GEUS Bulletin (Geological Survey of Denmark and Greenland)
Not a member yet
521 research outputs found
Sort by
Use of geochemistry in groundwater vulnerability mapping in Denmark
No full textThe principal aim of mapping ground-water vulnerability in Denmark is to ensure optimal protection of present and future drinking-water resources. Groundwater vulnerability mapping of areas up to 1000 km2 has been taking place over the past seven years. The scale of mapping has been adjusted to meet the demands for details of regulation of land use requested by Danish legislation. Groundwater vulnerability mapping comprises analyses and integration of geological, geophysical, hydrological and geochemical data. This paper focuses on the geochemical reactions between groundwater and sediment. Geochemical knowledge may sometimes not be fully and systematically utilised in groundwater vulnerability mapping. This paper presents different geochemical approaches and demonstrates how these can be successfully integrated with geological, geophysical and hydrological data
Diagenesis influencing the porosity of Upper Jurassic reservoir sandstones, Danish North Sea
No full textUpper Jurassic quartz-rich sandstones in the North Sea Basin are important reservoir rocks for oil and gas, and one of the latest discoveries of oil in the Danish sector was made in the area of the Hejre wells that penetrated such sediments (Fig. 1). The reservoir properties of sandstones are strongly influenced by diagenetic alteration, i.e. the mineralogical changes that take place during burial of the sediments. The diagenetic features depend on the source area, depositional setting, facies architecture and burial history of the sediment. The major diagenetic features influencing porosity in Upper Jurassic reservoir sandstones are feldspar dissolution and precipitation, precipitation of illite, calcite and quartz, and quartz stylolite formation. With regard to the Upper Jurassic sandstones in the Danish sector of the North Sea, the important question is: how can porosity be preserved in sediments buried at depths of more than 5 km? The Hejre-2 well penetrated the Upper Jurassic sediments (Fig. 2) before reaching pre-Upper Jurassic volcaniclastic conglomerates. The diagenetic features were studied in thin sections of core samples with traditional petrographic techniques using transmitted light microscopy supplemented by scanning electron microscopy (SEM) of rock chips and thin sections
New zircon ages from the Tasiusarsuaq terrane, southern West Greenland
No full textIn the last three field seasons the Geological Survey of Denmark and Greenland (GEUS) has undertaken mapping in the south-eastern part of the Nuuk region in southern West Greenland, and here we present new zircon ages that help constrain the northern boundary of the Tasiusarsuaq terrane. The Archaean geology of the Nuuk region is commonly interpreted as a tectonic collage assembled through lateral accretion and collision of oceanic and continental slivers and blocks (e.g. Friend & Nutman 2005). Popular jargon describes these as terranes, bounded by faults or mylonite zones and characterised by rocks of contrasting origin on either side of their tectonic boundaries (Coney et al. 1980). The Isukasia and Færingehavn terranes (Figs 1, 2) are the oldest terranes at ≥3.75 Ga, and extend from the outer part of Godthåbsfjord in the south-west to the margin of the Inland Ice in the north-east, but they might not have a common geological history (Friend & Nutman 2005). The Tre Brødre terrane is mainly represented by the Ikkatoq gneiss and occurs in close spatial relationship with the Færingehavn terrane, and also as a pronounced thrust unit along the Qarliit Nunaat thrust between the Færingehavn and Tasiusarsuaq terranes (Fig. 1; Nutman et al. 1989). The terrane boundaries in the inner fjord region near the Inland Ice margin are less well constrained; the Tre Brødre terrane extends into the region from the south-west, the Kapisilik terrane is defined from the northern and eastern part and borders the Tasiusarsuaq terrane to the south and possibly to the east. The terrane accretion is believed to have taken place in two events. The first terrane accretion is defined from the northern part of the region, and possibly involves the Isukasia, Kapisilik and Akia terranes. The thermal event stitching these terranes is dated to c. 2.99–2.95 Ga (Fig. 2; Hanmer et al. 2002; Friend & Nutman 2005). The second accretion phase of the major continental blocks is believed to have occurred at around 2.725–2.71 Ga. This second event is well described, and includes