surface‑bedrock interactions
4.2 Evaluation of primary data
4.2.1 Regolith and Quaternary geology
Thorough descriptions and evaluations of the input data and the methods used are presented in /Hedenström et al. 2008/. The description of the Quaternary deposits is focused on the spatial dis-tribution of the different units, together with a description of their physical and chemical properties.
The physical properties are used as input data for the hydrogeological modelling /Johansson 2008/, whereas the chemical properties contribute to the biological models of the upper geosphere and to the hydrogeochemical modelling /Tröjbom et al. 2007/.
Below follows a condensed description of the Quaternary geology and the distribution and properties of regolith in the Forsmark area. The description consists of a brief overview of the geological evolution of the area and the spatial distribution and properties of the different units. For a detailed description of the properties and distribution of the regolith at Forsmark, see /Hedenström and Sohlenius 2008/ and for the geological and historical development of the site, see chapter 3 as well as chapters 4 and 6 in /Söderbäck (ed) 2008/. A description of the semi-3D regolith depth model is found in /Hedenström et al. 2008/.
Distribution
Quaternary deposits cover c. 90% of the ground surface within the model domain (Figure 4-1).
The average modelled regolith depth at Forsmark, including the bedrock outcrops, is 5.6 m
1620000
1620000
1625000
1625000
1630000
1630000
1635000
1635000
1640000
1640000
1645000
1645000
6693000
6698000 6698000
6703000 6703000
6708000 6708000
±
0 1 2 4km
2008-02-11, 10:00
Quaternary deposits
Consent I 2007/1092
© Lantmäteriverket Gävle 2007.
Peat and gyttja Clay-silt (not specified) Postglacial clay and clay gyttja Glacial clay
Glaciofluvial sediment Till
Clayey till or boulder clay Precambrian bedrock
/Hedenström et al. 2008/. The spatial distribution of the regolith is highly dependent on the bedrock morphology. The more elevated areas in the south-western part are dominated by till and bedrock outcrops, whereas the fine-grained sediments are concentrated in the deeper areas offshore. Exposed bedrock or bedrock with only a thin regolith (< 0.5 m) occupies c. 9% of the area in the regional model area and only c. 5% of the central part (Table 4-1). Areas with low frequency of outcrops are e.g. the eastern part of Storskäret, west of Lake Bolundsfjärden, and the major part of the marine area. Areas with high frequency of bedrock outcrops are e.g. north of Bruksdammen, along the present shoreline and on several of the small islands. Many of the outcrops are abraded on the northern side, indicating active ice with a dominant flow direction from 350–360°, while an older ice-movement direction from the north-west is preserved on lee side positions.
All known regolith in the Forsmark regional model area has been deposited during, or after the Weichselian glaciation, i.e. during the last 115,000 years /Söderbäck (ed) 2008/. The oldest deposits are of glacial origin, i.e. deposited directly from the Weichselian ice sheet, or by water from the melting ice. The regolith in Forsmark, as in general in north-eastern Uppland, is characterised by a flat upper surface, young and un-weathered soils, high content of calcium carbonate in gravel and fine fractions, and the occurrence of till with high clay content. The calcium carbonate has its origin from Paleozoic limestone on the sea bottom north of Forsmark, incorporated into and deposited by the glaciers. The soils are typically poorly developed and dominated by Regosols, Gleysols and Histosols /Lundin et al. 2004/.
Till is the dominant Quaternary deposit at Forsmark (Figure 4-1 and Table 4-1). In the terrestrial area, the till has been divided into three areas, cf. /Sohlenius et al. 2004/. Till area I constitutes the major part of the Forsmark terrestrial area, especially in the western and southern parts of the model area. In this area, sandy till with a medium frequency of superficial boulders dominates. The average depth to bedrock within Till area I is 3.5 m (Figure 4-2). Till area II is dominated by clayey till and boulder clay, i.e. a clay content > 5% of the matrix. Clayey till dominates at Storskäret in the eastern part of the model area, and the major part of the arable land in Forsmark is located within this area.
The average depth of the clayey till/boulder clay is 5.8 m /Hedenström et al. 2008/. Till area III is located in the eastern part of the investigated area, close to the Börstilåsen esker, and is characterised by a surface layer with a high frequency of large boulders.
The stratigraphical relation between the till units is more complex than the surface distribution.
Sandy till has been observed on top of clayey or silty till in the western part whereas the reverse has been recorded in the east. At the transition between Till area I and II, the two till types have been nested into each other in a more or less random way /Hedenström and Sohlenius 2008/. A unit consisting of a hard clayey till has been observed under sandy till at several sites within Till area I.
Table 4‑1. The proportions (%) of the area covered with different Quaternary deposits and bedrock exposures, overall and in subareas of the Forsmark area. The subareas are described in Figure 2‑3 in /Hedenström et al. 2008/. Terrestrial refers to all areas excluding those covered by water. Detailed terrestrial refers to the central part of the model area /Sohlenius et al. 2004/.
All areas Terrestrial Detailed terrestrial Marine area
Bedrock exposures 9 13 5 6
Glacial clay 25 4 4 41
Postglacial clay (including
gyttja clay and gyttja) 11 4 4 17
Postglacial sand and gravel 4 2 4 6
Till (sandy/clayey) 48.5 (46/2.5) 65 (58/7) 74 (63/11) 30
Glaciofluvial sediment 0.5 1 2 0
Peat 1 8 3 –
Artificial fill 1 3 4 –
Glaciofluvial sediments in Forsmark are concentrated in one small esker, the Börstilåsen esker.
