UPPER PLEISTOCENE COMPARATIVE OSL, U/Th AND 14C DATINGS OF SEDIMENTARY SEQUENCES AND CORRELATIVE MORPHODYNAMICAL IMPLICATIONS IN THE SOUTH-WESTERN ANTI-ATLAS (OUED NOUN, 29o N, MOROCCO)

UPPER PLEISTOCENE COMPARATIVE OSL, U/Th AND 14C
DATINGS OF SEDIMENTARY SEQUENCES AND CORRELATIVE
MORPHODYNAMICAL IMPLICATIONS IN THE SOUTH-WESTERN
ANTI-ATLAS (OUED NOUN, 29o N, MOROCCO)

André WEISROCK1, Luc WENGLER2, Julien MATHIEU1,
Abderrahmane OUAMMOU3, Michel FONTUGNE4, Norbert MERCIER4,
Jean-Louis REYSS4, Hélène VALLADAS4 et Pascale GUERY5

- ABSTRACT:

The lower Oued Noun valley, in the arid region of the Atlantic SW Anti-Atlas, contains an extensive Soltanian (= Upper Pleistocene proparte)
terrace. The paper presents dates for these deposits and an interpretation of their fluvial dynamic and morpho-climatic geneses in this distal
part of the valley.
The Soltanian terrace, about 30 m thick at Fort Oued Noun, consists of 3 units: a basal deposit of coarse gravel buried by slope deposits and
travertine (Unit 1); a main unit, more than 20 m thick (Unit 2) that consists of at least 7 repeated sequences each comprising basal fine gravels and
sands, associated with detrital travertine, overlain by stromatolithic tufa and finally by a thick accumulation of sandy silts. These silts were deposited
by suspension and decantation in shallow non-turbulent water bodies. The gravels, sands and travertines aremore prominent at the bottom and in
the upper parts of Unit 2 whilst silts dominate the mid members.Middle Paleolithic artefacts and bones of large mammals are found throughout this
Unit.
Unit 3, at the top of the formation, comprises red silts that differ from those of Unit 2, containing more aeolian grains and more palygorskyte
and being spatially associated with adjacent tributaries fans.
Radiometric dates were obtained on travertine (U/Th), on quartz grains extracted from sediments (OSL) and on Melanopsis and Charcoal
(14C). U/Th results show three travertine constructions at ca 90, 55-50 and 25-20 ka B.P. The 90 ka dates, however, are not in correct stratigraphic
position. The OSL dates suggest that the period of silt aggradation of Unit 2 occured between ca. 50 and 25 ka B.P., the main part of them being deposited
between 40 and 30 ka B.P. 14C dates from the upper part of Unit 2 and the base of Unit 3, range between ca 28 and 18 ka B.P.
These dates, together with geomorphological and sedimentological analyses, indicate that the valley floor had been lowered to its present
depth before ca. 90 ka B.P. However, the slope deposits, older travertines and the basal gravel (Unit 1), classic fluvial and colluvial deposits, are not
yet securely dated but they possibly correspond with wetter episodes in O.I.S.stages 5b, 5a and 4, or even later. The silts that form the bulk of Unit 2
were deposited into shallow swamps during biostasic episodes of O.I.S. 3 and were associated with high groundwater levels. Then, large mammals
found grass and water along the valley and were hunted by Middle Paleolithic people. The water table remained high after 30 ka B.P. and the gravelbed
channels of the local tributaries were active during the 30-20 ka B.P. period. After ca. 20 ka, sediments suggest more varied conditions in this
part of the valley, vegetation disappeared and soils were reworked into local colluvial fans, with concomitant aeolian deposits (Unit 3, O.I.S. 2).
Finally, deep linear incision occured, dissecting the Soltanian aggraded valley floor during the early Holocene.
Key-words: Arid zone, sediment dating, U/Th, OSL, 14C, carbonates, silts, fluvial, aeolian and lacustrine sedimentation, Upper Pleistocene,
Morocco.
RÉSUMÉ
DATATIONS OSL, U/Th ET 14C CROISÉES DE SÉQUENCES SÉDIMENTAIRES DU PLÉISTOCÈNE SUPÉRIEUR ET LEURS
IMPLICATIONS MORPHODYNAMIQUES (OUED NOUN, SUD-OUEST DE L’ANTI-ATLAS, 29o N, MAROC)
La basse vallée de l’oued Noun, aujourd’hui située dans le domaine aride du versant sud de l’Anti-Atlas, montre une terrasse soltanienne
(Pléistocène supérieur pro-parte) bien développée. Les objectifs de cet article sont de déterminer avec précision les âges des sédiments et de comprendre
les changements de la dynamique fluviale, avec ses phases successives d’aggradation et d’érosion, à la fin du Pléistocène supérieur.
La formation soltanienne, épaisse de 30 m à Fort Oued Noun, se compose de 3 unités : un dépôt conglomératique de base surmonté par des
dépôts de pente et des travertins (Unité 1) ; un dépôt principal de plus de 20 m (Unité 2), qui consiste en au moins 7 séquences répétitives montrant
chacune à la base des petits galets et des sables associés à des travertins détritiques, surmontés de travertins stromatolithiques construits finalement
recouverts d’accumulations épaisses de limons. Ces limons ont été déposés par suspension et décantation dans des eaux calmes et peu
profondes. Les galets, sables et travertins sont surtout fréquents à la base et au sommet de l’Unité 2, tandis que les limons dominent dans sa partie
Quaternaire, 17, (1), 2006, p. 45-59
Manuscrit reçu le 09/07/2005, accepté le 12/02/2006
1 Université Nancy2, Département de Géographie, C.E.R.PA, B.P. 33-97, 54015 Nancy cedex, France. weisrock@univ-nancy2.fr
2 Université de Perpignan, UMR 5590 CNRS, Préhistoire et paléoenvironnements quaternaires du Bassin méditerranéen, 52 avenue de Villeneuve,
66860 Perpignan cedex, France
3 Université Ibn Zohr, Faculté des Lettres, Département de Géographie, B.P. 29 S, Agadir, Maroc
4 Laboratoire des Sciences du Climat et de l’Environnement, CNRS-CEA, Avenue de la Terrasse, 91198 Gif/Yvette, France
5 IAURIF, 15 rue Falguière, 75740 Paris cedex 15, France

