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Aeolian deposition and soil parent materials in northern Nigeria
Geoderma, 24 (1980) 241--255 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
241
A E O L I A N DEPOSITION AND SOIL P A R E N T MATERIALS IN N O R T H E R N NIGERIA
J.G. BENNETT
Land Resources Development Centre, Overseas Development Administration, Surbiton (Great Britain) (Received June 21, 1978; accepted December 3, 1979)
ABSTRACT
Bennett, J.G,, 1980. Aeolian Deposition and soil parent materials in northern Nigeria. Geoderma: 24: 241--255. The introduction considers the nature of the problem of " d r i f t " soils in northern Nigeria. The physical environment of the survey area is described briefly and the percentages of silt, very fine sand and fine sand occurring in the soils are mapped. Textural belts are identified and the profile distribution frequencies within the belts are tabulated. Parent material origins are discussed and attempts are made to reconcile the apparently aeolian nature of the soil materials, in terms of the particle-size distribution pattern, with the various soil profile morphologies of today.
INTRODUCTION
The Land Resources Development Centre of the Overseas Development Administration of the Foreign and Commonwealth Office recently completed a land resources appraisal of an area in central northern Nigeria. This paper is concerned with the northernmost part of that area shown on the location map (Fig. 1). The subject of aeolian deposition, or " d r i f t " as it is widely and loosely termed in Nigeria, is controversial. Although the presence of wind-blown material in northern Nigeria has been generally accepted, there has been no a t t e m p t to define the properties, range and distribution of the material in the present area. Falconer (1911) used the term "drift" in the geological sense of any superficial deposit while emphasising the importance of water movement. More recently, workers from the Institute for Agricultural Research at Samaru have referred to any supposed wind-blown deposit as " d r i f t " or "aeolian drift" (Tomlinson, 1961; Higgins, 1963; Klinkenberg and Higgins, 1968). Vine (1953) referred to a northerly group of soils formed from "wind-sorted desert sands which accumulated in a geologically recent period when the Sahara desert encroached hundreds o f miles south of its present limit". He also referred to "silty fine sands of the Zaria group of soils". Lawes 0016-7061/80/0000--0000/$ 02.25 © 1980 Elsevier Scientific Publishing Company
242
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Fig. 1. Location map. (1962) regarded these latter as being derived from loess and referred to "the loess plains of northern Nigeria". Sombroek and Zonneveld (1971), working to the west of the present area, recognised different dune forms and determined their separate origins. They also termed finer textured material, occurring further south, "Funtua loess" thus supporting Vine's primary separation. Sand dunes form a ready means of identifying aeolian deposition but they are largely absent from the area of the present survey, with the exception of a few degraded linear forms in the north and the occurrence of the western extremity of the ancient erg of Hausaland (Grove, 1958) along the eastern edge. The use of land forms as a means o f rapid identification of aeolian deposition by workers to the east and west is therefore largely excluded and soil profile characteristics alone remained for consideration by the survey team when assessing the influence of aeolian deposition on the soils of most of the area. The team entered the area from the south and were alerted to changes occurring in the parent materials of the soils by the increasing silt content. Subsequently, there was a tendency to regard morphologically distinct upper horizons as aeolian in conformity with the generally accepted view of the soils. This was, however, irreconcilable with the wide variety of profile forms encountered and their relative distributions.
243
Profile characteristics, therefore, having proved largely inadequate to the task, this paper attempts to show the extent and form of aeolian influence by mapping the distribution of particle-size classes of comparable horizons of the soils, irrespective of profile morphology or geomorphic position of the site. THE PHYSICAL ENVIRONMENT
The area is situated within the northern Guinea and Sudan vegetation zones (Keay, 1949). Rainfall ranges from 1200 mm in the south to less than 650 mm in the north and falls within a single season extending from May/June to September. The highest 10-day mean daily maximum temperature of 40°C occurs in the north during April and the lowest, 27°C, in the south in August. The highest 10-day mean daily minimum of 25°C occurs in the north during May and the lowest, 12°C, also in the north during January (Kowal and Knabe, 1972). The prevailing wind during the dry season, known locally as the Harmattan, is from the northeast and often carries dust. (Kalu, 1979). The terrain consists of gently undulating plains which rise gradually towards the south and west. The geomorphology, according to King (1967), relates to the early Tertiary "African" surface in the south and west and to the later "post-African" surface in the lower regions. Ironpan-capped mesas are associated with the higher areas. The plains are underlain by Basement Complex granites, gneisses, migmatites and schists (McCurry, 1976). Cretaceous sediments occur in the far north (Du Preez, 1958) and Pleistocene lacustrine sediments on the extreme eastern fringe of the area (Grove, 1958). Isolated Jurassic granite hills occur in the south. The soils are varied and the pattern of distribution can be complex. Except for relatively minor features of prominence such as valley bottoms, relict sand dunes, ironstone-capped mesas and granite hills, landform correlates poorly with soils. Soils per se were not mapped by the survey on which this paper is based. Considered from a regional viewpoint, however, certain soil characteristics are widespread and others exhibit regional trends. (1) A coarse-textured surface horizon is ubiquitous. It may be due to the coarse texture of the parent material in the north, but is more often related to clay removal in the south. (2) Secondary pedogenic ironstone occurs widely in various forms, at various depths, with various thicknesses and with little predictability. Quartz gravel is also widespread. (3) Chemical weathering and pedogenesis increase Southwards, with clay translocation and iron segregation leading t~ greater horizon differentiation. Clay illuviation north of Kano is mostly manifested by clay lamellae. (4) P.lanosols and related profile forms, often sodium-affected, occur locally in the central part of the area. Many of the soils are therefore pedogenetically well developed and although
244
pronounced discontinuities occur in several forms, they are not always -- and indeed are often clearly not -- directly related to aeolian deposition. A blanket surface deposit of aeolian material of uniform properties such as occurs on the Jos Plateau to the south (Macload et al. 1971) does not occur here. Examples are given below of profiles of a Cambic Arenosol from the north and a Ferric Luvisol, representing a typical plains soil in the south. Description is based on the FAO Guidelines for Soil Description but horizon designations are from FAO-UNESCO (1974). Cambic Arenosol
0--12 cm 12--33 cm
Aul Au2
33--80 cm
Bwl
80--153 cm
Bul
153 cm +
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1OYR 4/4 (moist); sand; single grain; loose. 1OYR 4/4 (moist); loamy sand; single grain to massive, slightly hard; less than 5% subrounded ironstone concretions. 5YR 5/6 (moist); loamy sand; single grain to massive; hard; less than 5% subrounded ironstone concretions. 7.5YR 5/6 (moist); fine sandy loam; weak, coarse, subangular blocky; very hard. continuous vesicular indurated ironstone or ironpan.
