S. van Heteren1*, R. Kooij2,3, R.G. Nieboer3, K.M. Cohen2, A.J.F. van der Spek2,4
1 TNO - Geological Survey of the Netherlands, * sytze.vanheteren@tno.nl
2Faculty of Geosciences, Utrecht University
3Deltares, Applied Geology and Geophysics Department
4Deltares, Applied Morphology Department
Introduction
Some 5000 years ago, Bergen Inlet formed one of the largest gaps in the barrier coast of the western Netherlands. It connected an enormous tidal basin to the North Sea, and dominated coastal development in a wide adjacent area for thousands of years. At its peak, the prism of this tidal inlet was similar to that of Texel Inlet before closure of the Zuiderzee. The demise of the inlet has been reconstructed using a wealth of morphological and subsurface data on land. After a little more than 4000 years of activity, the inlet closed around 3500 cal BP, leaving stacked tidal-channel and channel-fill facies of different age.
Objective and Methods
A large part of the deposits left by Bergen Inlet is located offshore, where their lateral extent and variability are not yet understood. A survey just offshore the province of Noord-Holland yielded a set of seismic lines (850 km covering 500 km2) and vibrocores (23). Supplemented by 19 recently obtained radiocarbon dates, this dataset sheds light on how the seaward half of the inlet developed. Despite some blanking of the signal caused by shallow gas formed in organic-bearing deposits, the seismic profiles provide valuable, laterally continuous images that are missing on land.
Results
Two key elements of a fragmentary Holocene sequence are large-scale tidal-channel deposits associated with the active inlet, and a clayey fill representing inlet demise. The base of sandy inlet-related units reaches its deepest point about 8 km seaward of the modern shoreline, where it is more than 10 m deeper than below the land. The base of the younger inlet fill is at least 5 m shallower than onshore. Assuming that this deepest point denotes the inlet throat, this implies about 10 km of landward coastline migration while the inlet reduced in size. The new radiocarbon dates put some initial constraints on offshore-onshore facies connections but more time control is needed to fully understand the rise and fall of Bergen Inlet.