Page 48

moghrébienne.
depressions reach this southern area, like those in
March 2002. Generally, anticyclonic arid conditions
persist throughout the year and following occasional
high flows, the lower Oued Noun reduces to a succession
of “gueltas”, small shallow ponds of clear water
coming from the groundwater and from karstic springs
that depend on the groundwater level. In addition,
coastal fogs retain early morning humidity in this low
part of the valley that supports a steppe vegetation of
Euphorbia, with some relict Argania spinosa.
The Gorge today comprises steep flanking dissected
slopes carrying thin stony soils and regolith and a floor
comprising the Soltanian silts. Today these latter are
intrically dissected into gullies, ravines and canyons
with piping a common process of erosion and gully
head extension. The Soltanian terrace is deeply incised
almost to the level of the original floor and silty terraces
of Holocene age, also gullied, flank the modern
wadi bed. The terrace inner margins at the foot of the
valley slopes merge with a gravel-covered low-level
pediment surface now moderately dissected by rills; in
some places, this gravel sheet extends over the top of
the silts.
Sedimentation in the present channel floor shows
seasonal variations and has different morphogenetic
meanings: coarse gravel channel clasts are of only local
origin, issuing from slope-wash of weathered material
on adjacent hill slopes and local tributaries which build
ephemeral fans at their confluences; silts and fine
sands, which constitute the main bulk of contemporary
channel sediments, are also related with the occasional
flood events both those generated locally and also
those coming from the whole basin; other fine silts are
deposited in the gueltas by prevailing NNW winds.
Small dunes sourced from the local silts and fine sands,
are built against well-exposed slopes of the valley. Under
the present climatic conditions in the Oued Noun
valley, we observe at the edge of some gueltas only
very small concretions and no cascading travertine
formations, nor stromatolithic and oncolithic deposits,
unlike those of the Atlantic Atlas, further north
(Weisrock, 1980), which require current regular fresh
water flows and extensive active karstic dissolution
throughout the catchment there.
3 - THE “SOLTANIAN” TERRACE
AT FORT OUED NOUN
The Upper Pleistocene terrace, called Soltanian in
Morocco (Beaudet, 1971; Biberson, 1971), is very well
developed in the lowland reaches of the oued Noun,
where it forms a continuous valley floor filling up to
30 m thick. It constitutes a significant aggradation that
occured during low marine levels (Weisrock &
Rognon, 1977). At the presentmouth of the oued Noun,
the isobath of – 50 m lies at more than 20 km from the
present shoreline (Oliva, 1977). The Soltanian terrace
at Fort Oud Noun was built at about 65 km from the
past shoreline, but the sediments probably feathered
out within 5 km of the present shoreline. Clearly, the
Soltanian aggradation was upstream, catchment controlled.
3.1 - MAIN UNITS
Sections were examined at Fort Oued Noun: lower
units being accessibly exposed in high river cliffs on
the right bank, the upper units in adjacent gullies and
rills (fig. 2). On the basis of majormorpho-sedimentary
discontinuities, 3 main, very unequal units were distinguished.
Unit 1 (figs 2 and 4: layer 1, ca. 2 m thick) is a basal
fluvial unit reposing on the valley floor, that here is the
bedrock. It consists of a conglomerate with ca. 8 cm
mean b axis, well-rounded pebbles in a silty carbonated
hard-cemented matrix. This unit is the remnant of fluvial
bed-load deposits on a valley-floor that was as
deep as today, but with a wider channel. Laterally, this
conglomerate is buried by calcreted slope deposits
from adjacent hill slopes, (on the left bank of the wadi),
and associated well-laminated travertines. At the present
mouth of Oued Noun (Foum Assaka), the valley
floor shows fossilised marine terrace deposits from the
Ouljian (= I.O.S. 5e) high marine level (Oliva, 1977;
Weisrock et al., 1999) which are also overlain by laminated
calcrusts and travertines. Thus, Unit 1 may be a
deposit of possible Ouljian or immediate post-Ouljian
age, as seen further north of Agadir (Sabelberg, 1978;
Weisrock, 1980).
Unit 2 (layers 2 to 35), more than 20 m thick, is the
main unit of the Soltanian terrace.
It consists of at least 7 multibedded, repeated
cyclothemic sequences. Each sequence comprises shallow
channels of different sizes, their infills and overlying
deposits. The channels are discontinuous, rarely
more than 4 mdeep and up to 15 mwide. They are filled
first by bed-load deposits of rounded small pebbles and
gravels, and secondly by sandy and silty sedimentary
bodies. The upper parts of the infilling show travertine
development, first of clastic fine grained detritic facies,
with oncoliths and carbonated reed and roots remnants,
secondly of built laminated facies, with Charophytes
and Stromatolithic layers, containing numerous
Melanopsis. These travertines are not so hard as those
of Unit 1, and can extend over several tens of meter in
length and are more than one meter thick. The detritic
facies is always channel width limited; the built laminated
facies can be often related to small steep-sloped
tributaries, probably flowing over the guelta shore. The
channels, their infillings and the travertines are finally
overlain by more or less thick accumulation of silts and
fine sands that display many laterally transitionnal facies,
from the main channel sediments to distal,
overbank facies. These silts are mostly grey-coloured
(7.5 YR 4,5 to 6 Munsell).
In Unit 2, three parts can be distinguished (fig. 2): at
the bottom, (layers 2 to 8), channel bar facies and relatively
hard-cemented travertines are prominent; the

