Resumen de Les series du Toarcien-Aalenien du SW du Moyen-Atlas (Maroc); precisions stratigraphiques et signification paleogeographique
Hassan El Arabi, Benali Ouahhabi, André Charriere
Toarcian and Aalenian deposits from south west of the Middle Atlas named "Couches du Seloum", were classically considered as a "border facies" of the Middle Atlas trough, which was extensively developed in the northern part of the folded Middle Atlas. Both studies realized on the border of the Bekrit-Timahdite syncline and the Iguer Awragh-Afennourir syncline, allow to specify the stratigraphy, the spatial distribution of deposits and to reconsider the paleogeographic organization of this "border" in the integral context of the northern Middle Atlas trough during the Toarcian and the lower Bajocian interval. A lithostratigraphic succession of the "Couches du Selloum" raised in the western part of Bekrit-Timahdite syncline shows the superposition of three units. Unit 1 is made of clay, reddish and gray marl, locally gypseous, assigned to the lower Toarcian. Unit 2 is essentially composed of a subnodular calcareous series, which consists of discontinuous, wavy and upward-thickening beds. The upper offshore is the most frequent environment. It is mainly made of crinoidal calcarenites, yielding some ammonites and brachiopods rich levels. The Middle Toarcian (Bifrons zone and Gradata zone) was identified in the lower part of this unit. Most of the upper Toarcian zones were distinguished (Speciosum zone, Meneghinii zone and Aalensis zone), they are represented in the upper part by brachiopods (Stroudithyris) and ammonites (Hammatoceras, Dumortieria and Catulloceras) rich set. On several lateral sections, the uppermost part of these pseudo nodular limestones is dated from Opalinum zone (levels with Leioceras and Tmetoceras). Unit 3 shows several facies: bioclastic calcarenites arranged in decimetrics beds, showing cross stratifications; microsparitic limestones in thick beds with silexites. The macrofauna is scarce, often represented by some crinoidal and mollusks fragments; some Zoophycos appear in the top bed of the set. The following deposits illustrate the evolution from the shoreface to the upper offshore. This unit is overlain by a sedimentary discontinuity, materialized by a boring surface, some pockets of accumulations with belemnites and a cover of ferruginous nodules. Ammonites drowned on the discontinuity surface or in the basis of the first interbedded marls which overlies this surface sign the Aalenian-Bajocian transition (top of the Concavum zone and the basis of Discites zone). Immediately above we found some index fauna of the Laeviuscula zone. The series overlaying the "Couches du Selloum" comprises thinning up alternations of limestone/marl, passing progressively to the "Marnes de Boulmane" Formation. Lateral variations and interpretations; SW to NE variations: The thickness and facies variations observed in the Bekrit-Timahdite syncline (C1 to C9), denote the existence of marine intervals generally more dilated, with some marly deposits developed towards the SW; on the contrary towards the NE, series are exclusively calcareous, reduced in thickness, and often condensed in their upper part. The same evolution is observed between the deposits of the Iguer Awragh syncline to the SW and those of Afennourir in the NE [Benshili et al., 1999]. SE to NW variations: Some observations realized in a Toarcian-Aalenian outcrop recently discovered in the sector of Outghalline (C11), shows that an uplifted area separates the marly basin of Bekrit from the Iguer Awragh one. Such variations denote a partition of the Toarcian Selloum basin, located in south west of the Middle Atlas trough, into several entities which are regrouped under the denomination of "Blocks of Selloum"; two NE-SW trending notably subsiding gutters are recognized, the Iguer Awragh-Afennourir and the Bekrit-Timahdite, separated by a shoal (ridge of Ijadrane). Paleogeographic framework of the SW border of the Middle Atlas trough: A synthesis of the geometry of the "Blocks of Selloum" and some paleogeographic and structural features of the NW border of the Middle Atlas trough, as they appear in a recent synthesis [Benshili, 1989; Elmi et al., 1989; Benshili et Elmi, 1994; Charriere, 1990; Fedan, 1993; Laadila, 1996; Sabaoui, 1998; El Arabi et al., 1999], is shown in 3 episodes from Toarcian to Aalenian. The southwestern part of the Middle Atlas studied here, does not appear in any case as a coastal area surrounding the Middle Atlas trough, but like a gulf occupied by marine deposits which is isolated from the Middle Atlas trough by the shoal areas called the "Haut-fond de Boulmane". In the Middle Atlas trough, the sequences prograde in a NE direction and show blocks tilting generally towards the same direction. On the other hand, in the "Blocks of Selloum" the tilting acts toward the SW, against the direction of the global Tethyan polarity.
