GEUS Bulletin (Geological Survey of Denmark and Greenland)
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Seismology: neotectonics and structure of the Baltic Shield
No full textRecent Danish seismological projects involving neotectonic investigations and structural studies have determined the edge of the Baltic Shield underlying Denmark. The most active earthquake zones in Denmark are located in northwestern Jylland and adjoining offshore areas, and in the region around Kattegat, Øresund and north-east Sjælland (Fig. 1). This pattern was originally recognised by Lehmann (1956) and has been confirmed by several later studies, e.g. Gregersen et al. (1998). Recent, more detailed investigations have documented that changes in the pattern of earthquake activity have occurred within a short time span. The most pronounced example of change – possibly related to exploitation of hydrocarbons – is an activity recorded in the Central Graben area of the North Sea that was first documented by Gregersen et al. (1998). The south-western margin of the Precambrian Baltic Shield separates areas of different earthquake activity (Fig. 1; Gregersen et al. 1991). Although lithospheric stresses are more or less uniform in northern Europe, there are pronounced differences in the behaviour of the lithosphere across Denmark. The north-eastern area underlain by the Baltic Shield experiences brittle failure as recorded by common earthquakes, whereas earthquakes are virtually absent in the region southwest of the shield (Fig. 1). The margin of the Baltic Shield as defined by earthquake activity is not identical with that distinguished structurally in sedimentary studies (EUGENO-S Working Group 1988; Vejbæk & Britze 1994), in crustal studies (Abramovitz & Thybo 2000), or by recent studies of the structure of the subcrustal lithosphere (Gregersen et al. 2002; Shomali et al. 2002). The physical edge of the Baltic Shield cannot be uniquely determined on the basis of seismological studies. The earthquakes recorded, although of low magnitude, do give information about the released stresses. The earthquakes seem to be a response to a dominant NW–SE compression, also apparent elsewhere in Scandinavia and northern Europe (Slunga et al. 1984; Slunga 1989; Gregersen 1992; Müller et al. 1992). These stresses are part of the large-scale stress systems associated with continued plate motion pattern (Gregersen & Basham 1989; Zoback et al. 1989). In contrast to present low-magnitude earthquakes, postglacial sediments in northern Scandinavia have preserved features interpreted as caused by earthquakes of magnitudes around 7; these major, c. 9000 years old earthquakes are believed to be related to the post-glacial uplift of Scandinavia (e.g. Arvidsson et al. 1991; Gregersen 2002). Earthquakes are always related to fault activity, but attempts to link recent earthquakes occurring in and around Denmark to geologically known faults have only been partly successful (Gregersen et al. 1996). The most significant fault zone in Denmark, the Sorgenfrei–Tornquist Zone, is only locally active. Recent geodetic and seismic investigations demonstrate that the two sides of the Sorgenfrei–Tornquist Zone are characterised by different patterns of deformation, but the zone itself is not defined by a present-day seismicity trend crossing the central parts of Denmark (Fig. 1)
Greenstone belts in the central Godthåbsfjord region, southern West Greenland
No full textIn 2004 the Geological Survey of Denmark and Greenland (GEUS) initiated a study of the origin and tectono-metamorphic evolution of greenstone belts and important regional structures in the central Godthåbsfjord region, southern West Greenland (Fig. 1; Hollis et al. 2004). Like other Archaean belts worldwide, these greenstone belts are locally host to gold mineralisation. Their complexity requires a combination of detailed geological mapping, geochemistry, petrographic work and geochronological studies to develop models of their geological setting, evolution and gold mineralisation
Small-scale mining – hazards and opportunities in Kyrgyzstan and Mongolia
No full textSmall-scale mining is the main source of income for about 100 million people in Asia, Africa and South America. However, the processing of raw materials during this mining activity results in the release of large amounts of mercury to the environment, creating serious environmental problems. Small-scale mining, or artisanal mining, is exploitation using only shovels, picks and hammers, carried out by individuals or small groups. A wide variety of commodities are exploited in this way, ranging from gold, diamonds, precious stones, tin, coal, dimension stones and slate. Small-scale mining is often carried out by labourers with virtually no knowledge of safety procedures. Tunnel cave-ins leading to loss of life are common, and the widespread use of mercury in gold extraction causes many long-term health problems for the miners. It is estimated that about 650 tonnes of mercury are annually released during small-scale mining to the environment, and this figure is likely to increase in the future. Mercury is highly toxic and its use causes health problems not only for the miners, but also to the entire population in areas where small-scale mining takes place. Some miners are aware of the dangers of using mercury, but have no knowledge of recycling procedures. Several international organisations, such as the World Bank, UNIDO (United Nations Industrial Development Organisation), ILO (International Labour Organisation) and UNDP (United Nations Development Programme), have launched programmes to examine the problems associated with small-scale mining. Progress so far has been slow, and much more international awareness of the global mercury pollution of the environment from smallscale mining is required. The Geological Survey of Denmark and Greenland (GEUS) has worked as consultant to the World Bank on projects involving small-scale mining in Kyrgyzstan, Mongolia and Laos, and has also undertaken programmes concerning small-scale mining in Lesotho for UNDP and in Tanzania for the Danish International Development Agency (DANIDA). This paper reports on some of the initiatives carried out in Kyrgyzstan and Mongolia, to secure and sustain the small-scale mining industry in these regions
