 25th August, 1989 (revised30th November, 1989).

B o r n 0300-9483.

Routine SEM analysis of a sequence of late Quat- ernary (<50,000 B.P.) sediments in a sequence of aeolian, fluviatile, and volcaniclastic deposits, and in weathered regolith, of the Karisoke Area, Virunga Volcanoes (Fig. l ) , revealed glacially- crushed quartz sand grains. Because there are no known tills in the Virunga Mountains (latitude 1"30'S), the most likely source area is the Ruwen- zori Massif (latitude OO), -150 km to the north. Duplicate samples were studied through the entire KARl section (Fig. 2) to determine the prevalence and range of microtextures on the glacial grains. Other studies of glacial grains have concentrated mainly on middle and high-latitude localities (Krinsley & Doornkamp 1973). In the tropical mountains most studies of microtextures on quartz grains involved local aeolian retransport of glacially-crushed grains (Mahaney 1987; Mahaney et al. 1988a, 1988b; Vortischetal. 1987). Inthis paper the first evidence for long-distance aeolian transport of glacially-crushed sand grains in the Virunga Mountains is discussed and evaluated.

Field area The Karisoke Area (Fig. 1) is located in the saddle between Visoke (3,711 m) and Karisimbi

(4,507 m) volcanoes on the Zaire-Rwanda border. The bedrock is basalt of relatively recent age (-56,000 year K/Ar date; Harris, pers. comm. 1982). Most outcrops of residual regolith carry weathering profiles ( paleosols) dating from the last glaciation. Relative-age dating of buried paleosols in these weathered regoliths shows that the most intense weathering occurred most likely during the Kalambo Interstadial(35,000-25,000 B.P.) of the last glaciation. Radiocarbon-dating of a younger horizon in the sequence (Fig. 2) shows that weathering during the Holocene has been less intense.

There are five main volcanoes on the Rwandan side of the border- Muhabura (4,127 m), Gahinga (3,474 m), Sabinyo (3,634 m), in addition to Visoke and Karisimbi. Most volcanic summits are too low to have had ice on them during the Pleistocene. Only Muhabura and Karisimbi are high enough to fall within the paleosnowline and both summits show only the effects of periglacial activity. No tills were found during a reconnais- sance survey in July, 1987. An air photo recon- naissance of the Virungas by S. Hastenrath and H. Osmaston (pers. comm. 1986,1987) did not reveal any glacigenic landform features either.

On the Zaire side of the Virunga Mountains, Nyiragongo (3,470 m) and Nyamuragira (3,060 m)

82 William C. Mahaney BOREAS 19 (1990)

Fig. 1. Virunga Volcanoes and location of site KARl in the Visoke-Karisimbi Saddle Area, Northwestern Rwanda.

are too low to have had Pleistocene ice covers. Mount Mikeno (4,437 m) is the oldest summit, a resistant plug of an older Pleistocene volcano heav- ily dissected by fluvial erosion and mass wasting (Mahaney, pers. aerial obs. 1987). While some summits were undoubtedly above the paleosnowline (estimated at 3,800 m) during the last glaciation, none are known to have supported even small cirque glaciers.

Climatic parameters at the field site are not known with precision, but precipitation today is estimated at 2,000 mm/year and mean annual temperature near the timberline ecotone (-3,000 m) is approximately 10°C. As in other tropical alpine localities the diurnal range of temperature probably exceeds the annual range (Coe 1967).

Vegetation at the field site is Hagenia forest (dominated by Hagenia abyssinica and Hypericum) which terminates upward into the Afroalpine belt (tussock grass, Lobelia spp., and Dendrosenecio spp.). The area is famous for its local inhabitants - Gorilla gorilla beringei - that

number about 400 individuals in several distinct groups. The work reported upon here is part of an ecological project designed to study interactions between the local fauna (including humans) and the montane environment.

Methods Samples were collected in bulk (<2rnrn) and analyzed for particle size distributions. Sands were wet sieved and separated from silt and clay (<63 pm fraction) following procedures established by Day (1965). Samples were pretreated with H 2 0 2 to remove organic matter and sodium pyrophosphate to deflocculate clays prior to particle size analysis. All sand samples were sonified at low energy in distilled H 2 0 to remove clay particles. Standard size classes follow the Wentworth scale of Folk (1968).

