1 UFR "Oceanology– Geodynamic and Valorisation of Sedimentary basins". Department of Earth Sciences, Faculty of Sciences, Mohammed V–Agdal University BP 1014 RP Rabat, Morocco, E–mail: c.choukri@mailcity.com / hamoumi@fsr.ac.ma

Key words: Deltas. Emerged lands. Ordovician. High Atlas. Morocco.

The studied successions: Aït Lahsen and Imini, belong to the High Atlas of Marrakech (Figure 1A and B). These areas were considered as parts of a wave and storm dominated distal shelf (Destombes, 1963; Destombes, 1971; Cornée et al., 1987; Ounaïmi, 1989).

This paper reports the results of recent sedimentological analysis in the High Atlas of Marrakech, with new interpretations (Chacrone, 2000).

The High Atlas of Marrakech is limited to the East by the corridor of Argana; to the West by the Mesozoïc cover of the central High Atlas; to the North by the plain of Haouz; and to the South by the plain of Souss and Ouarzazate (Figure 1A and B). Studied successions (Figure 1B and D) are : 1– The Aït Lahsen succession, attributed to the Arenig, and located in the N.W of the western High Atlas. This succession is 200m thick, its stratigraphic framwork has been established by Cornée et al. (1987), who recognize the F10C formation. 2– The Imini succession (150m thick), attributed to the Arenig (Destombes, 1963), and located in the central High Atlas, its stratigraphic framwork has been established by Destombes (1963), who recognize the upper Fezouata formation.

The stratonomic analysis of Aït Lahsen (western High Atlas) and Imini (central High Atlas) successions, allows to recognize various facies which refer to deltaic environments. These facies consist mainly of interbedded centimetric mudstones and decimetric fine to medium grained sandstones.The mudstones are highly bioturbated.

Facies F1 (Aït Lahsen succession) and F2 (Imini succession) are about 30 centimetres thick, they correspond to an elementary sequence, characterized by a microconglomeratic lenticular level, with erosive and sparsely bioturbated base, which shows impacts of trailed objects as flute casts and groove casts. The sandy layer exhibits a current cross bedding in facies F1 : this litage corresponds to an unidirectionnal fluvial current (Allen, 1968; Bhattacharya and Walker, 1991), and sigmoïdal laminations in F2. These facies are interpreted as plain delta channels.

Facies F3, 15cm thick, has a scoured base with flute–casts and groove–casts. The internal structure display a ripple cross laminations: this litage is produced by wave action (De Raaf et al., 1977). The top of the sand layer is modelled by an asymmetrical ripples of unidirectionnal current (Allen, 1968). Lateraly, this facies show some synsedimentary faulting. Facies F4, 10cm thick, consist of centimetric sandy beds with an erosive and highly bioturbated base. The internal structure shows an asymetrical ripples cross lamination and convolute bedding in the top correspond to an unidirectionnal fluvial current. Facies F5, 15cm thick, display an hummockey cross stratification, this facies is typical of a storm activity (Harms, 1975; Reineck and Singh, 1980). Facies F6, 20 cm thick, is sparsly bioturbated, and shows an erosive basis with load casts. The internal structure display a lower parallel laminations and a current cross bedding (Allen, 1968; Reineck and Singh, 1980). The summit surface show a polygonal ripples. Lateraly, this facies show some synsedimentary faulting. The facies F3, F4, F5 and F6 consist of alternating decimetric coarse to medium sandstone beds, and centimetric mudstone beds. The sandstone beds display wave influence (ripple cross laminations with opposed directions, asymmetrical ripple cross lamination, and lower parallel laminations) with the interplay of a fluvial unidirectional current as : asymmetrical ripples, current cross bedding , lingoïde ripples, and flute casts on the base (Allen, 1968). The sand layers exhibit also some storm facies as hummocky cross stratification and polygonal ripples (Harms, 1975; Reineck and Singh, 1980). Synsedimentary deformation structures as convolute laminations and synsedimentary faultings are also present. These facies are regarded as front delta deposits in a wave and storm–dominated delta. Facies F7 consists of centimetric sandy layers with an erosive and highly bioturbated base. The internal structure shows unidirectionnal cross–laminae. These layers are alternating with metric and highly bioturbated mudstones. Facies F7 is regarded as a prodelta facies, owing to the fact that it is situated between the front delta facies (F3 to F6) and the distal shelf facies (F8). Facies F8 is constitued by metric highly bioturbed mudstone strata, displaying some sandy–limestone nodules, which indicate a distal storm activity (Reineck and Singh, 1980; Guillocheau, 1983; Hamoumi, 1988). This facies indicate a distal shelf facies. Facies F1 to F8 are organized in coarsening–uppward sequences (Figure 1D).

Figure 1. A. Location of the High Atlas of Marrackech. B. Location of the studied series outcrops in High Atlas of Marrackech: 1- Aït Lahsen; 2- Tizi-n-Tichka; 3- Imini. Map established by Cornée (1989). C. Sedimentary facies: 1- interbed (pelite); 2- climbing ripples; 3- wavy laminae ripple; 4- current cross bedding; 5- conglomerate; 6- sigmoïdal lamination; 7- asymetric current ripple; 8- wavy cross bedding; 9- flute cast; 10- convolute bedding; 11- hummocky cross stratification; 12- planar lamination; 13- siltstone strata; 14- nodules. D. Delta sequences, AL: Aït Lahsen; IM: Imini. ag- Age; ft- formation; lith- lithostratigraphical column; fa- facies; off- offshore; pr- Prodelta; fr- front delta; pl- delta plain; sq- depositional sequence; rsl- relative sea-level (r- rise, f- fall); HST- high stand systems tracts; TST- transgressive systems tracts; SB2- sequence boundaries type 2.

The deltaic environments is confirmed also by the regional cartography and correlations between the sequential deposits in the two studied series, which show an hectometric thick and lenticular sedimentary bodies (Chacrone, 2000).

Applying the sequence stratigraphic concepts defined by Vail et al. (1987), three depositional sequences can be distinguished in Aït Lahsen succession (Figure 1D): Sa1 (80 m thick), Sa2 (50 m thick), Sa3 (70 m thick), and two depositional sequences in Imini succession: Sb1(100 m thick), Sb2 (50 m thick). These sequences are bounded by regionally extended type 2 sequence boundaries (SB2). Sequences are subdivised into two system tracts, from bottom to top: the transgressive systems tracts (TST), where the relative sea level fall, it is composed of offshore deposit (Facies F8) followed by prodelta deposit (Facies F7), and the high stand systems tracts (HST), where the relative sea level rise, it make up front delta deposits (F3 to F6) and plain delta channels (F1 and 2).

The paleocurrent analysis indicates transport from the West in the Aït Lahsen area, and from the SW in the Imini area during the Arenig. The source emerged lands correspond respectivley to the present–day location of the Argana corridor and the Siroua Massif.

The detailed sedimentological analysis of the studied successions shows a wave and storm dominated delta during the Arenig, in the regions of Aït Lahsen and Imini (western and central High Atlas). These deltaic systems are very thick. They were deposited in a greatly subsiding basin. The synsedimentary deformations (convolute laminations and synsedimentary faults) attest the instability of the basin during the lower ordovician in the High Atlas (Chacrone, 2000). The emerged lands during the Arenig, attested by the paleocurrent measurments are in agreement with the model of Hamoumi (1995) which shows the existence of two basins (mesetian basin and atlasic basin) separated by emerged lands in Morocco during the Ordovician.

Accepted: June 15, 2003