Fluvial systems of southwestern Morocco and their evolution since the late Pleistocene André Weisrock, Laurent Wahl, Abderrahmane Ouammou, Lahoucine Chakir 10° W
2
Essaouira O.
O. Tensift Ks
ob
3
The present fluvial system
9° W
id ad J. H
1
Chichaoua
Marrakech
4 O. T
idzi
5
LA T TA U HA
zoullen O. Igue
Essaouira
7
31° N
A
T
11° W O. Tamri
0m -15
AT
S A L
N A L
TI
E U Q
O. Tamghart Agadir
S
JToubkal 4165
Aoulouz
Taroudannt Igli
O. Souss
S U Biougra
S
O
J. Igdet 3619
J. Tichka J.Tichka 3350
O. Nokheïl
6
Floods in southwestern Morocco are very scarce and occur irregularly : after 10 mm of rain, rill wash occur over the area and concentrated flow begin in some wadis; when the flood is effective in the main wadi, its effects, except for rill incision on the slopes, are mainly the construction of inner-fans at the mouths of the tributaries, a widening of the braided channel reach, and a delivery of the suspended load to the ocean. Consequently, occurrence of vertical and lateral channel erosion in the whole catchment, colluvial deposition on slopes, widening of the braided channels in the median zone, and silt deposition in the distal zone collectively characterise the predominant processes in present-day conditions. Two other noteworthy characteristics are the various types of calcareous indurations, and links with aeolian deposits.
30° N O.
a Ma ass M O.
ss
a
u
A
N
T
I
AT
Adrar n'Aklim 2534
Tafraoute
2344
O.
sint Tis
st osut drdoru am O. .TaTm O o O. Ado u d
Tiznit
S A L
Ifni O . If n i
O
.N
O. Noun
ou
Ifrane
n
Assaka
Bou-Izakarn
O .A
o
re
o ra
Tarhjijt
Goulimine
29° N aa
Dr
50 km
x = 29 05' 57'' N y = 10 20' 37'' W z = 50 m
Lateral erosion: Late Holocene terrace and main road washed away by the November 3, 1987 flood at Tamri (picture by A. Weisrock).
Datations (ka B.P) Unités
Oued Noun, cote 50
Couches
Epaisseur en mètres
The catchments of southwestern Morocco, western Sahara excepted. 1: mountainous massifs; 2: front of the Hamada du Draa; 3: summit; 4 : perennial river; 5: wadi; 6: mountain and drainage divides; 7: meteorological station.
5b
5
limons et sables en lamines stratifiées avec charbons de bois et Helix sp.
5a 4
4
lentilles de petits galets limonssableux sableuxroses limons rosessur surlitlitde degraviers graviers
8
Description lithologique sommaire OSL
C 14
4,6 + 0,9
limons sableux roses
3e 6
limons sableux jaune rougeâtre à filaments calcaires
3d 3 3c 3b
sables limoneuxà encroûtement calcaire sables limoneux roses et lits de graviers schisteux à filaments calcaires
3a
lentilles de galets centimétriques
4 2b 2a 1b 2
0
2
limons sableux roses à Melanopsis et Unlos sables graveleux à Hélicidés limons sableux rougeâtres
1a
1
lentilles de galets hétérométriques à matrice sablo-graveleuse
0
0
grès jaune cimenté conglomérat cimenté
Excessive width of the bed: the Wadi Souss inner fan at Igli (picture by A. Weisrock, 2000). The braided channel reach (on the right), with five main threads, is separated from the tributary channels (left corner) by a low, partially cultivated terrace. Downstream, the inner fan is cut by an unique sinuous channel.
Characteristic reaches of an inner fan: an example from Wadi Nokheïl, 20 km east of Taroudannt. 1: mountainous front; 2: main channel; 3: fan apex; 4: braided channel reach; 5: small fans; 6: ephemeral rills and sheet wash reach; 7: headcut reach; 8: contour lines.
Holocene terraces The Holocene terraces, similar to the present-day deposits, generally show the classic succession of basal units of coarse pebbles and upper sandy and silty units. The degree of sediment induration is highly variable, depending on topographic position, proximity of limestone source rocks, water circulation, and vegetation cover. This is the rule for all the deposits after ca. 4 ka, with an increase in inter-bedded aeolian sands, and also, but only in a few cases, at ca. 10 ka. However, the 5 ka-old sedimentary units are mainly silty, suggesting slackwater flood deposits, with charcoal and the presence of Melanopsis and Unios which indicate still, shallow waters. Remnants of pedogenic phases and Neolithic tools are found throughout this middle Holocene unit. Dated deposits are scarce between 10 and 6 ka, a fact that suggests an episode of deep vertical incision by river channels.
10 + 1,5
The Holocene terrace at the benchmark 50, Wadi Noun, after J. Mathieu et al. (2004), modified.
Relations between Holocene and Soltanian deposits at Wadi Noun. 1: Cambrian; 2: pebbles; 3: sand; 4: silt; 5: sandy silt; 6: silt with calcareous nodules; 7: silt and clay with organic matter; 8: travertine; 9: Helix and Melanopsis; 10: ostrich egg; 11: charcoal; 12: age, in yr B.P.; 13: main incisional scars, respectively : A: late Soltanian; B: young Holocene; C: late Holocene; D: present.