Paleosol designations follow the nomenclature ofBirkeland(l984) andtheSoilSurvey Staff (1951,

BOREAS 19 (1990)

Fig. 2. KARl section showing bedrock and various volcaniclastic (VT = tuff ), alluvial, and aeolian deposits and clay and silt distributions with depth. Oxidized horizons = ox, buried horizons = b, and roman numerals = lithic discontinuities. Quartz grains are highest in units I11 and IV (down to IVAb). somewhat less common in unit 1, and absent in unit IVB2b and VCb.

Glacially-crushed quartz grains, Rwanda 83

KARl PERCENTAGE 10 20 30 40 so , w

I I L - - - - 1

A4 ROOTS I I

I SLOPE WASH I ll WATERLAID T U F F 8 ASH I I27 ALLUVIUM 1p WEATHERED TUFF 8 LOESS I7 WEATHERED BASALT

CLAY (<2pm), SILT (63-2pm)

- - ---

I 70

1 8 0 LYC20xb

YCb

190

m

210

220

230

2 4 0

2 M

260 UNWEATHERED BASALT

270

I 1 I

L

r------- I I I I I I

1975). The mineralogical analysis of the <2 prn fraction follows procedures established by Whittig (1965).

Stratigraphy and quartz grain

The KARl section (Fig. 2) consists of a sequence of weathered regolith (basalt), aeolian and fluvial

sediments and tuff dating from the late Pleisto- cene. The lowest weathering intensity occursin the lowest buried paleosol (units IV and V) comprised of residual regolith, tuff and loess (Fig. 2). The highest degree of weathering is in the IIIB group of sediments of fluvial origin, presumably correlating with the Kalambo Interstadial (c . 35,000- 25,000 B.P.; Coetzee 1967), whichappearstohave been wetter and probably warmer than earlier and

microtextures


later Pleistocene paleoclimates (Cooremans & Mahaney in press). The postglacial period (units I and 11) was apparently warmer and wetter than today, with maximum slope instability occumng in middle Holocene time (perhaps coeval with the emplacement of the Darwin Foundation on Mount Kenya between -8,000 and -4,000B.P.; Mahaney 1989). The presence of glacially-crushed sand grains in the aeolian and fluvial beds suggests direct aeolian transport to the site in some cases, followed by local reworking of sediments by fluvial transport in others.

The particle size distributions in the KARl section (Fig. 2) reflect more the origin of the different sediments rather than postdepositional weathering. The time frame is too short to expect considerable weathering. Indeed, most variations in silt and clay appear to reflect the mode of emplacement rather than in situ weathering. Silt and clay distributions are highest in the aeolian and fluvial beds and lowest in the weathered regolith. The increase of clay in the IIIB2b horizon may have resulted from increased translocation of fine- grained material in the section during the Kalambo Interstadial. Many grains are fresh and largely unweathered as revealed by scanning electron microscopy. Other weathered volcanic feldspar and glass grains are considered to have been preweathered possibly in nearby older deposits. Most neoformed clay minerals (mainly metahalloysite) and other weathering products (mainly gibbsite) are found on these older volcanic glass grains [see Vortisch et al. (1987) for a discussion of similar weathering products on Mount Kenya 800 km to the east].

The X-ray mineralogy of these sediments showed mainly small amounts of quartz and larger amounts of feldspar along with trace amounts of gibbsite, and trace to small amounts of hematite and goethite. Unlike other volcanic alpine areas in the tropics (Mahaney et al. 1988a), quartz reflec- tions in the <2 pm fraction did not show an abnor- mally high relative intensity of the 100 reflection, indicating that all of the wind-blown quartz is probably of hydrothermal origin (Eslinger et at. 1973). Quartz in the nearby Ruwenzori Mountains is predominantly from the African basement, e.g. hydrothermal (Osmaston 1989).

All quartz sand grains studied by SEM in the KARl section showed subangular to subrounded shapes, mostly with distinct fracture faces, but without pronounced crushing. From the samples studied, a number of representative photomicro-

graphs were selected for analysis and discussion. The samples from the surface (ground soil) A1 horizon show distinctly fresher fracture faces (Fig. 3), which could result from reworking of clasts from outcrops upslope by slope wash processes. Crushing features cover only a small percentage of grain surfaces in all horizons. The enlargement in Fig. 3 shows mainly low frequency type crushing (bottom) becoming slightly higher toward the top. This, together with the absence of very sharp edges and only moderate relief features, seems to be indicative of mountain glaciation (lower shear stresses and short transport distances). The crushing wave frequency may relate to differences in vibrational energy released under moderate shear stress in thin alpiEe ice (200-500m thick) (K. Fecher, pers. comm. 1988) during the Pleistocene.

