|
Karen Dybkjaer, Geological Survey of Denmark and Greenland (Denmark)
Stefan Piasecki, Geological Survey of Denmark and Greenland (Denmark)
Erik Skovbjerg Rasmussen, Geological Survey of Denmark and Greenland (Denmark)
|
|
The need for finding non-polluted groundwater (drinking water) has increased dramatically over the last ten years and consequently the interest in deep lying sand reservoirs (aquifers) has increased. In large areas of onshore Jylland, in western Denmark, the uppermost Oligocene - Miocene succession is located approximately 50-300 m below the surface. The succession comprises several coarse grained intervals which serve as high-quality aquifers. A general geological model of these sand reservoirs and interbedded muds was published in 1961, but over the last eight years intensive studies have revealed a much more detailed and complex succession and a strongly revised model has been generated.
The uppermost Oligocene - Miocene succession comprises a complex series of marine mud deposits interfingering with several large delta lobes of sand and gravel. As these sand bodies cannot be discriminated based upon lithological criteria, dinoflagellate cyst stratigraphy has played an important role in this task. A strict biostratigraphic framework has been set up based on palynological analyses of dinoflagellate floras in a large number of boreholes throughout the region. A series of stratigraphic events (first appearances, last occurrences and acmes of dinoflagellate cyst species) occurring in most of the studied boreholes have been used for correlation of both mud and sand layers. Parts of the Miocene succession comprise proximal deposits dominated by terrestrial palynomorphs. However, rare dinoflagellate cysts occur within subtle marine intervals and provide a rigid tool for dating and correlation. The fully marine parts of the succession provide excellent dinoflagellate cyst floras which correlate with contemporaneous North Atlantic floras. Lateral variation in the dinoflagellate cyst assemblages are ascribed to proximal-distal changes in the depositional environment.
The water supply authorities now use the new geological model when deciding where and how deep to drill their boreholes and the local counties use the model to plan which areas it is particularly important to avoid, in order to protect the aquifers against pollution (industry, agriculture, airports etc.).
|
