Le Crétacé moyen de la transversale Agadir-Nador (Maroc): précisions stratigraphiques et sedimentologiques

Abstract

A stratigraphic and sedimentological study of a transverse section from Agadir to Nador reveals three distinct stages in the geodynamic and palaeogeographic evolution of this part of Morocco. During the Albian, thick sediments infilled a basin in the Lower Cretaceous carbonate platform. Marls were deposited in the Atlantic Agadir-Essaouira domain, terrigenous conglomerates, sandstones and silts in the Central Atlas domain and carbonate and terrigenous sediments in the south Rif and eastern Rif foreland (North Atlas) domain. The facies change laterally from a fluvio-terrestrial to an infralittoral, restricted environment. Sediments in the Agadir-Essaouira domain are derived from the Atlantic and those from the North Atlas from the Neotethys. The Central Atlas domain appears to represent an intermediate sand-flat, sediments being derived in the south from the Atlantic and in the north from either the Atlantic or Neotethys. Uppermost Albian sequences, which are mostly thin, comprise carbonate and gypsiferous sediments deposited in quiet lagoonal environments. Facies in the Atlantic and Central Atlas regions are identical. The derivation of the sediments in the three domains is consistent with those for the earlier Albian deposits. There appears to have been a comparable sedimentary evolution along the transverse section during the Cenomanian. Initial sequences tend to be regressive but later they are clearly transgressive. The sediments are essentially marly. As for the Albian sequence, derivation of the sediments of the Agadir-Essaouira domain was again from the Atlantic and that of the North Atlas region from the Neotethys. In the central Atlas, an Atlantic provenance is apparent as far as the southern Ait Attab and El Koubbat syncline where there was a shoal centred on the present Middle Atlas, sediments being deposited in restricted environments on either side of this. A connection between the Atlantic and Tethys through the Albian is difficult to prove because of the lack of marine faunas in the Central Atlas domain and the occurrence of important facies variations in the Atlantic and North Atlas. However, during the latest Albian the uniformity of the facies in the Atlantic and North Atlas suggests that it did exist. A connection between the Atlantic and Tethys is confirmed for the late Cenomanian-Turonian by the presence of a similar sedimentological geodynamic and palaeogeographical evolution throughout the three domains.