Soft-sediment deformation and synsedimentary tectonics of Vallesian deposits in the northern Middle Atlas (Morocco): Implications for Paleoenvironmental modelling

Abstract The first Neogene deposits in the northern Middle Atlas are the Vallesian lacustrine and palustrine series of the Oued Zraa and Oued Mdaz basins. The first is part of the tabular Middle Atlas and consists of a terrigenous, conglomeratic, and micro-conglomeratic formation at the base, which is topped by volcanic and volcano-sedimentary layers sealed by marl-limestone alternations. The second basin belongs to the folded Middle Atlas, defined by micro-conglomeratic and silty-sandy deposits at the bottom, topped by marly layers and surmounted by white limestone formations. These deposits are affected by ductile and brittle soft-sediment deformation structures (SSDS), such as clastic veins, normal and hidden reverse faults, boudinages, symmetrical and asymmetrical folds, and stress patterns. The trigger mechanism investigation for these deformations, combined with the tectonic and microtectonic analyses carried out in the two basins, reveals that these hydroplastic deformations or seismites are seismic in origin. These tectonic activity correlate to the Vallesian tectonic phase in the Middle Atlas range, with NW-SE to WNW-ESE σ1 stress axis and NE-SW to NNE-SSW stress σ3 axis. This tectonic activity led to the NW-SE normal faults activation and the Oued Zraa and Oued Mdaz basins development. The two basins opening, which are separated by NW-SE to WNW-ESE transverse faults, is associated with the formation of NW-SE oriented clastic veins and hydroplastic boudinage. This tectonic phase is divided into two episodes: NE-SW extensional episode and NW-SE compressional one. This leads to the appearance of NE-SW reverse faults, symmetrical and asymmetrical synsedimentary folds, with axial planes striking in NE-SW direction.