Holocene deposits at the lower shoreface and inner shelf of the Dutch coast

Abstract

To further detail insights into the composition and distribution of Holocene and late Pleistocene deposits at the surface and in the shallow subsurface of the lower shoreface of the Dutch coast, vibrocores were collected in three coastal sections. The study areas Noordwijk, Terschelling and Ameland Inlet represent contrasting settings: closed Holland coast vs. segmented Wadden coast (Noordwijk – Terschelling) and lower shoreface of a barrier island vs. lower shoreface of an ebb-tidal delta (Terschelling – Ameland Inlet).Six different depositional environments were distinguished: 1. the active layer, 2. seabed deposits, 3. lower-shoreface deposits, 4. ebb-delta channel deposits, 5. tidal channel deposits, and 6. alluvial (river) channel deposits. The several dm-thick active layer forms the mobile top of the seabed-, lower-shoreface- and ebb-delta channel deposits. Ebb-delta channel deposits (probably grading into terminal-lobe deposits) are restricted to the Terschelling and Ameland Inlet areas, fluvial deposits to the Noordwijk site. The front of the Ameland Inlet ebb-tidal delta is steep and consists of material supplied by the main ebb channel. These ebb-delta channel deposits are reworked by waves and currents, they grade seawards into seabed deposits. The low-gradient shoreface of the Terschelling site consists of a thin active layer on top of ebb-delta channel deposits. At the Noordwijk site fluvial deposits with incised bodies of tidal channel sand underlie a steep shoreface and a ridge-swale topography farther offshore.The ebb-delta channel deposits at the shoreface of Terschelling are similar to those at the front of the ebb-tidal delta of Ameland Inlet. Moreover, the tidal channel deposits that are common in the Noordwijk area, occur in only one core at Terschelling. This indicates that the deposits underlying the shoreface of Terschelling were formed in the ebb-tidal delta of a precursor of Ameland Inlet and not in the transgressive setting of a retreating barrier island. These deposits were possibly formed as part of the ebb delta of the Middelzee, a large medieval predecessor of the Ameland tidal basin, this needs to be confirmed by dating.Reworking of the shoreface of the prograded Subboreal beach barriers at Noordwijk at water depths of 12.5–13.5 m produced a 1.1-m-thick series of fining-upwards storm beds, including the active layer. At the shoreface of Terschelling storm beds are missing at these depths and only an active layer 0.2 m thick occurs. This suggests that the largest part of reworked sediment at Terschelling is carried off, which implies large-scale erosion of the shoreface. This needs further investigation.