Perric Luvisol
0--12 cm
A1
12--28 cm
2Abl
28--63 cm
2Btsl
63--104 cm
2Bts2
104--124 cm
2Bsl
1OYR 4/3 (moist); sandy loam; very weak, medium, subangular blocky; soft. 1OYR 4/2 (moist); sandy loam; massive; hard; less than 5% subrounded ironstone concretions. 1OYR 5/6 (moist), 15% 2.5YR 4/6 mottles; clay loam; weak, medium, subangular blocky; slightly hard; less than 5% angular quartz gravel, less than 5% subrounded ironstone concretions; weak thin clay skins. 1OYR 5/4 (moist), 20% 2.5YR 4/6 mottles, 5% 7.5YR 5/6 mottles; clay loam; very weak, coarse, subangular blocky; very hard; weak thin clayskins. 1OYR 5/4 (moist); 15% 2.5YR 4/6 mottles, 10% 7.5YR 5/6 mottles; clay loam; weak, coarse, subangular blocky; very hard.
DATA COLLECTION AND ANALYSIS
The soil survey was carried out by sampling sites, selected with the aid of air photographs, within upland (i.e., non-valley bottom) facets of land systems (Brink et al., 1966).
245 Samples from the natural horizons of over 500 profiles were analysed by the Tropical Soils Analysis Unit of the Land Resources Development Centre to assist with the characterisation and classification of the soils. Organic matter was first removed by hydrogen peroxide digestion. Dispersion was by vibration in an ultrasonic cleaning tank for 15 min with continuous stirring using Calgon (sodium hexametaphosphate and sodium carbonate) as dispersing agent. The 4~-1 tank accomodated eight samples simultaneously, each sample consisting of 10 g of soil in 200 ml. Vibration frequency was 25 kHz at 7°
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Fig. 2. Soil sample sites. Fine sand % (100--200 urn) in the surface horizon.
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Fig. 3. Soil sample sites. Fine sand % ( 1 0 0 - - 2 0 0 urn) in the horizon of maximum clay content.
150 Watt. Clay and fine silt were determined by weighing samples of suspension extracted by a pressure box. This modification of the pipette method consists o f an "air-tight box, with fitted pipettes, into which dispersed samples are introduced. At the appropriate time air is pumped into the box, thereby forcing out samples of suspension through the pipettes. Sand was collected by wet sieving and fractionated by dry sieving; coarse silt was obtained by difference. The particle-size classes separated were clay (< 2 pm), fine silt (2--20 pm),
247
coarse silt ( 2 0 - - 5 0 Urn), very fine sand ( 5 0 - - 1 0 0 um), fine sand ( 1 0 0 - - 2 0 0 um), medium sand ( 2 0 0 - - 5 0 0 pm) and coarse sand ( 5 0 0 - - 2 0 0 0 pm). The silt fractions were subsequently combined because of the very low fine silt c o n t e n t For each profile the percentages of clay, silt, very fine, fine, medium and coarse sand were plotted on separate maps. A separate map was plotted in each case for the surface horizon and the horizon of m a x i m u m clay content irrespective of the depth at which the latter occurred. This was considered to 7°
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Fig. 4. Soil sample sites. Very fine sand % ( 5 0 - - 1 0 0 urn) in the surface horizon.
248
afford a basis for comparison between soils of the part of the profile most recently affected by aeolian deposition and that most developed pedogenetically and in which the character of the aeolian material (if any) is most altered. For simplification, the actual percentages were grouped in 10% class intervals, i.e., 0--9%, 10--19%, 20--29%, etc. Only the most informative maps, those showing the distribution of silt, very fine sand and fine sand, are presented here (Figs. 2--7). 7°
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Fig. 5. Soil sample sites. Very fine sand % (50--100 um) in the horizon of maximum clay content.
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Fig. 6. Soil sample sites. Silt % (2--50 ~ m ) in the surface horizon.
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Fig. 7. Soil sample sites. Silt % (2--50 ~m) in the horizon of maximum clay content.
