Part 1 Extended Summaries
The Precise Age of the Fezouata Lagerstätte, Lower Ordovician, Morocco
1 Faculté Pluridisciplinaire de Nador, Labo OLMAN-RL, FPN 300, Selouane 67200, Nador, Morocco;akodadmfpn@hotmail.fr;
2 Département de Géologie, Faculté des Sciences et Techniques, Université Cadi-Ayyad, BP 549, 40000 Marrakesh, Morocco; elhariri@fstg-marrakech.ac.ma;
3 Instituto de Geociencias CSIC, UCM and Departamento de Paleontología, Facultad de Ciencias Geológicas, José Antonio Novais 12, E-28040 Madrid, Spain; jcgrapto@ucm.es;
4 UMR CNRS 5276 LGLTPE, Géode, Université Lyon 1, 2 rue Dubois, F-69622 Villeurbanne, France;bertrand.lefebvre@univ-lyon1.fr, emmanuel.martin@univ-lyon1.fr;
5 Key Laboratory of Economic Stratigraphy and Palaeogeography (NIGP, CAS), Nanjing, China; oliver.lehnert@fau.de;
6 GeoZentrum Nordbayern, Lithosphere Dynamics, Friedrich-Alexander University of Erlangen-N甃rnberg, Schloßgarten 5, D-91054 Erlangen, Germany;
7 Univ. Lille, CNRS, UMR 8198 - Evo-Eco-Paleo, F-59000 Lille, France;hendrik-nowak@web.de, thomas.servais@univ-lille1.fr;
8 Naturmuseum S甃dtirol, Bindergasse 1, I-39100 Bozen, Italy.
1 Introduction
The Fezouata Lagerstätte,discovered in the Lower Ordovician of Morocco,is a Konservat-Lagerstätte of prime scientific importance (e.g. Van Roy et al., 2015; Lefebvre et al., 2016a). It provides access not only to the ‘shelly' (skeletonised) part of fossil assemblages, but also to non-biomineralised to lightly sclerotised organisms and to exceptionally preserved soft tissues of a complex ecosystem, mixing typical faunal elements of both the‘Cambrian Explosion' and the ‘Great Ordovician Biodiversification Event' (GOBE). The Fezouata Lagerstätte is present at different intervals in the Fezouata Shale, a formation ranging from the lower Tremadocian to the upper Floian (Lower Ordovician) (e.g. Martin et al., 2016a; Fig. 1).
Recent papers revised the history of research of the Fezouata Lagerstätte(Lefebvre et al., 2016a, b), the biostratigraphy of different fossil groups, including graptolites (Gutiérrez-Marco & Martin, 2016), trilobites (Martin et al., 2016b), palynomorphs, including acritarchs and chitinozoans (Nowak et al., 2016), and conodonts (Lehnert et al., 2016), and summarized the biostratigraphic constraints to recalibrate the age of the Lagerstätte(Fig.1;Lefebvre et al.,in press).
The aim of the present work is to provide precise age information for the Fezouata Lagerstätte, by using the most recent biostratigraphical results based on several fossil groups.
Figure 1 Precise stratigraphical position of the lower interval with exceptional preservation of the Fezouata Lagerstätte(Central Anti-Atlas,Morocco).Correlation chart is based on Lower Ordovician regional biostratigraphic scales, and established on Moroccan graptolites (high-latitude Gondwana;Gutiérrez-Marco & Martin, 2016), British acritarchs (Avalonia; Molyneux et al., 2007), and conodonts from both NW Argentina (Gondwana; Albanesi et al., 2011) and Baltoscandia (Baltica; Webby et al., 2004). For each biostratigraphic scale, the estimated position of the lower interval with exceptional preservation of the Fezouata Shale is indicated in grey. Stage slices are based on Bergström et al. (2009).The figure is adopted from Lefebvre et al.(in press).
2 Lithostratigraphy
Until recently, the distribution of exceptionally preserved fossils in the Fezouata Shale was unclear, due to the very extensive surface of outcrops, the monotonous lithology of the Fezouata Shale (mostly siltstones) and the extremely patchy distribution of productive sites, which are scattered across the Ternata plain, north of Zagora. Over the course of several field campaigns in recent years, a synthetic stratigraphic column was produced, and fossiliferous levels yielding exceptionally preserved fossils were plotted along it. It can now be concluded that exceptional preservation is restricted to a few, thin, discontinuous, lens-shaped horizons occurring in two distinct parts of the Fezouata Shale: a lower interval (260-330 m above the base of the Formation in the Zagora area) and an upper interval (570-620 m) (Lefebvre et al., 2016a, b).
3 Biostratigraphy
The lower interval (260-330 m) with exceptional preservation is now accurately dated with graptolites as belonging to the Araneograptus murrayi graptolite Biozone, with only a few levels with exceptional preservation extending into the base of the overlying Hunnegraptus copiosus Zone(Tr3,late Tremadocian)(Gutiérrez-Marco&Martin,2016). Acritarchs confirm this age assignment, with the presence of all diagnostic elements of the messaoudensis-trifidum acritarch assemblage(Nowak et al.,2016).The acritarchs found in the lower interval with exceptional preservation are typical of sub-assemblages 1 and 2 of the messaoudensis-trifidum acritarch assemblage(Molyneux et al.,2007).International correlation with other localities of the Gondwanan margin allows to attribute these levels to the late Tremadocian (Tr3), confirming perfectly the graptolite data. Chitinozoans (Nowak et al., 2016) and conodonts (Lehnert et al., 2016) from these levels also point to a late Tremadocian to (early) Floian age, but international correlations for these intervals are not yet perfectly accurate, although correlation with conodont assemblages from Argentina also supports the attribution to the late Tremadocian.
A middle Floian age can be attributed to the upper interval (570-620 m) with exceptional preservation based on graptolites, because the 570-620 m interval, which comprises the upper interval with exceptional preservation, can be assigned to the middle Floian(Fl2)? Baltograptus jacksoni graptolite Biozone(Gutiérrez-Marco&Martin,2016).
4 Conclusion
Integrated biostratigraphical studies, essentially based on investigations of graptolites, acritarchs and conodonts, indicate that the lower interval with exceptional preservation in the Fezouata Shale can be attributed to the late Tremadocian, whereas the upper interval corresponds to the middle Floian.
Acknowledgements This work was supported by the ANR (Agence Nationale de la Recherche, France; grant number ANR-11-BS56-0025), the INSU (Institut National des Sciences de l'Univers, France, CNRS), the Spanish Project CGL2012-39471/BTE, the French Ministère de l'Enseignement Supérieur et de la Recherche, the Hauts de France Region and the European Funds for Regional Economical Development. The authors are particularly grateful to Jacques Destombes, Bernard Pittet, Thijs Vandenbroucke, Peter Van Roy and Romain Vaucher for providing support information on the localities, biostratigraphy and environmental interpretations. This is a contribution to the IGCP Project 653, the RALI (Rise of Animal LIfe) Project, the CPER research project CLIMIBIO and the TelluS-SYSTER project ‘Vers de nouvelles découvertes de gisements à préservation exceptionnelle dans l'Ordovicien du Maroc'.
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