Coastal cliff with Miocene deposits.
The beach Søndbjerg Strand is located on the east coast of Thyholm about 1 km from the village of Søndbjerg. The cliff extends for about 300 m along the coast (Figure 46-1 and 46-2). The quality of exposure varies, and the profile is best studied after erosion during stormy weather. Most of the deposits are of Miocene age and illustrate the development of the area from a marine environment to a river plain as the coast migrated across the area. The Miocene sediments are overlain by Quaternary deposits that locally contain good examples of glaciotectonic folding and thrusting. The main geological value of the locality is that it contains a complete sequence of early Miocene deposits that bears witness to the evolution of the area from marine to a flood plain environment. This is the only place in GPWJ (apart from a few quarries) with exposures of Miocene deposits.
During Miocene times (from about 24 to 5 million years ago) the distribution of land and sea in the Danish area varied because of climatic changes, variation in the sediment supply, and subsidence of the North Sea basin. About 22 million years ago, during the early Miocene, the coastline was to the northeast of the Søndbjerg area and marine deposits accumulated here. Søndbjerg contains a complete sequence that shows a change in sedimentation pattern due to an advancing coastline. The deposits change from coastal marine sand to spit and isthmus deposits, through barrier and lagoon sediments to flood plain and marsh deposits.
A large part of the sequence consists of relatively fine-grained marine sand that was deposited on an outer beach plain. This sand is referred to the Billund Formation (Hvidbjerg Member). Elsewhere in the geopark this sand is encountered at a considerable depth (e.g. site 47 Hygum Bakke) but at Søndbjerg the Miocene deposits are elevated because of the underlying Uglev salt dome (see site 42 Odby Klint). The cliff also contains rhythmically alternating layers of sand and clay that were deposited in a lagoon affected by the tide, very similar to the modern Nissum Fjord (site 38). These sequences contain several layers of more coarse-grained material that were formed when the barrier that separated the lagoon from the sea was inundated by seawater. There were anaerobic conditions in parts of the lagoon that gave rise to several continuous layers of clay and silt with high organic contents. The layers formed in the lagoon are overlain by fine-grained marine sand, indicating that the area was again inundated by the sea, presumably because of a rise in sea level (Figure 46-3).
The marine and lagoon sequence of deposits is overlain by coarse sand and gravel that were deposited on a flood plain supplied by braided rivers that flowed to the coast that was further to the south. These sediments contain fragments of brown coal (lignite) that formed in a sub-tropical marshy area that was probably similar to the present Everglades in the USA. All the flood plain deposits can be referred to the Billund Formation (Addit Member).
The Miocene sediments are overlain by Quaternary meltwater deposits and clayey till. At the top of this sequence there is a compact basal till from the final Weichselian glacial advance that crossed the area (see sites 19 Odby basal moraine and 42 Odby Klint). The middle section of the Quaternary sequence locally contains good examples of folds and thrust sheets, whereas the northern section contains meltwater sediments with well-developed flow structures.
Most of this stretch of coast is preserved and a large part is a protected nature area. The locality is part of National Coastal Landscape NK 87 Kås Bredning – Skibsted Fjord. The majority of the area is owned by the Danish Nature Agency and there are excellent facilities for visitors, including marked paths, tables and benches and primitive overnight accommodation.