Upper Devonian Griottes Limestone in NW Sahara (Algeria): Facies, environments and geodynamic significance

[摘要] On the NW margin of the Algerian Sahara in the Bechar (Ben-Zireg), Ougarta (Marhouma) and Gourara (Charouine) basins, the Upper Devonian "griottes" nodular limestone was deposited during the Frasnian and the Famennian. These alternating clayey-sand-limestone deposits are rich in ammonoids. These facies vary laterally and in thickness in these three areas. These facies variations are directly related to depositional environmental conditions (hydrodynamism, basin physiography, sedimentary gaps). In the three studied areas, the griotte facies were deposited on a deep seabed platform and in basins under tectono-eustatic control. In the Ben-Zireg area platform facies are throughout the studied stratigraphic interval. In the Marhouma area, the facies change through time first as distal ramp sedimentation (Frasnian Ia), then shifting to an apron slope sedimentation with allodapic facies (Famennian II, VI and V), and then in basinal settings (Famennian II and VI). Finally, at the end of the Famennian, the series ended with sandy-clay slope sedimentation (lower member of the Ouarourout sandstone), and next platform sedimentation (upper member of Ouarourout sandstone) topped by an emersion surface. In the Charouine area, basin clays (hemipelagites) dominate the facies. The presence of asphaltene impregnations in the griotte facies and in centimetric slabs to metric blocks of the facies posterior to nodular limestones implies the effects of early diagenesis. This process played an important role in the sulphate reduction and the anaerobic oxidation of methane (biogenic and / or thermal). The segregation of sedimentary areas with griotte facies seems to be influenced by basement fragmentation. Indeed, the griotte facies closely follow the Pan-African flexure. In Ougarta, the "South Ougartian" fault corresponds to the extension of the mega-lineament of Tibesti. The reactivation of Precambrian basement faults during the Late Devonian may have been responsible for the observed contrasting facies.

[学科分类] 地质学