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Ammonites from Lias-Dogger of n’Zala (Central High Atlas, Morocco)

El Hariri K., Henriques M., Chafiki D., Duarte L., Ibouh H.

Volumina Jurassica

2006

Vol. 4, no. 1

170-171

This work aims at presenting the n’Zala section as a reference section characterizing the Lias/Dogger interval in the Moroccan Central High Atlas, both from the biostratigraphic point of view and the knowledge of some Ammonitina associations (recorded there). The n’Zala section is situated on the northern side of Jbel Aouja, near n’Zala Village, about thirty kilometers north of the village at the main road n° 21. The marly-limestone succession partly corresponds to the lower part of the Agoudim Formation. The studied interval covers about 60 metres and its major part consists of decimetric to metric thick alternation of micritic limestones and fissile marls. A study of 250 specimens belonging to over 30 taxa of Ammonitina, Lytoceratina and Phylloceratina makes it possible to recognize biostratigraphic units characterizing the Lias-Dogger interval at n’Zala, particularly the Meneghini Biozone (top), the Aalensis Biozone, the Opalinum Biozone (Opalinum and Comptum subzones) and the base of Murchisonae Biozone (Haugi Subzone). The Meneghini Biozone (about 15 m thick) at the base of the section is characterized by abundance of Catulloceras sp. associated with some Vacekia sp. and some Cotteswoldia sp. One can also notice the presence of Hammatoceratinae (particularly notable Hammatoceras aff insigne) associated with Lytoceratina and Phylloceratina. At the base of the Aalensis Biozone (about 12 m thick), we did not record Pleydellia mactra, the species index of the Mactra Biozone, but an abundance of Tmetoceras scissum, associated with Vacekia sp. along with Lytoceratina and P. aalensis, which appear in the lowest levels of the zone. Below, we found representatives of the species index of the Aalensis Subzone, Pleydellia aalensis, associated with Tmetoceras scissum and Vacekia sp. but without P. mactra. Note that Lytoceratina and Phylloceratina are also present there. The base of the Opalinum Biozone is characterized by the first appearance of forms belonging to the Cypholioceras – Leioceras dimorphic couple. The Opalinum Subzone (about 10 m thick) is defined by striated forms of Cypholioceras sp. along with Leioceras sp., Vacekia sp., Phylloceratina and Lytoceratina. The base of the Comptum Subzone (about 8 m thick) is marked by the presence of the costated representatives of Leioceras (Leioceras cf. comptum, Leioceras costusum), which are associated with Tmetoceras (Tmetoceras scissum and Tmetoceras sp.) along with Planammatoceras, Hammatoceras and Lytoceratina. The base of Murchisonae Biozone is defined by the first appearance of Graphoceratidae (Ancolioceras opalinoides, Ludwigia aff. haugi) characterizing the Haugi Subzone (about 30 m thick). The Tmetoceras sp. and Vacekia sp. persist in the biozone along with the Hammatoceratidae, the Phylloceratina and Lytoceratina. In the n’Zala section, the ammonite associations of the Lias-Dogger interval show marked biostratigraphic and paleogeographic changes. In the Aalensis Biozone, the ammonite associations show a clear Tethyan affinities as indicated by common occurrence of taxa belonging to Hammatocerataceae, Lytoceratina and Phylloceratina. The Opalinum and Murchisonae biozones show rather NW European influences, with a predominance of representatives of Leioceratinae and Graphoceratina consequently allowing the recognition of biostratigraphic units at the level of the subzones.