C A T E N A I VOL. 6, 43 - 62 I BRAUNSCHWEIG 1979

CYCLIC ALLUVIAL FAN SEDIMENTATION ON THE FLANKS OF FIXED DUNES, JANJARI, CENTRAL NIGER

M.R. Talbot Department of Earth Sciences

University of Leeds, Leeds LS2 9JT, G.B.

M.A.J. Williams School of Earth Sciences, Macquarie University

North Ryde, New South Wales 2113, Australia

SUMMARY

Al luv ia l fans forming along the flanks of f ixed dunes in central Niger have de- veloped in cycl ic fashion. Periods of fan aggradation al ternate with periods of fan degradation and with periods of surface s t a b i l i t y , when l i t t l e sediment is removed from the dunes for redeposition on the fans. The d i f fe ren t processes pro- bably accompany changes in the biocl imatic regime. Surface s t a b i l i t y , with pedo- genesis, occurs during re la t i ve l y humid intervals when both dunes and fans carry a f a i r l y dense and permanent cover of vegetation. Fan degradation on the other hand is promoted by rather arid intervals l i ke the present day. Aggradation oc- curs when there is at least a greater incidence of intense rainstorms than occur at present. Human influence on the current cycle of degradation appears to be on no greater scale than during ea r l i e r cycles. There seems as yet no reason to sus- pect that with suitable cl imat ic amelioration this region of the Sahel should not again return to a condition of greater surface s t a b i l i t y .

ZUSAMMENFASSUNG

Schwemmf~cher in Randbereichen von f ixierten DUnen in Mittelgebirgen weisen einen zyklischen Bildungsmechanismus auf. Perioden mit Schwennf~cherakkumulation wech- seln mit Abtragungsperioden sowie mit Perioden der Oberfl~chenstabilit~t, wenn we- nig Sediment im DUnenbereich abgetragen wird, das auf den Schwemmf~chern abgela- gert werden k~nnte. Diese unterschiedlichen Prozesse begleiten vermutlich Wechsel d~.s bioklimatischen Milieus. Oberfl~chenstabilit~t mit Bodenbildung ist kennzeich- nend fur relat iv humide Intervalle, wenn sowohl DUne wie F~cher von relativ dich- ter Vegetation bedeckt sind. Auf den Schwemmf~chern findet Abtragung in ariden Intervallen statt, z.B. in der Gegenwart. Schwemmf~cherakkumulation t r i t t ein bei h~ufigerem Auftreten von Starkregen als heute. Der anthropogene EinfluB scheint relat iv schwach zu sein. Es gibt keinen Grund f{Jr die Annahme, dab dieses Begiet des Sahel bei ausreichender Klimaverbesserung nicht wieder einen Zustand mit gr~- Berer Oberfl~chenstabilit~t aufweisen k~nnte.

1. INTRODUCTION

Apart from i ts obvious impact on human food and water resources (LAYA 1975, WINS- TANLEY 1975), a decade of drought in the Sahel has also brought into focus the ac-

43

• ~,, :;~,7 4.El ..:;:.-- (

~. ~ " . _ . . . . . . • . . . . . . . . . / ,; j ,r"

-16'N "':~,;~ '~ k "~r'~"~ "~' . Tchi~-Tabarader~ 16" N -

"~"-. C : ' . - : : ' " .-,:' . . . . ~ " J ",~'~ ; :" ~ . . ' : d ~ y a r e a ~ b a ' ""-'t'i., .: ~u ,a~

; 'J; "~': % a , , . ~ , , - . - /

/, ,, /"

' ~ ,.,~'" (( " ~ . . ~ ( " ~ " " - ~ " ,,-.::.: defunct ., ; 1 ~ - - ~" watercourses

• : ......... / ~ i , . ~ ;: / ~ n I im i l o f ,.^o,,lo . . . . . . - ' : : . . ~ : l . . ~ ~ ,', J " ~ "Z"__'" ~ road, ~ l e i s t o c e n e dunes ~ " . . . .

, , : : : : ,:, .,,~!"i~i::~:::' :--:: :: ::::.;~,....../.

Fig. i: Location map. Square outlines area covered by Photo I.

\ /

l

"4" "

..,~ .% ~ ~

D

" . . . ~ ; : . , " .,.. ' • ~ : i ~_ i

7 ! : '

c'#

/ower upper terrace terrace fan (erosional bench) dune

mainly silts and clays 0-3- (T5km wide

°°°;2na'

present ephemeral ~~...': .," : :" watercourse . . . . . : : ...: ....... : , ' . . . , . ".'" "," ~ :,..,..,

[ ~ p t I ~ e B P ~ ~ I I ! " -: :'.-." ;., ~.'..','."~ j "..'.'.......': : .." ."..... :.'...'.....

L : . . ; ' - . . : ; . . . : . : : . ' , . : ' :~ : ' . ' : . : . " . . , . . . . . . . . , , - . . , ; . , . - . . : . , r . . . " . . . . . ' . . . . " . . . . . ' . ' , •

~ i i ~ i i i ! ) i ! ~ i . ~ i ! i i i i ! i ! . } i i i i . ) . ) i i i i ~ i i i ~ i i i ~ i . i . i : fill.if.ill I ' ; . . ' . : . : . ? : ' > ":'." :,"' "~.:'.': :'.' ".-..'.~. ;'.'.'.""" ~,"..'..',:--: "'.'-'-'. :'.;c ~':','<';.;': .;'. ":" :" "'. ! ..... • .....:.': ....'~,'.~-:::. " . " v . : : . . ~ . ; ' . . ' : ; . . ' . " "" ~ ", .'~'. : , . . " : , . . . . : " " ' . " . . " " . . . . . . . . . ' .~ . ' :~ , : , '" ", ;C ,. ~ . , ' " . " : . . . " " . ' . : ." :

• " ", .... "," i" ," * -f ' • "'"" " " ~ i :* """; ' ""'";'~ -,'~'" " ",'." ",; ""I ", r . .* .... : ":."';'i,'" . * ; " ' ,*'," "'" . " , ; ' " " " ' "" " " ; . ; " "'.

I ( 1"0-1"3 km • I

40

30

20

Fig. 2: Diagrammatic cross-section of half the Janjari dallol, which in the study area is about 2 km wide. Predominantly fine-grained nature of upper i. 5 - 2.0 m of terrace proved by augering, iO m vertical scale at right.

=_

o l d e r f a n d e p o s i t s ~ . f i x e d d u n e

t. ~ ~/ f i x e d d u n "~

/ 0m/ / l -

~ '"'J depth ~ j 5 T M -- ~ ~ s t rU~ot~ya ~ldp:he2 ~tke (~ ~dO frn ~ t s

Fig. 3: Typical fan. Stippled - massive sands; lines - parallel lamination (see photo 3); solid - older fan sediments. Slope angles in degrees. P2, P4 and P5 refer to the positions of Photos 2, 4 and 5. Channel is entrenched into fan surface between points A and B.

÷ Photo i: Satellite photograph of study area (see Figure i for location). Fixed dunes (pale areas) occupy much of centre and upper parts of photo, with traces of large barchanoid structures (B) visible near top. Note also impersistent wind-aligned sand ribbons, some containing smaller barchan- like features, on Continental Terminal plateau at bottom left. D - dal- ioi along either side of which recently active fans were observed. M - mesas capped by lateritised sandstones of the Continental Terminal. Skylab SI90-B image, EROS Data Center No. G30BO86167.

