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Importance of the Mellala section (Traras Mountains, NW Algeria) for the correlations of the Pliensbachian/Toarcian boundary

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Elmi S. , Marok A. , Sebane A. , Almeras Y.

tom Vol. 4, no. 1

158-160

The Mellala profile has a great importance as international reference section because it exposes a continuous and very fossiliferous record through the Pliensbachian/Toarcian boundary (Fig. 1). This boundary is situated within the hemipelagic Bayada Formation (alternating marls and marly limestones) yielding Zoophycos and Steinmannia bronni (shells and “filaments”). It has been deposited in a small subbasin (umbilicus) that was strongly subsiding and deepening during the Late Pliensbachian and the Early Toarcian. The sequence evolution is globally stratodecreasing and deepening upward until the middle part of the Early Toarcian (early Levisoni Zone). The Bayada Fm. begins in the Upper Domerian. The Emaciatum Zone is divided in two parts. The lower (Solare Subzone, thickness 8 m) is documented by several Pleuroceras solare (Phillips) occurring alone in the lower part of the subzone, but associated with Emaciaticeras upwards. Brachiopods include Phymatothyris kerkyrae (Renz) and Quadratirhynchia quadrata Buckman. The upper part (Elisa Subzone) is characterized by Tauromeniceras elisa (Fucini), Canavaria finitima (Fucini), Paltarpites bettonii (Fucini) associated with numerous P. kerkyreae (Renz) and rare Lobothyris punctata (Sowerby). The main part of the brachiopod assemblage consists of small sized species: Nannirhynchia pygmoea (Davidson), Koninckella liasina (Davidson) and Cadomella cf. moorei (Davidson). This is the Koninckella fauna (previously named Leptaena fauna). Leioceratoides gr. serotinus (Bettoni) has been found in the upper part of the Elisa Subzone. The base of the lowermost Toarcian (Mirabile Subzone) is marked by a decimetric bed (n° 38) with Paltarpites paltus (Buckman) but without Eodactylites at the present state of the research. The following calcareous bed (n° 40) has yielded several Dactylioceras (Eodactylites) polymorphum (Fucini) and D. (E.) pseudocommune (Fucini). The brachiopods are represented by Liospiriferina subquadrata (Seguenza), Lobothyris sp. and the Koninckella fauna: C. moorei, N. pygmoea and K. liasina. Upwards, the Eodactylites becomes abundant at 10 to 12 m, especially in the bed n° 44. At 3.50 m under the top of the subzone, a marker-bed (48) corresponding to a lenticular level of bioclastic quartziferous limestones occurs. The overlying 15 m are attributed to the Semicelatum Subzone (Tethyan nomenclature). At the base, there occurs a level with D. (Orthodactylites) crosbeyi (Simpson) associated with Lobothyris arcta (Dubar) which allows a good correlation with the Clevelandicum Subzone (or Horizon) of northwestern Europe. The topmost 5 m of the deposits are dated to the Levisoni Zone with Eleganticeras sp. The anoxic event at the beginning of this Zone is only indicated by an abnormal variability of the foraminifera. The presence of Lenticulina obonensis (Cubaynes) mg Planularia indicates stressing conditions.