Sedimentary history of the island Læsø, Denmark

Coastal sections on the island Lzesø in the northern Kattegat, Denmark, expose many well-preserved sedimentary structures. These structures have been studied in order to gain information about the postglacial sedimentary history of the island. Furthermore, numerous beach ridges make it possible to reconstruct the size and shape of the island at all stages of its development. During early postglacial transgressions and alternating regressions the island was represented by a shoal. During the transgres-sions the shoal was inhabited by the heart urchin Echinocardium cordatum, whereas it was inhabited by the lugworm Arenicola marina during regressions. Heart urchin-burrowed layers are found in three horizons with lugworm-burrowed beds below and above the upper heart urchin-burrowed horizon. The burrows of the two animals are found in different sedimentary facies, thus also suggesting different hydrographic conditions by means of physical sedimentary structures. In places the lugworm-burrowed horizons are characterized by irregular erosion surfaces. Similar structures are found today at the pseudo-tidal flats south of Læsø, where they are formed by partial erosion of algal mats. The alternation of different sedimentary structures including trace fossils and irregular erosion surfaces is thus believed to reflect postglacial transgressions and regressions