Bulletin of the Geological Society of Denmark • Volume 44/2

Earthquakes in Denmark

Gregersen S., Hjelme, J. & Hjortenberg, E.: Earthquakes in Denmark. Bulletin of the Geological Society of Denmark, Vol. 44, pp.115-127. Copenhagen 1998-02-28.

Abstract: Within the last two decades the sensitivity to small earthquakes has been much improved in Denmark. Two to ten earthquakes are recorded each year of magnitudes 1½ to 4½. The seismicity pattern seen in recent data basically confirms the patterns noted from previous instrumental locations, as well as from felt areas of older dates. This means earthquake activity cutting off the earthquake zones of western Norway and of southern Sweden: (1) In north-western Jylland, and in the Skagerrak Sea the earthquake zone cuts off a zone of earthquakes along the western coast of Norway. At least some of these earthquakes in Jylland and Skagerrak occur at depths 30-40 km, close to Moho. (2) In north-eastern Sjaelland and in the Kattegat Sea, as well as around Bornholm the earthquake activity occurs in the upper crust, at depths shallower than 15 km. This appears as the south-western boundary of the scattered activity in south-western Sweden.

In general terms this can be considered the south-western rheological edge of the Fennoscandian Shield. The north-western earthquake zone is along the middle axis of the Norwegian-Danish Basin, and the eastern earthquake zone is in the Tornquist Zone. The two earthquake zones are not connected. This can not be ascribed to lack of sensitivity, so the Fennoscandian Border Zone can not be termed active as such. The central part of Denmark is aseismic; and the same is true for the south-western part of Denmark and northern Germany.

In the North Sea the graben area is the most active. The Viking Graben in the north has a significant earthquake activity, and the Central Graben, which goes through the Danish sector of the North Sea has small, but noticeable activity. On the British side of the graben there are additional active areas.

The stress field responsible for these earthquakes is rather uniform across the Fennoscandian Border Zone with scattered exceptions. It reflects the general NW-SE compression of northern Europe between the North Atlantic spreading ridge and the Alpine collision between Europe and Africa.

Key words: Earthquakes, Denmark, seismicity, stress field.

Address: Søren Gregersen Jørgen Hjelme & Erik Hjortenberg, Geodynamics Department, Kort & Matrikelstyrelsen, Rentemestervej 8, DK-2400 Copenhagen NV, Denmark. 18 November 1996.

The Broad Yoldia, Megayoldia thraciaeformis, in northernmost Baffin Bay: radiocarbon ages and paleoceanographic implications

Blake, W., Jr.: The Broad Yoldia, Megayoldia thraciaeformis, in northernmost Baffin Bay: radiocarbon ages and paleoceanographic implications. Bulletin of the Geological Society of Denmark, Vol. 44, pp.129-138, Copenhagen, 1997-02-28.
Abstract: Core 008 collected in 663 m of water (77^o16.0'N, 74^o19.9'W) on C.S.S. Hudson Cruise 91-039 to northernmost Baffin Bay, contained the Broad Yoldia, Megayoldia thraciaeformis, at depths of 527-529, 461-464 and 72-73 cm. Radiocarbon age determinations yielded results between 4190 ± 60 years (Ua-4118) and 2885 ± 60 years (Ua-2832), showing that this seldom mentioned pelecypod may have lived in northernmost Baffin Bay for some 1300 ¹⁴C-years or 1600 calibrated years. Furthermore, because this same species was collected alive close to the site of Core 008 in 1971, it may have persisted at this high latitude for much of the past 4000 years, or it may have disappeared, to re-establish itself in the area much more recently. As this pelecypod is quite abundant in deep water with positive temperatures off southern Greenland, it is classified as subarctic. Thus its presence in a deep trench well to the northwest of the marine subarctic boundary provides convincing evidence that wedges of warm Atlantic water have penetrated north of latitude 77° for significant portions of the last 4000 years.

Key words: Broad Yoldia, Megayoldia thraciaeformis, radiocarbon age, paleoceanography, Baffin Bay.

Address: Weston Blake, Jr., Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario KIA 0E8, Canada. July 23,1997.

