Geological Survey of Denmark and Greenland Bulletin 35, 2016, 9-12 9© 2016 GEUS. Geological Survey of Denmark and Greenland Bulletin 35, 9–12 . Open access: www.geus.dk/publications/bull A motorway was constructed in 2010–2016 through the suburbs of the city of Silkeborg (Fig. 1). The Danish Road Directorate wished to climate-proof the motorway against adverse future climate changes. The directorate collaborat- ed with the Geological Survey of Denmark and Greenland (GEUS) to study the hydrological conditions. Studies of historical and projected climate-change-driven variations in groundwater levels in relation to urbanised hydrologi- cal fluxes were conducted by Kidmose et al. (2013, 2015). During the construction of the motorway, Miocene and Quaternary deposits were exposed in the slopes of the Gudenå valley and late-glacial glaciofluvial deposits were found in the valley floor. This paper focuses on the Mio- cene sediments and their influence on the local hydrologi- cal conditions. At Silkeborg the Gudenå valley is c. 35 m deep (Fig. 1). The surrounding terrain is a till plain. In the slope of the valley, glaciofluvial sand is found below the till. Miocene deposits are found below the glaciofluvial sand. The floor of the Gudenå valley is covered by c. 15 m thick glacioflu- vial deposits, which rest on Miocene deposits. In borehole no. DGU 87.907 49 m of Miocene deposits belonging to the Vejle Fjord Formation are recorded, consisting primar- ily of marine clay with minor occurrences of sandy deposits. About 12 km south of Silkeborg lower Miocene deposits are seen in outcrops and boreholes (Fig. 2). Here the fluvial Addit Member of the Billund Formation (Rasmussen at al. 2010) is separated from the underlying marine Vejle Fjord Formation by a sharp erosional contact (Rasmussen 2014). Miocene deposits at Silkeborg, Jylland, and their influence on hydrology Peter Roll Jakobsen, Erik Skovbjerg Rasmussen, Karen Dybkjær and Jacob Kidmose 1 km Silkeborg Gudenå Clayey till Sandy till Glaciofluvial sand Outwash plain Peat Aeolian sand Miocene deposits Well no. 87.907 Fig. 1. Geological map of the Silkeborg area. Dashed line: motorway. 1 km 0 L e v e l (m ) 50 50 100 100 Continental sand and gravel Marine clay Marine sand Coal Pre-Miocene EastWest Addit Mb Billund Fm Vejle Fjord Fm ? 8 7 .9 0 7 F ig . 3 Quaternary deposits Continental clay Fig. 2. East–west profile about 12 km south of Silkeborg (from Rasmus- sen 2014). A composite log from Silkeborg (Fig. 3) and data from well no. DGU 87.907 are shown to the left. Planar cross-bedding Parallel bedding Clay Trace fossil Sample for palynology Hummocky cross-bedding Sedimentary structures Lithology Fig. 4A Fig. 4C Fig. 4D Fig. 4E Fig. 4B A q u it an ia n A d d it M b , B il lu n d F m V e jl e F jo rd F o rm at io n 0 m 1 m 2 m 3 m 4 m 5 m 6 m 7 m 8 m 9 m Sand Gravel Silt Fine Medium Coarse Fig. 3. Composite sedimentological log of the temporarily exposed Miocene deposits. 1010 Sedimentology The section along the motorway comprised 9 m of Mio- cene deposits (Fig. 3). The lower part is characterised by cross-stratified medium-grained sand, dipping c. 30° to- wards the north (Fig. 4A). A few trace fossils (Skolithos?) are seen. The cross-stratified sand is sharply overlain by wave-formed, coarse-grained ripples. The crests of the rip- ples strike SE–NW, and crest-to-crest spacing is in the range of 250 cm with amplitudes up to 35 cm (Fig. 4B). The ripples show tangential cross-bedding towards the SW. In a nearby exposure, tidal bundles form the base of the section. The presence of clay layers varies systemati- cally and is commonly characterised by double clay layers (Fig. 4A). Dips of cross-bedding are both SW and NE. The coarse-grained ripples are in turn overlain by a dark brown mud. The mud is succeeded by silt and fine-grained sand, c. 1.5 m thick. Hummocky cross-stratifications (HCS) are common, especially in the upper part of the section. These are superimposed by 3 m of dark brown mud (Fig. 4C) showing a slight increase in grain size upwards where hummocky cross-stratified sands are common (Fig. 4D). A sharp boundary separates the mud from an overlying 2 