251
RESULTS
Each map shows a very clear linear distribution pattern orientated broadly along an east--west axis. To emphasize and describe the nature of the distribution, trend lines were drawn subjectively on the maps dividing the area into textural belts, these being areas of the greatest uniformity of particle size. The composition of the belts in terms of profile distribution in each 10%-class is given in Table I. TABLE I Profile distribution-frequency 10%-intervals Textural belt
(%) b y t e x t u r a l b e l t o f p a r t i c l e - s i z e c l a s s e s ( f r a c t i o n s ) in
Particle-size class intervals 0--9%
10--19%
20-29%
30-39%
1 2 3 4 5
F i n e sand in the surface h o r i z o n - - F i g . 2 4 15 (47) (47) (62) 36 2 (84) 15 I 3 (50) 39
1 2 3 4 5
F i n e sand in the h o r i z o n o f m a x i m u m 2 12 16 2 25 (41) 17 (67) 15 (95) 5 14 (50)
1 2 3 4 5 6 7
V e r y f i n e s a n d in the surface h o r i z o n - - F i g . 4 5 (77) 18 18 (73) 9 36 (61) 11 1 8 2 9 (46) 25 (51) 22
40--49% (40) 6
3 (76) (41) 39 2
2 3 4 5 6 1 2 3 4 5
S i l t in the s u r f a c e h o r i z o w - F i g . 6 (83) 15 2 11 (69) 18 14 (62) 13 4 6
1
2 3 4 5 6
2 23 (63) 21
Silt in the horizon of m a x i m u m clay content--Fig. 7 (72) 26 2 9 (72) 17 2 14 (58) 26 2 23 (68) 3 30 13 (71)
5
15
5
V e r y fine s a n d in the h o r i z o n o f m a x i m u m clay c o n t e n t - - F i g . 5 24 (65) 11 11 (78) 11 10 (76) 14 9 8 (37) 36 15 (49) 36 7 (79) 12 2
1
36
60--69%
8
clay c o n t e n t - - F i g . 3 20 (33) 23 9 1
31
50--59%
1 24 (54)
2 7 (67) 13
13 39 2
15
11 2
252 Examination of the data shows the belts to have some obvious properties. (1) In each belt the frequency distribution of profiles in the 10% class intervals shows a single peak either at the low, medium or high content of the range. (2) For all maps the highest-peaked profile frequency distributions tend to occur in belts with low contents of the particle-size class being mapped. Broad peaks occur in belts of high content. (3) The data indicate pronounced regional trends in particle-size distributions. Silt is seen to increase regularly towards the south; very fine sand to increase regularly southwards and then decrease, and fine sand to decrease towards the south. The exception to this is fine-sand belt 5 which shows the controlling influence of the high fine-sand content of the Pleistocene lacustrine sediments, and silt in belt 6, Fig. 7, which is perhaps an indication that this is (or was) the area of greatest silt deposition. A comparison may be made between surface and subsoil horizons of softs in those belts which have coincident boundaries. Silt belt I has slightly more silt in the subsoil than at the surface but belts 2 and 3 show little difference. The peak-frequency distribution is the same in b o t h horizons, 0--10% belt 1, 10--20% belt 2 and 20--30% belt 3. All fine-sand belts show more sand at the surface than in the subsoil but the difference is less towards the south. C o m m o n peak-frequency distributions again occur, 40--50% belt 1, 20--30% belt 2, 10--20% belt 3 and 0--10% belt 4. The regularity of particle-size class distribution, both within and between belts, at the surface and in the subsoil (properties 1 and 3 above), indicates a closer regional relationship between soils than would be expected to result from pedogenic processes acting upon in-situ or locally derived parent materials. DISCUSSION OF PARENT-MATERIAL ORIGIN During the Quaternary period climatic changes occurred which brought a b o u t alternating periods of both wetter and drier conditions south of the Sahara Desert than the present climate (Grove and Warren, 1968; Burke et al., 1971), or alternating periods of m o r p h o d y n a m i c activity and stability (Rohdenburg, 1970). During stable phases, deep chemical weathering would have occurred with downwasting of the land surface, the removal of soil components mainly in solution and the concentration of less soluble substances. During active phases, loose surface material would be subjected to wind action, occasional high-intensity rainfall would bring a b o u t mass movement of unstabilised surfaces, previously segregated iron would harden on exposure, and exposed rock would be subjected to physical .weathering. Aeolian movement of material into or o u t of an area would be a corollary of such changes taking place over a wider region. Although there has been climatic fluctuation and corresponding environmental change, the north has overall been drier than the south and the sandstone in the north has given rise
253
to a coarser-textured weathering p r o d u c t than the Basement Complex rocks in the south. Conditions in the north have favoured d u n e formation and presumably the removal of fine material southwards. Climatic shifts have resulted in dune encroachment southwards or degradation northwards, depending on the direction of the shift, and correspondingly the dust accretion zone has become a source area for the removal of dust further south or has extended northwards. A site may, therefore, have been acted u p o n during several periods by wind or water, erosional or depositional, or any combination of these processes. Parent materials of upland soils over most of the area (neglecting the small area of Pleistocene deposits) can be seen as a function of a number of materials plus process combinations. If, as a model, we consider a land surface without regolith, the composition of the soils' parent materials after an initial stablephase--active-phase sequence is some combination of the following, the composition at any site depending on the history of that particular site. (1) In-situ weathered Basement Complex and Cretaceous derived material. (2) Basement Complex and Cretaceous-derived material of local origin transported by coUuvial processes. (3) Basement Complex and Cretaceous-derived material of local origin transported by wind action. (4) Material of u n k n o w n origin imported from further afield by wind action. (5) Material originating locally from the erosion of ironpan cappings by colluvial processes (a variant of 2). A second stable-phase--active-phase sequence would produce the same combination of processes, the effects of which would be superimposed on the soils parent materials, monophase or polyphase, produced b y the first sequence. The second sequence would then tend to homogenise the results of the first sequence while creating discontinuities anew. This chain of events would continue through successive sequences. Parent materials of present-day profile forms can be associated with each of these individual processes to greater or lesser extents depending on the properties of particular materials. (1) In-situ weathering can be clearly identified in the gradually altered parent rock at the bases of many profiles. It is most evident in the north and southeast of the survey area, where overlying deposits have been stripped or thinned or where deposition has been slight. (2) Local colluvial material occurs widely. Clear evidence is the presence of quartz gravel or detrital ironpan gravel through the provile. The occurrence of aeolian material throughout the profile (a statement this paper attempts to make), is also p r o o f of homogenisation, possibly b y faunal activity in part, but more significantly b y mass movement (see 5). The wide distribution range of a particle-size class in areas of maximum deposition (property 2) may also be explained by local surface movement of material. (3) Wind action, by particle movement along the surface or saltation, on coarse-textured Cretaceous material is assumed to have brought a b o u t dune formation in the north of the area. Subsequent degradation, perhaps involving
254
water in a similar manner to that described by Talbot and Williams (1978) has resulted in a relatively even cover of coarse material with only vestigial dune formations remaining in parts. The Cambic Arenosol previously described is an example of a soil formed in this t y p e of material. (4) The transport of material in aeolian suspension is an active process at the present time. It is proposed here t h a t the very fine sand and silt distribution patterns are evidence of the significance of the process in the past and that the progressive change southwards from coarse to fine particle size indicates a source area to the north. The very fine sand has presumably not travelled far and some may have originated in the north of the survey area. Sombroek and Zonneveld (1971, p.71) identify a possible source of their F u n t u a loess south of Lake Chad and east of its deposition zone. Kalu (1979) gives the source area of present-day Saharan dust as the Bilma--Faya Largeau area much further north. Soils formed in material covering ironstone-capped mesas in the south of the area are of this type. (5) Material originating from the destruction of ironpan caps has been mentioned already as evidence of widespread local reworking. From the a m o u n t of detrital material encountered, the ironpan mesas of t o d a y must be remnants of much larger ironpan formations. Material similar to t h a t in (4), resting on previously more extensive ironpan caps, will also have been redistributed downslope in this way. CONCLUSIONS
The textural distribution patterns support the findings of previous workers to the extent t h a t soils t h r o u g h o u t the area contain elements of aeolian origin, and that aeolian material is a significant c o m p o n e n t o f the pedologicaUy welldeveloped soils with argillic horizons in the south as well as of the coarsetextured soils in the north. The results do, however, support impressions gained during the survey of the inadvisability of applying the term " d r i f t " , meaning wind-deposited, to some soils (or parts of soils) on the basis of a cursory field examination and the identification of certain "aeolian" properties such as silt, while regarding others as " n o n - d r i f t " on the basis of "nonaeolian" properties such as ironstone gravel. Components of various origins and with varied and indeterminate histories have contributed in various amounts to parent materials homogenised to greater or lesser extents. The exceptions to this perhaps are sand covers in the north of the area, providing the dune degradation process has not introduced extraneous material, and silty mesa covers in the south. It may be appropriate to regard the latter as loess. ACKNOWLEDGEMENTS
I would like to t h a n k the Cartography Section and the Data Management Unit o f the Land Resources Development Centre for their assistance with mapping and data handling.