Quartz grains recovered from the fluvial beds (unit 111) (Fig. 4A, B) show a mix of subangular to subrounded clasts. The occasional angular clast showed mainly low frequency linear crushing features (Fig. 4A), as well as small abrasion scars that may owe their origin to water transport. Other grains, presumably rounded by water transport (Fig. 4B), carry low frequency glacial-crushing features as well. The mix of volcanic glass, volcanic feldspar and quartz shown in Fig. 4B is representative of samplesstudiedin all the aeolian and fluvial beds of KARl .

In the older aeolian beds of KARl, subangular quartz grains (Fig. 5A, B) show mainly low frequency linear crushing features and rather dull edges. Some crushing features appear on fresh fracture faces (Fig. SA), whereas on other grains the degree of dissolution etching suggests glacial transport --$ weathering + aeolian transport + emplacement in KAR1. The absence of coatings on these quartz grains is considered representative of weathering over relatively recent time (i.e. late Quaternary) (Vortisch et al. 1987).

The percentage of quartz grains is noticeably lower in the IVAb weathered tuff and loess horizon of KARl. However, glacially-crushed quartz is still common (Fig. 6A, B) along with volcanic minerals, including high percentages of volcanic glass. Once again relatively dull edges and low frequency crushing features dominate on all grains studied.

The lower thickness and shorter transport distances of alpine ice might affect the frequency with which crushing planes occur on individual grains (Mahaney et al. 1988a). Because only low frequency crushing was observed on all grains

BOREAS 19 (1990) Glacially-crushed quartz grains, Rwanda 85

Fig. 3. Fractured and glacially- crushed quartz grain in the A1 horizon of KARl with variable higher (top of enlargement) and lower frequency crushing (bottom of enlargement). Grain is flanked by preweathered volcanic feldspar clasts.

studied, it is likely the source area was one capable of supporting valley glaciers. Moreover, the observation of dull clast edges on most grains studied tends to give the impression of restricted transport distances and low shear stress (Mahaney et al. 1988a).

Glacially-crushed grains in KARl bear striking similarities with grains studied on Mount Kenya (-800 km to the east) (Mahaney et al. 1988a). Overall, thousands of grains from Mount Kenya, studied with the scanning electron microscope, showed predominantly linear crushing features, low frequency type crushing, dull edges, and a low percentage of grain area covered by glacial microtextures (Mahaney et al. in press). Other similarities among the two Afroalpine areas include the tendency for preweathered grains to preserve their old etched surfaces despite the effects of glacial corrasion (Mahaney etal. 1988a). In some cases it is possible to identify two or more separate stages of crushing and weathering on individual grains. Thus, there is the possibility of gaining considerable insight into glacial crushing and interglacial weathering episodes that predate the emplacement of sediments under investi- gation.

Possible source area and climatic implications The most likely source area for glacially-crushed

grains in deposits of the Virunga Volcanoes are the Ruwenzori Mountains, 150 km to the north. Aeolian processes provide the only means of trans- porting these grains (63-250 pm), most probably within the trade wind system. The annual shift in the intertropical convergence zone (ITCZ) occurs over the Virunga Volcanoes during the summer months bringing dry arid air from the Sahara Desert (Coetzee & van Zinderen Bakker 1989). It is possible that glacially-crushed grains were entrained in this paleowind system and delivered to the site together with other non-glacial grains as a blanket of aeolian sediment. Because higher percentages of quartz were found in the buried Pleistocene beds relative to Holocene horizons, it is probable that the trade winds were stronger during the last glaciation when temperature gradients between the equatorial and polar zones were inten- sified.

Detailed particle size analysis showed that glacially-crushed grains were restricted to the smaller diameters of the fine sand fraction (<175pm). Indeed, the wind velocity required to set fine and very fine sand in motion is about 5 m/s (11 mph) (Bagnold 1941). Assuming velocity updrafts are 1/5 of this minimum speed (1 m/s) (Easterbrook 1969), sands with average diameters less than 200 pm should be maintained in motion and win- nowed from coarser grains. While wind speeds in the trade wind belt are lower over land (68 m/s) relativetotheoceans(7-12 m/s) (Rieh11954), they are known to be relatively constant (Donn 1965)


and of sufficient strength to entrain and transport fine and very fine sand particles.