45

companying problems of environmental degradation - destruct ion of natural vegeta- t ion with consequent increase in surface gu l l y ing and de f la t ion - now usual ly re- ferred to as d e s e r t i f i c a t i o n (GROVE 1974, RAPP 1974, DRESCH 1975). Recent discus- sion has been concerned not only with the obvious e f fec ts of c l ima t i c change but also with the ro le man may have played in accentuating and accelerat ing the cur- rent , and perhaps i r r e v e r s i b l e , dese r t i f i ca t i on of large areas of northern A f r i ca . (CLOUDSLEY-THOMPSON 1974, RAPP 1974, DRESCH 1975, HARE 1977, DREGNE 1977). Clear- l y the question of whether recent environmental changes r e a l l y are i r r e v e r s i b l e can only be s a t i s f a c t o r i l y resolved i f and when there is c l ima t i c amel iorat ion. In the meantime, however, some attempt can be made to determine whether the pre- sent s i tua t ion is unique wi th in the period of human exp lo i ta t i on of the Sahel and i f not, to what extent environmental recovery was possible a f t e r previous phases of dese r t i f i ca t i on . We present here evidence which shows that surface degradation on a scale comparable to that cur ren t l y in progress has occurred in the Sahel zone of Niger on at least two previous occasions during the la te Holocene, well w i th in the span of human occupation of the region. Despite the rather severe surface ero- sion that took place during both these e a r l i e r phases of degradation, the surface was able subsequently to return to a state of r e l a t i v e s t a b i l i t y .

In an e a r l i e r paper we described the development of a series of small a l l u v i a l fans which occur in central Niger at the lower end of gu l l i es incised into the f lanks of f ixed dunes (TALBOT & WILLIAMS 1978). We proposed that in th is sort of t e r ra in act ive gu l l i es and fans are probably confined to a geographical ly r e s t r i c - ted morphoclimatic zone. In the present paper we describe the sequence of sediments exposed where channels have become incised into the surface of the a l l u v i a l fans. These sections reveal that the fans have developed in h ighly episodic fashion.

2. PHYSIDGRAPHIC SETTING

For much of i t s length, the main north-south road through Niger, from Tahaoua to Agadez (Fig. I ) , t raverses the extensive f i e l ds of f ixed dunes which cover large areas of central Niger. Between Tabala and Abalak the road fo l lows a broad dry va l l ey (known l o c a l l y as a d a l l o l ) , which is cut into one such dune f i e l d . This va l l ey is f lanked by two terraces, one at c. 1 - 2 m, and the other at c. 15 - 20 m above the da l lo l f l oo r (Fig. 2). The dunes here are not p a r t i c u l a r l y we l l - preserved, but s a t e l l i t e photographs (espec ia l l y Skylab images G30B086167 and 8) suggest they may have been barchanoid in form, I - 1.2 km across the horns and shaped by a roughly east to west wind. Much of the dune f i e l d has an i r r e g u l a r , hummocky topography, but the da l lo l has cut a r e l a t i v e l y s t ra igh t course across the aeolian landforms, at a s l i g h t l y obl ique angle to the main dune trend (Pho- to I ) . The a l l u v i a l fans under considerat ion here occur at 0.3 - 0.5 km in te rva l s along the f lanks of f ixed dunes bordering the da l lo l in the v i c i n i t y of the small v i l l age of Janjar i (15°18'N, 6°Ol 'E). The fans are up to 250 m long and 150 m wide. Figure 2 provides a generalised section across the da l lo l and Figure 3 shows a typ ica l fan. A deta i led descr ipt ion of the surface morphology of the fans can be found in TALBOT & WILLIAMS (1978).

The local cl imate is characterised by high mean temperatures; minimum, January 23.7°C; maximum, June 33.4°C ( f igures for Tahaoua, from BERNUS 1974) and a strong- l y seasonal r a i n f a l l regime. Median r a i n f a l l is about 350 mm, the bulk of t h i s f a l l i n g in July and August (RODIER 1975).

At present the region carr ies a mixed population of nomadic herdsmen and sedentary ag r i cu l t u r i s t s . ;The l a t t e r l i v e along, and confine t h e i r cu l t i va ted crops to the main da l lo l f l oo rs , where water is most read i l y ava i lab le from wel ls and seasonal pools. Sheep, goats and camels graze seasonal pastures in the da l l o l s and on the f ixed dunes. I t is probable that human exp lo i ta t i on of the l a t t e r areas has been essen t ia l l y unchanged fo r a very long time indeed (TALBOT, in press).

46

3. FIELD METHODS

Selected fans were mapped by pace and compass traverses and inc l inat ions were measured with a Sunto clinometer. Fan, channel and dune sediments were examined in natural exposures and by augering.

4. CHRONOLOGY

The presence of impressed and undecorated potsherds, and of a microblade industry of ear ly or pre-Neol i thic age on the dune summits (A.B. SMITH, personal communi- cation, Jan. 1975), implies that the dunes were already in existence by early Neol i thic times• By analogy with the detai led late Quaternary sequence established by SERVANT (1973) for the Chad basin, i t is possible that the Janjari dunes were formed or reactivated during the very arid phase at the end of the Pleistocene, between 20000 B.P. and 12000 B.P.. The Neol i thic in eastern Niger has been dated to between 7000 B.P. and 4000 B.P. (CLARK 1971, MALEY et a l . 1971, CLARK et a l . 1973, ROSET 1974) and the mic ro l i th ic industr ies to between ca. 7000 B.P. and at least I0000 B.P. (CLARK et a l . 1973). I f our suggested end-Pleistocene age for the dunes is correct, the i r s tab i l i sa t ion would have coincided with the early Holocene interval of r is ing lake leve ls , increased plant cover and more abundant ra in fa l l evident in eastern Niger, Chad and Mauritania (WILLIAMS 1971, SERVANT 1973, CHAMARD 1973, ROGNON 1976, MALEY 1977, TALBOT, in press). Such an estimate is cer ta in ly in accord with the l imi ted archaeological evidence we have.

No Neol i thic material was found in si tu in, or on the alluvium along the centre of the da l l o l , so these deposits may---b~ younger than Neol i th ic. A radiocarbon date of 335 ± 60 years B.P. (N-2129) o ~ i n e d from a f i replace in s i tu in the soi l capping the lower terrace indicates that the l a t t e r feature w a s ~ l ~ y formed by that time. Two fur ther dates, 155 ± 70 years B.P. (N-2130) and 225 ± 60 B.P. (N-2131), were obtained from hearths within a s imi lar soi l crust in the v i c i n i t y of In Wag- geur (6°40'E, 16°08'N). The good state of preservation of the hearths indicates that unt i l quite recently there must have been a high degree of surface s t a b i l i t y , with minimal gul ly ing and sheet erosion•

The fans can c lear ly be no older than the dal lo l into which they are being b u i l t and i t would appear probable that incision of th is va l ley , or at least the deve- lopment of i t s present morphology, commenced during the same la test Pleistocene - ear ly Holocene humid phase. In fact the fans may be rather younger features as the i r development may have been impeded by the r e l a t i v e l y dense vegetation cover of the ear ly Holocene (MALEY 1977, TALBOT & WILLIAMS 1978, TALBOT, in press). Dis- section of the f ixed dunes by gul ly erosion may have begun once the climate of the Sahel started i t s decline towards the present semi-arid condition, a process which has taken place only during the las t 3000 - 4000 years (WILLIAMS 1971, SERVANT 1973, MUNSON 1974, ROGNON 1976, MALEY 1977, TALBOT, in press).

