Paleoúdolí Doubravník–Borač ve světle elektrické odporové tomografie (ERT), jeho 3D model a vazba na hydrogeologický rajon 2242 (Kuřimská kotlina)

Abstract

There is a depression of the relief between Doubravnik and Borac villages, filled mainly with Lower Badenian sediments, to a lesser extent also with sediments of alluvial cones, loess and slope (colluvial) deposits. Current reflections on its origin oscillate between the idea of an abandoned pre-Badenian valley of the Svratka River after a depression caused by purely tectonic processes. Their disadvantage is that they rely on a minimum of objective data, especially in terms of thickness of sediments and the shape of the buried relief. The aim of this study is to increase knowledge about the tectonic structure, extent and thickness of sediments. Morphostructural analysis, field research and especially geophysical measurements (ERT) on 13 profiles with lengths of 81 to 595 m and a depth range of up to 70 m were used for this purpose. The work confirmed that the studied area is probably a paleo-valley of the Svratka River, which is divided by tectonic faults into several segments. The most significant appears to be the tectonic zone in the area between Borac and Maňova. In the interpreted results of ERT measurements, it manifests itself in a rapid change in the thickness of calcareous clays as on the D05 profile, the bottom of the valley was found at about 285 m a. s. l., while from other profiles towards Borac it is certain that the bottom altitude is at least 30 m lower. What is the position of the rock bottom of the depression at the site of the divide with the D14 profile could not be determined, but it is definitely situated at a depth greater than 310 m a. s. l. From the profiles led directly in Doubravnik (D01, D02, D15) it is clear that the deepest part of the valley filled with Lower Badenian sediments is situated W of the church, but towards the north it turns to the east. The bottom was not found, but it is lower than 270 m a. s. l. Overall, it is possible to summarize that within the section between Doubravnik and Borac, the deepest shape of the relief is formed by depression with curves. Its position is different within today's shape of the relief. The situation is similar in the direct parts of the Svratka valley north and south of this area of interest (between Nedvědice and Doubravnik and between Borac and Stěpanovice). Therefore, the hypothesis of a paleo-valley of the Svratka River seems to be very probable. The diversion of today's flow of the Svratka River between Doubravnik and Borac to the current valley was caused by horst uplift and the overall change in the slope of the area after Badenian regression. There are also some hydrogeological impacts of this study. It is clear, the groundwater source (well HBV-1) is not situated in the deepest part of the valley, where sandy layers can be assumed, similar to that in Nedvědice (north from the area of interest). Besides groundwater flow under Badenian clays into hydrogeological district 2242 – Kuřimska kotlina (south from the area of interest) from more distant parts of the Svratka River drainage area can be supposed, as the bottom of the Tertiary paleo-valley between Doubravnik and Borac is not raised above the level of the floodplain. Also a significant failure of the underlying rocks at the contact of the Svratka Crystalline complex and the Moravicum (crystalline complex) certainly allows an uninterrupted overflow of groundwater under the insulator of Badenian clays.