Barremian intracontinental rift and Aptian Atlantic transgression in the northern boundary of the Central High Atlas (Morocco)

https://doi.org/10.1016/j.jafrearsci.2020.104094Get rights and content

Highlights

  • Identification of an E-W intracontinental Barremian graben in the Aghbala-Naour area.

  • Evidence of an Aptian paleogeographic S–N zoning from marine to continental deposits.

  • Geodynamic evolution from Barremian intracontinental rift to Aptian marine gulf.

  • Extensional faulting between Atlantic & Tethyan Oceans, across the High Atlas domain.

Abstract

The northern boundary of the Central High Atlas was affected by a transgression during the Aptian that reached the junction zone between the Middle and the High Atlas. In this sector (Naour-Aghbala) which corresponds to the presumed closure zone of this Aptian Atlantic marine trough, the sedimentary record reveals a strong dependence on the regional structural framework. The Barremian continental sedimentation is concentrated in an elongated W-E highly subsiding zone, limited to the south by the polyphase Aghbala-Afourer Fault Zone (AAFZ), which corresponds to the contact between the main Atlasic Belt and its northern boundary (Beni Mellal Atlas). The Aptian deposits preserve similar littoral marine characters from West to East without showing any confined facies, which could evoke the eastern limit of the gulf. On the other hand, towards the North, the Aptian layers rapidly change from marine to lagoonal then to continental facies. The paleogeographic boundary of the Aptian shoreline, oriented E-W, is locally controlled by the North El Ksiba Fault (NKF) in the North. The Aptian Atlantic transgression, closely linked to the narrow Barremian graben, shows a wide northward extension of the marine sedimentary area.

The Barremian graben highlighted in the Aghbala-Naour area represents only a segment of a major east-west intracontinental rift that continued eastward with the more recent frontal thrust of the eastern Moroccan High Atlas. This faulted structure was the penetration axis of a double transgression: from the Tethys to the East and from the Atlantic Ocean to the West on the emerged Atlasic domain.

Introduction

The widespread Cenomanian-Turonian transgression on the West African Craton was locally preceded by marine transgressive episodes, notably in the NW part of the African continent along the Moroccan Atlas system (Fig. 1A) located at the junction of Atlantic and Tethyan influences. In this domain, an Aptian Atlantic marine transgression was first observed at the northern boundary of the Central High Atlas (Fig. 1B) determining a narrow gulf (Choubert and Faure- Muret, 1960–1962). An Aptian marine episode was later reported in the central Middle Atlas (Charrière and Vila, 1991) and considered to be of Tethyan origin. More recently, a new Tethyan extension, the “Marginal Folds gulf” (Upper Barremian -? Aptian) has been discovered (Haddoumi et al., 2019) at the northern boundary of the eastern High Atlas. The Aptian transgression on the northern boundary of the Central High Atlas is attested by preserved marine deposits in different synclinal basins (Fig. 1B). The distribution of these deposits in a SSW-NNE oriented narrow strip indicates a westward trending to the Atlantic Ocean. Since the paper of Choubert and Faure Muret (1960–62) all paleogeographic reconstructions (Dercourt et al., 2000; Guiraud et al., 2005; Frizon de Lamotte et al., 2008, 2009; Haddoumi et al., 2019) indicate an eastward closure of this gulf. Various cartographic works (Rolley, 1977; Jenny, 1985; Monbaron, 1985; Le Marrec, 1985; Fadile, 2003) and university PhD theses (Souhel, 1987, 1996; Lowner, 2009) provided data about the basins located on the northern boundary of the Central High Atlas. We have extended these works to the east, especially in the Naour and Aghbala areas, which represent the presumed closure zone of the eastward Atlantic gulf. This key-sector is also in the transitional zone between the Middle Atlas and the High Atlas (Figs. 1 and 2).

The main objective of this paper is firstly to precise the paleogeography of the Aptian Sea by evaluating its possible extension towards the East and/or the North. Towards the South or the Southeast, in the Atlas belt the whole post-Jurassic succession has been eroded or was never deposited. The second aim, carried out in parallel, attempts to establish whether there was a structural-sedimentary framework associated with this transgressive episode. To this purpose, we will analyze the nature and the distribution of the Barremian and Aptian continental and marine deposits, then we will propose a dynamic reconstruction of the structural framework that preceded and accompanied the Aptian transgression.

Section snippets

Geological setting

In most synclinal basins of the Central High Atlas northern boundary, the marine Aptian layers constitute a stratigraphic reference level, about 40 m thick, essentially formed by greyish marls and beige or yellowish limestones (Fig. 3). This “Aptian bar” (Rolley 1973) redefined as “Aït Tafelt Formation” (Souhel, 1987), is clearly identified on the different geological maps at 1/100.000: Afourer (Rolley, 1977), Beni Mellal (Monbaron, 1985), Azilal (Jenny, 1985), Demnat (Le Marrec, 1985) and

The Barremian and Aptian sedimentary record in the Naour area

A detailed geological mapping has been carried out at the eastern periclinal end of the Naour syncline basin where three major radiating faults can be observed (Fig. 4A). The first one, the AAFZ fault which limits the syncline to the South, affects all the Mesozoic and Cenozoic formations (Liassic to Miocene). The second one, the ANF fault (Fig. 5D–G) does not affect the Aptian limestones. The last one, the North Naour Fault (NNF) (Fig. 5A–C) crosses the Aptian strata (about 20 m of offsetting)

A new Barremian intracontinental rift (Figs. 12 and 13A)

The Naour area study (Fig. 4) demonstrates clearly that several blocks of contrasting stratigraphic composition are separated by different faults: one of them is clearly a pre-Aptian fault (ANF); the other one (NNF) shows a main pre-Aptian offset and a small post Aptian offset. The subsidence is particularly important during the Bathonian-? Callovian (Guettioua Fm) and the Barremian (Jbel Sidal Fm) periods. The sedimentary record synthesis is schematized in Fig. 12 which highlights clearly two

Conclusions

Our researches in the transition zone between the Central High Atlas and the Middle Atlas provide new data of different kinds.

Contributions to the Lower Cretaceous stratigraphy and paleogeography of Morocco. The distinction between the Jurassic and Cretaceous “continental Red beds” reveals that the Barremian sedimentation (Jbel Sidal Fm) has a previously unsuspected importance in the studied area. This continental sedimentation which occurs on a previously structured and eroded Jurassic

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We gratefully acknowledge Professor Joseph Canérot from the Toulouse University and Professor Giovanna Moratti from CNR institute of Earth Sciences and Georesources, Section of Florence, for reviewing our work and we greatly appreciate their constructive and helpful comments that allowed us to improve our manuscript. We would like sincerely to express our thanks to Professor El Mostafa Ettachfini (Chouaïb Doukkali University) who examined the foraminifera and to Said Oussou (English literature,

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