PhD defence: Lasse Sander

Lasse Sander defends his PhD thesis: 

Beach ridge and lagoon systems as indicator of sea-level changes: The Holocene evolution of the coastal landscape in Denmark

Beach ridge and lagoon systems

Professor Morten Pejrup, IGN
Head of Department Lars Henrik Nielsen, Department of Stratigraphy, GEUS

Assessment Committee
Associate Professor Troels Aagaard, IGN (chairman)
Professor Helmut Brückner, Universität zu Köln
Associate Professor Maria Jensen, University Centre in Svalbard

After the PhD defence there will be a reception in ’Rød Stue’, Øster Voldgade 10, Area 6, First floor – and everybody is welcome.

This thesis investigates the possibilities of reconstructing Holocene relative sea-level (RSL) developments from coastal lagoon systems and beach ridges in a periglacial soft-sediment setting. The focus of this study lies on the sedimentological analysis, morphological description, and absolute dating of Holocene marine deposits on the island of Samsø, southern Kattegat Sea, Denmark. Coastal lagoons and beach ridges are common features of the coastal landscape and are found distributed along and across gradients of isostatic uplift. Their widespread occurrence gives these systems certain relevance as sedimentary archives. We here investigate the potential of combining these genetically independent, though complementary sedimentary environments to reconstruct a composite RSL dataset covering the entire transgressive-regressive sea-level history of the area.

The present geomorphology, surface geology, and land-sea distribution of the glacial land-scapes around Samsø are primarily influenced by short periods of ice advance in the mid- and late Weichselian. The involved processes and deposits determine the presence of basins within an undulating topography as well as the presence of large amounts of unconsolidated sedi-ments for the formation of coastal landforms. Our data show that fine-grained marine sedi-mentation commenced synchronously in near-coastal basins in different elevations which is interpreted as an indication of a period of rapid RSL rise between 7.5 – 7.3 kyr BP. Absolute ages established on the marine deposits of the lagoon environments are evidence that condi-tions of marine sedimentation prevailed until at least 4 kyr BP; direct RSL indications for that period are however poorly constrained. A succession of beach ridges within a wide strandplain on Samsø was dated to between 4.8 kyr BP and present. The elevations of downlapping reflec-tions in the prograded beach-ridge system are used as a continuous record of RSL develop-ment since the mid-Holocene. Our data indicates a relatively stable RSL at approximately 2.2 m above mean sea-level (MSL) between 4.8 – 3.5 kyr BP. This highstand period ended abruptly around 3.5 kyr BP with a marked RSL fall of approx. 1.3 m. In the lagoonal environments, facies transitions from organic-rich marine mud to marine sand are interpreted as winnowing lags resulting from a lowering of the wave-base. In a fossil lagoon system, the sand yields consistent ages of approx. 4.1 kyr BP and the transition thus agrees with the timing of the marked RSL fall evidenced for the beach-ridge system. Over the last 3.3 kyr BP, RSL on Samsø experienced minor fluctuations with an overall falling trend with average rates comparable to modern rates of isostatic uplift.

It is concluded that the combination of coastal lagoons and beach ridges as sedimentary ar-chives for the reconstruction of Holocene RSL development bears great potential in the ice-marginal areas of the late Quaternary glaciations in SW Scandinavia and in other coastal land-scapes in periglacial soft-sediment settings.

The thesis is available from the PhD administration office 04.1.409