Geological Survey of Denmark and Greenland Bulletin 35, 2016, 31-34 31© 2016 GEUS. Geological Survey of Denmark and Greenland Bulletin 35, 31–34. Open access: www.geus.dk/publications/bull The pre-Quaternary sediments and rocks in Denmark gen- erally have a low content of radioactive minerals and ele- ments. Uranium, thorium and radium are built into min- eral structures or are, for example, adsorbed on the surface of clay minerals, Fe-minerals or organic material. Radon (222Rn) is a radioactive noble insoluble gas with a half-life of 3.8 days. It belongs to the uranium (238U) decay chain where radon is formed from radium (226Ra). When Rn is formed by radioactive decay from Ra, the emanation process sends part of the radon produced into the pore spaces of rocks and soils. From here, the radon can enter and accumulate in buildings. The source of the radio- active materials in Danish sediments and rocks is primarily from weathered Precambrian crystalline rocks from Nor- way, Sweden, Finland and the Danish island of Bornholm. Physical and chemical weathering disintegrates these rocks and rivers transport the material into the Danish–Norwe- gian and Danish–Polish sedimentary basins. Several studies have analysed and described the radioac- tive content of Danish sediments and crystalline rocks (e.g. Damkjær & Korsbech 1985, 1988; Gravesen et al. 1996, 1999; Gravesen & Jakobsen 2010) and investigations have demonstrated a relationship between sediments and rocks and Rn levels in Danish buildings (Andersen et al. 2001). This paper addresses the radioactive content of sediments and rocks with the highest radioactive levels in Denmark and the highest recorded radon emanations: Precambrian crystalline rocks on Bornholm and Late Paleocene clays in north-western Jylland (Fig 1). The data were collected by Gravesen et al. (1999) at the Geological Survey of Den- mark and Greenland (GEUS) with the aim of characteris- ing and mapping Rn in Danish rocks and sediments. Methods and data In Thisted, a 4 m long trench was dug, four shallow bore- holes were drilled and older borehole data in GEUS’ Jupi- ter database were studied. The trench was dug to a depth of c. 2 m into till deposits and limestone (Fig. 2) and the lithology, structures and macro-pores were described and samples collected. The four 5–6 m deep boreholes were drilled close to the trench. A total of 48 samples of the tills, clay and limestone were described and collected every 30 cm. The samples from the trench each weighed more than 500 g whereas the core samples weighed 300–500 g. Studies and sampling were carried out at the Bornholm outcrops of granites and diabase at Allinge-Sandvig and Tejn (Fig. 1, loc. 3–4) at the northern end of the island and of the crystalline rocks at Birkely (near the farm of Val- lensgård) to the south of Almindingen (Fig. 1, loc. 5). The lithology, structures, weathering, and faults and fractures Pre-Quaternary rocks and sediments with a high level of radioactivity in Denmark Peter Gravesen and Peter Roll Jakobsen 1 2 3 4 5 10°E Sweden 57°N 55°N 10°E 100 km Denmark Sweden Jylland Germany 57°N 55°N Bornholm S o rge n f re i–T o rnqu i s t Fau l t Zone Fig. 1. Map of Denmark with the highest levels of radioactivity meas- ured in surface rocks. 1: Thisted. 2: Erslev. 3: Allinge. 4: Tejn. 5: Birkely. 2 m 1 m 0 1 m 2 m 3 m 4 m Fractures Limestone (Danian) Black Clay (Paleocene) Sand Sandy till Illuvial zone Topsoil Fig. 2. Geological section in the trench from the eastern part of Thisted. 3232 were described. A total of 10 samples of 500–1000 g were collected from the three localities. The chemical and physical components of the matrix and the petrographical components of the clast material of the samples were analysed. The U concentration was mea- sured by instrumental neutron activation analysis by Acti- vation Laboratories Ltd, Canada. The Ra content was mea- sured with a germanium detector at the National Institute of Radiation Protection, Denmark. The Rn emanation rate was determined using the closed-chamber method using ZnS(Ag) scintillation cells at the Risø National Labora- tory, Technical University of Denmark. Relationship between Danian and Late Paleocene deposits in Thisted The distribution of Danian – Late Paleocene (65.5–55.8 Ma) deposits in north-western Jylland in the area of Mors and Thisted is related to two salt diapirs, the Erslev salt diapir (Pedersen et al. 2013) and the Thisted salt dome (Hansen & Håkansson 1980). Sediment distribution around the two salt structures is similar at the pre-Quaternary surface with Maastrichtian chalk in the middle surrounded by Danian limestone and with Palaeogene clays bordering the lime- stone. The layers are generally inclined away from the centre of the salt structures. Different aspects of the radioactive components in the areas were investigated by Damkjær & Korsbech (1985) and Gravesen et al. (1996). The Quaternary cover above the Danian limestone is rather thin (0–2 m) in the Thi- sted area. A topsoil layer