Contributions to the geological mapping of Mors, Denmark – A study based on a large-scale TEM survey FLEMMING JØRGENSEN, PETER B. E. SANDERSEN, ESBEN AUKEN, HOLGER LYKKE-ANDERSEN & KURT SØRENSEN Jørgensen, F., Sandersen, P.B.E., Auken, E., Lykke-Andersen, H. & Sørensen, K. 2005–11–15. Contri- butions to the geological mapping of Mors, Denmark – A study based on a large-scale TEM survey. Bulletin of the Geological Society of Denmark, Vol. 52, pp. 53–75. © 2005 by Geological Society of Den- mark. ISSN 0011–6297. Recent improvements of the transient electromagnetic (TEM) method provide unprecedented capa- bilities of imaging geological features in the uppermost few hundred metres of the subsurface. This is documented with an example from the island of Mors, Northern Jutland, Denmark, where 2655 TEM soundings have been acquired as a part of Viborg County’s hydrogeological investigation programme. Horizontal aspects of the interpreted TEM soundings are presented in thematic maps revealing variations of resistivities in specified depth intervals and depths to the resistivity base- ment, i.e. the deepest recorded low-resistive layer. Vertical aspects are displayed in cross-sections. The dense coverage of TEM soundings, combined with borehole logs, allows the precise delineation of major geological features. These comprise the Mors salt diapir, intricate systems of buried Qua- ternary valleys and several glaciotectonic complexes, all contributing to a complicated geological framework for the island. The layers above the salt diapir, which have been subject to severe ero- sion, are distinctly imaged in the thematic maps, even though they are frequently dissected by buried valleys. At least four generations of buried valleys can be identified on the basis of their preferred orientations; these were mainly formed during early glaciations. Large parts of the island were glaciotectonically deformed during the Late Weichselian subsequent to valley formation, and it is suggested that the presence of deep valleys may have affected the process of glaciotectonic deformation. Keywords: Transient electromagnetic; TEM; hydrogeophysics; salt diapirs; buried valleys; glaciotec- tonic deformation; subglacial; Quaternary; Pleistocene; Mors; Denmark. Flemming Jørgensen [[email protected]], Vejle Amt, Damhaven 12, DK-7100 Vejle, Denmark. Peter B.E. San- dersen [[email protected]], Watertech a/s, Søndergade 53, DK-8000 Aarhus C, Denmark. Esben Auken [[email protected]], Hydrogeophysics Group, Department of Earth Sciences, University of Aarhus, DK-8200 Aarhus N, Denmark. Holger Lykke-Andersen [[email protected]], Department of Earth Sciences, University of Aarhus, DK-8200 Aarhus N, Denmark. Kurt Sørensen [[email protected]], Hydrogeo- physics Group, Department of Earth Sciences, University of Aarhus, DK-8200 Aarhus N, Denmark. In order to outline aquifers and the vulnerability of terpretation of modelled data (e.g. Poulsen & Chri- water resources, large areas of Denmark have been stensen 1999; GeoFysikSamarbejdet 2003; Jørgensen covered by TEM (Transient ElectroMagnetic) surveys et al. 2003a, 2003b). The quality of the collected data since the early 1990s. During this time the TEM meth- has also been considerably improved in recent years. od has undergone, and is still undergoing, intense Optimized handling of instruments, deletion of noise- development as a tool for hydrogeological investi- infected and coupled soundings and the identifica- gations. This development has primarily taken place tion of defective instruments have led to significant within three main areas: 1) technical development improvements of survey results. Experience shows (e.g. Danielsen et al. 2003; Sørensen & Auken 2004; that surveys of large areas with dense data coverage Sørensen et al. 2004a), 2) advances in the handling, provide a solid base for the construction of geologi- modelling and presentation of data (e.g. Effersø et cal models. al. 1999; GeoFysikSamarbejdet 2003; Auken & Chri- Traditionally, TEM surveys carried out as part of stiansen 2004), and 3) advances in the geological in- the Danish counties’ hydrogeological investigations Jørgensen et al.: Geological mapping of Mors, Denmark – Study on a large-scale TEM survey · 53 DGF Bulletin 52-2.PMD 53 31-10-2005, 10:37 Fig. 1. Location map. have not been subjected to detailed geological inter- sam & Elkraft 1981; Larsen & Baumann 1982; Klint pretation and have generally not contributed to re- & Pedersen 1995; Pedersen 1996, 2000; Korsager 2002; search in, for example, sedimentology and tertiary Jørgensen & Sandersen 2004), who have mainly fo- and Quaternary geology. The on-going refinements cused on the individual elements and not on the re- of the TEM methods are, however, expected to lead lations between them. to significant contributions in above-mentioned fields of research topics. Here we demonstrate how a large- scale TEM survey can be used as the basis for geo- logical interpretation. We show that various types of Study area - the island of Mors geological structures can be defined and