The Upper Cenomanian–Lower Turonian of the Preafrican Trough, Morocco
Highlights
► Environments range from the peritidal zone and inner ramp to the deep open platform. ► OAE2, is well recorded by δ13C excursions even in shallow water. ► Four depositional sequences during Upper Cenomanian–Lower Turonian interval. ► Dysoxic conditions prevailed into the Early Turonian, after the OAE2. ► The major drowning phase is recorded at the Cenomanian–Turonian boundary.
Introduction
The shallow carbonate platform of the Preafrican Trough develops in the Upper Cenomanian–Lower Turonian, and forms part of the Saharan platform. It is located in an area subject to Tethyan influences, recorded in Tunisia (Caron et al., 2006, Faouzi Zagrarni et al., 2008; Jati, 2007) and Algeria (Grosheny et al., 2008; Herkat, 1999, Herkat, 2004, Jati, 2007), as well as Atlantic influences, which are expressed in the Agadir Basin (Andreu, 1989, Andreu, 1991, Gertsch et al., 2010; Jati, 2007) and the Essaouira Basin (Ettachfini, 1993).
Previous studies mainly concerned the paleontology and/or biostratigraphy of the Preafrican Trough (Cavin et al., 2010, Ettachfini, 2006, Ettachfini and Andreu, 2004, Ferrandini, 1988, Ferrandini et al., 1983, Kennedy et al., 2008, Meister and Rhalmi, 2002, Rhalmi, 2000). The establishment, development and disappearance of the platform remain poorly known. Its development takes place during the OAE2, an important event in Earth history causing major palaeoenvironmental and palaeocological disturbances (see references on Gertsch et al., 2010), generally well recorded in the ocean basins owing to the deposition and preservation of black shales, but poorly expressed on the platforms.
The present study aims to define the main stages of platform development, to determine the record of the OAE2 in the sedimentation, and to identify the sedimentary disturbances in relation with this event.
Thus, the planktonic foraminifera and ammonite distribution are studied here to establish the biostratigraphic framework. Based on sedimentary facies and palaeocological analysis, we propose an interpretation of the depositional environments, while measurements of δ13C indicate the shallow-water response to global organic productivity. Sequence stratigraphy analysis is applied to subdivide the succession into depositional sequences reflecting sea-level fluctuations. Lastly, correlations between the studied sections in the Preafrican Trough allow us to define the morphology of the platform during the main stages of its development.
Section snippets
Geographical and geological setting
The Preafrican Trough extends over the provinces of Souss Massa-Drâa and Meknes Tafilalet, in southern Morocco. It forms a wide triangular plain between the Moroccan High Atlas and Anti-Atlas mountain range systems (Fig. 1).
The Preafrican Trough includes, from west to east, the Souss, Ouarzazate and Errachidia-Boudnib-Erfoud basins (Ambroggi and Choubert, 1952). The south Atlasic fault bounds the Jurassic basins of the High Atlas in the north of the trough, the Precambrian and Paleozoïc
Methods
The material investigated comes from the carbonates, which were systematically sampled at the nine studied sections. The Ziz and Goulmima sections are characterized by a continuous stratigraphic record, and were chosen as reference sections of the Upper Cenomanian–Lower Turonian. They were supplemented with data from the other sections.
Microfossils and microfacies were studied from 700 thin-sections. Generic attributions of the benthic foraminifera are based on Loeblich and Tappan (1988),
Biostratigraphy
The biostratigraphic framework of the succession is mainly based on foraminifera and ammonites. The planktonic foraminifera are identified by M.J.W. and the ammonites by Ch. M. The ammonite zonation is established according to the Southern Europe biostratigraphy (Hardenbol et al., 1998). The biostratigraphic succession proposed by Ettachfini and Andreu (2004) is supplemented.
Conclusion
The biostratigraphy framework of the platform of the Preafrican Trough is based on foraminifera and ammonites. Three planktonic foraminifera zones are recognized: the Rotalipora cushmani Total Range Zone in the Upper Cenomanian, the W. archaeocretacea Partial Range Zone in the Cenomanian–Turonian boundary interval, and the H. helvetica Total Range Zone in the Lower Turonian. Two ammonite zones are recorded: the Guerangeri Zone in the lower Upper Cenomanian, and the Nodosoides Zone in the upper
Acknowledgments
We thank Michael Carpenter and Christiane Cavaré-Hester, respectively, for his correction of the English version and for her assistance in the line drawing of figures. The authors are grateful to Brian Jones and an anonymous reviewer for improving the manuscript with their critical reviews and suggestions.
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