anatexis and emplacement of continental crust-derived granites, which are associated with contemporaneous metamorphism (Friend et al. 1996). Figure 2 outlines regional plutonic, metamorphic and supracrustal events. Individual terranes were formed during relatively short time periods with active geological processes of creation and recycling of continental crust, and most of the terranes follow a similar pattern of development. The first plutonic events consisted of primitive magmas and produced tonalite–trondhjemite–granodiorite (TTG) and dioritic gneisses. Younger, more evolved granitic magmas were often intruded simultaneously with high-grade metamorphism. This development may reflect a stabilisation of the individual terranes
Base Quaternary in the Danish parts of the North Sea and Skagerrak
No full textOver the years, several maps of the base Quaternary surface of the Danish area have been published. However, the maps have either been local in character (e.g. Håkansson & Pedersen 1992; Huuse et al. 2001) or have concentrated on special topics such as tunnel valleys (e.g. Huuse & Lykke-Andersen 2000) or glaciotectonic features (e.g. Klint & Pedersen 1995; Andersen et al. 2005). The only published map of a more regional character is that of Binzer & Stockmarr (1994) that covers onshore Denmark and eastern Danish waters. Here we present for the first time a regional map of the base Quaternary surface for the entire Danish sector of the North Sea and Skagerrak based on interpretations of reflection seismic data at the Geological Survey of Denmark and Greenland (GEUS) (Fig. 1). The new map has been depth-converted and merged with the onshore map of Binzer & Stockmarr (1994) and thus the first map covering the entire Danish land and sea areas has been compiled. The definition of the base Quaternary is a current issue of debate. In this article, we follow Gradstein et al. (2004) who place the base Quaternary at base Gelasian, which is dated to 2.59 Ma. In parts of the studied area, glacial tectonic features in the form of thrust complexes can be seen on the seismic data. Here the base Quaternary surface has been placed at the base of the dislocated thrust units, corresponding to the basal décollement horizon. The base Quaternary surface is of both academic and practical interest. The depth to the base Quaternary surface and its morphology are of interest to the understanding of the Quaternary development of the region, but are also important in relation to offshore constructions such as oil and gas platforms, pipelines and wind mills
Environmental response to the cold climate event 8200 years ago as recorded at Højby Sø, Denmark
No full textThe need for accurate predictions of future environmental change under conditions of global warming has led to a great interest in the most pronounced climate change known from the Holocene: an abrupt cooling event around 8200 years before present (present = A.D. 1950), also known as the ‘8.2 ka cooling event’ (ka = kilo-annum = 1000 years). This event has been recorded as a negative δ18O excursion in the central Greenland ice cores (lasting 160 years with the lowest temperature at 8150 B.P.; Johnsen et al. 1992; Dansgaard 1993; Alley et al. 1997; Thomas et al. 2007) and in a variety of other palaeoclimatic archives including lake sediments, ocean cores, speleothems, tree rings, and glacier oscillations from most of the Northern Hemisphere (e.g. Alley & Ágústsdóttir 2005; Rohling & Pälike 2005). In Greenland the maximum cooling was estimated to be 6 ± 2°C (Alley et al. 1997) while in southern Fennoscandia and the Baltic countries pollenbased quantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around 1.5°C (e.g. Seppä et al. 2007). Today there is a general consensus that the primary cause of the cooling event was the final collapse of the Laurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassiz into the North Atlantic Ocean around 8400 B.P. (Fig. 1; Barber et al. 1999; Kleiven et al. 2008). This freshwater outflow, estimated to amount to c. 164,000 km3 of water, reduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to the North Atlantic region, resulting in an atmospheric cooling (Barber et al. 1999; Clark et al. 2001; Teller et al. 2002). The climatic consequences of this meltwater flood are assumed to be a good geological analogue for future climate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warming models (e.g. Wood et al. 2003; IPCC 2007) and is also currently being registered in the region (Curry et al. 2003). In an ongoing project, the influence of the 8.2 ka cooling event on a Danish terrestrial and lake ecosystem is being investigated using a variety of biological and geochemical proxy data from a sediment core extracted from Højby Sø, north-west Sjælland (Fig. 2). Here we present data on changes in lake hydrology and terrestrial vegetation in response to climate change, inferred from macrofossil data and pollen analysis, respectively
