Sedimentary infill of a microtidal sand flat during the last two millennia,Gulf of Gabes,Tunisia

Abstract

This study outlines the key morpho-sedimentary phases that have occurred over the last two millennia, which have significantly contributed to the infill of a microtidal sandy flat in an arid coast (Gulf of Gabes, Tunisia). To better understand the vertical sedimentary changes in relation to late-Holocene sea-level changes, two sediment trenches were excavated and analyzed. The sedimentary succession, which occurs in conjunction with the trend of rising sea levels, provides a modern analog for sedimentary basin infill during periods of sea-level rise. Stratigraphy of the studied sedimentary succession goes from subtidal to intertidal facies and end with microbial mats. This succession begins with a buried beachrock layer, dated to 1700 ± 35 cal B.P., 250 AD, which contains fragments of Phoenician pottery. The beachrock is covered by 14 cm of fine to medium bioclastic deposit. The middle part of this sedimentary succession has a regular surface, which marks the boundary between Holocene sands and finer-grained bioclastic sediments, deposited from the 1980s onward, which are 10 cm thick. These recent sediments are capped by a 6–8 cm layer of microbial mat. This mat forms a rigid substrate that supports the growth of halophytic vegetations; these pioneering plants, in turn, stabilize the marsh by trapping additional sediments. The uppermost section of the highstand systems tract displays desiccation polygons, indicating the end depositional cycle within the tidal flat. Monitoring sediments flux during at least two full neap-spring tidal cycles shows that the accretion rate of the marsh deposit reaches 4 mm/year and may exceeds the effects of relative sea level rise. The observed aggradation in the microtidal arid tidal flat demonstrate that rising sea levels do not necessarily lead to coastal erosion, as sediment accumulation can still occur under such conditions.