Nøvling: An unusual salt structure on the southern margin of the Danish Zechstein Basin

Madirazza, I. & Jacobsen, B. H.: Nøvling: An unusual salt structure on the southern margin of the Danish Zechstein Basin.Bulletin of the Geological Society of Denmark, Vol. 44, pp.139-149. Copenhagen, 1998-02-28.

Abstract: The Nøvling salt structure is located on the southern margin of the Danish Zechstein Basin. The present study of this structure is based on a detailed gravity survey, reflection and refraction seismic data, and well information. Apart from the exploratory well Nøvling-1, and a shallower well Herning-1, a number of wells, mainly 20-30 m deep, were drilled in this general area (by the Geological Survey of Denmark). Through this combined study using geological and geophysical methods it is possible to distinguish several stages in the development of the Nøvling structure. It is shown that these stages are closely related to faulting at the base Zechstein level. The faulting, which apparently started as normal faulting in the Triassic, triggered the first salt movements from the deeper parts of the basin to this peripheral area. The faulting was reactivated during the late Jurassic, resulting in the formation of a salt pillow. The tectonic regime changed in the post-Paleocene tertiary when transpressional movements along a prominent basement wrench fault became dominant. This ultimately resulted in the formation of a positive flower structure and the uplifting of the Saale Quaternary surface. At shallow depths the salt became dissolved creating a topographic depression above the flower structure. The present depression was formed at the beginning of the last late glacial time. Quaternary landforms in the Nøvling area are therefore greatly influenced by recent tectonic events.

Key words: Nøvling, Zechstein salt, Danish Basin, transpression, geophysics, neotectonics.

Address: Ivan Madirazza and Bo Holm Jacobsen, Department of Earth Sciences, Finlandsgade 8, DK-8200 Aarhus N, Denmark.12 September 1997.

A deep seismic reflection line in the Danish Central Graben

Klinkby L., Balling, N. & Liboriussen, J.: A deep seismic reflection line in the Danish Central Graben.Bulletin of the Geological Society of Denmark, Vol. 44, pp.151-159. Copenhagen, 1998-02-28.

Abstract: Results are presented from a deep seismic reflection line, recorded to16 seconds travel time, across the Coffee Soil Fault in the southern part of the Danish Central Graben. Data processing has been focused on the deeper parts of the sedimentary sequences, the crystalline crust and the crust-mantle boundary. Depth migration is performed down to 15 km depth, and time migration is performed on the entire section. The Coffee Soil Fault is clearly imaged as a normal fault dipping about 45° from about 2.5 to 9 km depth. Interpretation of the sedimentary sequences is constrained by well data and shows an almost uniform 2 km thick unit of post-chalk deposits. The thickness of the Mesozoic sequences is to a large extent controlled by faulting and highly influenced by movements of Zechstein salt. They exceed 4 km of thickness in the halfgraben immediately west of the Coffee Soil Fault. Large block-faulted pre-Zechstein units of more than 2 km thickness are seen indicating a total depth of at least 8 km to the crystalline basement. The crystalline crust is generally non-reflective. Around 11 seconds two-way travel time (about 28 km depth) reflectivity interpreted as the crust-mantle boundary (Moho) is observed. Comparison with other deep seismic profiles across the Central Graben is taken to indicate a local crustal thinning by a factor of 1.5 associated with graben formation by extension.

Key words: Deep seismic reflections, Central Graben, data processing, Coffee Soil Fault, crustal thinning.

Address: L. Klinkby (lkl@geoserver1.aau.dk) & N. Balling, Department of Earth Sciences, Geophysical Laboratory, University of Aarhus, F'inlandsgade 8, DK-8200 Århus N, Denmark. J. Liboriussen, Mærsk Olie og Gas AS, Esplanaden 50, DK-l263 København K, Denmark. September lst, 1997.

The sandstone dyke swarm of Vang, Bornholm (Denmark)

Katzung G. & Obst, K.: The sandstone dyke swarm of Vang, Bornholm (Denmark).Bulletin of the Geological Society of Denmark, Vol. 44, pp.161-171, Copenhagen, 1997-02-28.