Page 46

moyenne. Dans toute l’Unité 2, on trouve des ossements de grands mammifères et des outillages lithiques du Paléolithique moyen. Au sommet de la
formation, des limons rouges supérieurs (Unité 3) diffèrent des précédents et appartiennent à des cônes déposés par les affluents locaux, avec davantage
de grains éoliens et de palygorskyte.
Les datations radiométriques ont été obtenues pour des travertins (U/Th), des grains de quartz extraits des sédiments (OSL), des charbons et
Melanopsis (14C). Les datations U/Th montrent trois périodes de construction de travertins autour de 90, 55-50 et 25-20 ka B.P. Les résultats à 90 ka
ne sont toutefois pas en bon accord avec la stratigraphie. D’après les résultats OSL, la période d’aggradation des silts de l’Unité 2 se situe entre environ
50 et 25 ka B.P., la majeure partie de ces silts se déposant entre 40 et 30 ka B.P. Les datations 14C se rangent entre 28 et 18 ka B.P. Elles concernent
la partie supérieure de l’Unité 2 et la partie basale de l’Unité 3.
Ces résultats, associés aux observations géomorphologiques et sédimentologiques, montrent que la vallée était déjà creusée autour de 90 ka
B.P. Les dépôts de pente, les plus anciens travertins et le dépôt basal graveleux, ayant tous une signature de dépôts colluviaux ou alluviaux classiques,
ne sont pas datés avec sûreté ; ils peuvent correspondre à des épisodes humides des stages isotopiques 5b, 5a et 4, mais peuvent aussi être plus
récents. Les silts qui forment la masse de l’Unité 2 ne peuvent être reliés uniquement à une activité fluviale et ont été déposés dans des étendues
d’eaux calmes liées à une élévation concomitante du niveau des nappes phréatiques durant des épisodes biostasiques du stage isotopique 3. De
grands mammifères ont alors pu trouver de l’eau et de l’herbe dans la vallée et être chassés par les hommes du Paléolithique moyen. La nappe phréatique
s’est maintenue à un niveau élevé après 30 ka B.P et les chenaux à graviers des affluents locaux sont restés actifs durant la période 30-20 ka
B.P. Après cette date, l’instabilité s’est accrue ; dans cette partie de la vallée, la végétation s’est raréfiée, les sols ont été remaniés, édifiant des cônes
locaux où s’observent aussi des influences éoliennes (Unité 3, stage isotopique 2). Enfin, une forte incision linéaire s’est produite à l’Holocène inférieur,
disséquant l’accumulation soltanienne.
Mots-clefs : Zone aride, datation de sédiments, U/Th, OSL, 14C, carbonates, limons, sédimentation fluviale, éolienne et lacustre, Pléistocène supérieur,
Maroc.
1 - INTRODUCTION
Earlier studies of Upper Pleistocene sediments along
the northern edge of the Sahara desert focused on the
interior eastern valleys (Alimen et al., 1966; Conrad,
1969; Williams, 1970; Littmann & Schmitt, 1989) or
on dune coastal areas (Rognon & Coudé-Gaussen,
1987, 1996; Petit-Maire et al., 1987). These studies did
not focus exclusively on fluvial morphodynamics, generally
did not consider alluvial deposits older than the
limit of the 14C method, and came to varying conclusions.
For some authors, the 40 - 20 ka period there was
more humid than the present, with large flood events,
whilst for others, it was on the contrary almost a dry period.
More recent studies, in eastern of Morocco
(Wengler et al., 1992; Wengler, 1993) or in the Anti-
Atlas (Ouammou, 1994; Weisrock et al., 1994; Thorp
et al., 2002) suggest whilst conditions during this period
may have been generally arid or semi-arid, some
phases were more humid before 40 ka, at ca. 30 and
perhaps ca. 20 ka B.P. In the coastal areas of southern
Morocco and the western Anti-Atlas, these more humid
phases were not characterised by erosive catastrophic
floods, but by floods of very low energy into
shallow valley marshes or lakes dammed by silts and/or
by travertine accumulation. However, many published
dates are not secure: for example, the 14C dates from
Helix and Melanopsis shells in the Oued Tamdroust
(Ouammou, 1994), could not be correlated with the
Middle Paleolithic tools found in the sedimentary
sequences.
The main foci of this paper are:
1) to securely determine the ages of the large sedimentary
accumulations that form a very prominent and
extensive “Soltanian” (= Upper Pleistocene pro-parte)
terrace in the lower part of the Oued Noun valley
(= Oued Assaka, Wengler et al., 2002), in the south
western Anti-Atlas. This is done for a ca. 30 m thick
section near Fort Oued Noun (29o05’46’’N;
10o20’24’’W). For this, comparisons are made between
the results of three dating techniques applied to samples
from the same section: OSL on quartz grains, U/Th on
travertine deposits and 14C on charcoal and molluscs.
2) to establish the sequence of fluvial and other
morphodynamic processes that are responsible for
these unusual thicknesses of deposits in the Soltanian
Moroccan terraces, and
3) to compare this fluvial history with other documented
fluvial sequences in the Mediterranean basin
(Macklin et al., 2002) and with global climatic change
records which are available for the surrounding areas
of: north-west Africa and the Sahara (Rognon, 1987,
1996), and the North Atlantic Ocean (Bond et al.,
1993; Cortijo, 1998).
2 - THE LOWER OUED NOUN VALLEY AND
THE PRESENT FLUVIAL SYSTEM (fig. 1)
The Oued Noun drains part of the southwestern Anti-
Atlas mountains - the massifs of Kerdous and Ifni, and
the western Jebel Bani. Its tributaries focus in the
Guelmim basin, join those Oued Seyad and it then
passes through the southern end of the Ifni Massif in a
narrow incised bedrock gorge of ca. 21 km before exiting
into the Atlantic Ocean at Foum Assaka. Whereas
the Guelmim Basin above Tiliouine varies from 7 to
21 km in width, the Noun - Assaka gorge is only 360 -
480 m wide although there are several reaches where
less resistant bedrock and lateral tributary confluences
permit local widening, as at Assaka village and Fort
Oued Noun. Numerous short bedrock tributaries enter
the gorge reach, but only four drain larger areas of the
Ifni hills and mountains to the north and south. Rainfall
is less than 150 mm/year over the Guelmim lowlands,
but increases to 300 mm/yr in the northern mountain
rim and generates only episodic river flows. High
magnitude flows throughout the length of the catchment
today are very rare and only exceptionally reach
this lower part of the valley, when high intensity rainfalls
cause floods with large amounts of suspended
silty sediments. Such floods occur only when cyclonic

