Magnetostratigraphy of the Zobzit and Koudiat Zarga sections (Taza-Guercif basin, Morocco): implications for the evolution of the Rifian Corridor

Abstract

Magnetostratigraphic analyses for two Neogene `post-nappes' successions of the Taza-Guercif basin enable a reliable correlation of the sedimentary sequence to the astronomical polarity time scale. Rock magnetic analyses indicate that hematite is the dominant carrier of the magnetisation in the marine marls of the Melloulou Formation, whereas both magnetite and hematite contribute to the NRM of the near-shore and continental sediments of the Kef Ed Deba and Bou Irhardaiene Formations. Anisotropy of magnetic susceptibility (AMS) measurements indicate that the maximum axes of the magnetic fabric are aligned in a direction SW-NE. This suggests that the AMS is tectonically induced, related to SE-NW compression, in agreement with the major fold and fault systems in the basin. Our magnetostratigraphic correlation shows that the oldest marine sediments in the basin, which are related to the development of the Rifian Corridor, are dated at approximately 8 Ma. Between 7.2 and 7.1 Ma, just after the Tortonian/Messinian boundary, an important shallowing of the Taza-Guercif basin takes place. This shallowing phase is primarily related to active tectonics, although a small glacio-eustatic sea level lowering also took place. Our results indicate that at least the Taza-Guercif basin, and perhaps the entire Rifian Corridor, became emerged at an age between 6.7 and 6.0 Ma. Continental deposits, separated from the underlying deposits by a considerable hiatus of 700 kyr, extend into the Pliocene (up to 4.7 Ma).