Paper Number: 4958
The Hirnantia fauna and the stratigraphic assessment of the uppermost Ordovician in the central Anti-Atlas (Morocco)
Villas, E.1, Colmenar, J.2, Gutiérrez-Marco, J.C.3, García-Bellido, D.C.4 , Lorenzo, S.5, Pereira, S.6,7,8 and Álvaro, J.J. 3
1 Associate Professor, University of Zaragoza, Faculty of Sciences, Zaragoza, Spain, villas@unizar.es.
2 Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark.
3 Research Scientist, Spanish Research Council, Institute of Geosciences CSIC-UCM, Madrid, Spain.
4 ARC Future Fellow, University of Adelaide, Faculty of Sciences, Adelaide, Australia.
5 Associate Professor, University of Castilla-La Mancha, Faculty of Mines, Almadén, C. Real, Spain.
6 Departamento de Geologia and Instituto Dom Luiz, Universidade de Lisboa, Lisboa, Portugal.
7 Departamento de Geologia, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal.
8 Centro de Geociências, Universidade de Coimbra, Coimbra, Portugal.
The pandemic brachiopod Hirnantia fauna characterizes the lower part of the Hirnantian (Hi1 Stage Slice of the uppermost Ordovician Global Stage). In the Moroccan Anti-Atlas, its occurrence in glacially-related successions allowed contrasting the onset of several Late Ordovician glacial events. In the Central Anti-Atlas, the Hirnantian is lithostratigraphically represented by the Second Bani Group, a sandstone-dominated succession. In the region immediately west of Tagounite (in the vicinity of Zagora), the group is subdivided into two formations, the basal Lower Second Bani Formation and the overlying Upper Second Bani Formation, separated by a major erosive (glaciogenic) unconformity. The Lower Second Bani Formation is composed of two members, the lower mostly made of clayey sandstones and the upper consisting of thick-bedded quartzitic sandstones (Fig. 1).
Figure 1: Aït-Isioul section, west of Tagounite (Morocco). Upper Ordovician formations with the situation of the sampled levels in the Lower Second Bani Formation.
We have revisited the stratotype of the Lower Formation of the Second Bani Group, west of Tagounite, and sampled brachiopods from its uppermost quartzitic horizons (from Destombes’ horizon F upsection). In addition to those brachiopods listed by Destombes [1] from the region, we have found Plectothyrella sp. and Kinnella sp. Both genera are exclusive of the pandemic Hirnantia fauna, which allows confirmation of a Hirnantian age for the upper member of the Lower Second Bani Fm. By contrast, the lower member of the Lower Second Bani (concretely, Destombes’ G bryozoan limestone horizon, rich in the echinoderms Maghrebocystis and Herpetocystis, and horizons slightly above it), has yielded Katian brachiopods and trilobites, such as Eostropheodonta intermedia, Destombesium sp., Hirnantia sp. (species different than H. sagittifera), Actinopeltis aff. insocialis, Mucronaspis termieri and Cekovia aff. loredensis. Thus, following the present-day lithostratigraphic framework, in the vicinity of Tagounite the Katian/Hirnantian boundary lies in the Lower Formation of the Bani Group and lithostratigraphically correlative horizons, such as those reported in Bou Ingarf [2]. The latter were there mainly dated as a result of lithostratigraphic and sequential correlations and, based on correlation with the Tagounite area, are in need of revision. The age of the lower part of the chitinozoan Tanuchinita elongata Biozone should be re-evaluated, and the suggested delayed onset of the Hirnantian glaciation [2] reconsidered.
This is a contribution to the project CGL2012-39471/BTE of the Spanish MINECO.
References:
[1] Destombes J et al. (1985) In: Lower Palaeozoic of North-western and West Central Africa, 91-336.
[2] Loi A et al. (2010) Palaeogeo Palaeoclim Palaeoecol 296: 332-358.