Page 50

3.2 - CMDIAGRAM OF THE UNIT 2 AND MORPHODYNAMIC
IMPLICATIONS (fig. 3)
To obtain an understanding of the transport processes
associated with the deposition of the sandy and silty
members, particularly those in Unit 2, grain size
properties of samples from the main silty and sandy
beds were analysed on a Passega CM diagram (Passega,
1957, 1964, 1977; Passega & Byramjee, 1969).
Following particle size analysis, the C (coarsest 1
percentile in microns) and the M (median size in
microns) values were plotted on a log: log graph. The
plots suggest three groups in a general continuum of
values (fig. 3).
Group 1, the coarsest samples 31-2a, 31-2f, 31-2g
and 33e, show an M range of 45-120 microns and a Cu
range of 1-4 mm. These sediments would be classified
as a type III “graded” (grains maintained in the flow by
turbulence) suspension with bed rolled grains by a
moderately turbulent flow, and 33e with higher

Page 51

turbulence. These could be interpreted as deposition
less distal in relation to the flood flow than the other
samples, and from a flow that had higher turbulence
than the others. The samples come from two within
channel sedimentary bodies (31 and 33) in the upper
part of Unit 2.
Group 2, fine sandy silts, the finest samples, possesses
an M range of 2-14 microns and a Cu range of
45-82 microns. These would be classed as type VIII,
finest “uniform” (sediment distributed through the
whole depth of flow) suspension from a non-turbulent,
perhaps non flowing body of water such as a shallow
lake. Such a lake might form in a pre existing guelta or
lie at the distal end of a waning flood, perhaps dammed
behind a dune or a downstream sill of calcrete and travertine.
Morphomicroscopy of some of the grains may
have been Aeolian derived from reworking of earlier or
adjacent fine silts and sands. All these samples came
from layers that belong to the middle, dominantly silty
fine sand part of Unit 2, except no 32b which came from
the upper part.
The remaining samples fall into Passega’s classes VI,
VII and VIII of “uniform” suspensions. Type VII samples
towards the upper right in this group, 31-2g and
35, would be classed as a low turbulence “uniform”
suspension either within a channel towards the end in
time of a flood, or laterally on the adjacent flood plain;
35 would indicate moderate turbulence. They contain
some medium sized sands that might have been part of
a far travelled or local tributary derived load. The remainder
in this group, those mid to bottom right with
Cus above 100 microns – are mainly type VIII – fine
“uniform” suspension in flows with little turbulence,
probably deposited from slowly moving water onto
floodplains, marshes and swamps or in secondary
abandoned channels.
These Group 2 samples came from the fine-grained
silty fine sandy, clayey deposits making up the bulk of
the main middle portion of Unit 2. There are numerous
Middle Palaeolithic tools and bones through these sediments
suggesting habited, moist, vegetated environments.
This group also includes layer 36 from the non-
Soltanian top layers.
The genesis of Unit 2.
1) The lower part of Unit 2 (layers 2 – 8) contains highenergy
deposits, beds of suspension load sediments
and several interbedded travertines and calcretes
(no3, 5 and 3-5 on the left bank of the wadi). A conspicuous
feature is an irregular layer of angular schist
gravels in a reddish-yellow silty matrix (layer no8)
derived from local hill slopes, that marks a specific
but short lived input during this longer period of
aggradation.
2) In its middle part (layers 9 – 28), Unit 2 begins with
an “uniform” suspended load deposit possibly
containing aeolian transported fines (no9). As aggradation
proceeded, the floodplain became wider.
During this phase, large floods carrying suspended
silt loads reached this part of the valley:
a) bed nos 10, 18 and 26 are shallow channel infills
with massive structure and sometimes small gravels
(no26) indicating sediment transport by
traction with an “uniform” suspended load under
upper regime flow. They are interpreted as
flood deposits in active channels.
b) bed nos12a, 20a, 28a are thicker ( 2,50, 1 and
2,30 m respectively) and the CM plots suggest
deposition from uniform suspended load during
periods of maximum overbank inundation. However,
in the case of 12a it is likely that the coarser
fraction may have a wind blown source. In
the narrower sections of the valley gorge, dams
from earlier sediments or temporary dunes may
have occured with subsequent extension of
marshes and gueltas.
c) bed nos 12b to 17a, 20b, 32b are thin, show lamellar
structure and often contain clay. They
were probably laid down in abandoned channels
and gueltas from the settling out of suspension
load under slack water conditions.
In this middle part of Unit 2, the successive shallow
channel-, overbank- and floodplain-deposits
represent three repeated aggradation sequences.
Flows were generally valley floor wide and had
differing discharges, velocities, turbulences and
sediment sourcing. Some may have been near the
end of the downvalley flow extent; other may
have proceeded further down gorge. The flows
were generally very turbid despite sedimentation
that must have occured in the Guelmim basin
upstream of the gorge. We may suppose that during
at least three periods, the valley floor saw a
succession of shallow palaeo-gueltas.
3) In its upper part (layers 29 – 35), Unit 2 shows an increase
of gravelly and sandy deposits (no31, 33 and
even 35) oversealed by travertines.We suggest these
indicate a new sedimentary style associated with
shifting gravel-bed channels towards the end and on
the top of themain phase of the silty aggradation and
especially near the flanking valley slopes. Groundwater
levels, however, remained high and some very
fine slack water suspension load beds were laid
down (e.g. no32). In some places the fine gravels
may have been locally reworked as discontinuous
“shorelines” around palaeo-gueltas.
Other morphological and chemical evidence for
high groundwater table levels are CaCO3 indurations
of the steep faces of beds 12 and 28, gypsum and salt
crystallisations in numerous beds and hydromorphic
features; calified bioliths of root channels indicate
vegetation in moist soils. We may suppose that Unit 2
is a record of i) flood events at the end of the flow extent,
probably widely separated in time, and ii) almost
continuous sedimentation in the gueltas, albeit with a
very low rate, between the flood events.