m thick section of medium- to coarse-grained sand and gravel. This coarse-grained section is composed of tabular co-sets of cross-stratified beds dipping towards the south (Fig. 4E). Fig. 4. Details of the Miocene deposits. The stratigraphic positions of the photos are indicated on Fig. 3. A: Cross-stratified medium grained sand. Double clay layers are indicated with small arrows. Trace fossil is indicated with larger arrow. B: Wave-formed, coarse grained ripples. C: Thick dark mud succession. D: Hummocky cross-stratification (HCS). E: Tabular co-sets of cross-stratified beds. B C D E A 11 Bio- and chronostratigraphy and depositional environment In order to confirm the Miocene age of the described suc- cession and to achieve a more precise dating, two sediment samples were selected for palynological analysis. The strati- graphic positions of the samples are shown in Fig. 3. One of the samples was almost barren, while the other contained a rich assemblage of organic particles dominated by bisaccate and non-saccate pollen. In addition, the sample contained a moderately rich and diverse dinoflagellate cyst (dinocyst) assemblage together with a few wood particles, cuticle, acritarchs and freshwater algae. This assemblage indicates a marine, inner neritic depositional environment with a high influx of freshwater (Tyson 1995). The dinocyst assemblage is dominated by two species of the genus Homotryblium: H.? additense (Fig. 5A) and H. plectilum. Among several other dinocyst taxa, a single specimen of the stratigraphically important species Chirop- teridium galea was found (Fig. 5B). The dinocyst assem- blage refers the sample to the Chiropteridium galea Zone (Dybkjær & Piasecki 2010). This dinocyst zone is dated to the early Aquitanian (earliest Miocene) and the age of the sample is 23.03–22.36 Ma. Palaeogeography The sand and gravel in the lower part of the section were deposited during an overall regression of the Billund For- mation in the early Miocene (Rasmussen et al. 2010). The gravel was probably originally deposited in a fluvial envi- ronment during the most extended regression. The cross- stratified sand in the lower part was formed in a marine bar that migrated landwards. The tidal bundles were formed by both ebb and flood currents, as indicated by the bipolar dips of cross-bedding, in an adjacent tidal inlet. The over- laying wave-formed, coarse-grained ripples were formed by marine reworking (Leckie 1988) of the coarse-grained flu- vial sediments laid down during maximum regression and now forms a transgressive lag (Plint 1988). The deposition- al water depth of the coarse-grained ripples may lie in the range of 15 to 60 m (Leckie 1988) – most likely in the lower end as the sea-level changes during this part of the Miocene was c. 25 m (Miller et al. 2005). The strike of the crests of the coarse-grained rippels, SE–NW, indicates the trend of the palaeo-shoreline (Leckie 1988). The succeeding mud and HCS-dominated silt and fine-grained sand were de- posited in slightly deeper water, in the offshore transition zone. The mud-dominated part with few intercalations of HCS’s was deposited offshore near the storm wave base. The assemblage of organic particles indicates that the sediment was deposited in a marine depositional setting near the coast. The two Homotryblium species further indi- cate that the palaeoenvironment was marine but probably with lowered salinity (Dybkjær 2004). These interpreta- tions support the sedimentological interpretations and palaeogeographic maps for the earliest Miocene of Jylland, indicating that large river and delta systems existed, which transported large amounts of freshwater and sediment from the north to the middle part of Jylland (Rasmussen et al. 2010). 