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 word: Sedimentary evolution. Sedimentation control. Western High Atlas. Ordovician. Morocco.

The Western paleozoic High Atlas Ordovician deposits were subject to many works, essentially focus on lithostratigraphy (see references in Cornée, 1989; Cornée et al., 1987; Chacrone, 2000). The first detailed sedimentological study carry out in this domain (Chacrone, 2000) was realised in the framework of a large program, which interest with the whole of moroccan Ordovician successions, in the goal to precise the paleogeographic reconstructions, and the sedimentation control. The studied successions in this work (1– Aït Lahsen; 2– Bas Seksaoua; 3– Adrar Amesnir, 4– Adrar Walma; and 5– Adrar Tigeurni) belong to the structural domain of the Western Paleozoic High Atlas. Their lithostratigraphic framework was established by Roch (1930); Schaer (1981); Destombes et al.  (1985); Cornée et al. (1987); and Cornée (1989).

The facies associations of the studied successions reflects sedimentation in various environments: nearshore environments, delta, and siliciclastic shelves.

The littoral environment is recognized in the Ashgill of Aït Lahsen and Adra Walma. In these areas, the sedimentation exhibites typical tidal facies and stuctures as: mud couplets, neap/spring cycles, reactivation surfaces, tidal bundles, tangential and sigmoidal cross–bedding, and mud drap (see references in Terwindt, 1981; Dalrymple, 1992), as well as flaser–bedding, wavy–bedding, and Skolithos ichnofacies.

Deltaic environment is recognize in the Arenig of Aït Lahsen. This succession consist of coarsenning–upward sequences, displaying front delta and prodelta facies. The Front delta facies consists of alternating decimetric coarse to medium sandstone beds, and centimetric mudstone beds (Battacharya and Walker, 1991). 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. The prodelta deposits are composed of bioturbed mudstones and siltstones. The paleocurrents measurement indicate transport path from the West of the Aït Lahsen area during the Arenig . This sources of apport provened from emerged land, who correspond to the present–day location of the Argana corridor. The deltaic system was 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 Western High Atlas (Chacrone, 2000).

Figure 1. A. place of the the western paleozoic High Atlas in Morocco. B. Place of the studied areas in the western paleozoic High Atlas. Geological map established by Cornée (1989): 1– Aït Lahsen, 2– Bas Seksaoua, 3– Adrar Amesnir, 4– Adrar Walma, and 5– Adrar Tiguerni. C. Correlations between western paleozoic High Atlas outcrops (see legends in Figure1B). Lithology: 1–silstone; 2– interbedded silstone and sandstone; 3– fine to medium grained sandstone; 4– medium grained sandstone; 5– coarse grained sandstone; 6– conglomerates. Age: A: lower to upper Arenig; A/Lv: upper Arénig to Llanvirn; Lv/L: upper Llanvirn to Llandeilo; L/C: upper Llandeilo to Caradoc; C/As: upper Caradoc to Ashgill.

Shelf environment is recognized in all studied areas, it consist of storm dominated shoreface, transition zone, and offshore. The shoreface successions display amalgamated sandstone with hummocky cross–stratifications (Harms, 1975). The transition zone successions are composed of highly bioturbated argillaceous sandstones (Leckie and Krystink, 1989). The offshore successions are characterized by typical facies as: Storm gradded layers (Reineck and Singh, 1972), mudstone with isolated sandy limestone nodules, lenses or lenticular beds with sometimes a basal shelly lag (Guillocheau, 1983; Hamoumi, 1988).

Ordovician sediments of western paleozoic High Atlas successions consist essentially of siliciclastic sediments with minor intercalations of: calcareous sediments and oolitic ironstones.

Siliciclastic sediments were derived from the panafrican belt and its old cover , they consist of: mudstones, siltstones, quartzarenite, poorly sorted arenite, and microconglomeratic mudstones. They are composed of quartz, plagioclase, microcline, rocks fragments (sedimentary, metamorphic and volcanic rocks), muscovite, chlorite, zircon, sphene and iron minerals, scattered within argillaceous matrix or cemented by chlorite, silice or hematite. The quartz grains of the poorly sorted arenite and the microconglomeratic mudstone that characterize especially the Upper Ordovician successions display a wide variety in size and shapes. The largest grains which can reach 4mm are well rounded (eolian) with some conchoidal fractures, the smaller grains (30–50mm) are angular or acicular, but all intermediate forms and sizes exist. These quartz are similar to those recognize by Hamoumi (1988) in Upper Ordovician glacial sediments of Morocco. The mixed siliciclastic–carbonate sediments consists of sandy limestones, silty limestones, calcareous sandstones, and calcareous siltstones. They are composed of siliciclastic phase displaying the same mineralogical composition as the siliciclastic sediments and a calcareous cement. The carbonate phase is related to an intrabasinal source (shell banks). The oolitic ironstones and basal transgressive sand are related to transgressive/regressive cycles (Hamoumi, 1988; Hamoumi, 1996).

The lateral and vertical variations of the deposit sequences (Figure 1C) show that the western Paleozoic High Atlas basin was controled by a differential subsidence, caused by the reactivation of panafrican fracture systems, and induced synsedimentary faults, which structured the basin in horsts and grabens.

The sedimentary evolution is summarised as: The Arenig began with a transgression which induced the shoreface erosion. In this period the sedimentation occurs in a highly subsiding basin, submitted to a synsedimentary tectonic during rise sea level stage; wave dominated delta builded up in the shelf margin, when rate of sedimentation is high. But when the rate of sedimentation is low, repeated reworking by storms currents and waves leads to the formation of erosional surfaces with lag deposits and condensed beds in offshore.

The Llanvirn commenced with a transgressive stage, which the sedimentation occurs in a storm dominated offshore. During this period, the basin was submitted to a subsidence. During the Llandeilo, the sedimentation was controlled by tectonic in the basin. This event is marked by soft sediment horizon, synsedimentary faults, and tectono–eustatism. Their was evolution from storm dominated offshore into storm dominated shoreface.

During Caradoc/Ashgill, sedimentation occurs in storm dominated shoreface and tides dominated nearshore, under the interplay of glacioeustatism, rate of sedimentation, and tectonic.

The sedimentological studies of the western paleozoic High Atlas, during Ordovician, allow to precise facies and sedimentary environments, nature of sediments, sources and sedimentation control. The facies and sedimentary environments recognized are similar to those recorded in Moroccan North Gondwana margin (Hamoumi, 1988). They consists of: tidal dominated littoral; wave and storm dominated delta, tidal dominated delta; wave and storm dominated shelf. The nature of sediments is: siliciclastic, mixted limestone–siliciclastic, and particular deposits (basal transgressive sand, and oolitic ironstones). The siliciclastic sediments are from extrabasinal source: the panafrican belt during Lower and Middle Ordovician and from the Saharian ice sheet during Upper Ordovician. The carbonate phase is related to an intrabasinal source (shell banks). The particular deposits (basal transgressive sand, and oolitic ironstones) are related to transgressive/regressive cycles. 

The sedimentation record all the events recognise in the moroccan North Gondwana shelf. The tectonic events related to the reactivation of panafrican faults (Cornée and Ferrandini, 1985; Chacrone, 2000) induced synsedimentary faults, which structured the basin in horsts and grabens, and tectonoeustatism. The Upper Ordovician glaciation effects is record by glaciomarine sediments and glacioeustatism. The paleocurrents measurement indicate the existence of an emerged land to the West of Morocco during the lower Ordovician. This result is in agreement with the model of Hamoumi (1995) which propose the existence of an emerged lands to the West of Morocco during Ordovician.

Accepted: June 15, 2003