Importance of the Mellala section (Traras Mountains, northwestern Algeria) for the correlation of the Pliensbachian-Toarcian boundary

Abstract

In the Mellala profile, located in the Traras Mountains (Tlemcen Domain, NW Algeria), the hemipelagic
Bayada Fm. includes the transition from the Pliensbachian to the Toarcian within a continuous succession of alternating marls and marly limestones, whose thickness reaches approximatively 50 m.
The Bayada Fm. begins in the Upper Domerian. The Solare Subzone of the Emaciatum Zone is proved
by the occurence of several Pleuroceras solare (Phillips) in the lower part, associated with Emaciaticeras upwards.
The Elisa Subzone is characterized by Tauromeniceras elisa (Fucini), Canavaria finitima (Fucini), Paltarpites
bettonii (Fucini) associated with numerous Phymatothyris kerkyreae (Renz) and rare Lobothyris punctata
(Sowerby). Leioceratoides gr. serotinus (Bettoni) has been found in the upper part of the subzone.
The base of the lowermost Toarcian (Mirabile Horizon, Paltus Subzone) is marked by a decimetric bed (no. 38) with Paltarpites paltus (Buckman) but which has not yielded Eodactylites in the present state of the research.
The following calcareous bed (no. 40) has yielded several Dactylioceras (Eodactylites) polymorphum (Fucini), D. (E.) mirabile (Fucini) and D. (E.) pseudocommune (Fucini). Upwards, Eodactylites is abundant over 10 to12 m, especially in Bed no. 44. At 3.50 m below the top, a marker-bed corresponds to a lenticular level of bioclastic quartz-rich limestones that marks the limit between the two members of the formation. The overlying 15 m are attributed to the Semicelatum Subzone (Tethyan nomenclature). At the base a level with D. (Orthodactylites) crosbeyi (Simpson) allows a good correlation with the Clevelandicum Subzone (or Horizon) of Northwestern Europe. The topmost 5 m of the outcrop are dated to the Levisoni Zone with Eleganticeras sp. The anoxic event of the beginning of this Zone is indicated only by an abnormal variability of the foraminifera. The presence of Lenticulina obonensis Ruget indicates stressing conditions.
The occurrence, from the Upper Domerian to the Polymorphum Zone, of a fauna comprising the brachiopod
Koninckella is important because it confirms that the environment was restricted within a deep, strongly subsiding basin (the “umbilicus”).