Lower-middle Visean transgressive carbonates in Morocco: Palaeobiogeographic insights

摘要

Foraminifera have been used to revise the biostratigraphy of all known Mississippian outcrops in Morocco cited either as early Visean (or its equivalent V2a or V2a/b chronozones or the regional Cfm1 biozones), or mid Visean (or the foraminiferal zones V2b-V3a or Cfm 2-Cfm 3 biozones). This group of microfossils occur in shallow-water limestones within siliciclastic-dominant successions. Most of the previously determined lower Visean ages for the outcrops are now interpreted either as middle or even late Visean. Similarly, some outcrops originally assigned to the mid Visean are now considered as late Visean. Only two successions can be confirmed as belonging to the early Visean in the Western Meseta. The first succession, of uppermost lower Visean age (Cf4 gamma-6 subzones or MFZ11 zone), occurs in the lower part of the Sidi Sebaa Formation in the Mdakra Massif (south of the Sidi Bettache Basin). The second succession occurs in a small area in the lower part of the Khenifra Nappe (in the so-called Hvs1' Formation), which is assigned to the base of the Visean (Cf4 alpha 2 subzone or MFZ9 zone). Middle Visean outcrops are more common, occurring in the western part of the Western Meseta (Mdakra and Khatouat massifs), north-central Western Meseta (Tilouine, Bouqachmir, El Goulib, Jbel Bouechchot), as well as in the South Western Meseta (Haouz-High Atlas of Marrakech) and the central High Atlas (Skoura). In contrast, the Visean succession in the AntiAtlas domain is more continuous, from early to mid Visean. This scarcity of lower Visean outcrops suggests that the Moroccan Meseta was mostly an uplifted region at that time. The recognition of the early Visean in the Mdakra Massif suggests a marine connection from the west or northwest, linked with the Rheic Ocean, with an opposite orientation (from the southeast) for the main transgressions in the Meseta in the late Visean. Taking into consideration previous tectonic reconstructions of Morocco, the presence of early Visean marine rocks in the Khenifra region can only be explained by a southward connection with the Sahara across the Tinerhir and Tafilalt regions linking with the Palaeotethys Ocean, which later covered areas within the High Atlas during the mid Visean. This connection during the early Visean, suggests a relatively close position between the Moroccan Meseta and Gondwana, in spite of the disparity in the lithological successions; this is also recognized in the mid Visean. Thus, it appears that the Moroccan Meseta had not yet achieved its final emplacement completely juxtaposed to Gondwana.