overlies Weichselian glacial sandy tills, clayey tills, and just above the limestone is a strongly calcareous till (about 60–80% CaCO3). These tills contain vertical fractures at 5–10 cm intervals. The Late Danian limestone is partly cemented and can be classified as a calcilutitic or calcarenitic limestone ac- cording to grain size. The trenches demonstrated more complex geological structures than expected (Fig. 2). The top surface of the Danian limestone was eroded and brec- ciated during the Late Paleocene and cut by horizontal and vertical fractures with pronounced karst features and inclined fractures filled with black structureless Paleocene clay which contains Selandian dinoflagellate cysts. The clay also contains limestone clasts and is probably redeposited. At the bottom of the karst structures, several clay-filled fractures reach at least 10–15 cm downwards (Fig. 2). In a few shallow boreholes at Thisted, more than 50 cm of black clay has been encountered indicating non-eroded remnants of the black clay, and black clay material is also enclosed in the till at some locations. The content of radioactive material in the Thisted sedi- ments is presented in Table 1 (Data from Damkjær & Kors- bech 1988 and Gravesen et al. 1999). The U, Ra and Rn values for the Danian sediments are comparable with levels for these sediments in other parts of the country (Damkjær & Korsbech 1985) but the high value for the radon emana- tion of the black clay can only be compared with levels of Paleocene black clay from the Erslev area which has even higher values (Table 1). High levels of radon emanations are also known from Cambrian–Ordovician alum shale: 16 atoms/kg/s, Eocene diatomites: 22 atoms/kg/s, Miocene U-bearing heavy sand: 52 atoms/kg/s, and Miocene black clay: 38.9 atoms/kg/s (Damkjær & Korsbech 1985). Discussion Karst in Danish limestone and chalk is found in many areas of northern Jylland but the occurrence in the Thi- sted trench is noteworthy due to its radioactivity. Karst is formed by acidic water percolating through fractures in the limestone and dissolving parts of it. The erosion of the pre- Quaternary surface started at the inversion and uplift of the Sorgenfrei–Tornquist zone at the end of Cretaceous– Paleocene time followed by Cenozoic sub-areal erosion (Stenestad 2006). The erosion had removed the Late Pale- ocene clay from most of the Thisted area and redeposited part of the material in the shallow karst holes in the lime- stone. In a search for the origin of Rn in buildings Damkjær & Korsbech (1988) investigated sediments from the Erslev area and suggested that redeposited black clay in tills was the source of the high levels of radon. However, the results showed that the high values were in the limestone areas. The later investigation of Andersen et al. (2001) demon- Allinge Hammer granite 4.2 86.9 − (weathered) Hammer granite 3.0 63.5 − Tejn Vang granite 3.8 66.0 − Birkely Almindingen granite 3.4 41–51 9.6 (weathered) Almindingen granite 4.3 50.7 8.8 Diabase (weathered) 4.2 104.2 39.6 Clay: weathered diabase 8.5 − − Locality Lithology and age Uranium Radium Radon ppm Bq/kg Atoms/kg/s Thisted Weichselian sandy till 0.8−1.9 18.7−24.7 7.5−10.4 Late Paleocene clay − − 42.4 Danian limestone < 0.6 2.8−3.3 0.12−0.8 Erslev Paleocene clay 2.72−26.3 38−300 11.2−130 Table 1. Measured radioactive components 33 strated relatively high levels of Rn in buildings sitting on limestone in Thisted although the radioactivity of lime- stones at both localities was among the lowest in Danish sediments. The present investigation from Thisted based on the trench and shallow boreholes shows that the high levels of Rn emanation from isolated or redeposited Late Paleocene black clays can probably be the source of the high Rn levels in the buildings in this region. In Sussex, England, high Rn levels in houses are also partly caused by redepos- ited material on top of low-radioactive chalk (Killip 2004). Precambrian basement rocks: radioactivity and weathering Bornholm is situated in the Sorgenfrei–Tornquist Fault Zone south of Sweden (Fig. 1). The Precambrian basement of northern and eastern Bornholm consists of granitic and gneissic rocks which contain abundant leucogranitic bod- ies, pegmatites and aplites, besides more than 250 mafic dykes that cut these crystalline rocks. The Kampeløkke Å locality at Allinge consists of medi- um-grained Hammer granite with a c. 20 cm thick crust of weathered granite (Fig. 3). The Hammer granite com- prises 41% K-feldspar, 18% plagioclase, 33% quartz, 1% hornblende, 4% biotite, and accessory Ti-magnetite, apa- tite, epidote, allanite and fluorite (Micheelsen 1961). Large crystals of black gadolinite, a REE-Fe-Be silicate mineral, are found in the area with abundant small red spots of Fe2O3 on its crystal surfaces. The granite is cut by vertical and horizontal fractures. The content of radioactive com- pounds is seen in Table 1. In the Hammer granite the U content is between 3.0 and 4.2 ppm and the Ra content is