Information from the site shows that c. 5–7 m glaciofluvial sand, gravel and stones are located directly on the bedrock. The finest particles, clay and silt, were deposited in deeper water, on top of till /Elhammer and Sandkvist 2005/. Hence, the major part of the glacial clay is found in the offshore area, especially in the areas with a water depth greater than 6 m. In the marine area, the average depth of glacial clay is c. 3 m /Elhammer and Sandkvist 2005/. In the terrestrial part, the areas covered with glacial clay are concentrated in local depressions such as the bottom of lakes and small ponds. Generally, glacial clay in terrestrial areas, especially those not associated with the larger Figure 4‑2. The total modelled regolith depth from /Hedenström et al. 2008, Figure 4-1/. Generally, the regolith is deeper in the marine area (average c. 8 m) than in the terrestrial part (average c. 4 m). The average regolith depth within the whole model domain is c. 6 m.
After the deglaciation c. 8800 BC, the water level in the Forsmark area was c. 150 m higher than at present /Påsse 2001/. Since the most elevated areas in Forsmark are only c. 25 m above sea level (m.a.s.l.), the Forsmark area has been situated below the Baltic Sea until the last few thousand years.
Thus, the formation, erosion and relocation of postglacial deposits have mainly been taking place prior to the emergence from the Baltic Sea. Postglacial gravel and sand are frequently superimposed on glacial clay, interpreted to mainly represent deposition after erosion and transport by currents on the sea floor. Postglacial clay, including clay gyttja, is predominantly found in the deeper parts of valleys on the sea floor (Figure 4-1) and only minor occurrences are documented in the terrestrial area. In the marine area, the average depth of postglacial clay is 0.9 m /Elhammer and Sandkvist 2005/. The ongoing isostatic uplift transfers sedimentary basins to sheltered positions, favouring the accumulation of organic sediments. Clay gyttja is frequent in the surface of the wetlands located at low altitudes, e.g. along the shores of Lake Fiskarfjärden and Lake Gällsboträsket. Gyttja is formed in lakes and consist mainly of remnants from plants that had grown in the lake. In areas with calcare-ous soils, such as the Forsmark area, calcarecalcare-ous gyttja is formed when lime-saturated groundwater enters the lake and/or by biological precipitation by algae. Peatlands in the Forsmark area are generally young and nutrient rich. Bogs do occur but are few and still young, whereas rich fens are the dominating type of wetlands. Peat is found most frequently in the south-western part of the area, i.e. the most elevated parts that have been above the sea level long enough for infilling of basins and peat to form. Stenrösmossen and the mire at Rönningarna are two examples of mires, which, at least partly, are developed into bogs with an average depth of the peat layer of 1.4 m.
Properties
Table 4-2 presents a summary of grain-size distribution, sorting coefficient (D60/D10) and calcium carbonate (CaCO3) content of the minerogenic deposits. The clay content in till samples is between 0.9% (sandy till) and 25.9% (boulder clay) and the CaCO3 is between 1% (clayey till) and 34% (sandy till). The higher the sorting coefficient (D60/D10) is, the more poorly sorted are the minerogenic deposits.
Table 4‑2. Summary of some physical parameters of the most common minerogenic deposits in Forsmark. The ratio D60/D10 (where D60 is the particle diameter corresponding to 60% finer on the grain‑size curve, and D10 is the particle diameter corresponding to 10% finer on the grain‑size curve) is also called coefficient of uniformity. The smaller the coefficient of uniformity, the more uniform the material. SD = standard deviation.
Deposit Gravel %
(SD) Sand %
(SD) Silt %
(SD) Clay %
(SD) Porosity %
(SD) D60/D10
(SD) CaCO3 %
(SD) Sandy and sandy silty till
(Till area I), n=66 22.1
(7.8) 50.8
(10.1) 23.5 (8.9) 3.6
(1.1) 9.47 46.4
(29.5) 19.1
(5.7 (n=62)) Clayey till and boulder clay
(Till area II), n=103 15.8
(8.1) 41.2
(6.8) 32.3
(7.2) 10.8
(4.4) 8.5 104.0
(155.4) 23.4
(6.6) Gravely till (Till area II),
n=15 44.8
(8.8) 40.7
(10.7) 12.7 (5.4) 2.4
(1.1) 132.6
(105.7) 17.5
(5.3)
Gravel, n=5 53.1
(6.7) 44.2
(6.1) 1.5
(1.2) 1.2
(1.1) 16.8
(4.1)
Sand, n=15 16.9
(16.2) 68.0
(14.1) 13.0 (15.4) 2.7
(2.2) 10.6
(8.1)
Silt, n=4 0.2
(0.5) 15.9
(13.7) 68.9
(12.6) 15.0
(6.8) 9.9
(not valid)
Clay, n=30 0.6
(1.8) 3.0
(7.2) 41.6
(16.1) 54.8
(13.8) 18.0
(13.3 (n=29))
The bulk density of the upper 60 cm of the regolith was investigated in a number of horizons in the soil type inventory /Lundin et al. 2004/. In the upper horizon (0–10 cm), bulk density is low;
0.4–1.5 g/cm3. The density increases downward to 1.4–2.3 g/cm3 (50–60 cm) /Lundin et al. 2004/.
Bulk density measured in two trenches located in Till area I was 1.5–2 g/cm3 in the top soil layers, whereas in the deeper layers it was 1.9 and 2.3 g/cm3, respectively. Porosity is c. 30–40% in the surface and c. 10–20% at c. 2 m depth. The contents of carbon, nitrogen, sulphur and phosphorous in marine and lacustrine sediments and in peat are presented in Table 4-3. In general, the porosity is higher (37–90%) in the water-laid sediments than in the till, whereas the bulk density is generally lower in the organic sediments.