5. ALLUVIAL FAN SEDIMENTATION

I t was clear from our v i s i t to the area during the 1974/75 dry season that the a l - luv ia l fans around Janjari had been active during the 1974 rainy season. Segments on the lower slopes of the fans were quite devoid of grass, in strong contrast to the adjacent dunes and older fan surfaces. Evidence of dune erosion and concomi- tant deposition on the fans was l imi ted to a r e l a t i v e l y small area, presumably because a pa r t i cu la r l y intense rainstorm affected th is region (TALBOT & WILLIAMS 1978). Sediments deposited during th is most recent phase of a c t i v i t y w i l l be dis- cussed f i r s t , followed by a description of the older fan sediments•

5.1. }~ !m~!e~_~u~!~9_ !~Z~

Sediment accumulation resul t ing from a storm during the 1974 rainy season was con-

47

99"9 -

9 9 -

9 5 - -

9 0 - -

~ 8 0 - -

~ 7 0 - -

~" 6 0 - -

5 0 - -

40--

3 0 - -

--~ 2 0 - -

u = 10--

5 - -

1 -

0 , 1 -

/ /

~ IS /

i / /

2

i "7 "I

,: / '.~

:'!

I I I I f I -1 0 1 2 3 4

phi units

Fig. 4: Cumulative size distribution curves for: fixed dune sands (solid line); older fan sands (dotted line); newly deposited sands (dashed line).

f ined to the gu l l i es and a seg- ment of each fan below the in - tersect ion point (sensu HOOKE 1967), where a t h i ~ e t of sand was deposited over the hard set- t ing so i l that caps most of the upper and lower terrace and the fans (Fig. 2; Photo 2). This sheet of sediment usual ly dies out downslope across the inner part of the lower ter race, but occasional ly the sands i n t e r f i n g e r with s i l t s and muds that accumula- ted in shallow depressions on t h i s , terrace. Apart from the s ing le , s t rongly entrenched channel, the fan surface upslope from the in - tersect ion point car r ies no re- cent ly accumulated sediments and is general ly devoid of evidence of recent f l u v i a l a c t i v i t y . Sedi- ment thicknesses are shown in Figure 3 and grain sizes summari- zed in Figure 4.

Within channels the most prominent large-scale sedimentary structures are long i tud ina l bars which can be over 30 m long, but are ra re l y more than 5 m wide and 30 cm high. These bars are not normally pre- served in t h e i r o r ig ina l form, t h e i r tops and margins usual ly be- ing eroded into a ser ies of small terraces during the waning stages of f low (Photo 3). Poorly defined bars are also present around the apex of the newly-deposited sedi- ment fan, along with shallow, impersistent channels that are sometimes flanked by low lev~es, 5 - I0 cm high. I t is probable that immediately a f t e r the period of discharge the main feeder

channel contained other, smaller bedforms. Remnants of l inguoid r ipp les could s t i l l be discerned, but the major i ty of smal l -scale f l u v i a l bedforms had been destroyed by the time of our i nves t iga t ion , pa r t l y as a resu l t of aeolian rewor- king, but mainly because of wandering domestic animals.

Para l le l lamination is the commonest in terna l s t ruc ture , occurring both in channel and fan deposits (Photo 3). Ind iv idual laminae extend over several metres, are 0.5 - 2.0 cm th ick , and are e i ther homogeneous in texture or show normal or re- versed grading. Within graded uni ts the grain size var ies from coarse to medium sand and the sand laminae may be separated by 0.1 - 0.2 cm th ick s i l t drapes. This f i n e l y laminated st ructure appears to be s im i l a r to what PICARD & HIGH (1973) have cal led horizontal para l le l s t r a t i f i c a t i o n .

Bars also contain l e n t i c u l a r uni ts of trough cross-bedded sands. Planar cross- bedded sands with sets 15 cm or less th i ck , occur in the channel sands, par t icu-

48

i

Photo 2: View of a fan, with parent dune behind. Note wind rippling of fan surface in foreground. See ] [gure 3 for exact location of photo.

Photo 3: Scoured flank of small bar, showing parallel lamination picked out by thin

silt drapes. Length of pick head, 25 cm.

49

• ; ~.; "L~.~ :;"

. - . . . . . . ... • ...- . . . . . ...' --,, ....... .;....~..-. :. :.'.:!.'... :.~:.:::'.::."~.

- . . . . . > - : - . : : , : ~ : : . : . . . . . , . . . . . ~ . - . . . : . ~ , 'T : : ; : . . ' . . . ~...~ : . . : . ~ . .hL : . . i ' : . : ' ' : ' ; : ~ ' " - . : : : , ; . ' : . ~ . : . . . . . . . . . . . . . . _ . - -

i - . t

15tom, ~ ~ ' " " ~ : ~ " ~ - ~ ~ - - - " ' ' " . . . . . . . ' . . . . ": :":S'" " "' ~ ""

< D O W N S T R E A M

Fig. 5: Cross-section through bar showing two units of upstream-dipping cross-bed- ded sands. These, and the underlying unit of downstream-dipping cross-beds, are separated by silt-draped scour surfaces.

l a r l y in the lower hal f of the stream course. The cross st rata,which general ly dip at low angles (< I0 °) were invar iably truncated pr ior to bur ia l . PICARD & HIGH (1973) suggest that these structures are character is t ic of longitudinal and point bar deposits. Ear l ier phases of bar development, subsequently eroded during waning flow, are revealed by buried, s i l t draped micro-terraces.

In many of the cross-bedded units laminae dip downstream, but in addit ion there are commonly present len t i cu la r units of cross-bedded sands within whichthe la- mination is planar or convex upwards and dips consistent l~ upstream (Fig. 5). As the channel gradient averages 3 o and can be as steep as 8 u i t seems most unl ike ly that the upstream dip of these cross-strata is due to f low reversal. Neither does i t seem l i ke l y that ref lected currents, such as PICARD (1970) has postulated, are responsible, since upstream dips are often present in s t ra ight channel sections. These cross-bedded units must therefore be the resul t of antidune ac t i v i t y . Ant i - dune development would cer ta in ly be an expected consequence of the v io len t , sedi- ment-laden discharge that doubtless characterise these channels when in spate (TALBOT & WILLIAMS 1978).

On the surface of the fans, wind is responsible for extensive r ipp l ing and i t can be anticipated that during the long dry season aeolian reworking w i l l progressive- ly ob l i te ra te the f luv ia l character is t ics of the fan surface. Indeed, i t is l i k e l y that much of the sediment w i l l be completely removed by def la t ion. By January 1975, no more than about f ive months af ter the previous rains, i t was apparent that con- siberable sediment red is t r ibu t ion had already taken place and the windiest period (March - Apr i l ) was s t i l l to come. Dry season def lat ion is almost cer ta in ly an im- portant reason why there has been l i t t l e or no net accumulation on the fan surface during recent times (see below).

The channel deposits, with abundant bedforms and structures indicat ing high velo-

50

city flows and large sediment transport rates, are typical products of sedimenta- tion in ephemeral streams (McKEE et al. 1967, WILLIAMS, G.E. 1971, PICARD & HIGH 1973). Si l t drapes within the predominantly sandy sediments, particularly those with horizontal parallel strat i f icat ion, indicate that discharge within the chan- nels must have been characterised by pulses of sand-charged flow interspersed with periods of less intense current act ivi ty. FROSTICK & REID (1977) suggest that in ephemeral streams in northern Kenya each graded lamina w~thin a set of parallel laminated sands was the product of individual flood waves entering the main channel from tributaries. A precisely analogous explanation cannot satisfac- to r i l y explain the pulse-discharge of the Janjari channels, as these lack signi- ficant tributaries (TALBOT & WILLIAMS 1978). The pulses were perhaps related to variations in the intensity of the storm and the differing arrival times at the channel head of flood waves generated on the upper dune and terrace.

