Sedimentological, mineralogical and geochemical investigation of Essaouira’s coastal dune system: potential origin and provenance

Abstract

Quaternary coastal sand dunes occur frequently along the Atlantic coast of Morocco and result from the combined effect of river discharge, littoral drift, winds, vegetation, and other meteorological conditions. In this paper, we investigate the textural and compositional parameters of the coastal dune sands of Essaouira across the Ksob riversides, as well as their provenance. Morphologically, they occur as present-day active dunes, Holocene fossil dunes and Pleistocene aeolianite deposits. To investigate the grain provenance, we have opted for laboratory analyses including grain size, mineralogy, microtextures, and micromorphology of quartz grains. Sands from Holocene fossil and active dunes revealed very fine to medium mean grain size and a moderate sorting, while those of the Pleistocene aeolianites showed fine to very fine mean grain size and very poor sorting. Studied dunes showed the dominance of grain shapes varying from sub-rounded to fresh angular. Regarding mineralogy, calcite bioclasts are prevalent in sands of the right bank of the river, while quartz is more abundant in dunes located at the left bank. In the active dunes of the left bank, other carbonates were also detected in bioclasts such as aragonite, dolomite, and magnesian calcite, suggesting a marine provenance. The siliciclastic grains of the studied generations of aeolian dunes are likely originating from Triassic, Jurassic, and Quaternary units crossed by the Ksob River. In fact, the results revealed the presence of minerals such as albite, plagioclase, microcline, and orthoclase which could have been sourced from Triassic basalts and gypsum which is often associated to Jurassic evaporitic rocks. This study allowed to suggest the probable origin of the sands that form these dunes based on the detailed study of their grains, which has not been studied on previous research works of the area. In addition, this research is of particular relevance as it fits into the current research trend related to understanding the evolution of Plio-Quaternary coastal dune systems in the world and could be applicable to other dune systems of the Atlantic coast of Morocco in the future.