"Chaotic" rocks in the boundary zone between Ordovician and Carboniferous in the eastern Jebilet (central Morocco): tectonic or sedimentary origin?
Matsen Broothaers1, Manuel Sintubin1 & Mohammed Nefly2
1 Structural Geology & Tectonics Group, K.U.Leuven, Redingenstraat 16, 3000 Leuven, Belgium
2
Département de Géologie, Faculté des Sciences Ben M’sik, Université Hassan II Mohammedia, Casablanca, Moroccomatsen.broothaers@geo.kuleuven.ac.be
The Jebilet are an Hercynian inlier located north of the High Atlas chain, and forming part of the Meseta. The eastern Jebilet are largely built up of Lower Carboniferous turbidite series, surrounding several units of Ordovician to Devonian rocks. Between the turbidites and the latter units, a formation of chaotic appearance is described . To date, the units are interpreted as synsedimentary gravity nappes that slided into the basin prior to tectonic deformation in the Late Carboniferous . In this view, the "chaotic" formation is considered as an olistostrome, developed in front of the advancing nappes.
Research near the southeastern boundary zone of one of the Ordovician units (Bou Marhara Unit) shows a more complex structural architecture, both with respect to the nature of the "chaotic" formation and the presumed allochthony of the Ordovician units. Here, the "chaotic" formation is present in a NE-SW oriented depression, separating the Ordovician to the north from the Carboniferous to the south. This zone shows a markedly different tectonic style compared to the Ordovician and the Carboniferous. Structures in both Ordovician and Carboniferous are truncated by this zone. Within this zone, several tectofacies can be distinguished. They are not continuous over the area. They sometimes show a gradual transition (stratigraphical contact?), whereas elsewhere they are truncated (tectonic contact?). The central part of the zone consists of a black pelitic facies, showing an anastomosing cleavage. It contains discontinuous packages of sandstone beds, occurring randomly within the zone. These beds show sharp to isoclinal folds. Isolated fold hinges are also observed. The regional, NE-SW oriented, cleavage appears as axial-planar to these folds. In the middle of this facies, a level of sandstone beds, which is folded and appears to be displaced by faults, is present.
These structures may have a tectonic origin, as a result of large-scale shearing, which lead to the folding and faulting of the sandstone levels. The discontinuous occurrence of the sandstones may, however, also have resulted from sliding and/or slumping, eventually overprinted and reoriented by a tectonic deformation of minor importance. The presence of tectonic contacts favours a tectonic origin of the zone. They also make it difficult to attest whether or not we are dealing with parts of a normal sedimentary sequence containing slumps and slides.