5.2. g ! ~ _ f ~ _ ~ i ~ Where the present channels are entrenched into the fan surface i .e . between A and B on Figure 3, bank sections show an interest ing sequence of older fan deposits. Up to three cycles of episodic fan development are exposed, each representing a period of fan dissect ion, followed by aggradation and f i n a l l y surface s t a b i l i t y with soil formation and bioturbation (Photos 4 and 5). The soi ls are hard-sett ing, organical ly stained, but with an otherwise uniform sandy texture. They are inva- r iab ly somewhat truncated by erosion (Photos 4 and 5). The buried soil horizons seem to be rather extensive and at the time of production probably covered much of the fan, jus t as the most recent soil forms much of the present fan surface (Fig. 2). Subsequent dissection of the fans loca l ly caused scouring to depths of over i metre, the sands deposited during the next phase of aggradation accumula- t ing on this scoured surface (Photos 4 and 5). Minor d iscont inu i t ies , traceable for short distances in bank exposures, are apparent within each set of a l luv ia l sediment. These are neither so d i s t i nc t nor so persistent l a te ra l l y as the dis- cont inu i t ies associated with the soi l horizons (Photo 4).

As Figure 4 shows, the older sediments are l i t t l e d i f fe ren t from those deposited during the recent period of f luv ia l ac t i v i t y . The medium sands are, however, red- brown in colour and s l i gh t l y indurated. Lamination is generally rather i nd is t i nc t , but len t i cu la r units of shallow trough and low-angle planar cross-bedding are pre- sent. The l a t t e r frequent ly dip upslope and i t is clear that antidune cross-bed- ding is widely preserved in these older sediments. A structure notable for i t s absence is the f ine paral le l lamination, so common in the recently deposited sands. This absence tends to confirm PICARD & HIGH's (1973) suggestion that paral lel s t r a t i f i ca t i on in ephemeral stream deposits has a low preservation potential since i t mainly occurs at the top of sedimentation uni ts, from which i t is readi ly re- moved by subsequent flood events and by wind.

Depositional processes during the periods of fan aggradation must have been essen- t i a l l y simi lar to those in operation during the 1974 f lood, the older sands also accumulating during periods of b r ie f , but intense, run-of f from the upper parts of the f ixed dune. In contrast to the modern fans, however, there must also have been episodic accumulation on the mid-fan and fan-head areas.

6. DISCUSSION

From their distinctive morphology there is l i t t l e doubt that the sediment bodies described here can truly be called alluvial fans. In detai l , however, they di f fer from piedmont alluvial fans, largely as a result of their derivation from a homo- geneous source of sediment. Janjari fan sediments are uniformly well-sorted, show no down-slope variations in grain size nor abrupt vertical changes in lithology. Debris flows and sieve deposits are absent (no coarse clasts available), as are mudflows (essentially no fines in source area) (cf. reviews of piedmont fans by

51

Photo 4: Three cycles of fan sedimentation exposed in bank of entrenched channel. Oldest cycle (darkest sediment) deeply incised prior to deposition of next cycle. Note animal burrow, just right of knife, piping paler sand down into earlier cycle, and minor discontinuities within the bodies of alluvial sand. Length of knife handle 8 cm. See Fig. 3 for exact location of photo.

Photo 5: Entrenched channel to fan shown in Fig. 3. Earlier cycles of fan aggrada- tion in bank at left foreground. Surface of oldest exposed cycle deeply scoured prior to next phase of aggradation. Fan spreads out from intersection point just below figure.

T 0.25 - t - 0 m

ated sands, may have

|.

often rough and ping up-

ng older to 0-75m

nent down

Plant colonisation, sand stabilisation I and soil development under hi~]her rainfall ancl~r more favourable balance.

Seasonal rainfall, frequent intense storms, episodic aggradation. Development of antidunes at peak discharge,

Dissection of fan by runoff and deflation following period of surface stability and soil formation, Episodic droughts probable,

Fig. 6: Summary of main features (left) and interpretation (right) of a typical cycle.

BULL 1972, 1977, COOKE & WARREN 1973). In other respects, however, the Janjari fans display features which occur widely on the much larger and heterogeneous fans of classical piedmont sett ings.

Par t icu lar ly noteworthy is the way in which the Janjari fans have accumulated in episodic fashion.

6.1. ~!!~_~_~!~!2~

A typical cycle is summarised in Figure 6. Channel sections in a l l the fans exam- ined by us show the uppermost 1.5 - 2.0 m of these sediment bodies to contain three such cycles, the soi l that caps the youngest of these forming the present fan surface (Fig. 3). The close s im i la r i t y between bank sections exposed in seve- ral d i f fe ren t fans implies that the fans have developed in unison.

Cyclic development of a l luv ia l fans is by no means uncommon, being widely documen- ted from both recent and ancient examples (e.g. BULL 1972, 1977, COOKE & WARREN 1973, STEELE et al . 1977). Explanations of th is phenomenon can be divided into two groups (see reviews in COOKE & WARREN 1973, and WASSON 1977): those favouring tec- tonic control and those favouring some sort of c l imat ic control . The former seems highly improbable as an explanation for the Janjari cycles. Evidence for marked Pleistocene-Recent tectonic ac t i v i t y in the stable, cratonic area of central Niger is completely lacking. The physiographic r e l i e f responsible for fan development is in any case quite c lear ly of essent ia l ly depositional or ig in . However, before accepting a cl imatic explanation for the cycles, some attent ion must be given to a th i rd poss ib i l i t y - that the cycles are not external ly control led, but are an in- t r i ns i c part of sediment accumulation on a l luv ia l fans - i .e . that a l luv ia l fan sedimentation is autocycl ic.

In recent years SCHUMM (1973, 1976, 1978) has revived and refined the concept of geomorphic thresholds, pointing out, in ter a l ia , that threshold ef fects could play

53

a p a r t i c u l a r l y prominent ro le in a l l u v i a l fan development. B r i e f l y , th is is envis- aged to proceed as fo l lows.

Sediment deposited from the parent stream accumulates at the apex, p r e f e r e n t i a l l y bui ld ing up th is area of the fan. U l t imate ly the slope here w i l l exceed a s t a b i l i - ty threshold, whereupon the stream, perhaps as a resu l t of a storm, cuts down in to the fan-head, in the process r ed i s t r i bu t i ng some of the e a r l i e r deposited sediment to lower parts of the fan. Accumulation is then confined to areas downslope from the in tersect ion point un t i l such time as aggradation of the now-entrenched chan- nel permits renewed sedimentation at the fan apex, thereby i n i t i a t i n g a new cycle. During periods of entrenchment the area of fan upslope from the in te rsec t ion point receives l i t t l e or no a l l u v i a l sediment.

There is one immediate object ion to applying the threshold model to the Janjar i fans. I t has been pointed out above that trenching should occur only a f t e r the fan apex has passed a s t a b i l i t y threshold. The present entrenched state of the fans implies that they must a l l have reached th is threshold more or less at the same time. This seems inherent ly un l i ke l y , fo r despite the basic un i fo rmi ty in se t t ing and sediment source, the fans show marked var ia t ions in form, slope and size of catchment (TALBOT & WILLIAMS 1978). I t is rather improbable, therefore, that they should a l l have reached the same stage of development, p a r t i c u l a r l y when other possible sources of v a r i a b i l i t y such as aspect (north or south slope of d a l l o l ) and the patchiness of r a i n f a l l in semi-arid areas are also borne in mind.

A p p l i c a b i l i t y of the threshold model may be tested fu r ther . I t can be demonstrated that fans whose cycle is cont ro l led by threshold e f fec ts should develop in pre- d ic table fashion, producing a d i s t i n c t i v e s t ra t ig raphy. A hypothet ical cross-sec- t ion through such a fan is shown in Figure 7a, with a typ ica l section through a Janjar i fan for comparison (Fig. 7b). There are some notable d i f ferences between the two:

i . On the Janjar i fan the youngest so i l covers the whole fan surface, as well as extending up onto the adjacent dune and down onto the da l lo l ter race. The older so i l horizons also appear to extend downfan well beyond the present in te rsec t ion point. Hiatus surfaces resu l t ing from threshold e f fec ts would be less extensive (Fig. 7a).

i i . Despite the entrenched channel, the lower fan is not aggrading at present. Only a th in veneer of sediment was deposited during the 1974 period of a c t i v i t y and much of th is was c l ea r l y in the process of being removed by de f la t i on as the succeeding dry season progressed (TALBOT & WILLIAMS 1978). A f te r a period of sta- b i l i t y the fan surface is now being degraded.

i i i . I f the Janjar i fans are cont ro l led by threshold a f fec ts and trenching had occurred only recent ly - as might be expected in view of the small volume of new- l y deposited sediment on the lower fan - then sediment deposited immediately p r io r to trenching should be found at the fan apex. There were no signs of any loose a l - l uv ia l sediment on the upper part of the fans. C lear ly the area around the apex has been starved of sediment for some considerable time.

