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Sedimentary history of the island Læsø, Denmark JENS MORTEN HANSEN Hansen, J. M.: Sedimentary history of the island Læsø, Denmark. Bull. geol. Soc. Denmark, vol. 26, pp. DGF 217-236. Copenhagen, December 2nd 1977. 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. Jens Morten Hansen, Henningsens Allé 2, DK-2900 Hellerup, Denmark. August 10th, 1977. The island Læsø in the northern Kattegat, Den­ dence from beach ridges and other sedimentary mark, was built up mainly during postglacial structures provides a detailed history of the transgressions and regressions. The sea cliffs forces and events involved. The cliff at Bansten provide extensive sections of sedimentary struc­ Bakke (fig. 2), which cuts through an old tongue tures including well-preserved trace fossils. of land, shows a continuous sequence of sedi­ Numerous beach ridges make it possible to re­ ments from the initial transgressions as far as the construct size and shape of the island at all stages maximum extent of series of Subboreal trans­ of its development (fig. 1). The combined evi­ gressions. In addition, a study of the recent Fig. 1. Previous shorelines at Læsø. Continuous lines: beach ridges. Slope signature: dead and recent cliffs. Stippled: pre­ vious extension of Læsø. 218 J. M. Hansen: Sedimentary history of Læsø A Nordre Rønner 'Borfelt 5 km Fig. 2. Locality map. I 1 1 1 1 1 pseudo-tidal flats south of Læsø provides an ex­ along the north and west coast and in a pit 3 planation of some of the structures involved and km west of Byrum. Thickness 0-13 m. adds significantly to the conclusions that can be 4. Beach sediment. Exposed in the coastal sec­ drawn. tions along the north and west coast and in Previous work has been carried out by Jessen several pits. Thickness 0-5 m. Radiocarbon (1897, 1920 & 1936), who gave an outline of the age: Tooth of sperm whale (Physeter general geology. Michelsen (1967) dealt with the catodon) found north of Byrum, 4.5 m above Foraminifera from the late glacial Yoldia Clay sea level, 980 ± 80 years BC (Tauber, in litt.). and from the lowermost part of the sand and 5. Aeolian sand. Thickness 0-17 m. Main part gave (1968) a brief outline of the beach ridge formed in the 18th century. sytem. Morner (1969) visited the island as a part of a major work on the late and postglacial his­ The present paper deals with the Yoldia Clay, tory of the Kattegat. laying special emphasis on the overlaying marine The island is built up of 5 sedimentary units sand and beach sediments, which contain the (mentioned in sedimentary order): well-preserved sediment structures. 1. The Yoldia Clay. Exposed north of Vestrø Havn, at Bansten Bakke and in several clay pits in the southern part of the island. Thick­ The Yoldia Clay ness unknown. Radiocarbon age: 10.960 ± 180 years BC (Tauber 1966). The late glacial Yoldia Clay underlies the entire 2. Residual gravel and blocks resting on the northern Kattegat sea and covers extensive parts Yoldia Clay. Thickness 0-0.5 m. of northern Jylland, and forms the platform upon 3. Marine sand. Exposed in the coastal sections which the postglacial sand of Læsø has been Bulletin of the Geological Society of Denmark, vol. 26 1977 219 f f Fig. 3. Model (stereopair) showing the supposed relation be­ tween time and shape of Læsø. Each step roughly corresponds to 500 years. Notice that the steps do not correspond to con­ tours, since the isostatic recovery of the northern part of Læsø has been 2-3 m greater than of the southern part (cf. Mertz 1924). built up. It has a variable lithology, ranging from calcarea and Balanus sp., whereas Portlandia sticky, silty clay to more or less sandy silt or arctica is rare. However, fossils are generally even to muddy and stony sand. In places the Yol- few and those that occur are often crushed, dia Clay is very rich in stones and boulders some probably by boulders as they hit and deformed of which are very large (about 50 tons). In the the sediment or by ice bergs ploughing through southern part of Læsø, where the postglacial the sediment. Both in borings and exposures the sand sheet is very thin, these boulders are Yoldia Clay is overlain by a thin layer of gravel strewn over considerable areas, probably as a and stones obviously derived from the clay. This result of outwash during the Ancylus time and layer is equivalent to the boulders strewn on the during the postglacial transgressions (Michelsen southern part of Læsø. 1967). Jessen (1897) suggested that the boulders were derived from drift ice, an idea that might be supported by the fact that the Yoldia Clay is The postglacial sand specifically rich in boulders where it formed highs at the sea bottom. The small islands and Briefly the postglacial sand can be divided into shoals north of Læsø (Nordre Rønner and an older and a younger part with reference to Borfelt) are almost entirely built up of boulders topography. The older part forms the triangular of quite unusual size in spite of the total lack of raised part of the island, which to the south is tills or till-ridges on Læsø and its neighbour­ surrounded by the younger part, and to the north hood. Therefore it seems likely that the Yoldia by the sea. The two parts of different age are Clay was formed as some kind of drop-till while separated by fossil cliffs and big beach ridges. the late Weichselian ice stood near the Swedish The older part of the sand formation is up to 13 west coast or even farther westwards. Morner m thick and elevated 6 to 11 m above sea level, (1969) assumed the presence of a till-ridge run­ whereas the younger part is much thinner, rang­ ning north-south through Læsø. The presence of ing mostly from 0 to 3 m, and is not elevated this till-ridge is speculative and not supported by more than 4.5 m above sea level. During earlier primary observations. If present, the till-ridge stages Læsø had a shape much like that of the must be buried under several metres of Yoldia island Anholt in the middle of the Kattegat. As Clay. seen in figs. 1 & 3, the eastern end of this older Locally the Yoldia Clay contains an abundant part of Læsø is cut by the present shoreline. The fauna dominated by Saxicava arctica, Macoma section at this locality shows the previous exist- 220 J. M. Hansen: Sedimentary history of Læsø + 2m +lra j(fitttf"""""tttirt•"""'- I Recent & fossil shorelines Surface of Yoldia clay Yoldia clay exposed '"• ll/l, (I, 5 km Fig. 4. Surface of the Yoldia Clay and early stages ofLcesø's development. ence of a tongue of land very like the eastern part trast, the growth of the tongue of land followed a of Anholt and orientated in the same direction. series of linear beach ridges. The island of Anholt is built up on the lee side During a postglacial transgression maximum, of a moraine knoll (Jessen 1897 & Schou 1945). probably a Subboreal transgression (Iversen However, no such structure exists or has existed 1937 & 1967) (see discussion later), the fossil at Læsø. On the contrary, Læsø seems initially cliffs and big linear beach ridges surrounding the to have been built up on the lee side of a high but older triangular part of Læsø were formed. Sub­ submerged part of the Yoldia Clay (fig. 4). The sequent shallowing of the sea produced a new triangular "core" of Læsø from which point regime of currents around Læsø. This is evident growth started, is situated immediately east of from the fact that the recurved spits on the the highest part of the Yoldia Clay under the southwestern part of Læsø during these stages sand. From this point Læsø grew northeast­ migrated northwestwards, whereas they mig­ wards forming a long tongue of land according to rated southeastwards during the older stages (cf. the dominating wind direction, and also west- fig. 1). It is clear that the sea shallowed consid­ and southwards in response to the position of the erably after the Subboreal transgression because Yoldia Clay platform. As seen in fig. 1 this of the fact that the marine sand surrounding the growth took the form of a succession of recurved old triangle is only elevated maximally 4.5 m spits on the SW side of the island.

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