between 63.5 and 86.9 Bq/kg. The Møllebæk locality at Tejn consists of coarse-grained Vang granite with very coarse-grained pegmatites covered by 10–20 cm thick weathered granite. The granite contains vertical and horizontal fractures. The Vang granite is com- posed of 33% K-feldspar, 22% plagioclase, 27% quartz, 5% hornblende, 6% biotite, 3% Ti-magnetite, 1% titanite, 1% apatite and subordinate allanite (Micheelsen 1961). The pegmatites have only a low content of dark miner- als (averaging 1%) but 45–60% K-feldspar and 30–40% quartz. The content of radioactive components is compa- rable to that of the Hammer granite. The Birkely locality (1½ km north of the farm Val- lensgård) is a small quarry with medium-grained Almind- ingen granite. The mineralogical composition is nearly the same as for the Hammer granite. Figure 4 shows a section with a strongly fractured and faulted part with several part- ly weathered olivine diabase dykes and weathered granite. These faults and fractures are orientated WNW–ESE, the same direction as the major fault between the crystalline rocks and the younger sediments (Micheelsen 1961), and the faults have slickensides demonstrating movement. The diabase dykes mainly contain olivine and pyroxene but biotite, epidote and hornblende also occur (Callisen 1934). Alteration of these minerals produced serpentine, chlorite and calcite. The diabase dykes are orientated NW– SE as are many other diabase dykes and faults in these rocks. Along its contacts with weathered granite, the dia- base is altered to green clayey material of chlorite or serper- tine (Fig. 4). The rocks in the area are partly weathered and Fe-bearing minerals are oxidised to yellow-brown, clayey iron-rich weathering products on the fracture surfaces. The content of radioactive components is shown in Table 1. Discussion In this study, the U content of the Hammer, Almindingen and Vang granites was found to be below 5 ppm, but higher values (6–16 ppm) have previously been recorded (Johans- son et al. 2016). The olivine diabase dyke yielded a com- parable U content, about 4.2 ppm. The granites and dia- bases have high levels of U, Ra and emanations of Rn, and examples of weathered granites and diabases with higher levels are found. Some of these rocks are among those with the highest radon emanations known in Denmark. The 380 180 160 140 120 100 80 60 40 20 0 cm Kampeløkke Å Allinge Møllebæk Tejn Hammer granite Vang granite Scree Sandy and gravelly till Weathered granite Hammer and Vang granite Fracture Pegmatite Fig. 3. Geological sections from Allinge and Tejn. 3434 weathered diabase yields Rn emanation levels comparable to the Paleocene clay from Thisted. According to leaching investigations of Pliler & Adams (1962), granitic rocks become depleted in U during the first phase of acid chemical weathering, but during the sub- sequent weathering phases, the top of the weathered rocks become altered and now predominantly consist of resistant U-bearing minerals. The weathering of the ground surface and shallow fractures of the rocks is mainly due to hydro- lysis by percolating surface water and fluctuating low tem- peratures. The mafic minerals in diabases, such as hornblende, py- roxene and biotite, can be altered to green chlorite or serpen- tine and eventually form a clayey material hosting the U. The crystalline rocks are the primary source of radon in buildings on northern Bornholm, and especially where cellar walls consist of unweathered or weathered rocks the Rn concentrations in the buildings can be high (Andersen et al. 2001). Conclusions The black Paleocene clay on Danian limestone in Thisted and the crystalline rocks on Bornholm are among the Dan- ish deposits with the highest contents of U and Ra, and they also have the highest Rn emanation rates. Other sedi- ments at this emanation level are fine-grained Cambrian– Ordovician alum shale, Eocene diatomites and Miocene black and brown clays and sand. Most of these sediments, like the Thisted clay deposits, have limited distribution and are found below thick Quaternary layers and can be difficult to locate. Acknowledgements The Danish Ministry of Health is thanked for financial support. References Andersen, C.E., Ulbak, K., Damkjær, A. & Gravesen, P. 2001: Radon i danske boliger. 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Pliler, R. & Adams, J.A.S. 1962: The distribution of thorium and ura- nium in a Pennsylvanian weathering profile. Geochimica et Cosmo- chimica Acta 26, 1137–1146. Stenestad, E. 2006: Fluviokarst in the top of the Maastrichtian chalk at Rørdal, northern Jutland, Denmark. Bulletin of the Geological Society of Denmark 53, 93–110. Schmidt equal area projection N = 22 10 m Weathered granite Granite Diabase Diabase, clayey Fractures Fault Fig. 4. Geological section from the Birkely quarry, south of Almind- ingen, Bornholm. The orientation of faults and fractures, and diabase sheets (green) are shown in the diagram. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: pg@geus.dk