These di f ferences point to one major contrast between the two types of fan. On the Janjar i fans each hiatus surface is fan-wide in extent . This can only happen when there are s i g n i f i c a n t var ia t ions in sediment supply, fan-wide hiatus surfaces forming whi le l i t t l e or no sediment reaches the fan. On threshold-dominated fans, sediment supply is essen t i a l l y constant; i t is merely the locus of accumulation that var ies , so hiatus surfaces are less extensive, being confined to those areas of fan temporar i ly starved of sediment. The contrast is thus of fundamental impor- tance; geomorphic thresholds at the fan apex have c l ea r l y not been responsible for development of the major hiatus surfaces wi th in the Janjar i fans. This should not be taken to imply that threshold e f fec ts had no ro le to play in the more or less uniform d i s t r i b u t i o n of sediment over the fans. Some of the minor d i scon t i nu i t y surfaces v i s i b l e wi th in each set of cross-bedded sands could have or ig inated from

54

Present zone of ~ccumulation Sediment-starved apex

Intersection points ,~

- I Incised c h a n n e l ~ ! /

Local hiatus surfaces formed _ / on temporarily abandoned . . ~ ¢ ~ / ~ ~ ~ K~I/~ / segments of fan ~ ~ " f ~,_|-!

(A) THRESHOLD-CONTROLLED FAN

Rate of sediment supply constant, only locus of deposition varies

Incised channel Major hiatus surfaces cover whole fan. / ~ ~ /~', Little or no net accumulation / ~ / - ~'t downslope from present intersection / ~ . ///:'

• 11 ~

• / s~-/

, -: ",'/. ,7-- ) q /'-'- "" ' "" - "

(B) JANJARI FAN

Major changes in rate of sediment supply

Fig. 7: Diagrammatic cross-sections through a threshold-controlled fan and a Jan- jari fan. Note how in (A) major hiatus surfaces are restricted to that area of fan upslope from the zone where periodically entrenched channels have intersected the fan surface. Sediment will always be reaching some part of the fan downslope from this zone. In (B) hiatus surfaces completely cover the fan, indicating the episod- ic occurrence of intervals when little or no sediment accumulated on the fans.

threshold induced events at the fan apex. Cl imat ic va r ia t i on emerges, however, as the mechanism most probably con t ro l l i ng the fan cycles. The three d i f f e r e n t stages of fan evolut ion represented wi th in each cycle may now be examined in the l i g h t of possible changes to the cl imate of th i s part of the Sahel.

6.2. ~ _ ~ 9 ~ Q ~

Episodes of fan aggradation require a r e l a t i v e l y frequent and abundant supply of sediment from the dunes. We have suggested elsewhere that s i g n i f i c a n t runof f from f ixed dunes tends to occur only a f t e r p a r t i c u l a r l y intense rainstorms. Such storms have been rather infrequent during recent years and over the same period there has hence been l i t t l e or no net accumulation on the fans. Phases of aggra- dation therefore imply at least a greater incidence of intense storms than occur at present. These may have accompanied a general increase in r a i n f a l l , as there appears to be a pos i t i ve cor re la t ion between storm i n tens i t y and to ta l p rec ip i t a - t ion , both in the Sahel (ORSTOM 1970) and other parts of semi-arid Af r ica (JACKSON 1972, NIEUWOLT 1974), but i t is also possible that there was no change in to ta l p rec ip i ta t i on . LEOPOLD (1951), for example, has demonstrated that in the semi-ar id south-western United States there have been per iodic var ia t ions in r a i n f a l l in ten- s i t y without any accompanying trend in mean p rec ip i t a t i on . These in te rva l s of e le- vated storm i n tens i t y seem, furthermore, to have been characterised by accelerated. surface lowering and headward migrat ion of gu l l i es (LEOPOLD 1951, 1976). In the case of the Janjar i fans, the need for high sediment y ie lds ce r t a i n l y suggests that aggradation is un l i ke l y to occur under g rea t l y elevated r a i n f a l l condi t ions. The almost inev i tab le increase in vegetation cover accompanying such a change would tend to impede surface erosion.

6.3. ~ Q g ~ § In contrast to the r e l a t i v e l y unstable surface condit ions that must character ise the periods of fan aggradation, the soi l horizons capping each cycle imply surface s t a b i l i t y with l i t t l e or no sediment accumulation on the fans. Buried so i l s are a common feature of a l l u v i a l fan deposits (DENNY 1967, WILLIAMS, G.E. 1970, 1973, BULL 1972, Fig. 2) and many workers bel ieve so i l development to be a natural con- sequence of the sh i f t s in depositiona.l locus that character ise the typ ica l a l l u - v ia l fan surface (BULL 1964, DENNY 1965, 1967, HUND & ~BEY 1966). While some areas of fan are aggrading i t is thought that there w i l l always be adjacent, in - act ive segments where a s t a b i l i s i n g plant cover can develop. In th is model pedo- genesis is therefore continuous rather than episodic, but whi le such a hypothesis may adequately explain the presence of so i l p ro f i l es w i th in some a l l u v i a l fan de- posi ts , i t should be remembered that in the case of these Janjar i examples each soi l appears to be continuous over most of the fan. This impl ies that major epi - sodes of so i l formation occurred when red i s t r i bu t i on of mater ial derived from erosion of the f ixed dunes was at a minimum. I t is l i k e l y that the s t a b i l i t y necessary for th is condit ion could only be achieved with the growth of a s u f f i c i e n t l y dense ve- getat ion cover to provide year-round protect ion to the ground surface. We have ar- gued elsewhere that in the Sahel plant cover capable of protect ing f i xed dunes from gu l ly ing occurs where annual r a i n f a l l exceeds 400 - 450 mm (TALBOT & WILLIAMS 1978). Thus the so i l horizons in the fans around Janjar i may have developed during periods when annual r a i n f a l l in the region was 115 % - 130 % or more of i t s pre- sent value.

6.4. F~_d~E~d~t~QD I t has been pointed out (sect ion 5) that during recent years l i t t l e or no sediment seems to have accumulated on the fans. Instead the most recent ly formed so i l and the underlying sediments have been exposed and dissected. Evident ly the fans are at present degrading rather than aggrading. During th is period of degradation se- diment y ie lds from the dunes must have been r e l a t i v e l y low, but in contrast to the

56

super f i c ia l l y s imi lar conditions that favour pedogenesis, degradation cannot have resulted from the effects of a s tab i l i s ing mantle of vegetation. On the contrary, dissection of the soi l implies a rather sparse vegetation cover and th is in turn suggests r e l a t i v e l y dry, rather than humid conditions. Thus the present state of the fans could be a re f lect ion of recent in tervals of a r i d i t y , as well as a low incidence of intense rainstorms. Periodic droughts during the 20th century (GROVE 1973, 1974, LAYA 1975, ROGNON 1975, ROCHE et a l . 1975) have had a drast ic ef fect on the vegetation of the Sahel and i t is probably during these intervals that de- gradation of the fan surface has been most marked. Droughts may also help keep ac- cumulation rates of the fans at a minimum by favouring the removal by def lat ion of any sediment deposited during the occasional runoff event.