Even though Nicholson & Flohn (1980) pos- tulate a stronger Hadley Cell circulation and increased subsidence in the subtropical high pressure cells that produced reduced seasonal move- ment of the Intertropical Convergence Zone (ITCZ) during the Pleistocene, its quasi-mean position is within the Virunga-Ruwenzori areas. During the early postglacial period, subtropical high pressure zones were probably displaced northward, in response to the decay of the Fenno- Scandinavia Ice Sheet and lowering of the equatorial-polar temperature gradient.

Fig. 4 . 0 A . Subangular glacially-crushed (arrows) quartz grains in the IIIB2b horizon of the KARl section. Enlargement shows tears and gouges that could be percussion cracks. 0 B. Subrounded quartz clast with linear crushing features (arrows).

An alternative hypothesis for the origin of glacially-crushed sand clasts in the Virunga Mountains relies on the documented shift of vegetation belts (Coetzee 1967; Mahaney 1989) from palynological and paleopedological evidence. Lower timberline and increased Afroalpine area during stadials of the last glaciation in the Ruwenzori Mountains might increase the source area of glacially-crushed grains. This hypothesis obviates the need for stronger paleowinds to deliver increased airfall influx of fine and very fine sand clasts. The lack of glacially-crushed sand clasts in the lower paleosol (lower units IV and V, Fig. 2), which contains wind-blown material emplaced most probably dur-


Fig. 5. 0 A. Quartz grain in the IVAb horizon (right) and (enlargement) with linear crushing features (arrows). 0 B. Weathered crushing features (arrows) on quartz grain.

ing the early stade of the last glaciation, suggests that even with a lower timberline an increase of the paleowind circulation may be required to entrain and deliver glacial grains. Tighter dating controls are needed on this lower paleosol, however, in order to prove or disprove this hypothesis. In addition, the presence of low frequency linear crushing microtextures supports crushing under expanded mountain ice with a probable thickness of c. 200-500 m. Holocene ice in the Ruwenzori Mountains is restricted to cirques with overall thicknesses <200 m. Therefore the data appear to

support the hypothesis of crushing followed by aeolian delivery during the last glaciation.

Conclusions The interpretations of glacial microtextures discussed herein lead to a number of conclusions. The data indicate that glacially-crushed sand grains from the Ruwenzori Mountains were transported to the Virunga Volcanoes by aeolian processes. Because a greater number of crushed particles


were found in Pleistocene sediments, it is likely the paleotrade winds were stronger during the last glaciation than during the Holocene and/or that an expanded Afroalpine belt increased the size of the source area for glacially-crushed grains. Low degrees of grain relief and edge sharpness appear representative of alpine glaciation (i.e. valley glaciers and low transport distances). Glacially- induced microtextures are dominated by linear features of low wave frequency covering only small surfaces of individual grains. Conchoidal fractures proved to be rather rare. Subparallel steps

Fig. 6. 0 A . Mixed clasts consisting of quartz (Qu), sanidine (Sa), amphibole (Am), and volcanic glass (VG) in the IVAb (weathered tuff and loess) horizon of KAR1. The quartz grain is crushed (arrows) in two places. 0 B. Glacially-crushed quartz grain (arrows) coated with Si.

(Krinsley & Takahashi 1982; Krinsley & Doorn- kamp 1973) and crescentic gouges, which appear common on grains emplaced by continental glaciation (Mahaney et al. 1988a), were not observed. Unlike similar grains studied on Mount Kenya, the presence of V-shaped percussion cracks (Krinsley & Donohue 1968) was rather rare even though many grains were preglacially rounded (possibly by water transport).

Acknowledgemenu. - I thank Klaus Fecher (Philipps University, Marburg F.R.G.) for assistance with the scanning electron micro-


scope. This research was completed with funding from the joint DFG-NSERC cooperative research program (1988) (with W. Andres and W. B. Vortisch). Sample preparation was completed in the Geomorphology and Pedology Laboratory at York Uni- versity. Janet Allin drafted the illustrations.

References Bagnold, R. A . 1941: The Physics of Blown Sand and Desert

Dunes. 265 pp. Methuen, London. Birkeland, P. W. 1984: Soilsand Geomorphology. 372pp. Oxford

University Press, New York. Coe, M. J. 1967: The Ecology of the Alpine Zone of Mt. Kenya.

