Architecture, evolution and depositional controls of a Late Holocene hypertidal recurved barrier-spit system
Résumé
Understanding barrier spit formation and evolution is increasingly important in order to assess how these soft-sediment coastal systems will respond and adapt to expected future environmental changes. Barrier spit development is the result of complex multiscale interactions between wave and tide dynamics, fluctuations in sea level, storm impact and sediment supply along with geological and morphological inheritance. The stratigraphy and internal architecture of barrier systems preserve information about the dominating processes and controls that governed past coastal evolution. In coastal areas with relative high levels of tidal energy and low levels of wave energy, tide-dominated deposits preferentially make up the sedimentary record and wave-dominated deposits are rarely preserved. Hence, these systems are often considered to hold little or no information of past changes in wind and storm climate. In this study we investigate a Holocene recurved barrier spit situated along the west coast of the Cotentin Peninsula in the English Channel, France. This coast is subject to a tide-dominated energy regime with a tidal range exceeding 14 m and a mean significant wave height of less than 0.5 m. Integrated analysis of core and ground-penetrating radar data suggest that the barrier spit has a complex sedimentary architecture consisting of tide and wave dominated sedimentary bodies. We present a new sedimentological model for recurved barrier spit development under hypertidal conditions. The model describes how the spit extents downdrift by clinoformal progradation resulting from littoral drift caused by longshore sediment transport and due to swash bar welding along the distal end of the barrier. Lateral and vertical growth of the spit terminus results from sediment convergence due to landward migration of swash bars and seaward migration of tidal dunes. Radiocarbon and optically stimulated luminescence datings reveal that spit evolution and progradation probably occurred in two stages from about 900 to 1200 years ago and from 300 to 400 years ago. We suggest that increased windy conditions during the Little Ice age and resulting increased littoral drift may be responsible for the latter period of spit progradation.