Mineralogical and geochemical criteria to identify the origin and the depositional environment of the upper Numidian babouchite siliceous rocks, northwestern Tunisia
The lower Miocene babouchite siliceous rocks from the Numidian Formation were widely deposited in the Northwestern of Tunisia, with a total thickness ranging from 20 to 50 m. The samples are principally composed of quartz and clay minerals (smectite, illite and kaolinite), with total lack of amorphous silica. The bulk chemical composition of the babouchite siliceous rocks is represented by high SiO2 (73.60-91.45%) and Al2O3 (5.78-10.55%) with negligible amounts of CaO, MgO, K2O and TiO2. The enrichment of these cherts by Mn and Cu, and the low content of the other trace elements are indicators of oxic condition in the water column. The ratio of Al/(Al + Fe + Mn) (0.65-0.87) is > 0.6 which is typical of a biogenic origin. Furthermore, the high ratios of Si/(Si + Al + Fe) justify the biogenic origin. All the analyzed samples from Babouch member fall into the non-hydrothermal sediment field of the Al-Fe-Mn diagram. Moreover, according to the environment discriminating diagrams, such as Al2O3/(100-SiO2) vs. Fe2O3/(100-SiO2), Al2O3/SiO2 vs. Fe2O3/SiO2 and Al2O3/(Al2O3+Fe2O3) vs. Fe2O3/TiO2, the babouchite siliceous rocks were deposited on a passive continental margin. The δ18O value of the chert, range from 24.9‰ to 27.9‰. The forming temperature determined by oxygen isotopic composition indicates formation at a slightly high temperature, about 65 °C.