255 NOTE Compatible particle-size analysis results are not available for deposits further south than shown on the maps, but field work indicated that a significant silt content does not extend much further south than 10 ° 30', and that t h e s o u t h e r n l i m i t o f s i g n i f i c a n t s i l t a c c r e t i o n lies b e t w e e n Z a r i a a n d K a d u n a .
REFERENCES Brink, A.B., Mabbutt, J.A., Webster, R. and Beckett, P.H.T., 1966. Report of the Working Group on Land Classification and Data Storage. Milit. Eng. Exp. Establ., Christchurch, England, Rep., 940. Burke, K., Durotoye, A.B.,and Whiteman, A.J., 1971. A dry phase south of the Sahara 20 000 years ago. W. Afr. J. Archaeol., 1: 1--8. Du Preez, J.W., 1958. The water supply of Katsina town Rec. Geol. Surv. Niger., 1955: 59--64. Falconer, J.D., 1911. The Geology and Geography of Northern Nigeria. MacMillan, London, FAO-UNESCO, 1974. Soil Map of the World, 1. Legend. Unesco, Paris. Grove, A.T., 1958. The ancient erg of Hausaland, and similar formations on the south side of the Sahara. Geogr. J., 124(4): 526--533. Grove, A.T., and Warren, A., 1968. Quaternary landform and climate on the south side of the Sahara. Geogr. J., 134(2): 194--208. Higgins, G.M., 1963. Upland soils of Samaru and Kano plains. Inst. Agric. Res., Samaru (Unnumbered Soil. Surv. Rep.). Kalu, A.E., 1979. The African dust plume: its characteristics and propagation across West Africa in winter. In: C. Morales (Editor), Saharan Dust. Wiley, New York, N.Y., (Published on behalf of the Scientific Committee on Problems of the Environment). Keay, R.W., 1949. An outline of Nigerian vegetation. Gov. Printer, Lagos. King, L.C., 1967. The Morphology of the Earth. Oliver and Boyd, Edinburgh. Klinkenberg, K. and Higgins, G.M., 1968. An outline of Northern Nigerian soils. Niger. J. Sci., 2(2): 91--115. Kowal, J.M. and Knabe, D.T., 1972. An Agroclimatological Atlas of the Northern States of Nigeria. Ahmadu Bello Univ. Press, Samaru, Zaria, Nigeria. Lawes, D.A., 1962. The influence of rainfall conservation on the fertility of the loess plain soil of northern Nigeria. Niger. Geogr. J., 5. Macload, W.N., Turner, D.C. and Wright, E.P., 1971. The geology of the Jos Plateau. Bull. Geol. Surv. Niger., 32. McCurry, P., 1976. The geology of the Precambrian to Lower Palaeozoic rocks of Northern Nigeria--a review. In: C.A. Kogbe (Editor), Geology of Nigeria. Elizabethan Publishing, Surulere (Lagos), pp. 15--39. Rohdenburg, H., 1970. Morphodynamische Aktivit~ten und Stabilit~tszeiten start Pluvialund Interpluvialzeiten. Eiszeitalt. Gegenwart, 21: 81--96. S0mbroek, W.G. and Zonneveld, I.S., 1971. Ancient dune fields and fluviatile deposits in the Rima-Sokoto river basin. Neth. Soil Surv. Inst., Wageningen, Soil Surv., Pap., 5. Talbot, M.R.~nd Williams, M.A.J., 1978. Erosion of fixed dunes in the Sahel, Central Niger. Earth Surf. Proc., 3(2): 107--113. Tomlinson, P.R., 1961. Report on the detailed soil survey of the Livestock Investigation Centre, Katsina, and the reconnaissance survey of the ~urrounding area. Inst. Agric. Res., Samaru, Soil Surv. Publ., 11. Vine, H., 195~. Notes on the main types of Nigerian soils. Gov. Printer, Lagos.
241
A E O L I A N DEPOSITION AND SOIL P A R E N T MATERIALS IN N O R T H E R N NIGERIA
J.G. BENNETT
Land Resources Development Centre, Overseas Development Administration, Surbiton (Great Britain) (Received June 21, 1978; accepted December 3, 1979)
ABSTRACT
Bennett, J.G,, 1980. Aeolian Deposition and soil parent materials in northern Nigeria. Geoderma: 24: 241--255. The introduction considers the nature of the problem of " d r i f t " soils in northern Nigeria. The physical environment of the survey area is described briefly and the percentages of silt, very fine sand and fine sand occurring in the soils are mapped. Textural belts are identified and the profile distribution frequencies within the belts are tabulated. Parent material origins are discussed and attempts are made to reconcile the apparently aeolian nature of the soil materials, in terms of the particle-size distribution pattern, with the various soil profile morphologies of today.