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4 - CHRONOLOGY MEASUREMENT RESULTS
Three dating methods were used, giving 19 age results
at the Fort Oued Noun section: 14C method on 8 samples
of charcoal and Melanopsis; Optically Stimulated Luminescence
(OSL) on quartz grains from 6 sediment samples,
and Uranium/Thorium (U/Th) method on 5
samples of travertines (tab. 1, 2, 3 and fig. 4).
4.1 - 14C DATES (tab. 1 and fig. 4)
The samples were collected from layers 21 to 35 in
the upper part of Unit 2. All but 3 came from channel
sandy facies within the Unit. Three samples came from
the top layer 35 (massive sandy silts).
Radiocarbon analysis was performed by β-counting
(using conventional method employing CO2 gas
proportional counters) at the L.S.C.E. of Gif-sur-
Yvette. Results are presented as conventional 14C
ages. All samples were prepared following the standard
procedure (Délibrias, 1985). The shells were
mechanically cleaned of adhering contaminants.
Leaching with dilute HCl was used to remove the portions
of the shell matrix suspected to have been affected
by exchange reactions (Vita-Finzi & Roberts,
1984). However, Melanopsis shells are very thin and
52
N°Gif Layer Reference Material Conventional
Age Y. B.P.
Delta
13C
Minimum14C
Y.B.P. age(1)
11487 35 ON99/11-3 Melanopsis 18 970+/–230 –6,13 ≥ 18 510
11486 33 ON99/11-2 Melanopsis 21 130+/–350 –5,33 ≥ 20 430
11488 35 ON99/11-4 Melanopsis 21 400+/–230 –6,23 ≥ 20 940
11758 35 ON8/3/02 Melanopsis 22 100+/–370 ≥ 24 000
11485 29 b ON99/11-1 Melanopsis 24 100+/–630 –7,29 ≥ 24 000
11212 21 OA98/11-9 Melanopsis >24 000 –7,82 ≥ 24 000
11648 31 ON11/22a Charcoal 27 820+/–580 –24,85 27 820+/–580
11650 31 ON22/2aval Charcoal 28030+/–530 –26,06 28 030+/–530
Tab. 1: 14C Dates of Fort Oued Noun section.
(1): Minimum 14C Y.B.P. age, except for charcoal samples
Fig. 4: Fort Oued Noun: Stratigraphic location of dated samples.
1 = Conglomerate; 2 = Angular schist gravel; 3 = Travertine; 4 = Sand; 5 = Silt; 6 = Calcrete; 7 = Cambrian bedrock (schist); 8 = Progressive limit
of layer.
Fig. 4 : Fort Oued Noun : position stratigraphique des échantillons datés.
1 = Conglomérat ; 2 = Gravier de schiste anguleux ; 3 = Travertin ; 4 = Sable ; 5 = Limon ; 6 = Croûte calcaire ; 7 = Substratum cambrien schisteux ; 8
= Limite de couche progressive.