Entrenched channels from a s t r ik ing feature on the modern fan surface (Photos 2, 5). Since these channels c lear ly cut the most recent soi l they are probably asso- ciated with the present phase of fan degradation, rather than being re l ics inhe- r i ted from an ea r l i e r stage of the fans history. At present, apart from post- dating the main phase of pedogenesis, we have no idea precisely when the exist ing channels were excavated. Comparison of the volume of sediment deposited on each fan in 1974 with the volume removed in cutt ing the channel indicates that in some cases the l a t t e r could have been excavated during the most recent period of ac t i - v i t y . Occasional ~ n t s of rooted vegetation on banks and bars suggest, however, that at least some of the channels have been in existence rather longer than th is .

Quite how channel entrenchment is related to those factors which induce fan degra- dation is not clear. Indeed, i t has been suggested that an entrenched channel may be a normal feature on a l l uv ia l fans, d i f fe ren t depositional processes being re- sponsible for fan aggradation above and below the intersection point (HOOKE 1967). This would not, however, seem to be the case on the Janjari fans. The very close s i m i l a r i t y in sediment texture from fanhead to toe implies that s imi lar deposit io- nal 9rocesses have prevailed throughout. HOOKE (1967) and other workers (LUSTIG 1965, BEATY 1974) have also emphasised the possible role mud or debris flows may play in promoting trenching. These, too, provide an improbable explanation for en- trenchment in the Janjari area, the almost wholly sandy nature of the source sedi- ment obviously precluding the development of such flows.

WASSON (1977) has suggested that the cutt ing of channels into Late Pleistocene a l l uv ia l fans in temperate Tasmania may be the resul t of vegetational changes that accompanied Late Pleistocene - Early Holocene c l imat ic amelioration. Increased ve- getation cover of the catchments reduced sediment y ie lds and as a resul t the sedi- ment-poor runoff reachingthe fans was able to i n i t i a t e channel incis ion. In the case of the Janjari fans, however, we have already noted that important runoff is unl ike ly to occur when the dunes are th ick ly vegetated. I t seems more l i k e l y that in th is region c l imat ic deter iorat ion, not amelioration, favours entrenchement. The surface of a drought-affected fan would be par t i cu la r l y prone to gul ly ing, entrenchment perhaps occurring when a wet year terminates a series of drought years. COOKE & REEVES (1976) have suggested that a s imi lar sequence of events may be of importance in the development of some Arizonan gu l l ies . BEAUMONT (1972) and WILLIAMS (1973) also believe that trenching may be related to periods of re la t i ve a r i d i t y .

Whatever the i r or ig in , the entrenched channels, once established under modern con- d i t ions, w i l l persist unt i l such time as the climate once again favours fan aggra- dation. Sequences exposed in channel banks indicate that the channel may then gra- dual ly be eliminated by a l l uv ia l back f i l l i n g (Photos 4 and 5).

The close resemblance between the surface of the modern fans and the discont inui ty surfaces which separate each of the sets of a l l uv ia l sand exposed in channel banks has already been noted. Whether these older erosion surfaces were produced under conditions s imi lar to those responsible for the present state of the fans is not altogether clear. Certainly each of the major d iscont inui t ies appears to post date

57

a period of extensive pedogenesis (Photo 4, 5), but we cannot be certain that channelling was not simply a product of the subsequent phase of aggradation. Al- most cer ta in ly some scouring did occur at this time, but in the fans examined by us, the major palaeochannels seem always to pre-date accumulation of the a l l uv ia l sediments that over l ie the truncated palaeosols (Photo 5). No palaeochannels on a scale comparable to the modern examples were observed to or ig inate within the a l - luvium. I t does seem possible, therefore, that these ea r l i e r d iscont inui ty sur- faces were also produced, at least in part, by in tervals when runoff from the dunes was rather infrequent and the region subject to episodic droughts. Climatic conditions would have been s imi lar to, or even d r ie r , than those that have pre- vailed in the region during the past few decades.

6.5. ~£!~_~ie~i~i~ The soi l forming much of the present surface of the fans is continuous with the soi l exposed on the lower terrace, from which we have obtained a radiocarbon age of 335 ± 60 B.P., thus indicat ing that the last major phase of fan aggradation may have occurred sometime pr ior to the 17th Century. The charcoal samples from In Waggeur (see section 4 above), are of s imi lar age and also come from a hard- sett ing uniform so i l . Limited geological and h is tor ica l evidence, suggests in- creased precip i tat ion throughout the Sahel zone towards 150 - 350 B.P. (GASSE 1975, NICHOLSON 1976, 1978). Since then conditions have been somewhat d r ie r , with gul ly ing and def lat ion the dominant surface processes, although preservation of the f i replaces, many of which are only now being destroyed, indicates surface sta- b i l i t y over the last I00 - 200 years.

Unfortunately no material suitable for dating could be found in any of the older deposits. However, as three depositional units are present within the top two metres of a sediment body only a few thousand years old, i t is probable that the depositional period is r e l a t i v e l y short, of the order of hundreds of years. I f our interpretat ion of the or ig in of the cycles is correct then the i r presence implies periodic changes of r a i n fa l l amounts, and possibly type, in th is region of the Sahel. Such a conclusion is in no way unexpected; abundant data presented by other workers c lear ly reveal marked f luctuat ions in the climate of the Sahel during the last several mil lennia (SERVANT 1973, ELOUARD 1973, CHAMARD 1973, MUNSON 1974, NICHOLSON 1976, 1978, MALEY 1977, TALBOT, in press). What is s t r ik ing about the sequence revealed by the Janjari fans is that evidence of the effects of r e la t i ve - ly s l igh t c l imat ic f luctuat ions may under favourable circumstances be preserved in the sedimentary record.

7. CONCLUSIONS

Al luv ia l fans are forming episodical ly on the flanks of f ixed dunes in an area of 300 - 350 mm mean annual r a i n f a l l . Under present conditions fan renewal is a re la- t i v e l y rare event and for the las t I00 - 150 years or so there seems to have been l i t t l e , i f any, addition to the volume of material stored in these sediment bo- dies. An increase in the incidence of intense rain storms, with or without a s l igh t increase in total p rec ip i ta t ion, would probably promote much more frequent fan a c t i v i t y and a return to fan aggradation, while a r ise in annual prec ip i ta t ion of 15 - 30 % would encourage the growth of a s u f f i c i e n t l y dense and permanent ve- getation cover to prevent any fur ther runoff and gul ly erosion on the f ixed dunes.

Archaeological evidence, buried soi ls and extrapolated sediment y ie ld rates (TAL- BOT & WILLIAMS 1978) suggest that the fans may have been formed over the last few mil lennia. Throughout th is period central Niger has probably been dominated by a semi-arid c l imat ic regime, but the cycl ic development of the fans, at least during the i r more recent h is tory, suggests that th is climate experienced periodic s l igh t var iat ions in both type and amoung of prec ip i ta t ion. Alternating phases of aggra- dation, pedogenesis and dissection have marked the evolution of other fans (e.g.

58

WILLIAMS, G.E. 1970, 1973), but around Janjari the per iod ic i ty of these events has been on the scale of hundreds rather than thousands of years, probably because of the par t i cu la r ly del icate balance that exists between sediment source areas and the biocl imatic regime. The exposed sedimentary cycles consequently carry a valu- able record of the climate of the Janjari region during the last few hundred years. According to our model of fan development, for example, the.current phase of fan dissection shows modern conditions to be rather more arid than they have been for some time previously. We can note also that during the time taken to accumulate the uppermost 2 metres of the fan, there have been, in addit ion to a period which seems roughly to have coincided with the L i t t l e Ice Age in Europe, at least two ear l i e r periods when the climate was humid enough to promote s ign i f i can t l y more permanent vegetation cover than can ex is t under present condit ions. There are clear paral le ls to be drawn between the sequence of c l imat ic f luctuat ions revealed by the fans and simi lar chronologies established for the last one or two mil lennia from other regions along the southern margin of the Sahara (e.g. SERVANT 1974, WICKENS 1975, MALEY 1977, NICHOLSON 1976, 1978). A series of radiometric dates from the ea r l i e r episodes of fan accumulation would obviously be of considerable value in fur ther ing our knowledge of the frequency of small-scale cl imatic change in th is part of the world.