INTRODUCTION
The Land Resources Development Centre of the Overseas Development Administration of the Foreign and Commonwealth Office recently completed a land resources appraisal of an area in central northern Nigeria. This paper is concerned with the northernmost part of that area shown on the location map (Fig. 1). The subject of aeolian deposition, or " d r i f t " as it is widely and loosely termed in Nigeria, is controversial. Although the presence of wind-blown material in northern Nigeria has been generally accepted, there has been no a t t e m p t to define the properties, range and distribution of the material in the present area. Falconer (1911) used the term "drift" in the geological sense of any superficial deposit while emphasising the importance of water movement. More recently, workers from the Institute for Agricultural Research at Samaru have referred to any supposed wind-blown deposit as " d r i f t " or "aeolian drift" (Tomlinson, 1961; Higgins, 1963; Klinkenberg and Higgins, 1968). Vine (1953) referred to a northerly group of soils formed from "wind-sorted desert sands which accumulated in a geologically recent period when the Sahara desert encroached hundreds o f miles south of its present limit". He also referred to "silty fine sands of the Zaria group of soils". Lawes 0016-7061/80/0000--0000/$ 02.25 © 1980 Elsevier Scientific Publishing Company
242
SOKOTO
x
~ +
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x :::::::::::::::::::::::
.
: : ill!!~:~
'
+
+
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~ t-
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KAOUNA
]~
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/~ ~
~,
x
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a
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- \
ABOJA
/
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~
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:: :
/
:::: :: :
/~"
÷ +
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./,
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:,............ , ; : ;to:~:aB: BouRdary URdary State Boundary
+
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Fig. 1. Location map. (1962) regarded these latter as being derived from loess and referred to "the loess plains of northern Nigeria". Sombroek and Zonneveld (1971), working to the west of the present area, recognised different dune forms and determined their separate origins. They also termed finer textured material, occurring further south, "Funtua loess" thus supporting Vine's primary separation. Sand dunes form a ready means of identifying aeolian deposition but they are largely absent from the area of the present survey, with the exception of a few degraded linear forms in the north and the occurrence of the western extremity of the ancient erg of Hausaland (Grove, 1958) along the eastern edge. The use of land forms as a means o f rapid identification of aeolian deposition by workers to the east and west is therefore largely excluded and soil profile characteristics alone remained for consideration by the survey team when assessing the influence of aeolian deposition on the soils of most of the area. The team entered the area from the south and were alerted to changes occurring in the parent materials of the soils by the increasing silt content. Subsequently, there was a tendency to regard morphologically distinct upper horizons as aeolian in conformity with the generally accepted view of the soils. This was, however, irreconcilable with the wide variety of profile forms encountered and their relative distributions.
243
Profile characteristics, therefore, having proved largely inadequate to the task, this paper attempts to show the extent and form of aeolian influence by mapping the distribution of particle-size classes of comparable horizons of the soils, irrespective of profile morphology or geomorphic position of the site. THE PHYSICAL ENVIRONMENT
The area is situated within the northern Guinea and Sudan vegetation zones (Keay, 1949). Rainfall ranges from 1200 mm in the south to less than 650 mm in the north and falls within a single season extending from May/June to September. The highest 10-day mean daily maximum temperature of 40°C occurs in the north during April and the lowest, 27°C, in the south in August. The highest 10-day mean daily minimum of 25°C occurs in the north during May and the lowest, 12°C, also in the north during January (Kowal and Knabe, 1972). The prevailing wind during the dry season, known locally as the Harmattan, is from the northeast and often carries dust. (Kalu, 1979). The terrain consists of gently undulating plains which rise gradually towards the south and west. The geomorphology, according to King (1967), relates to the early Tertiary "African" surface in the south and west and to the later "post-African" surface in the lower regions. Ironpan-capped mesas are associated with the higher areas. The plains are underlain by Basement Complex granites, gneisses, migmatites and schists (McCurry, 1976). Cretaceous sediments occur in the far north (Du Preez, 1958) and Pleistocene lacustrine sediments on the extreme eastern fringe of the area (Grove, 1958). Isolated Jurassic granite hills occur in the south. The soils are varied and the pattern of distribution can be complex. Except for relatively minor features of prominence such as valley bottoms, relict sand dunes, ironstone-capped mesas and granite hills, landform correlates poorly with soils. Soils per se were not mapped by the survey on which this paper is based. Considered from a regional viewpoint, however, certain soil characteristics are widespread and others exhibit regional trends. (1) A coarse-textured surface horizon is ubiquitous. It may be due to the coarse texture of the parent material in the north, but is more often related to clay removal in the south. (2) Secondary pedogenic ironstone occurs widely in various forms, at various depths, with various thicknesses and with little predictability. Quartz gravel is also widespread. (3) Chemical weathering and pedogenesis increase Southwards, with clay translocation and iron segregation leading t~ greater horizon differentiation. Clay illuviation north of Kano is mostly manifested by clay lamellae. (4) P.lanosols and related profile forms, often sodium-affected, occur locally in the central part of the area. Many of the soils are therefore pedogenetically well developed and although
244
pronounced discontinuities occur in several forms, they are not always -- and indeed are often clearly not -- directly related to aeolian deposition. A blanket surface deposit of aeolian material of uniform properties such as occurs on the Jos Plateau to the south (Macload et al. 1971) does not occur here. Examples are given below of profiles of a Cambic Arenosol from the north and a Ferric Luvisol, representing a typical plains soil in the south. Description is based on the FAO Guidelines for Soil Description but horizon designations are from FAO-UNESCO (1974). Cambic Arenosol
0--12 cm 12--33 cm
Aul Au2
33--80 cm
Bwl
80--153 cm
Bul
153 cm +
2ms
1OYR 4/4 (moist); sand; single grain; loose. 1OYR 4/4 (moist); loamy sand; single grain to massive, slightly hard; less than 5% subrounded ironstone concretions. 5YR 5/6 (moist); loamy sand; single grain to massive; hard; less than 5% subrounded ironstone concretions. 7.5YR 5/6 (moist); fine sandy loam; weak, coarse, subangular blocky; very hard. continuous vesicular indurated ironstone or ironpan.
