Impact of relative sea level and extreme climate events on the Southern Skhira coastline (Gulf of Gabes, Tunisia) during Holocene times: Ostracodes and foraminifera associations' response

Abstract

This study examines the evolution of the Skhira coast (Southeastern Tunisia) during Holocene times, by analyzing ostracodes, foraminifera and mollusk associations in subsurface sediments of both sebkhas of Dreîaa and El Guettiate. Nine biofacies represented by (1) five categories of lagoon namely widely opened, opened, brackish estuarine, closed and emerged; (2) the maximum of closure of environment revealed by continental and sebkha biofacies; (3) a peculiar two biofacies, formed by coarse sands or shelly clays and dominated by mollusk fragments and showing a mixture of lagoonal, marine and brackish taxa of mollusk, ostracods and foraminifera which provide evidence of high energy events (storms). The lower Holocene continental sediments are overlain by two transgressive categories of biofacies. The first one corresponds to a widely opened lagoon biofacies, related to the first Holocene marine transgression, around 7.46 ka cal BP. The second one, more important, consists of an opened lagoon biofacies between 6.67 and 5.91 ka cal BP. Sudden changes, during Holocene time, in the numerical distribution of foraminifera and ostracods species up cores are used to infer the change from widely opened lagoon, subjected to estuarine influences, to sebkha by passing by a close lagoonal environment. The enrichment of the sediment in quartz grains coupled with the action of the current drift authorized the build-up of sandy spits in front of the estuaries and the genesis of lagoon that preserve a rich assemblage of euryhaline foraminifera, ostracods and mollusks species. This process was interrupted by two distinct high-energy events. The break of sand spits induced the introduction of washover clay deposits at 5.418 ka cal BP and washover coarse sands at 644 a cal BP rich in mixture of brackish and marine macrofauna and microfauna.