Contribution of the anisotropy of magnetic susceptibility to detect the changes in the sedimentation infill of the Reggane basin during its hercynian structuration (Saharan plateform, Algeria)
Résumé
The drift of the West African Craton (WAC) from the southern hemisphere towards the northern hemisphere since Ordovician times is at the origin of large distensions where several basins formed, in particular the Reggane basin, deepened and infilled with sedimentary material coming from destruction of the post-proterozoic chains (Mauritanides, Ougarta, Ahnet-Mouydir,...). The distribution of marine and continental sediments recorded in the north Gondwana margin, the large volume of clastic material and the current directions made the WAC as a good source proxy for the basins infill surrounding Gondwanaland. The aim of this study is to address the anisotropy of magnetic susceptibility (AMS) tool to better focuse on the origin of these sediments and to track where did they come from. Most of the studied specimens (over 240) were sampled within different levels along a Silurian-Devonian cross section on one limb of the Bled El Mes synclinal structure. They are composed of sandstones, fine grained sandstones with tigillites, silicified sandstones and limestones and composed of grains which carry low bulk susceptibility values (in the range 2-100x10-6 SI) with prolate shape and a low degree of anisotropy (<2%), except for Lower Devonian specimens where oblate and prolate shapes occurred. Most of these specimens contain high coercivity grains (high SIRM values), probably of detritic origin, since AF technique is not suitable to fully demagnetize the specimens, whereas thermal demagnetization often shows changes in magnetic mineralogy between 300°C and 400°C. Thermomagnetic curves show changes in mineralogy at the same temperature range, while Morin transition is in agreement with the presence of oxides. Magnetic carriers, which contribute to remanence are goethite, pyrrhotite, magnetite and hematite, also contribute to magnetic fabric. Knowing that Hercynian phase which was the major regional phase and that a late minor Barremian phase, recorded during the doleritic sills intrusion within the basin, occurred, a tentative schematic scenario concerning the driving tectonics may then be proposed: 1- During Silurian times, K3 direction is steep and well grouped, suggesting that the sedimentary magnetic fabric is preserved and show a northwards transport. Most of the specimens are prolate in shape, while bulk magnetic susceptibility is ranging within values 42-83x10-6 SI. 2- At the Silurian-Devonian limit, a change in the nature of clasts was recorded: from silicified sandstones and limestones with bioturbations to red sandstones with tigillites. An eastwards change in the transport direction is recognized, shown by an easterly cluster of shallow K3, either related to deposits during folding or at the vicinity of the basin border. Tectonics may have played a significant role at this stage since clockwise of the WAC is in agreement with this orientation transport. 3- During Lower Devonian, the increase in the sedimentation is nearly similar to that described for the Silurian, ie towards a NNW trend. 4- From Lower to Middle Devonian, a slight change may have occurred in the sedimentation transport, similar to that of Silurian age. 5- From Middle to Late Devonian, changes in the grain size may have occurred, showing an easterly cluster of shallow K3, the same behaviour as for the Silurian-Devonian limit.