Perric Luvisol
0--12 cm
A1
12--28 cm
2Abl
28--63 cm
2Btsl
63--104 cm
2Bts2
104--124 cm
2Bsl
1OYR 4/3 (moist); sandy loam; very weak, medium, subangular blocky; soft. 1OYR 4/2 (moist); sandy loam; massive; hard; less than 5% subrounded ironstone concretions. 1OYR 5/6 (moist), 15% 2.5YR 4/6 mottles; clay loam; weak, medium, subangular blocky; slightly hard; less than 5% angular quartz gravel, less than 5% subrounded ironstone concretions; weak thin clay skins. 1OYR 5/4 (moist), 20% 2.5YR 4/6 mottles, 5% 7.5YR 5/6 mottles; clay loam; very weak, coarse, subangular blocky; very hard; weak thin clayskins. 1OYR 5/4 (moist); 15% 2.5YR 4/6 mottles, 10% 7.5YR 5/6 mottles; clay loam; weak, coarse, subangular blocky; very hard.
DATA COLLECTION AND ANALYSIS
The soil survey was carried out by sampling sites, selected with the aid of air photographs, within upland (i.e., non-valley bottom) facets of land systems (Brink et al., 1966).
245 Samples from the natural horizons of over 500 profiles were analysed by the Tropical Soils Analysis Unit of the Land Resources Development Centre to assist with the characterisation and classification of the soils. Organic matter was first removed by hydrogen peroxide digestion. Dispersion was by vibration in an ultrasonic cleaning tank for 15 min with continuous stirring using Calgon (sodium hexametaphosphate and sodium carbonate) as dispersing agent. The 4~-1 tank accomodated eight samples simultaneously, each sample consisting of 10 g of soil in 200 ml. Vibration frequency was 25 kHz at 7°
8" I
9" I
il 6
13" -
:J
,,
:*
•
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~o
,
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12"
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= "
r
o, o
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~ Zaria
,0oK~
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÷,
', t
o--9% 10--19% 2 0 - - 29 % ,
30--
39 %
40--49% 50--59% c~
60-- 69 % Textural
Belt
3
Fig. 2. Soil sample sites. Fine sand % (100--200 urn) in the surface horizon.
246 7"
9"
8" I
I
13"
SCALE
t,
•"
I" ~
108km
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12"
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0--9% 10--19% 20 -- 29 %
.
30--
39 %
40 -- 49 % 50--
59%
60--69%
3
Textural
Belt
Fig. 3. Soil sample sites. Fine sand % ( 1 0 0 - - 2 0 0 urn) in the horizon of maximum clay content.
150 Watt. Clay and fine silt were determined by weighing samples of suspension extracted by a pressure box. This modification of the pipette method consists o f an "air-tight box, with fitted pipettes, into which dispersed samples are introduced. At the appropriate time air is pumped into the box, thereby forcing out samples of suspension through the pipettes. Sand was collected by wet sieving and fractionated by dry sieving; coarse silt was obtained by difference. The particle-size classes separated were clay (< 2 pm), fine silt (2--20 pm),
247
coarse silt ( 2 0 - - 5 0 Urn), very fine sand ( 5 0 - - 1 0 0 um), fine sand ( 1 0 0 - - 2 0 0 um), medium sand ( 2 0 0 - - 5 0 0 pm) and coarse sand ( 5 0 0 - - 2 0 0 0 pm). The silt fractions were subsequently combined because of the very low fine silt c o n t e n t For each profile the percentages of clay, silt, very fine, fine, medium and coarse sand were plotted on separate maps. A separate map was plotted in each case for the surface horizon and the horizon of m a x i m u m clay content irrespective of the depth at which the latter occurred. This was considered to 7°
8"
9"
I
I
13"
110oKm
12°
~*
"
•
•
6
"
•
O-- 9% 1 0 - 19 % 20-- 29 % ,
30-
39 %
40-
49% 59% 60- 69 % 50-
o 3
Textural
Belt
Fig. 4. Soil sample sites. Very fine sand % ( 5 0 - - 1 0 0 urn) in the surface horizon.
248
afford a basis for comparison between soils of the part of the profile most recently affected by aeolian deposition and that most developed pedogenetically and in which the character of the aeolian material (if any) is most altered. For simplification, the actual percentages were grouped in 10% class intervals, i.e., 0--9%, 10--19%, 20--29%, etc. Only the most informative maps, those showing the distribution of silt, very fine sand and fine sand, are presented here (Figs. 2--7). 7°
8" I
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13"
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1tOOKm
12 °
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0--9% 10--19% 2 0 - 29 % 30-- 39 %
40--49% 50-- 5 9 % D
3
60--69%
Textural
Belt
Fig. 5. Soil sample sites. Very fine sand % (50--100 um) in the horizon of maximum clay content.
249 7°
8"
9"
I
I
13"
°,,m
12"
•.
¢
~.f.. •"
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;
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0--9% 10-- 19 % 2 0 - - 29 % ,
30--
39 %
40 -- 49 % 50-- 5 9 % 60--69%
3
Textural
Belt
Fig. 6. Soil sample sites. Silt % (2--50 ~ m ) in the surface horizon.
o
250 7°
8"
9"
I
I
13"
\
c¸~
I
SCALE 1" ~ 1 0 8 k m
5,°
12"
-~.
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.
-
_ . _ . 4
~+
.
2
+
0--9% 10-- 19% 2 0 - - 29 % 3 0 - - 39 % 4 0 -- 4 9 % 50-- 59% 60-- 69 % 3
Textural Belt
Fig. 7. Soil sample sites. Silt % (2--50 ~m) in the horizon of maximum clay content.