136 pp. Junk, The Hague. Coetzee, J . A. 1967: Pollen analytical studies in East and Sou-

thern Africa. Palaeoecology ofAfrica 3,146 pp. Coetzee, J. A . & van Zinderen Bakker, E. M. 1989: Palaeo-

climatology of East Africa during the last glacial maximum: a review of changing theories. I n Mahaney, W. C. (ed.): Quat- ernary and Environmental Research on East African Mountains, 189-198. A. A. Balkema, Rotterdam.

Cooremans, B. & Mahaney, W. C. in press. Palynology of the TV61 section on Mount Kenya, East Africa, Palaeoecology of Africa.

Day, P. 1965: Particle fractionation and particle size analysis. In Black, C. A . (ed.): Methods of SoilAnalysis, 545-567. Ameri- can Society Agronomy, Madison, Wisconsin.

Donn, W. L. 1965: Meteorology. 484 pp. McGraw-Hill, New York.

Easterbrook, D. J . 1969: Principles of Geomorphology. 462 pp. McGraw-Hill, New York.

Eslinger, E . V., Mayer, L. M., Durst, T . L., Hower, J. & Savin, S . M. 1973: An X-ray technique for distinguishing between detrital and secondary quartz in the fine-grained fraction of sedimentary rocks. Journal of Sedimentary Petrology 43,54& 543.

Folk, R. L. 1968: Petrology of Sedimentary Rocks. 110 pp. Hemphill Press, Austin, Texas.

Krinsley, D. H. & Doornkamp, J. C. 1973: Atlas of Quartz Sand Surface Textures. 91 pp. Cambridge University Press, Cam- bridge.

Krinsley, D. H. & Donahue, J . 1968: Environmental inter- pretation of sand grain surface textures by electron microscopy. Geological Society of America Bulletin 79,14>748.

Krinsley, D. & Takahashi, T. 1982: Applications of electron microscopy to geology. New York Academy of Sciences Trans- actions25,1,3-22.

Mahaney, W. C. 1987: Reinterpretation of moraines at -4ooo m in the Mount Kenya Afroalpine Area. Palaeogeography, Palaeoclimatology, Palaeoecology 60,41-51.

Mahaney, W. C. 1989: Quaternary glacial geology of Mount Kenya. I n Mahaney, W. C. (ed.): Quaternary and Environ- mental Research on East African Mountains, 121-140. Balkema, Rotterdam.

Mahaney, W. C., Vortisch, W. & Julig, P. 1988a: Relative differences between glacially crushed quartz transported by mountain and continental ice - some examples from North America and East Africa. American Journal of Science 288, 81&826.

Mahaney, W. C., Vortisch, W. & Julig, P. in press. Relative differences between glacially crushed quartz transported by mountain and continental ice - some examples from North American and East Africa, Reply. American Journal of Science, in press.

Mahaney, W. C., Vortisch, W. B. & Spence, J. R. 1988b: Pollen study and scanning electron microscopy of aeolian grains in a compound paleosol in the Mutonga Drainage, Mount Kenya. Journal of African Earth Sciences 7,718,895-902.

Nicholson, S. & Flohn H. 1980: African environmental and climatic changes and the general circulation in late Pleistocene and Holocene. Climatic Change2,31>348.

Osmaston, H. 1989: Glaciers, glaciations and equilibrium line altitudes on the Ruwenzori. In Mahaney, W. C. (ed.): Quat- ernary and Environmental Research on East African Mountains, 31-104, A . A. Balkema, Rotterdam.

Riehl, H. 1954: Tropical Meteorology. 392 pp. McGraw-Hill, New York.

Soilsurvey Staff, 1951:SoilSurvey Manual. 503pp. U.S. Govern- ment Printing Office, Washington.

Soil Survey Staff, 1975: Soil Taxonomy -Agriculture Handbook 436. 754 pp. U.S. Government Printing Office, Washington, D.C.

Vortisch, W. B., Mahaney, W. C. & Fecher, K. 1987: Lithology and weathering in a palaeosol sequence on Mount Kenya, East Africa. Geologica et Paleontologica 21,245-255.

Whittig, L. D. 1965: X-ray diffraction techniques for mineral identification and mineralogical composition. I n Black, C. A. (ed.): Methods of Soil Analysis, 671-696, American Society of Agronomy, Madison, Wisconsin.

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