I f deser t i f i ca t ion is to occur, i t must take place during the most arid phase of the cl imat ic cycle and since such phases have recurred episodical ly , the present c r i s is is un l ike ly to be unique. I t can perhaps be argued that human impact in the 20th century has been greater than in any ea r l i e r dry period, thereby lessening the a b i l i t y of the environment to recover, i f and when cl imat ic conditions improve. Human settlement and agr icu l tura l practices in recent years have cer ta in ly had a drast ic e f fec t on the f ixed dunes of some parts of the Sahel, causing local remo- b i l i sa t ion of the aeolian sands (e.g. COUREL 1977, TALBOT, in press). To what ex- t en t anthropogenic ef fects are responsible for the present state of the Janjari fans and dunes is not known, but i t cannot be assumed that these ef fects are grea- ter now than during ea r l i e r centuries. As we point out in section 2, i t is prob- able that human explo i ta t ion of f ixed dune areas has remained unchanged for a very long time. The Sahel has c lear ly been able to support re la t i ve ly large populations at times in the past (BROOKS 1970, MUNSON 1971, CHAMARD 1973, LEVTZION 1973) and these, l i ke the exist ing population, must have placed the environment under consi- derable stra in whenever the climate showed any tendency to deter iorate (see also TALBOT, in press). Nevertheless, whatever the role of human interference in the past, the Janjari fans and dunes were undoubtedly able to return to a state of re- la t ive s t a b i l i t y . The present phase of dissection seems to be on no greater scale than previous phases preserved in the sedimentary record, so as yet we have no reason to believe that recovery may not once again be possible. The same is prob- ably true for a l l but the most d ras t i ca l l y overgrazed and devegegated areas of the Sahel.

ACKNOWLEDGEMENTS

Grateful thanks go to Dr. Andrew B. Smith for inv i t ing us to par t ic ipate in his invest igat ions into prehis tor ic occupation in central Niger; to Dr. Michel Icole and his wife for generous help and hosp i ta l i t y while we were in Niamey; and to Akwalwal Nazim, the ll labakan Touareg community, and the v i l lagers of In Waggeur for the i r physical and moral support during out time in the Azaouak val ley.

We are much indebted to the Royal Society, the Royal Geographical Society and to Macquarie Universi ty for research funds, to Macquarie for special leave for M.A.J.W. and to the Universi ty of Ghana, Legon, for special leave for M.R.T. F inal ly we thank Dr. R.J. Wasson and Paul M. Bishop for the i r thorough and per- ceptive cr i t ic isms of ea r l i e r drafts of th is manuscript.

59

BIBLIOGRAPHY

BEATY, C.B. (1970): Age and estimated rate of accumulation of an a l luv ia l fan, White Mountains, Cal i forn ia, U.S.A. American J. Sci . , 268, 50 - 77.

BEAUMONT, P. (1972): Al luv ia l fans along the foo th i l l s of the ~ u r z Mountains, Iran. Palaeogeogr., Palaeoclimatol., Palaeoecol., 12, 251 - 273.

BERNUS, E. (1974): Les ll labakan (Niger), Une Tribu T o u a r ~ e sah~lienne et son aire de nomadisation. ORSTOM, Paris.

BROOKS, C.E.P. (1970): Climate Through the Ages. Dover, London. BULL, W.B. (1964): Geomorphology of segmented a l luv ia l fans in Western Fresno

County, Cal i fornia. U.S. Geol. Surv. Prof. Paper, 352-E, 89 - 129. BULL, W.B. (1972): Recognition of a l luv ia l fan deposits i n ~ s t r a t i g r a p h i c re-

cord. In: Rigby, J.K. & Hamblin, W.K. (Eds.), Recognition of Ancient Se- dimentary Environments. Soc. Econ. Palaeontologists and Mineralogists. Spec. Publ. 16, 63 - 83.

BULL, W.B. (1977): T ~ a l luv ia l fan environment. Progress in Physical Geography, 1/2, 222 - 270.

CHAMARD,T.C. (1973): Essai sur les pal~oclimats du sud-ouest saharien au Quater- naire r~cent. Actes du Colloque de Nouakchott sur la D~ser t i f icat ion, 17-19 Dec. 1973. ( in press, Inst. fond. Afr. noire, Dakar).

CLARK, J.D. (1971): An archaeological survey of northern A3"r and T~n~r~. Geogr. J " 137, 455 - 457.

CLARK, J.T. , WILLIAMS, M.A.J., SMITH, A.B. (1973): The geomorphology and archaeo- logy of Adrar Bous, central Sahara: a preliminary report. Quaternaria 1__77, 245 - 297.

CLOUDSLEY-THOMPSON, J.L. (1974): The expanding Sahara. Environmental Conservation, i , 5 - 13.

COOKE, R-/U. & REEVES, R.W. (1976): Arroyos and Environmental Change in the Ameri- can South-West. Clarendon Press, Oxford. 213 p.

COOKE, R.U. & WARREN, A. (1973): Geomorphology in Deserts. Batsford, London, 374 pp.

COUREL, M.-F. (1977): Etude G~omorphologique des Dunes du Sahel: Niger nord-occi- dental, Haute Volta septentr ionale. Th~se, Universit~ Paris 7, 284 pp.

DAVEAU, S. (1965): Dunes ravin~es et d~p6ts du Quaternaire r~cent dans le Sahel mauritanien. Revue G~ogr. Afr. occ. Dakar, I - 2, 7 - 47.

DENNY, C.S. (1965): Al luv ia l fans in the Death Valley region, Cal i fornia and Ne- vada. U.S. Geol. Survey Prof. Paper, 466, 62 pp.

DENNY, C.S. (1967): Fans and pediments. Am. J ~ c i . , 265, 81 - 105. DREGNE, H.E. (1977): Degradation by degrees. Geogr. ~ , 49, 708 - 710. DRESCH, J. (1975): Reflections on the future of the semi-ar-Td regions. African

Environment. Spec. Rept. I : 1 - 8. ELOUARD, P. (1973): Osci l lat ions ~limatiques de l'Holoc~ne 6 nos jours en Mauri-

tanie Atlantique et dans la vall~e du Senegal. Actes du Colloque de Nouak- chott sur la D~ser t i f icat ion, 17 - 19 December, 1973 (in press, Inst. fond. Afr. noire, Dakar).

FROSTICK, L.E. & REID, I. (1977): The or ig in of horizontal laminae in ephemeral stream channe l - f i l l . Sedimentology, 24, I - 9.

C~ASSE, F. (1975): L'~volut ion des lacs de l']~-i~ar central (Ethiopie et T .F .A. I . ) du Plio-Pl6istocene ~ l 'Ac tue l . Reconstitution des pal~omilieux lacustres

par t i r de l '~tude des diatom~es. D.Sc. Thesis, Universi ty of Paris. GROVE, A.T. (1973): A note on the remarkably low ra in fa l l of the Sudan zone in

1913. Savanna, 2, 133 - 138. GROVE, A.T. (1974): Deser t i f icat ion in the African environment. African A f fa i rs ,

73, 137 - 151. HARE, K. (1977): Man gets his deserts. Geogr. Mag., 49, 705 - 708. HOOKE, R.L. (1967): Processes on arid-region a l luv ia l fans. J. Geol., 75, 438 -

460. HUNT, C.B. & MABEY, D.R. (1966): Stratigraphy and structure, Death Valley, Cal i -

fornia. U.S. Geol. Survey Prof. Paper, 494-A, 162 pp. 6O

JACKSON, l .J . (1972): Mean dai ly ra in fa l l in tensi ty and number of raindays over Tanzania. Geograf. Annaler, Ser. A, 54, 369 - 373.

