Stratigraphy, Provenance, and Geodynamic setting of the Lower-Middle Jurassic succession in the Prerif foreland basin (Rif Chain, Morocco)

The Rif chain is located on the western edge of the Apennine-Maghrebian Chain. It is classically subdivided into three main superimposed tectonic domains: The Internal, Flysch Basin, and the External domains. The latter regroups three sub-domains: Intrarif, Mesorif, and Prerif. The present work is based on new geological mapping with lithostratigraphic logging and sampling for geochemical analysis from Lower-Middle Jurassic sedimentary successions belonging to the Prerif sub-domain. Four stratigraphic sections have been analyzed (Jbel Zerhoune, Dhar Nsour I and II, and Jbel Outita section). The Lower Jurassic successions are mainly characterized by alternating marls with limestones, bioclastic limestone, and calcareous sandstones, whereas the Middle Jurassic ones are mainly represented by calcareous sandstones and bioclastic limestones, both alternating with marly intervals.

The collected fifteen samples have been examined using Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA-ICP-MS) and X-Ray Fluorescent (XRF) for Elemental geochemical analysis to provide new insights about provenance, source rocks, paleoweathering, mechanical sorting/recycling, and geodynamic setting.

The elemental geochemistry (Major, Trace, and Rare Earth Elements) reveals that major oxides concentrations (SiO₂, Al₂O₃, MgO, and K₂O) are relatively close to the Post-Archean Australian Shales (PAAS) except for MnO and Na₂O, which show a depletion trend, while Cao and Sr display an enrichment, also Zr, Hf and REEs show low concentrations and indicate weak recycling processes. In order to characterize the provenance, we used a combination of the Cr/V vs. Y/Ni plot, the Cr/Th vs. Th/Sc plot, and the elemental ratios of provenance (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th). Various discriminant diagrams were used to reveal the paleoweathering intensity, sorting, and maturity of sediment during their source to sink fate. The Chemical Index of Alteration (CIA) shows values varying from 49.09 % to 77.01 %. Th/Sc versus Zr/Sc ratios and the Al2O3-Zr-TiO2 plots have been used to assess the sorting related to the fractionation of sediment during their transport. This is also corroborated by the relatively high values of the compositional variability index (ICV) of Jbel Zerhoune and Dhar Nsour (I and II) and Jbel Outita (ICV<1 and ICV>1, respectively). We also used multidimensional discriminant function diagrams to characterize the geodynamics setting on the analyzed samples.

The main expected results and related interpretations reveal that the provenance of the lower-Middle Jurassic sediments mainly indicates a supply from felsic source rock areas as attested by prevalent enrichment in Sr and probably from a minor mafic supply. The chemical index of alteration indicates a low to moderate degree of source area weathering. The Th/Sc versus Zr/Sc ratios and the Al2O3-Zr-TiO2 recycling plots, and the depletion of Hf and Zr reflect poor mechanical sorting and recycling processes which are confirmed by the high ICV values indicating that almost all samples are immature first-cycle sediments with unweathered detrital minerals. The analyzed sediments have been deposited within a passive margin controlled by a rifting/drifting geodynamic evolution of the northern African margin during the Early-Middle Jurassic.