Geochemistry of greenstones in the Tasiusarsuaq terrane, southern West Greenland
No full textTonalite-trondhjemite–granodiorite (TTG) gneisses and melanocratic to ultramafic greenstones dominate the Archaean basement of southern West Greenland. The greenstones are likely to represent different original environments, which is important as the mineral deposits they may host depend on this. For example, massive sulphide deposits associated with gold and base metals are commonly volcanogenic, while chrome, nickel and platinum group elements are more commonly associated with layered intrusions (Robb 2005). Current investigations by the Geological Survey of Denmark and Greenland (GEUS) in southern West Greenland are therefore focused on the origin of greenstones and their relationship to associated TTG gneisses. Here, we report on work in progress on greenstones within the Tasiusarsuaq terrane (Fig. 1; Friend et al. 1996). They differ from many other greenstone belts in southern West Greenland in their spatial association with the TTG gneisses. Unlike the Isua, Ivisârtoq and Storø greenstone belts in the central and northern Nuuk region, the Tasiusarsuaq greenstones are not proximal to terrane boundaries but form dismembered blocks and slivers within the terrane (Fig. 1). Contact relationships to the gneisses are almost exclusively tectonic, and primary textures are, with rare exceptions, obliterated by amphibolite to granulite facies metamorphism
Cenozoic palaeogeography and isochores predating the Neogene exhumation of the eastern North Sea Basin
No full textDenmark is a key region for studies of the Cenozoic development of Scandinavia because Paleocene to Upper Miocene sediments crop out across the country and because it is possible to correlate these occurrences with the up to 3 km thick Cenozoic succession of the North Sea Basin. However, the reason why the Cenozoic deposits occur close to the surface of the Earth in Denmark is that the sediments have been exhumed from their cover of younger rocks. This implies that a reconstruction of the Cenozoic development across Denmark – involving both burial and exhumation – must rely on sedimentological and seismic studies of preserved sediments as well as on physical parameters that may yield evidence of the postdepositional history of the sediments now at the surface. Only if the burial and exhumation history of the basins can be deciphered is it possible to infer the geological development in the Scandinavian hinterland where Cenozoic sediments are rarely preserved. We have identified four Mesozoic–Cenozoic palaeothermal phases related to burial and subsequent exhumation, and one phase reflecting climate change during the Eocene. This is based on new apatite fission-track analyses (AFTA) and vitrinite reflectance data from eight Danish wells (Japsen et al. 2007a). The study combined thermal history reconstruction with exhumation studies based on palaeoburial (sonic velo city), stratigraphic and seismic data (cf. Japsen & Bidstrup 1999; Green et al. 2002; Nielsen 2003; Rasmussen 2004; Japsen et al. 2007b). Two of the exhumation phases occurred during the mid-Jurassic and the mid-Cretaceous. In this study we focus on the Cenozoic development and on the early and late Neogene exhumation phases during which up to 1 km of sediments were removed across most of the Danish region (Fig. 1)
Fully automated analysis of grain chemistry, size and morphology by CCSEM: examples from cement produc tion and diamond exploration
No full textComputer-controlled scanning electron microscopy (CCSEM) combines the advantages of energy dispersive X-ray spectrometry (EDX) with those of digital image analysis of back-scattered electron (BSE) micrographs. CCSEM analysis of a wide range of geological or non-geological materials has been introduced at the Geological Survey of Denmark and Greenland (GEUS) as a fast and reliable method to determine both the chemistry of individual grains and bulk samples. The chemical analysis is combined with measurements of the two-dimensional size and morphology of every single grain. The CCSEM technique was developed in the early 1980s for characterisation of coal minerals (Huggins et al. 1980; Lee & Kelly 1980) and studies of synthetic crystals