20 μm20 μm A B Fig. 5. A: Homotryblium? additense. B: Chiropteridium galea. Fig. 6. Palaeogeographical reconstructions of the Silkeborg area. A: Early Aquitanian (earliest Miocene, Vejle Fjord Formation) tidal-dominated marine-barrier system. B: Late Aquitanian (Addit member, Billund Formation) f luvial environment. Grey line: motorway. A B1 km 1212 Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: prj@geus.dk The coarse-grained sand and gravel at the top of the stud- ied succession, that sharply overlie the marine deposits, were deposited in a fluvial environment, the Addit Mem- ber of the Billund Formation (Rasmussen et al. 2010; Fig. 6). The dramatic change in the depositional environment was partly a result of an eustatic sea-level fall and partly a result of inversion of the Norwegian–Danish Basin (Ras- mussen 2014). The latter resulted in marked incision in the middle and northern part of Jylland during the late Aquitanian. The Miocene succession in Silkeborg shows a strong resemblance to successions in boreholes and expo- sures about 12 km south of Silkeborg (Fig. 2). Hydrology On the floor of the Gudenå valley, wells with screens in the Miocene deposits have artesian hydraulic heads, whereas the hydraulic head in the overlying glaciofluvial sediments is in hydraulic contact with the Gudenå. This shows that the alternating Miocene layers of the Vejle Fjord Formation form a hydraulic barrier between deeper groundwater and the surficial glaciofluvial aquifer that is in contact with the motorway (Fig. 7). In the higher terrain the measured hydraulic head in the Quaternary glaciofluvial sand is very different from the hydraulic head measured in the underlying Miocene deposits. This is because the 3 m thick Miocene mud unit (Fig. 4C) acts as a barrier. The Addit Member, however, is in hydraulic contact with the glaciofluvial sand (Fig. 7). The hydraulic connection between the glaciofluvial sand found in the slopes of the valley and the glaciofluvial deposits in the valley floor is also affected by the Miocene deposits as the 3 m thick mud unit separates them. The hydrogeological relations between the Miocene de- posits and the Quaternary deposits illustrate the impor- tance of applying detailed field-site geological evidence to get an impression of the local groundwater flow. Acknowledgement The Danish Road Directorate is thanked for access to the field site and funding. References Dybkjær, K. 2004: Morphological and abundance variations in Homo- tryblium-cyst assemblages related to depositional environments; up- permost Oligocene – Lower Miocene, Jylland, Denmark. Palaeoge- ography, Palaeoclimatology, Palaeoecology 206, 41–58. Dybkjær, K. & Piasecki, S. 2010: Neogene dinocyst zonation in the eastern North Sea Basin, Denmark. Review of Palaeobotany and Pa- lynology 161, 1–29. Kidmose, J., Refsgaard, J.C., Troldborg, L., Seaby, L.P. & Escrivà, M.M. 2013: Climate change impact on groundwater levels: ensemble modelling of extreme values. Hydrology and Earth System Sciences 17, 1619–1634. Kidmose, J., Troldborg, L., Refsgaard J.C. & Bischoff, N. 2015: Cou- pling of a distributed hydrological model with an urban storm water model for impact analysis of forced infiltration. Journal of Hydrology 525, 506–520. Leckie, D. 1988: Wave-formed, coarse-grained ripples and their rela- tionship to hummocky cross-stratification. Journal of Sedimentary Research 58, 607–622. Miller, K.G. et al. 2005: The Phanerozoic record of sea-level changes. Science 310, 1293–1298. Plint, A.G. 1988: Sharp-based shoreface sequences and “offshore bars” in the Cardium Formation of Alberta; their relationship to relative changes in sea level: In: Wilgus, C.K. et al. (eds): Sea-level changes: an integrated approach: SEPM, Special Publication 42, 357–371. Rasmussen, E.S. 2014: Development of an incised-valley fill under the influence of tectonism and glacio-eustatic sea-level change: valley mor- phology, fluvial style and lithology. Journal of Sedimentary Research 84, 278–300. Rasmussen, E.S., Dybkjær, K. & Piasecki, S. 2010: Lithostratigraphy of the Upper Oligocene – Miocene succession in Denmark. Geological Survey of Denmark and Greenland Bulletin 22, 92 pp. Tyson, R. 1995: Sedimentary organic matter: organic facies and palyno- facies, 615 pp. London: Chapman & Hall. Clayey till Meltwater sand Diamicton Meltwater plain Sand Clay Quaternary deposits Miocene deposits W e ll n o . 8 7 .9 0 7 Addit Member Vejle Fjord Formation Sand Sand/clay Level (m) 60 50 40 30 20 10 0 –10 –20 Fig. 7. Conceptual geological model along the motorway alignment.