Colophon - Abstract - Introduction - Glacial tectonics - Location and construction of cross-section - Geological setting
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Quantifying the relationship between pollen sedimentation in lakes and land cover using historical maps
No full textPollen records from lake sediments have a great potential for providing information on the quantitative composition of past vegetation and land cover in the surrounding landscape. This can contribute to a better understanding of the development of the cultural landscape and interactions between human impact on the landscape and natural conditions like soil and climate. A good understanding of the history of cultural landscapes is necessary for choosing appropriate management strategies for areas dependent on cultural impact, such as heaths, meadows and dry pastures. It is also important for archaeological research concerning utilisation of the landscape in earlier periods. Furthermore, quantitative reconstructions are relevant for climate research. Here they can be used to test climate models, since model predictions of past climate can be translated into past vegetation, which can then be compared to pollen-based reconstructions. Past vegetation cover is also a necessary input to climate models, as it influences albedo, evapotranspiration and carbon storage and cycling. Quantifying vegetation from fossil pollen samples requires a detailed understanding of the way vegetation is reflected in pollen assemblages, including the approximate size of the area of vegetation represented. The relationship between pollen and vegetation is complicated by the fact that different plant species produce different amounts of pollen, and that pollen types are dispersed differently in the atmosphere, depending on their size, shape and weight. These pressing challenges in pollen analysis have attracted much attention in recent years. Models have been developed to describe and simulate species specific pollen dispersal, to quantitatively relate pollen proportions to plant abundance, as well as estimate pollen productivity and to quantify the pollen source area of different types of basins (Parsons & Prentice 1981; Prentice & Parsons 1983; Prentice 1985; Sugita 1993, 1994; Sugita et al. 1997, 1999; Broström 2002; Bunting et al. 2004). The Geological Survey of Denmark and Greenland (GEUS) has in recent years contributed to the development and validation of such models through the project AGRAR 2000 (Odgaard 1999; Nielsen 2003), where quantitative estimates of past land cover in different regions of Denmark were one of the main objectives, and through participation in the international research network POLLANDCAL (POLlen LANdscape CALibration), funded by NORDFORSK (Nordic Research Board), which focuses on further model development, validation and application
Identification of hydrocarbons in chalk reservoirs from surface seismic data: South Arne field, North Sea
No full textSeismic data are mainly used to map out structures in the subsurface, but are also increasingly used to detect differences in porosity and in the fluids that occupy the pore space in sedimentary rocks. Hydrocarbons are generally lighter than brine, and the bulk density and sonic velocity (speed of pressure waves or P-wave velocity) of hydrocarbon-bearing sedimentary rocks are therefore reduced compared to non-reservoir rocks. However, sound is transmitted in different wave forms through the rock, and the shear velocity (speed of shear waves or S-wave velocity) is hardly affected by the density of the pore fluid. In order to detect the presence of hydrocarbons from seismic data, it is thus necessary to investigate how porosity and pore fluids affect the acoustic properties of a sedimentary rock. Much previous research has focused on describing such effects in sandstone (see Mavko et al. 1998), and only in recent years have corresponding studies on the rock physics of chalk appeared (e.g. Walls et al. 1998; Røgen 2002; Fabricius 2003; Gommesen 2003; Japsen et al. 2004). In the North Sea, chalk of the Danian Ekofisk Formation and the Maastrichtian Tor Formation are important reservoir rocks. More information could no doubt be extracted from seismic data if the fundamental physical properties of chalk were better understood. The presence of gas in chalk is known to cause a phase reversal in the seismic signal (Megson 1992), but the presence of oil in chalk has only recently been demonstrated to have an effect on surface seismic data (Japsen et al. 2004). The need for a better link between chalk reservoir parameters and geophysical observations has, however, strongly increased since the discovery of the Halfdan field proved major reserves outside four-way dip closures (Jacobsen et al. 1999; Vejbæk & Kristensen 2000)
Climatic warming: a trigger for glacial iceberg surges (‘Heinrich events’) in the North Atlantic?
