Reconstructing the Holocene depositional environments along the northern coast of Sfax (Tunisia): Mineralogical and sedimentological approaches

A sedimentological and mineralogical study of sedimentary cores allows reconstructing the evolution of depositional environments along the Northern coast of Sfax (Tunisia). The aim is identifying the factors controlling the sedimentation from the Holocene to the Present. Three 30-m sediment cores collected by drilling at 30 meter water depth were analyzed for their color, magnetic susceptibility signal, grain size by laser diffraction, organic matter content by loss of ignition, carbonate content by calcimetry and mineralogy by X-ray diffraction on bulk powder and clay < 2 m. The three cores broadly present the same sedimentological and mineralogical features. Microscopical observations of petrographic slides allow to identifying six main sedimentary facies. Bulk mineralogical assemblages comprise clay minerals, quartz, calcite, gypsum and K-feldspars. The main change concerns the carbonate content that mimics the bioclaste abundance and dilute the detrital minerals (clay minerals, quartz and feldspars). The gypsum mainly occurs in the lower sedimentary columns (SC12 and SC9) and in the upper/middle of core SC6. The clay fraction is made of a mixture of kaolinite, illite, smectite and palygorskite with no clear variation through core depth. Both grain-size parameters and magnetic susceptibility profile indicates a sharp transition in the upper 2 to 5 m of the sedimentological columns. Coarse, sandy to gravely, sediments characterized by a low magnetic susceptibility signal are replaced by fine bioclastic-rich clayey sediments. The analysis of vertical succession of depositional facies showed a fluvial depositional environment (coastal plain) basically marked by fluvial channels and inundation plains at the bottom of all cores. Core-top sediments, however, record a littoral marine environment characterized by sand depositions rich in gastropods, lamellibranches and algæ. Depositional facies, sedimentological and mineralogical parameters are consistent with a transition from a continental, fluviatile, depositional environment, with some emersion phases marked by the gypsum precipitation to a marine littoral environment. Such evolution is consistent with a relative sea-level rise which flooded the fluvial system at the coastal plain during the Holocene, in agreement with sea-level fluctuations in southeast Tunisia during the Holocene.