Faune corallienne des récifs toarciens du Moyen Atlas marocain, première approche

• Autores: Bernard Lathuilière
• Localización: Bulletin de la Société Géologique de France, ISSN 0037-9409, Vol. 182, Nº. 6, 2011, págs. 533-544
• Idioma: francés
• Títulos paralelos:
  • Corallian fauna of Toarcian reefs of the Middle Atlas in Morocco, first approach
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• Resumen
  • English

    The Liassic is generally and rightly considered to be a time period in the history of the Earth when reefal activity was restricted in comparison to the intervals that preceded and followed it [Kiessling et al., 1999]. Extinctions at the end of the Rhaetian and at the beginning of the Toarcian are largely responsible for this [Kiessling et al., 2007; Lathuilière and Marchal, 2005; 2009]. The Toarcian stage, in particular, is a time when coral reef-building was extremely limited and the little that we know is clearly insufficient [Beauvais, 1986]. The reefs described in the Moroccan Middle Atlas by Elmi et al. [2002] open a window into the Toarcian coral fauna, still poorly known throughout the entire planet. They were described in the Upper Toarcian of the Middle Atlas of Morocco. They are located in the Awragh-Afennourir syncline (western Middle Atlas, just at south of the tabular Middle Atlas, fig. 1). They consist of bioherms, 1 to 12 m thick, placed on top of tilted blocks (fig. 2). These reefs lie on a formation consisting of yellow bioclastic limestones, dated as Levisoni Zone Gemma Subzone, and are covered by a terebratulid bed, the bottom of which is assigned to the Upper part of the Meneghinii Zone [Elmi et al. 2002]. These reefs occurred at a paleolatitude of 18°N [Besse and Courtillot, 1991].

    A set of 18 coral samples collected by S. Elmi is identified and analyzed in this paper. The material was sampled from several reefs. In most cases the specimens are free of debris. I did not have the opportunity to access the outcrops myself; the samples were entrusted to me by S. Elmi. His approach was to sample taxa representative of the diversity of these reefs. Consequently, I had at my disposal only a few specimens per taxon and therefore open nomenclature is often preferred, in order to avoid the proliferation of specific taxa before having a clearer understanding of intraspecific variability. The material is deposited in the collections of the University H. Poincaré in Nancy (UHP).

    The taxa belong to the genera Cladophyllia, Diplocoenia?, Montlivaltia (two species), Thecosmilia (two species), Isastrea?, Latomeandra, Periseris, Microphyllia (three species) and Proleptophyllia (two species).

    Comparisons are performed with the only general monograph on Liassic corals from Morocco [Beauvais, 1986]. Among the 82 taxa described by this author, only 19 taxa are identified in the "Upper Liasssic" from South Rifian ridges or from the High Atlas. The fauna described here is derived completely from the Middle Atlas. Only three of 15 genera are in common between the two studies : Montlivaltia, Isastrea and Microphyllia. Some differences may be explained by particular ecologies (e.g. Ellipsoidastraea, Cardiastraea), but these differences must not be overinterpreted because the number of specimens is low in both studies.

    The samples consist of 12 colonies and 7 solitary forms. They include massive and meandroid forms having a high level of colonial integration and providing a strong argument for the presence of symbiotic zooxanthellae [Coates and Jackson, on 1987; Coates and Oliver, on 1973]. Accordingly, these observations indicate the location of these constructions in the photic zone. Subject to a sedimentological field analysis, mud-supported matrices associated with corals suggest environments not exposed to fair weather waves. Bioerosion is moderate (Gastrochaenolites, Entobia and worm-like traces of possible sipunculids).

    A sample including branching corals shows microbial crusts, with abundant sponge spicules. We cannot exclude a temporal shift between coral building and microbial settlement [Olivier et al. 2006]. Nevertheless, one observation worthy of generalization, involves the frequent development of pennular morphologies in corals, which suggests a rather mesotrophic reefal ecosystem [Dupraz and Strasser, 2002; Gill et al., 2004; Olivier et al., 2004].

    Based on a recent compilation of the stratigraphic distributions of genera [Lathuilière and Marchal, 2009], the present study substantially modifies our knowledge of the distribution for some genera. The first appearances of the genera Cladophyllia, Latomeandra and Periseris are now older, perhaps also for Diplocoenia. Conversely, the last appearance of the genus Proleptophyllia, previously known as Pliensbachian, is younger.

    In this initial examination, the coral reef fauna from the Toarcian of Middle Atlas seems rather diverse (9 genera identified for 18 specimens). These reefs with zooxanthellate corals lived in optimal conditions with regard to the latitude and the depth. This fauna has more features in common with the faunae of Dogger than with that of Pliensbachian. A more complete sampling is obviously needed to improve the taxonomic precision, describe the structure of the populations better and define more precisely the paleoenvironment of these reefs, which remain an exception as much temporal as geographical.

  • français

    Des récifs ont été décrits dans le Toarcien supérieur du Moyen Atlas Marocain [Elmi et al., 2002]. Ils sont localisés dans le synclinal de Awragh-Afennourir (Moyen Atlas occidental) immédiatement au sud du Moyen Atlas tabulaire. Ce sont des biohermes placés au sommet de blocs basculés. Leur épaisseur varie de 1 à 12 m. Ils reposent sur une formation datée de la zone à Levisoni, sous zone à Gemma et sont couverts par des sédiments appartenant à la partie supérieure de la zone à Meneghini. Leur paléolatitude était de 18°N. Un ensemble de 18 échantillons de coraux collectés par S. Elmi sont ici analysés. Ils constituent un témoignage exceptionnel de faune corallienne récifale pour cet étage. Les taxons appartenant aux genres Cladophyllia, Diplocoenia ?, Montlivaltia (deux espèces), Thecosmilia (deux espèces), Isastrea ?, Latomeandra, Periseris, Microphyllia (trois espèces), et Proleptophyllia (deux espèces), ont été identifiés. La faune inclut 12 formes coloniales et 7 solitaires. Parmi les spécimens coloniaux, les formes méandroïdes avec un haut niveau d'intégration colonial sont bien représentées et fournissent un argument fort pour la présence de zooxanthelles symbiotiques. L'examen des textures micritiques et des croûtes microbialitiques, associé avec le développement important des microarchitectures pennulaires suggèrent un récif sous le niveau d'action des vagues de beau temps et des eaux plutôt mésotrophiques. Les coraux, leur forme et leur diversité, permettent de diagnostiquer des récifs de zone photique, à une profondeur et une latitude optimale. Les extensions stratigraphiques de plusieurs genres sont agrandies : nouvelle première occurrence pour Cladophyllia, Latomeandra et Periseris (peut être aussi Diplocoenia) et nouvelle dernière occurrence pour Proleptophyllia classiquement considéré comme Pliensbachien. La seule monographie disponible à caractère général sur les coraux du Lias du Maroc [Beauvais, 1986] identifie 15 taxons génériques pour le Toarcien. Tous proviennent du Haut Atlas et seulement 3 taxons sont communs avec les résultats présentés ici. La vue générale est celle d'une faune qui ressemble davantage à celle des communautés récifales du Dogger qu'à celle du Pliensbachien.