outlined, Geography and that the understanding and knowledge of near- surface geology can be significantly improved by a The study area is confined to the island of Mors, detailed study of TEM data. The aim of the paper is which is situated in the northwestern part of Jutland, two-fold: to demonstrate to what extent TEM data Denmark (Fig. 1). The island covers an area of about can be used in geological interpretations and to add 360 km2. It is 10–15 km wide and about 35 km long new information to the geology of Mors. with a SSW–NNE trend. The landscape is slightly The geology of Mors is spectacular in several re- undulating and mainly dominated by small hills, spects: Large-scale glaciotectonic complexes are shallow erosional channels and valleys (Fig. 2). In present in the northern part of the island; the Mors two areas the terrain reaches altitudes of more than salt diapir is located in the central area; and numer- 50 m above sea level (m a.s.l.): In the northwest where ous buried valleys incise the subsurface of the island. parallel ridges reach the highest point (88 m a.s.l.), These geological elements have previously been de- and in the central southern part where a local up- scribed by several authors (e.g. Gry 1940, 1979; El- land reaches 66 m a.s.l. 54 · Bulletin of the Geological Society of Denmark DGF Bulletin 52-2.PMD 54 31-10-2005, 10:37 during the Quaternary glaciations and 3) extensive systems of incised buried valleys. Halokinetic movements The Mors salt diapir is located in the central part of the island (Elsam & Elkraft 1981; Larsen & Baumann 1982). The diapir, which is elliptical in shape, covers an area of about 75 km2 with its long axis trending E-W. Upper Cretaceous white chalk and Danian limestone, together with Paleogene clay and diatomite are up- lifted over the salt diapir and along its flanks as a result of halokinetic movements. The top of the Zech- stein salt is situated at a depth of more than 400–500 m (Madirazza 1977; Elsam & Elkraft 1981). Erosion has removed some of the pre-Quaternary deposits over the central parts of the diapir so that Quater- nary sediments cover the Upper Cretaceous white chalk here. Away from the diapir the chalk is flanked by dipping layers of Danian limestone, followed by Paleogene diatomite and clay. This structural setting is evident in seismic sections (Elsam & Elkraft 1981; Larsen & Baumann 1982), in many boreholes that penetrate the central part as well as the flanks of the diapir (Gravesen 1990, 1993), and in several expo- Fig. 2. The topography of Mors. Landscapes in the Hanklit ar- sures (Andersen & Sjørring 1992). Pedersen (2000) ea characterized by composite-ridge systems are marked with black polygons. Grey lines mark ice-marginal moraines in the describes glaciotectonic deformation of Paleogene northeastern part of the island. Scale: 5 km between axis ticks. clay and diatomite on the northern flank of the struc- Coordinate system: UTM zone 32/ED50 (from Viborg Amt with ture. These deposits are here found at shallow depths data from National Survey and Cadastre). as a result of the uplift by the salt diapir. Geological setting of Mors Glaciotectonic deformation The sub-Quaternary strata comprise Upper Creta- Significant parts of the island have been affected by ceous white chalk (Maastrichtian) and Paleocene glaciotectonic deformation. The most prominent ex- limestone (Danian) covered by clays and diatomite ample is the Hanklit glaciotectonic complex in the (Paleocene–Eocene). The diatomite is comprised in northwestern part of the island (Fig. 2), where it is the Fur Formation (Pedersen & Surlyk 1983). These exposed in the cliff of Hanklit (Gry 1940; Klint & Pe- layers are followed by Oligocene micaceous clay and, dersen 1995). The system exposed in Hanklit is clas- to the south, also by Miocene clay, silt and sand (Gra- sified as a large composite-ridge system (Aber et al. vesen 1990, 1993). The Paleogene sediments are in 1989). More than 50 m thick thrust sheets of Paleo- general 50–250 m thick, but locally they are thinner gene clay and diatomite that are imbricated into or even absent. They have commonly been subjected thrust fault complexes are clearly seen in the terrain to deformation during the Quaternary glaciations, as several parallel arc-shaped ridges (Fig. 2). A south- but the most pronounced impact on the Paleogene ward moving glacier (the Main Advance, e.g. Kjær topography is by a series of incised Quaternary val- et al. 2003) is believed to have formed the complex leys. These valleys have mainly been filled with thick during a Late Weichselian stage (Gry 1940; Klint & sequences of glacial deposits. Where no valleys exist, Pedersen 1995). The ice movement direction subse- only relatively thin layers of glacial origin cover the quently shifted towards SW and W, and at least three pre-Quaternary formations. other glaciotectonic deformation complexes were The overall structure of the tertiary and Quater- formed in the northeastern part of the island (Gry nary formations on Mors is mainly controlled by 1) 1940, 1979; Pedersen 1996, 2000).