Page 53

fragile, secondary recrystallisation processes are
probable and could make the ages too young. As we
cannot be completely sure that acid leaching removes
all the possible recrystallisation, we prefer to express
the results as minimum ages, except of course for charcoal
samples.
4.2 - OSL DATES (tab. 2 and fig. 4)
The optical dating method (Aitken, 1998) was applied
to 6 sediment samples from Unit 2. Their precise
stratigraphic positions are indicated on figure 4, and
they span the full sedimentary sequence, from near the
base (layer 6) to the top layer 35. All samples are fine
silts, except those of layer 31 which are sands.
For each sample, middle-sized quartz grains (40-
50 μm) were extracted and isolated, according to the
procedure described in Mercier et al., (2003). To determine
the mean equivalent dose (ED), a single-aliquot
regenerative dose (SAR) protocol (Murray & Wintle,
2000) was applied to several sub-samples (at least 24
aliquots per sample). All luminescence analyses were
done with a Risoe TL/OSL DA-15 reader equipped
with blue light emitting diodes for optical stimulation,
and artificial doses were given by a calibrated Sr/Y-90
beta radioactive source fitted to the reader.
Radioisotopic contents of bulk sediment samples
were measured with a high-purity Ge detector; no disequilibrium
in the U-series and Th-series was detected.
Alpha and beta dose-rates were deduced from these
values using the conversion factors given by Adamiec
and Aitken (1998) and from attenuation factors relevant
to the 40-50 μm granulometric fraction (Mejdahl,
1979; Brennan et al., 1991). Quartz grains were assumed
free of radioelements. α-sensitivity of 5 μGy/a/103 σ was
used for all samples.
Gamma and cosmic dose-rates were deduced from
field measurements done with a portable gamma
spectrometer. Annual dose-rates were computed by assuming
a mean past water content of 10% (±3%) by
weight.
For each sample, at least 24 aliquots, each composed
of hundred of grains, were analysed in order to evaluate
the distribution of the individual equivalent doses. Depending
on the sample, the distributions indicated a
more or less important scatter of the individual values
around the arithmetic mean, probably due to incomplete
bleaching of the grains or because of the presence
53
Sample Layer U Th K Alpha Beta Gamma Cosmic Annual Equi.
Dose
Age
n ° (ppm) (ppm) (%) Doses (m Gy/a) + – (Gy) + – (ka) + –
OSL 19 35 2,96 8,32 2,32 130 2132 1162 200 3624 43 112 1,5 30,9 2,5
OSL 1 31 2,68 6,43 2,42 110 2767 1238 144 4259 130 119 1,2 27,9 2,4
OSL 2 31 2,76 6,03 2,28 75 2608 1183 148 4014 127 124 2 30,9 2,7
OSL 18 28 2,95 7,35 2,02 123 1904 1054 200 3281 42 128 2 39 3,2
OSL 3 14 3,83 7,44 1,77 144 2314 1031 124 3613 113 133 3 36,8 3,2
OSL 17 6 2,45 7,56 2,28 113 2025 1069 110 3317 42 163 5 49,1 4,3
Tab. 2: OSL dates of Fort Oued Noun section.
Travertines
Layers
238U ppm 234/238U mes 230/234U Age ka 234/238U 232Th ppm 230/232Th Age ka,
corrected*
+/–
35 0,854 +/– 0,020 4,931 +/– 0,096 0,250 +/– 0,003 30 +2,3 -2,2 5,275 +/– 0,104 0,924 +/–0,019 3,47 21,6 +4,3
33 3,07 +/– 0,053 3,73 +/– 0,042 0,313 +/– 0,003 38,9+1,6 –1,6 4,041 +/– 0,05 4,284 +/– 0,06 2,55 24,4 +1,6
–1,5
3 1,81 +/– 0,088 3,79 +/– 0,14 0,528 +/– 0,009 73,6 +7,9 –7,3 4,421 +/– 0,172 3,178 +/0,04 3,46 55,9 +9,3
–8,6
3, leaching 1,48 +/– 0,098 4,64 +/– 0, 245 0,445 +/– 0,011 59,0 +10 –9,1 5,291 +/– 0, 289 1,52 +/– 0,06 6,1 50,9 +10,8
–9,8
3-5 2,11 +/– 0,034 3,39 +/– 0,061 0,677 +/– 0,009 105 +8,7 –8,1 4,20 +/– 0,98 3,12 +/– 0,07 4,73 87,6 +9,1
–8,5
5 1,39 +/– 0,03 3,90 +/– 0,08 0,643 +/– 0,01 96,3 +11 -10 4,80 +/– 0,11 1,79 +/– 0,06 5,92 Isochron age
Sample 5
5 1,81 +/– 0,041 4,25 +/– 0,088 0,618 +/– 0,007 90,7 +8,3 –7,7 5,19 +/– 0,11 1,053 +/– 0,04 13,8 92,2
5 1,72 +/– 0,063 3,64 +/– 0,133 0,723 +/– 0,025 115 + 33 - 25 4,64 +/– 0,18 2,691 +/– 0,22 5,13
Waters
Hotel
Oued Noun
238U ppb
2,83 +/– 0,10
5,82 +/– 0,20
5,12 +/– 0,12
5,07 +/– 0,10
Tab. 3: U/Th dates of Fort Oued Noun section.
Errors quoted are one sigma counting statistics.
: corrected ages = corrected for detrital contamination with 230Th/232Th =1, measured in the sedimentary detrital fraction.