251
RESULTS
Each map shows a very clear linear distribution pattern orientated broadly along an east--west axis. To emphasize and describe the nature of the distribution, trend lines were drawn subjectively on the maps dividing the area into textural belts, these being areas of the greatest uniformity of particle size. The composition of the belts in terms of profile distribution in each 10%-class is given in Table I. TABLE I Profile distribution-frequency 10%-intervals Textural belt
(%) b y t e x t u r a l b e l t o f p a r t i c l e - s i z e c l a s s e s ( f r a c t i o n s ) in
Particle-size class intervals 0--9%
10--19%
20-29%
30-39%
1 2 3 4 5
F i n e sand in the surface h o r i z o n - - F i g . 2 4 15 (47) (47) (62) 36 2 (84) 15 I 3 (50) 39
1 2 3 4 5
F i n e sand in the h o r i z o n o f m a x i m u m 2 12 16 2 25 (41) 17 (67) 15 (95) 5 14 (50)
1 2 3 4 5 6 7
V e r y f i n e s a n d in the surface h o r i z o n - - F i g . 4 5 (77) 18 18 (73) 9 36 (61) 11 1 8 2 9 (46) 25 (51) 22
40--49% (40) 6
3 (76) (41) 39 2
2 3 4 5 6 1 2 3 4 5
S i l t in the s u r f a c e h o r i z o w - F i g . 6 (83) 15 2 11 (69) 18 14 (62) 13 4 6
1
2 3 4 5 6
2 23 (63) 21
Silt in the horizon of m a x i m u m clay content--Fig. 7 (72) 26 2 9 (72) 17 2 14 (58) 26 2 23 (68) 3 30 13 (71)
5
15
5
V e r y fine s a n d in the h o r i z o n o f m a x i m u m clay c o n t e n t - - F i g . 5 24 (65) 11 11 (78) 11 10 (76) 14 9 8 (37) 36 15 (49) 36 7 (79) 12 2
1
36
60--69%
8
clay c o n t e n t - - F i g . 3 20 (33) 23 9 1
31
50--59%
1 24 (54)
2 7 (67) 13
13 39 2
15
11 2
252 Examination of the data shows the belts to have some obvious properties. (1) In each belt the frequency distribution of profiles in the 10% class intervals shows a single peak either at the low, medium or high content of the range. (2) For all maps the highest-peaked profile frequency distributions tend to occur in belts with low contents of the particle-size class being mapped. Broad peaks occur in belts of high content. (3) The data indicate pronounced regional trends in particle-size distributions. Silt is seen to increase regularly towards the south; very fine sand to increase regularly southwards and then decrease, and fine sand to decrease towards the south. The exception to this is fine-sand belt 5 which shows the controlling influence of the high fine-sand content of the Pleistocene lacustrine sediments, and silt in belt 6, Fig. 7, which is perhaps an indication that this is (or was) the area of greatest silt deposition. A comparison may be made between surface and subsoil horizons of softs in those belts which have coincident boundaries. Silt belt I has slightly more silt in the subsoil than at the surface but belts 2 and 3 show little difference. The peak-frequency distribution is the same in b o t h horizons, 0--10% belt 1, 10--20% belt 2 and 20--30% belt 3. All fine-sand belts show more sand at the surface than in the subsoil but the difference is less towards the south. C o m m o n peak-frequency distributions again occur, 40--50% belt 1, 20--30% belt 2, 10--20% belt 3 and 0--10% belt 4. The regularity of particle-size class distribution, both within and between belts, at the surface and in the subsoil (properties 1 and 3 above), indicates a closer regional relationship between soils than would be expected to result from pedogenic processes acting upon in-situ or locally derived parent materials. DISCUSSION OF PARENT-MATERIAL ORIGIN During the Quaternary period climatic changes occurred which brought a b o u t alternating periods of both wetter and drier conditions south of the Sahara Desert than the present climate (Grove and Warren, 1968; Burke et al., 1971), or alternating periods of m o r p h o d y n a m i c activity and stability (Rohdenburg, 1970). During stable phases, deep chemical weathering would have occurred with downwasting of the land surface, the removal of soil components mainly in solution and the concentration of less soluble substances. During active phases, loose surface material would be subjected to wind action, occasional high-intensity rainfall would bring a b o u t mass movement of unstabilised surfaces, previously segregated iron would harden on exposure, and exposed rock would be subjected to physical .weathering. Aeolian movement of material into or o u t of an area would be a corollary of such changes taking place over a wider region. Although there has been climatic fluctuation and corresponding environmental change, the north has overall been drier than the south and the sandstone in the north has given rise
253
to a coarser-textured weathering p r o d u c t than the Basement Complex rocks in the south. Conditions in the north have favoured d u n e formation and presumably the removal of fine material southwards. Climatic shifts have resulted in dune encroachment southwards or degradation northwards, depending on the direction of the shift, and correspondingly the dust accretion zone has become a source area for the removal of dust further south or has extended northwards. A site may, therefore, have been acted u p o n during several periods by wind or water, erosional or depositional, or any combination of these processes. Parent materials of upland soils over most of the area (neglecting the small area of Pleistocene deposits) can be seen as a function of a number of materials plus process combinations. If, as a model, we consider a land surface without regolith, the composition of the soils' parent materials after an initial stablephase--active-phase sequence is some combination of the following, the composition at any site depending on the history of that particular site. (1) In-situ weathered Basement Complex and Cretaceous derived material. (2) Basement Complex and Cretaceous-derived material of local origin transported by coUuvial processes. (3) Basement Complex and Cretaceous-derived material of local origin transported by wind action. (4) Material of u n k n o w n origin imported from further afield by wind action. (5) Material originating locally from the erosion of ironpan cappings by colluvial processes (a variant of 2). A second stable-phase--active-phase sequence would produce the same combination of processes, the effects of which would be superimposed on the soils parent materials, monophase or polyphase, produced b y the first sequence. The second sequence would then tend to homogenise the results of the first sequence while creating discontinuities anew. This chain of events would continue through successive sequences. Parent materials of present-day profile forms can be associated with each of these individual processes to greater or lesser extents depending on the properties of particular materials. (1) In-situ weathering can be clearly identified in the gradually altered parent rock at the bases of many profiles. It is most evident in the north and southeast of the survey area, where overlying deposits have been stripped or thinned or where deposition has been slight. (2) Local colluvial material occurs widely. Clear evidence is the presence of quartz gravel or detrital ironpan gravel through the provile. The occurrence of aeolian material throughout the profile (a statement this paper attempts to make), is also p r o o f of homogenisation, possibly b y faunal activity in part, but more significantly b y mass movement (see 5). The wide distribution range of a particle-size class in areas of maximum deposition (property 2) may also be explained by local surface movement of material. (3) Wind action, by particle movement along the surface or saltation, on coarse-textured Cretaceous material is assumed to have brought a b o u t dune formation in the north of the area. Subsequent degradation, perhaps involving