Upper Pleistocene terraces
Fort Oued Noun x= 29° 05' 57'' y= 10° 20' 25'' z= 52 m
The Upper Pleistocene terraces (the so-called Soltanian terraces) exhibit more complicated assemblages. Generally, they comprise of coarse basal pebbly units, older than ca. 50 ka, and upper sandy and silty units; various calcrete occurrences are well developed throughout the profiles and even at the surface. A distinctive feature is the ubiquity of a thick upper unit which was deposited between ca. 25 and 17 ka, with colluvial sequences of mostly fine material in some cases interbedded with aeolian sands.The middle silty units are thick only in the southernmost valleys (Wadi Tamdroust, Wadi Noun and Wadi Tissint). They were deposited as repeated sequences of slackwater flood deposits and decantation sediments into shallow, still waters, between ca. 50 and 30 ka. Bones of large mammals and Middle Paleolithic tools are found throughout. These middle units correspond to repeated flood deposits causing temporary palustrine and (or) lacustrine conditions on the Saharan fringe, and perhaps to more frequent incursions of weather systems from the south. On the other hand, the upper colluvium indicates the progressive return of an arid climate across the whole area.
25
Unités et sous-unités
RAA D DU A HA M A D 0
Couches
O.
b 36 a
14
OSL
C
U/Th
schistes anguleux 3 limons sableux rouges
35
limons brun-rouge à Helix et Melanopsis
34
limons brun-rouge à nodules
33
travertins sur sables et galets
32
30
limons brun-rouge travertins sur galets et sables avec charbons de bois limons et sables roses
29
barre de petits galets; matrice rare
28
limons fins brun-rouge
27
limons bruns indurés
26
limons sableux brun-rouge clair à Helix et Melanopsis
25 24 23 22 21
croûte limons croûte limons brun-clair petits galets
20
limons brun-rouge encroûtés
2c
31
20
Datations (ka) B. P.
Description lithologique sommaire
15
24
18,5
30,9 + 2,5
21,6 + 4,3 24,4 + 1,5
20,4 27,8 + 0,5 27,9 + 2,4 30,9 + 2,7
28 + 0,5
24 39 + 3,2
petits galets
19
Epaisseur en mètres
limons rouges à structure lamellaire limons indurés
18
limons brun-rouge
17
argile, halite et gypse
16
10
8
6
2b
15
limons brun-jaune
14
limons fins brun-jaune à nodules
13
travertin
12
limons fins bruns à Helix et Melanopsis cristaux de halite et gypse
11
limons sableux roses à nodules et manchons racinaires
10
limons sableux roses à cristaux de gypse
9
limons fins jaunes à Helix
5 6 5
2a
The Soltanian terrace at «the Pylône» (Wadi Tamri) : a 18 m-thick upward sequence of alluvial, colluvial and aeolian deposits. (picture by A. Weisrock, 2003)
+ 9,1 92,2 / 87,6 - 8,5 isochrone
limons sableux jaunes à nodules et manchons racinaires
3
travertins sur galets et sables
2
limons fins roses durcis 1
49,1 + 4,3
2 séries de travertins sur sables et galets
4
2
1
36,2 + 3,2
graviers de schistes anguleux sur petits galets limons sableux jaunes à manchons racinaires, Helix et Melanopsis
7- 8
4
limons fins argileux brun-rouge à nodules carbonatés
+9,3 50,9 +10,8 - 9,8 55,9 - 8,6
conglomérat cimenté
0
The Soltanian infilling of Wadi Noun at Fort Oued Noun : 25 m thick slackwater flood silts, now dissected (picture by A. Weisrock, 1998).
The Soltanian terrace at Fort Oued Noun, after A. Weisrock et al., 2006, modified. See also fig. 3D. 1: Middle Palaeolithic tools.
Complex fluvial changes H0 H1
événements de Heinrich
As shown by the Wadi Noun example, fluvial metamorphosis in southwestern Morocco during the late Pleistocene and Holocene was not governed directly by eustatic fluctuations because the main aggradational phases (50–30 ka) occurred during a glacial regression. Conversely, severe linear incision occurred during the late Holocene marine transgression, with a wave of headward erosion propagating inland from the new base level. The middle Holocene terrace can only be of eustatic origin in the estuaries. The main reasons for the changes in fluvial sedimentation patterns in this dryland environment are subtle changes in climate that have unequally affected the different reaches of the catchments.
H2
H3
H4
H5
H6
1 Age ka
2 3
10
20
30
40
50
60
70
80
90
100
C 14
4
(cote 102*)
5 6
OSL
7 U / Th
8 9
?
?
événements
10
?
11
Dated alluvial units of Wadi Noun and correlative morphological events, after Weisrock et al., 2006, simplified. Dating control and morphological environments. 1: OSL, on incised channel deposit; 2: OSL, on slackwaterflood deposit ; 3: U/Th, on built stromatolitic travertine; 4: U/Th, on travertine in channel; 5: 14C, on charcoal in incised channel; 6: 14C on shell or ostrich egg, in carbonated deposit; 7: 14C, on shell in slackwater flood deposit. Deposits and landforms : 8: fan deposits; 9: aeolian supply; 10: main incision; 11: minor incision; 12: alluvial deposit; 13: travertine deposit.
12 13
T
numéro de la couche stade isotopique
35 T 33
1
2
31 31 31 31 (35)
14 28
6 3 (?) T
3
3 (?) T
4 (?)
4
3-5 T
5a
5b
5 T
