The Backbarrier Sediments of the Skallingen Peninsula, Denmark

Abstract

Geografisk Tidsskrift, Danish Journal of Geography 97: 11–32, 1997.

The Skallingen peninsula is formed by sand derived from the south-going littoral drift along the North Sea coast. The backbarrier has accumulated sand as a result of wash-over deposits. In the beginning of the 20^th century, fine grained sediment started to cover the surface in form of salt marsh deposits.

The sand on the peninsula consists primarily of fine sand with a mean grain size of 2.5–2.25 phi (0.177–0.210 mm). Exposed to waves in the tidal flat environment and to tidal currents in the salt marsh creek environment, the finest sand fractions are removed and deposited on the salt marsh. The remaining sand is sorted in accordance with the dynamics in the sub-environments. The change in grain-size characteristics during this dispersal of sand in the back barrier is analyzed by means of decomposition of the grain-size distributions into overlapping log-normal distributions.

The import of fine grained sediment is demonstrated, based on measurements of tidal dynamics and concentration of suspended matter in the mouth of a salt marsh creek.

The salt marsh formation is analyzed on the basis of²¹⁰Pb-datings and grain-size analysis of the surface layers in a line across the central part of the backbarrier environment. The deposition rate is observed to increase from south to north, when approaching the inner part of the tidal area. The grain-size distribution of the average material depositing on the backbarrier in the central part of the peninsula consists of 15% sand, 46% silt and 39% clay with a corresponding deposition rate of 0.07 g.cm⁻² y⁻¹. The dry density of the salt marsh clay is primarily controlled by variations in the content of organic matter and by compaction. Two power functions are formulated to describe variations in the dry density: D = 4.47·O^−0.81; 10<O<50; r=0.98 (surface samples) and D = 0.58^{*X 0.17}; 0.5<X<32.5; r=0.70, where D is the dry density (g/cm³), O is loss in ignition (%) and X depth below the surface.

Keywords:

• Estuarine sediments
• salt marsh
• grain-size distributions
• barrier islands