LAYA, D. (1975): Document interviews with f a~e rs and l ivestock owners in the Sahel. African Environment, I , 49 - 93.

LEOPOLD, L.B. (1951): Rainfall frequency: an aspect of cl imatic var iat ion. Trans. American Geophys. Union, 32, 347 - 357.

LEOPOLD, L.B. (1976): Reversal o ~ r o s i o n cycle and cl imatic change. Quat. Re- search 6, 557 - 562.

LEVTZION, N. (i~73): Ancient Ghana and Mali. Studies in African History, 7, Methuen, London, 283 pp.

LUSTIG, L.K. (1965): Clastic sedimentation in Deep Springs Val ley, Cal i fornia. U.S. Geol. Survey Prof. Paper, 352-F, 131 - 192.

MALEY, J. (1977): Analyses pol l iniques et ~ l~ocl imato logie des douzes derniers mil l~naires du bassin du Tchad (Afrique Centrale). Rech. Fran~aises sur le Quaternaire, INQUA 1977, Suppl. Bul l . AFEQ, 50, 187 - 197.

MALEY, J . , ROSET, J.P. & SERVANT, M. (1971): Nouveaux gTs-ements pr~historiques au Niger or ienta le ; local isat ion strat igraphique. Bull . ASEQUA, 31, 8 - 18.

McKEE, E.D., CROSBY, E.J. & BERRYHILL, H.L. (1967): Flood deposits in--l~ijou Creek, Colorado, June 1965. Jour. Sed. Petrology, 37, 829 - 851.

MUNSON, P.J. (1971): The T i ch i t t t rad i t ion : a late ~ e h i s t o r i c occupation of the southwestern Sahara. Ph.D. Thesis, Univ. l l l i n o i s , Urbana.

MUNSON, P.J. (1974): Late Holocene cl imatic chronology of the southwestern Sahara. Abs. 3rd Biennial Meeting of American Quaternary Association, Madison, Wisconsin.

NICHOLSON, S.E. (1976): A cl imat ic chronology for Afr ica: synthesis of geological, h is tor ica l and meteorological information and data. Ph.D. thesis, Univ. Wisconsin, Madison.

NICHOLSON, S.E. (1978): Climatic variat ions in the Sahel and other African regions during the past f i ve centuries. J. Arid Environments, i , 3 - 24.

NIEUWOLT, S. (1974): Rainstorm d is t r ibu t ion in Tanzania. Geograf. Annaler, Ser. A, 56, 241 - 250.

ORSTOM (~70) : Monographie hydrologique du bassin du Niger, 3. Le moyen Niger. ORSTOM, Paris, 45 p.

PICARD, M.D. (1970): Current direct ions in st ra ight to s l i gh t l y meandering ephe- meral streams. Jour. Sed. Petrology,40, 771 - 772.

PICARD, M.D. & HIGH, L.R. (1973): Sedimentary Structures of Ephemeral Streams. Developments in Sedimentology, 17, Elsevier, Amsterdam, 223 p.

RAPP, A. (1974): A review of desert izat ion in Afr ica: water, vegetation and man. Secretar iat for International Ecology, Report, I . Stockholm.

ROCHE, M., RODIER, J. & SIRCOULON, J. (1975): Les aspects hydrologiques de la s~cheresse r~cente en Afrique de l 'Ouest. In: 15 - 16 Meteorological and Hydrological Aspects of Continental Droughts. 16th General Assembly of Int . UN. Geodesy Geophysics, Grenoble, 25 August - 6 September, 1975.

RODIER, J. (1975): Evaluation de l'~coulement annuel dans le Sahel tropical a f r i - cain. Trav. et Doc. ORSTOM, 46.

ROGNON, P. (1975): Pal~ocl imatologie~t s~cheresse actuel le (1969 - 1974) au Sa- hel. G~ologues, 32, 55 - 69.

ROGNON, P. (1976): Essa i~ ' in t~ rp re ta t ion des variat ions climatiques au Sahara depuis 40,000 ans. Rev. G~og. Phys. G~ol. Dyn., 18, 251 - 282.

ROSET, J.-P. (1975): Un gisement n~oli thique ancien pres---a~e Fachi (Erg du T~n~r~). Cah. ORSTOM, s~r. Sci. Hum., I I , 105 - I I0 .

SCHUMM, S.A. (1973): Geomorphic th res~ Ids and complex response of drainage sys- tems. In: Morisawa, M. (Ed.), Publications in Geomorphology, State Univer- s i t y of New York, 299 - 310.

SCHUMM, S.A. (1976): Episodic erosion: a modif ication of the geomorphic cycle. In: ~elham, W.N. and Flemal, R.E. (Eds.), Theories of Landform Development. Publications in Geomorphology, State Universi ty of New York, 69 - 85.

61

SCHUMM, S.A. (1978): The Fluvial System. Wiley Interscience, New York, 338 p. SERVANT, M. (1973): S~quences continentales et variat ions climatiques: ~volution

du bassin du Tchad au C~noz~ique sup~rieur. D.Sc. Thesis, Univ. of Paris, 348 p.

SERVANT, M. (1974): Les variat ions climatiques des r~gions in ter t rop ica les du con- t inent a f r ica in depuis la f in du Pleistocene. Soc. Hydrotechnique de France, 13emes Journ~es de l 'hydrauloque, question i , rapport 8, I I , Paris.

STEEL, R.J., MAEHLE, S., NILSEN, H.,R~E, S.L., SPINNAGR, A. (1977): Coarsening- upward cycles in the alluvium of Hornelen Basin (Devonian) Norway: sedi- mentary response to tectonic events. Geol. Soc. America Bu l l . , 88, 1124 - 1134.

TALBOT, M.R. (in press): Environmental responses to climate change in the West African Sahel over the past 20,000 years. In: Williams, M. and Faure, H. (eds.), The Sahara and the Ni le; A.A. Balkema, The Hague.

TALBOT, M.R. & WILLIAMS, M.A.J. (1978): Erosion of f ixed dunes in the Sahel, cen- t ra l Niger. Earth Surface Processes, 3, 107 - 113.

WASSON, R.J. (1977): Catchment processes and the evolution of a l luv ia l fans in the lower Derwent val ley, Tasmania. Z. Geomorph. 21, 147 - 168.

WICKENS, G.E. (1975): Changes in the climate and vege~t ion of the Sudan since 20,000 B.P. Boissiera, 24, 43 - 65.

WILLIAMS, G.E. (1970): Piedmont~edimentation and late Quaternary chronology in the Biskra region of the northern Sahara. Z. Geomorph., supplem. 10, 40 - 6 3 .

WILLIAMS, G.E. (1971): Flood deposits of the sand-bed ephemeral streams of Central Austral ia. Sedimentology, 17, I - 40.

WILLIAMS, G.E. (1973): Late Quater~ry piedmont sedimentation, soi l formation and palaeoclimates in arid south Austral ia. Z. Geomorph. 17, 102 - 125.

WILLIAMS, M.A.J. (1971): Geomorphology and Quaternary g e o l o g ~ f Adrar Bous. Geogr. J . , 137, 448 - 455.

WINSTANLEY, D. ( 1 9 7 ~ T h e impact of regional c l imat ic f luctuat ions on man: some global implications. Proc. WMO/IAMAP Symposium on Long-Term Climatic Fluctuations. WMO, 421, 481 ~ 491.

62