Abstract: A very well exposed sandstone dyke swarm in the basement of Bornholm (Vang and Hammer granite) has been discovered recently north of Vang. It strikes 120° to 125° on average. Individual dykes are up to 23 cm thick. The fillings consist of pale, greyish to brownish, fine-grained to medium-grained quartz sandstone. It has a grain-supported structure, and the porespace is filled with argillaceous-limonitic material and carbonate (dolomite) cement. The opening and filling of the fissures were caused by normal extension movements in NNE-SSW direction in several steps, probably during the early Cambrian. The overlying Balka sands were implosively sucked down into the fissure vacuums caused by the sudden opening. The downwards moving of the water saturated sediments led to a loss of material above and to the creation of funnel structures. Tapering down ring structures in the Hardeberga sandstone at the east coast of Scania (Sweden) are interpreted as such funnel structures.

Key words: Bornholm, Tornquist Ocean, Lower Cambrian, extension tectonics, sedimentary dyke swarm, sandstone, funnel structure.

Address: Gerhard Katzung & Karsten Obst, Institut für Geologische Wissenschaften, Friedrich-Ludwig-Jahn-Strasse 17A, D-17489 Greifswald, Germany. 25 September 1996.

Uranium-series isotopes from Eemian lake deposits, Hollerup, Denmark

Israelson C., Björck, S., Hawkesworth C. J. & Noe-Nygaard N.: Uranium-series isotopes from Eemian lake deposits, Hollerup, Denmark. Bulletin Geological Society of Denmark, Vol. 44, pp.173-179. Copenhagen 1998-02-28.

Abstract: We report the results of a U-series isotope study on Eemian lake sediments from Hollerup western Denmark. The purpose is to examine the possibility of dating these, and similar, sediments with the ²³⁸U-²³⁰Th method. Two sedimentary facies were studied; carbonate-present, and virtually carbonate-free but organic-rich sediments. All carbonate-present sediments (5-95%) have (²³⁰Th/²³⁸U) values higher than unity and relatively radiogenic (²³⁰Th/²³²Th) values between 2 and 6.5, indicating postdeposition removal of U, probably by percolation of ground water. Carbonate-free sediments, but with high organic content (40-60%) had (²³⁰Th/²³⁸U) values lower than 1 and 'model ages' between 89 and 199 ky. The large spread in ages and variation in U content for these sediment samples that were deposited over a much shorter time interval, indicate a complex postdepositional migration pattern of U, probably involving several episodes of leaching and absorption of U from ground water. Our results suggest that lake sediments, such as those found in Hollerup, are vulnerable to mobilization of U and its decay products and care should be taken when interpreting U-series disequilibrium data from such sediments. Future studies will concentrate on sediments that have been more deeply buried and/or are less compacted and sealed from percolation of oxidized groundwater.

Key words: Eemian, interglaciation, Hollerup, U-Th dating, sediments.

Address: C. Israelson, S. Björck & N. Noe-Nygaard, Geological Institute, University of Copenhagen Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
C. J. Hawkesworth Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. 22 August 1997.

The oldest known proboscidean and the role of Africa in the radiation of modern orders of placentals

Gheerbrant E.: The oldest known proboscidean and the role of Africa in the radiation of modern orders of placentals. Bulletin of the Geological Society of Denmark, Vol. 44, pp.181-185. Copenhagen, 1998-02-28.

Abstract: Implications of the discovery of the oldest known proboscidean (Mammalia; Proboscidea) in the late Paleocene of Morocco, Phosphatherium escuilliei, are examined here in an overview of the paleobiogeographical framework of the African mammals. P. escuilliei evidences again the role of Africa in the radiation of modern orders of mammals and the early age of this radiation, which may be related to the extinctions at the K-T boundary. It supports definitely an African origin of the proboscideans, as for primates, hyracoideans, tubulidentates and macroscelideans. It examplifies a much higher antiquity of the African endemism of the placentals than was thought.

Key-words: Africa, Paleogene, radiation, mammals, proboscideans, endemism, Ouled Abdoun Basin.

Address: E. Gheerbrant, Laboratoire de Paléontologie des vertébrés (Case 106), Université Paris 6, 4 place Jussieu, 75252 Paris cédex, France.18 February 1997.