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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