No full textIn the present-day western North Atlantic, icebergs can be observed off north-east Canada, drifting south along the coast in the cold Labrador Current. Normally they melt in the area off Newfoundland where they reach warmer waters. Most of these icebergs originate from calving glaciers in West Greenland or in the Canadian Arctic. Jakobshavn Isbræ in West Greenland (Fig. 1) deserves particular mention as it is the fastest known ice stream in the world draining 6–7% of the entire Greenland ice sheet (Joughin et al. 2004). Southward drifting icebergs also occur along the east coast of Greenland (Fig. 2), but most of these melt when they approach the southernmost tip of Greenland. The iceberg limit in the north-western Atlantic varies from year to year, but isolated icebergs may reach far south of Newfoundland (Fig. 1). Many icebergs carry a load of rock debris and soil incorporated by their parent glacier that leads to deposition of ice rafted debris on the deep ocean sea floor. In the past decade the Geological Survey of Denmark and Greenland (GEUS) has initiated marine geological investigations in the North Atlantic on the late Quaternary variability of North Atlantic thermohaline circulation, with special focus on the possible link between climate change and variations in deep-water flow intensity (Kuijpers et al. 1998, 2002, 2003). Moreover, glaciological projects in Greenland undertaken by GEUS have significantly contributed to the current debate of present-day climatic warming. Notably work carried out in East Greenland fjords has provided crucial information relevant for the study of glacial iceberg surges in the North Atlantic (Reeh et al. 1999). These surges are suggested to have been triggered under the influence of extreme cold climate conditions, but the actual trigger mechanism involved has been a matter of much debate. Evidence from modern glacier process studies referred to above, combined with results of recent studies in the North Atlantic carried out by GEUS and partner institutions, has provided new insights into the possible trigger mechanism of these massive glacial iceberg surges. These new findings have great significance for the current climate debate, since they strongly suggest that ongoing ocean warming can trigger a sudden, massive break-up of ice shelves. Such processes may already be in progress in the Arctic (e.g. Vincent et al. 2004), where rapid ice-shelf disruption on the margin of the Canadian Arctic Ocean has been reported to be the result of significant warming over the past few decades. During this period intensified inflow of Atlantic water to the Eurasian sector of the Arctic has been noted. It is evident that for Antarctic ice shelves large-scale disruption and break-up may lead to significant destabilisation of the Antarctic ice sheet with the serious risk of a sudden, drastic sea-level rise
Prospecting for dimension stones in Greenland
No full textDimension stones, or ornamental stones, are naturally occurring rocks that have properties that make them suitable for decorative exterior and interior use in the building industry. Large parts of Greenland should have a good potential for finding occurrences of valuable dimension stones, and the Geological Survey of Denmark and Greenland (GEUS) and Greenland Resources A/S (GRAS) therefore carried out prospecting in several parts of Greenland in the summers of 2001–2004. The project is mainly financed by the Government of Greenland, but recently the European Union and Nuup Kommunea have also contributed