Page 54

of poorly bleached quartz grains. This last value was
choosen as the mean equivalent dose (tab. 2). However,
because of the observed scatter, it cannot be totally excluded
that this value is over or under-estimated. In
Table 2 are summarized the dosimetric data relevant to
the sediment samples. All errors are given at 1 σ level;
age uncertainties include statistical and systematic
errors.
4.3 - URANIUM/THORIUM DATES (tab. 3 and fig. 4)
Three travertine samples (no3, 5 and 35) belong to the
Fort Oued Noun main section, and two ( a calcrust, no3-
5, and 33) came from the left bank of the wadi, in equivalent
stratigraphic succession. The collected samples
thus come from the base (no3, 3-5, 5) and from the top
(no33, 35) of Unit 2. All samples, except no3-5, are true
travertines, of hard cemented laminated facies.
All samples were analyzed after total dissolution and
addition of a 232U/228Th tracer solution using a method
similar to that described by Ku (1976). Alpha counting
was carried out with solid-state detectors and errors are
expressed as one standard deviation based on counting
statistics.
Impure carbonates are generally considered as unreliable
species for Th/U dating (Ku & Liang 1984; Bischoff
& Fitzpatrick, 1991; Kaufmann, 1993) for two
main reasons: (i) presence of detrital 230Th in dirty samples
will increase the 230Th/234U ratio and make the
sample look older; and (ii) post deposition addition of
Uranium from groundwaters gives rejuvenated ages.
Most samples in Table 3 were corrected for a detrital
contribution using a 230 Th/232Th ratio of 1, measured in
sediments sampled in the studied area. For sample no5,
an isochron technique was attempted, using total sample
dissolution (Bischoff & Fitzpatrick, 1991). The
isochron age of 92.2 ka compares well with the corrected
age of 87,6 ka of no3-5, in the same stratigraphic
situation. Sample no3 was dated on the bulk sample and
on a leached (HNO3 1N) aliquot, both giving concordant
ages.
5 - DISCUSSION (fig. 4)
5.1 - UNIT 2, UPPER PART (Layers 29 - 35)
The 14C dates, from charcoal, show a securely dated
channel deposit (no 31) with an age of about 28 ka B.P,
but more imprecise results for the other channels (sampled
beds 29, 33 and 35) using Melanopsis shells,
whose ages lie between ≥ 28 ka B.P. – and ≥ 18,5 ka
B.P. The two OSL dates of channel 31 are concordant
with 14C results, but the date of 30,9 +/– 2,5 ka obtained
for layer 35 overlying channel deposit 33 is too old
when compared to 14C (≥ 24 ka) and U/Th
(21,6 + 4,3 ka and 24,4 + 1,6 – 1,5 ka). The spread of
dates from the three techniques might arise through an
overestimation of the OSL age; because of insufficient
bleaching as noted previously, and from the fact that
the carbonated secondary mineral dated by U/Th had
developed within that sediment after its deposition
(Nanson et al., 1991).
Despite these problems, the radiometric dates clearly
indicate that the upper part of Unit 2 (layer numbers 30
to 35) was deposited between ca 30 and 21 ka B.P., and
perhaps even up to 18,5 ka B.P. These ages fit well with
those obtained for the overlying Unit 3 dated by OSL at
17 ka B.P. at a neighbouring site (Mathieu, to be published).
5.2 - UNIT 2, MIDDLE PART (Layers 9 – 28)
OSL results indicate that the most important part of
the silty sedimentation (more than 10 m thick between
layer numbers 14 and 28) occured between 39,0 +/– 3,2
and 36,8 +/– 3,2 ka B.P. The errormargins don’t allow us
to choose between two hypotheses of duration: long (between
42 and 33 ka) or short (between 39 and 36 ka).
5.3 - UNIT 2, BOTTOM PART (Layers 2 – 8)
OSL and U/Th dates of this basal portion of Unit 2
give two groups of results. U/Th ages for layer no3
(laminated travertine) and an OSL age for layer no6
silts suggest an age range of 55,9 + 9,3 –8,6 ka to
49,1 +/– 4,3 ka B.P. However, the U/Th ages for calcrete/
travertine in layer no 5, of 92,2 ka, and in layer
no3-5, of 87,6 + 9,1 – 8,5 ka B.P., are not concordant
with the other dates (fig. 5). Sample layer no3-5 came
from the opposite side of the valley and was not in the
same stratigraphic succession as the other samples.
Therefore, either this date is correct and the stratigraphic
interpretation of layer no3-5 is wrong, or the
date is unsound. The U/Th isochron age of 92.2 ka for
sample no 5 remains inexplicable, although it is known
that the ages of indurated secondary carbonated deposits
may not necessarily fall in strict chronostratigraphic
order (Nanson et al., 1991). So, presently, we prefer to
consider the dates from layer numbers 3 (U/Th) and 6
(OSL), which are stratigraphically coherent, as
representating the time between carbonate closure in
the travertine (defined by U/Th dating) and overlying
silt sedimentation (defined by OSL dating). Therefore,
the age span of this lower part of Unit 2 lies possibly
before 55 ka and after 49 ka. This possibly means that
the O.I.S period 4 was characterised by a higher frequency
of channel forming sediment events than
O.I.S.3.
In spite of their uncertainty, the two results about
90 ka could not be abandoned: we have recorded similar
dates on calcrete/travertine deposits in apparently
similar stratigraphic positions in neighbouring sites,
as has M. Thorp further East (in litteris). These deposits
are possibly preserved remnants of older formations
at the base of the Soltanians, and may imply a beginning
of the infilling of the valley at O.I.S. 5b-5a,
as previously suggested with regard to the relation

Page 55

between Unit 1 and the Ouljian fossil beach (Supra, 3-
1, Unit 1).
6 - CHRONOLOGY AND DURATION OF
THE MAIN MORPHODYNAMICAL EVENTS
On the basis of these results, we suggest the following
sequence of morphodynamic events in the Oued
Noun valley (fig. 5 and 6):
6.1 - OXYGEN ISOTOPE STAGE 5 (128-75kabp)
The first alluviation phase is a high-energy fluvial
deposit, (Unit 1) for which the U/Th datings remain uncertain.
One hypothesis is that this conglomerate dates to
stage 5, as suggested above. This was a coarse sedimentation
into a deep valley, with the same valley floor
almost as to day. The size of the boulders and gravels
indicates high stream power and large floods, with deposits
of both proximal and distal origin. The classic
interglacial stage 5 is known to have been a period of
valley-floor incision in South Morocco (Weisrock &
Rognon, 1977), as in the whole Mediterranean area
(Macklin et al., 2002). Evidence for a major humid
phase during the last Interglacial in the Sahara has also
been evoked (Petit-Maire et al., 1994).
Alternatively, the whole of Unit 1, and the base of
Unit 2 can be considered as more recent (ca. 55 - 50 ka).