254
water in a similar manner to that described by Talbot and Williams (1978) has resulted in a relatively even cover of coarse material with only vestigial dune formations remaining in parts. The Cambic Arenosol previously described is an example of a soil formed in this t y p e of material. (4) The transport of material in aeolian suspension is an active process at the present time. It is proposed here t h a t the very fine sand and silt distribution patterns are evidence of the significance of the process in the past and that the progressive change southwards from coarse to fine particle size indicates a source area to the north. The very fine sand has presumably not travelled far and some may have originated in the north of the survey area. Sombroek and Zonneveld (1971, p.71) identify a possible source of their F u n t u a loess south of Lake Chad and east of its deposition zone. Kalu (1979) gives the source area of present-day Saharan dust as the Bilma--Faya Largeau area much further north. Soils formed in material covering ironstone-capped mesas in the south of the area are of this type. (5) Material originating from the destruction of ironpan caps has been mentioned already as evidence of widespread local reworking. From the a m o u n t of detrital material encountered, the ironpan mesas of t o d a y must be remnants of much larger ironpan formations. Material similar to t h a t in (4), resting on previously more extensive ironpan caps, will also have been redistributed downslope in this way. CONCLUSIONS
The textural distribution patterns support the findings of previous workers to the extent t h a t soils t h r o u g h o u t the area contain elements of aeolian origin, and that aeolian material is a significant c o m p o n e n t o f the pedologicaUy welldeveloped soils with argillic horizons in the south as well as of the coarsetextured soils in the north. The results do, however, support impressions gained during the survey of the inadvisability of applying the term " d r i f t " , meaning wind-deposited, to some soils (or parts of soils) on the basis of a cursory field examination and the identification of certain "aeolian" properties such as silt, while regarding others as " n o n - d r i f t " on the basis of "nonaeolian" properties such as ironstone gravel. Components of various origins and with varied and indeterminate histories have contributed in various amounts to parent materials homogenised to greater or lesser extents. The exceptions to this perhaps are sand covers in the north of the area, providing the dune degradation process has not introduced extraneous material, and silty mesa covers in the south. It may be appropriate to regard the latter as loess. ACKNOWLEDGEMENTS
I would like to t h a n k the Cartography Section and the Data Management Unit o f the Land Resources Development Centre for their assistance with mapping and data handling.
255 NOTE Compatible particle-size analysis results are not available for deposits further south than shown on the maps, but field work indicated that a significant silt content does not extend much further south than 10 ° 30', and that t h e s o u t h e r n l i m i t o f s i g n i f i c a n t s i l t a c c r e t i o n lies b e t w e e n Z a r i a a n d K a d u n a .
REFERENCES Brink, A.B., Mabbutt, J.A., Webster, R. and Beckett, P.H.T., 1966. Report of the Working Group on Land Classification and Data Storage. Milit. Eng. Exp. Establ., Christchurch, England, Rep., 940. Burke, K., Durotoye, A.B.,and Whiteman, A.J., 1971. A dry phase south of the Sahara 20 000 years ago. W. Afr. J. Archaeol., 1: 1--8. Du Preez, J.W., 1958. The water supply of Katsina town Rec. Geol. Surv. Niger., 1955: 59--64. Falconer, J.D., 1911. The Geology and Geography of Northern Nigeria. MacMillan, London, FAO-UNESCO, 1974. Soil Map of the World, 1. Legend. Unesco, Paris. Grove, A.T., 1958. The ancient erg of Hausaland, and similar formations on the south side of the Sahara. Geogr. J., 124(4): 526--533. Grove, A.T., and Warren, A., 1968. Quaternary landform and climate on the south side of the Sahara. Geogr. J., 134(2): 194--208. Higgins, G.M., 1963. Upland soils of Samaru and Kano plains. Inst. Agric. Res., Samaru (Unnumbered Soil. Surv. Rep.). Kalu, A.E., 1979. The African dust plume: its characteristics and propagation across West Africa in winter. In: C. Morales (Editor), Saharan Dust. Wiley, New York, N.Y., (Published on behalf of the Scientific Committee on Problems of the Environment). Keay, R.W., 1949. An outline of Nigerian vegetation. Gov. Printer, Lagos. King, L.C., 1967. The Morphology of the Earth. Oliver and Boyd, Edinburgh. Klinkenberg, K. and Higgins, G.M., 1968. An outline of Northern Nigerian soils. Niger. J. Sci., 2(2): 91--115. Kowal, J.M. and Knabe, D.T., 1972. An Agroclimatological Atlas of the Northern States of Nigeria. Ahmadu Bello Univ. Press, Samaru, Zaria, Nigeria. Lawes, D.A., 1962. The influence of rainfall conservation on the fertility of the loess plain soil of northern Nigeria. Niger. Geogr. J., 5. Macload, W.N., Turner, D.C. and Wright, E.P., 1971. The geology of the Jos Plateau. Bull. Geol. Surv. Niger., 32. McCurry, P., 1976. The geology of the Precambrian to Lower Palaeozoic rocks of Northern Nigeria--a review. In: C.A. Kogbe (Editor), Geology of Nigeria. Elizabethan Publishing, Surulere (Lagos), pp. 15--39. Rohdenburg, H., 1970. Morphodynamische Aktivit~ten und Stabilit~tszeiten start Pluvialund Interpluvialzeiten. Eiszeitalt. Gegenwart, 21: 81--96. S0mbroek, W.G. and Zonneveld, I.S., 1971. Ancient dune fields and fluviatile deposits in the Rima-Sokoto river basin. Neth. Soil Surv. Inst., Wageningen, Soil Surv., Pap., 5. Talbot, M.R.~nd Williams, M.A.J., 1978. Erosion of fixed dunes in the Sahel, Central Niger. Earth Surf. Proc., 3(2): 107--113. Tomlinson, P.R., 1961. Report on the detailed soil survey of the Livestock Investigation Centre, Katsina, and the reconnaissance survey of the ~urrounding area. Inst. Agric. Res., Samaru, Soil Surv. Publ., 11. Vine, H., 195~. Notes on the main types of Nigerian soils. Gov. Printer, Lagos.