for super-conductors and catalysts (Lin & Barnes 1984). Soon it found a broader application in the study of dust particles and fibres in lung tissue of mine workers (Friedrichs 1987), in the analyses of aerosols for air quality control and source emission characterisation (e.g. Heasman & Watt 1989) and the degree of sintering and consolidation of coal ash deposits (e.g. Huffman et al. 1994). CCSEM has been used in the earth sciences for the determination of the sediment budget of a lake (Yin & Johnson 1984), for the characterisation of soil and dust (Pirrie et al. 2004), for provenance analysis of ilmenite-bearing beach sands (Knudsen et al. 2005; Bernstein et al. 2008), and provenance studies on sandstones in oil-bearing basins (Frei et al. 2005). Other areas where CCSEM has been applied range widely and include characterisation of small inclusions, e.g. impurities in metal alloys or steel (Schwoeble et al. 1988), analyses of gun-shot residues (e.g. Steffen et al. 2007), and analyses of bladder stones obtained from a skeleton found in a Mesolithic cave-tomb (D’Alessio et al. 2005). In this paper, we demonstrate the benefits of the method with examples from the cement industry and from diamond prospecting
Evidence of stretching of the lithosphere under Denmark
No full textThe structure of the lithosphere under Denmark has been investigated in relation to adjacent regions of Sweden and Germany. The most interesting result of the study is that the 120 km thick lithosphere under Denmark appears to be a stretched version of the Swedish lithosphere, which is more than twice as thick. During the international project Teleseismic Tomography across the Tornquist Zone (Tor), field work and international interpretation were carried out between 1996 and 2002. Following the field work period, model velocity computations were undertaken based on observations of distant earthquakes (e.g. Arlitt 1999; Shomali et al. 2002; Voss et al. 2006), and recently an evaluation of the Tor results was completed (Nielsen 2007). The Tor project investigates deeper parts of the Earth than previous projects, and in particular the depth interval 50–300 km, which is below the crystalline crust. The investigations have included many geophysical features such as teleseismic P-wave tomography, Rayleigh wave velocities, shear wave splitting and wave scattering. We have distinguished between relatively high- and low-velocity zones, which also show variations in anisotropy and scatter characteristics. Generalised high-velocity zones correspond to the lithosphere, while generalised relatively low-velocity zones are equivalent to the asthenosphere. The main outcome of the combined studies is that the deep lithosphere can be divided into three blocks separated approximately along the national boundaries between Sweden and Denmark and between Denmark and Germany. The boundaries between the blocks are steep, almost vertical. The Denmark block has lithosphere properties between those to the north and south. Based on previous crustal studies and the Tor results, we suggest that the Denmark block has evolved by stretching. The details in the new evaluation are derived from teleseismic tomography. Here we present a synthesis of the many derived models in the light of the new evaluation
Geophysical imaging of porosity variations in the Danish North Sea chalk
No full textMore than 80% of the present-day oil and gas production in the Danish part of the North Sea is extracted from fields with chalk reservoirs of late Cretaceous (Maastrichtian) and early Paleocene (Danian) ages (Fig. 1). Seismic reflection and inversion data play a fundamental role in mapping and characterisation of intra-chalk structures and reservoir properties of the Chalk Group in the North Sea. The aim of seismic inversion is to transform seismic reflection data into quantitative rock properties such as acoustic impedance (AI) that provides information on reservoir properties enabling identification of porosity anomalies that may constitute potential reservoir compartments. Petrophysical analyses of well log data have shown a relationship between AI and porosity. Hence, AI variations can be transformed into porosity variations and used to support detailed interpretations of porous chalk units of possible reservoir quality. This paper presents an example of how the chalk team at the Geological Survey of Denmark and Greenland (GEUS) integrates geological, geophysical and petrophysical information, such as core data, well log data, seismic 3-D reflection and AI data, when assessing the hydrocarbon prospectivity of chalk fields