Page 56

6.2 - OXYGEN ISOTOPE STAGE 4 (75-64 kabp)
No member of Unit 2 was directly dated to OIS 4. If
Unit 1 (undated) belongs to OIS 5 and the base of Unit
2 to 55 ka B.P., there are no dated sediments belonging
to OIS 4. Alternatively, Unit 1 may be OIS 4, a climatic
period known to be cool and dry in the Mediterranean
and the Atlantic (Heinrich 6), (Bond et al., 1993).
6.3 - OXYGEN ISOTOPE STAGE 3 (64-32 kabp)
The main aggradational phase, Unit 2, consisting of
repeated cyclothemic sequences only slightly disturbed
by minor erosional phases, occurred during this
Stage.
This aggradational phase begins at ca. 55 ka with
travertine deposits on gravels (layer 3), and the main
silty sand layers commence after ca. 49 ka B.P. (layer
no6), with an uniform suspended load of tractive flow.
Minor erosional phases, characterised by shallow small
channel or gully features, occur in this lower part of
Unit 2. They include colluvium and laterally derived
slope debris (layer no8) suggesting that glacis processes
were active on the lower valley side slopes,
transferring sediments to the valley floor. Such sheetflood
processes may have been associated with a drier
environment, but the duration of this episode remains
still unknown.
Above layer no8 occurs the most important
aggradational sequence, 12,50 m thick, with dates from
about 45 to 30 ka ( layers 9 to 28). This phase saw an increase
of fine sediment supply, mostly between ca.37
and 33 ka. Sedimentological records and faunistic remnants
(mostly of Bovini) indicate a local open biostasic
environment, like a steppic prairie or a dry-forest
(Wengler et al., 2002). This is probably one of the most
important palaeoclimatic inferences from this site, indicating
a more frequent occurrence of subtropical
rains in the desert margin (Rognon, 1996). An important
alluviation phase with this age is widely reported
from the Mediterranean basin (Besançon et al., 1997;
Macklin et al., 2002), but with an opposite, rhexistasic,
process significance.
The upper part of Unit 2 shows several small meandering
channel fills containing small pebbles, calcreted
root remnants and oncoliths, starting with layer no29.
Channel fill layer no31 gives dates of ca. 30,9 - 27,8 ka
B.P., dates comparable with those for channel gravels
in the Soltanian silts in the Anti-Atlas valleys further
east (Thorp et al., 2002). It may coïncide with Heinrich
event 3. The upper part of the sequence, layer numbers
29 - 32, therefore shows more variability in sedimentation
but not of such a degree as to suggest any major climate
change: tree charcoal, bones of Bovini andMiddle
Paleolithic tools are still present as they are in the layers
below. Recurrent travertine constructions and silt
deposits initiated by small shallow wandering gravel
bedded channels continue at a high aggraded valley
level during the 30 – ca. 24 ka period, suggesting little
significant change.
6.4 - OXYGEN ISOTOPE STAGE 2 (32-13 kabp)
This stage begins with a small channel (layer no33
eroded into layer no32). It is similar to layer no31 but
dates to ca. 24 - 20 ka and possibly coïncides with
Heinrich event 2. Further silt alluviation layers, numbers
34 and 35, overly these channels in the floodplain,
and pebbly bars of reworked coarse material appeared
close to the valley slopes. The top of this sedimentary
aggradation was here reached at ca. 20 ka B.P., as in
other parts of SouthMorocco (Boudad et al., 2003), but
not in the same catchment further east where it continues
till ca 11 ka B.P. (Andres, 1977; Thorp, in litteris).
The last phase of OIS 2 sedimentation, Unit 3, layer
no36, consists of both active erosion and deposition
and indicates the end of the main cyclothemic
aggradational phase. Thick sandy and silty fans,
sourced from the glacis slopes of the tributary catchments,
and also containing aeolian quartz grains and
palygorskyte, were deposited at their confluences with
the main valley after 20 ka B.P. These latest sediments
repose on the surface of layer no35 with a date of ca.
18,5 ka B.P. 2 km upstream, the base of this colluvial
fan aggradation is dated at 17 ka B.P (OSL date communicated
by N. Mercier), and at ca. 14 ka further
north in the Anti-Atlas, (Weisrock et al. , 1994). At this
time it is possible that an increase in aridity and a reduction
in vegetation cover led to increase soil erosion
by sheet-floods over the catchment slopes. On the valley
floor in Oued Noun, the gueltas became shallower,
more restricted and finally dried out, probably by 12 or
10 ka B.P.
At the end of OIS 2 and the beginning of OIS 1,
ca 10 ka B.P., deep linear incision of the whole infilling
commenced (Mathieu et al., 2004).
7 - CONCLUSIONS
7.1. Despite two radiometric ages that are not in
chronostratigraphical order, comparative dating methods
tested on the same section and even on the same layers
give generally consistent results. All but 2 of the 20 dates
lie in the range 55,9 – 18,5 ka B.P., that is OIS stages 3
and 2. Apart from the two out of sequence dates of 92,2
and 87,6 ka B.P., there are either no sediments or no dates
for OIS stages 4 and 5. The basal sedimentary Unit 1 consists
of a coarse cobble fluvial gravel but remains
undated.
7.2. The main unit (Unit 2) of the Oued Noun Soltanian
formation was deposited between ca.55 and 25 ka B.P.
The most important part of this silty aggradation (more
than 12,5 m thick) occured during OIS.3, between ca. 40
and 30 ka B.P. It comprises repeated sequences of shallow
channels, their small gravel and sands infills, their
travertine caps and overlying silts. Some of the silts contain
aeolian grains, some were deposited in former
gueltas, and most were fluvial overbank sediments. This
period was, in this area at the northern boundary of the
Sahara, a biostasic phase, with a more regular water

Page 58

supply than today and ground water levels may have risen
whilst the valley floor aggraded.
7.3. Aridity and flow variability increased after ca. 20 ka.,
when climate changed significantly from a biostasic state,
with probably more frequent subtropical rains than today,
to an arid state similar to a mediterranean seasonnal-type
climate. After ca. 18,5 ka B.P., Unit 3 – shallow silty
sandy fans with aeolian imputs and sourced from the local
tributaries – was deposited over Unit 2. This period of reduced
vegetation, enhanced aridity and accelerated soil
erosion continued until ca 12 ka B.P. after which, in
OIS.1, the aggradation fills were deeply incised to their
present depths.
ACKNOWLEDGMENTS
This work was supported by the French CNRS programme
“ECLIPSE” and the French-moroccan cooperation agreement AI
87/03. The authors are greatly indebted to Martin Thorp (University
of Dublin, Ireland) for his helpfull criticism and comments, and his
entire rewriting of the englishmanuscript. Thanks are due also to another
anonymous rewriter and to Hassan Benhalima (University of
Agadir), who has always facilitated our studies in Morocco.
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