Terciérní říční sedimenty v širším okolí Jihlavy

Abstract

Tertiary deposits have been known around Jihlava city for many years. However, they have never been thoroughly studied. Most of the known occurrences are situated in the Jihlava Furrow. During the drilling research, these Tertiary deposits were newly detected in three wells: HGM-1, V1 and V4. Facies analysis, petrography and gamma spectrometry were performed on samples taken from all three wells. The model of basement of Cenozoic deposits has been constructed. Based on facies analysis, 8 lithofacies (7 studied deposits and 1 eluvium) have been described. The study of lithofacies has made the recognition of the depositional environments of the Tertiary deposits possible. The depositional environments of these deposits are interpreted as fluvial channelized gravels and non-channelized deposits (flood plain, crevasse splays, oxbow lake and lake). The channelized gravels have been found by the well HGM-1 in the overburden of the crystalline rocks of the Moldanubian. Their thickness is relatively low (1.7 m) and the channels were probably not deep. The non-channelized deposits are formed by clays, silts and fine-grained sands. In the upper part of the succession a layer of organic deposits (peat) has been detected. The results of petrography have shown that material originated from local sources, especially Moldanubian Unit and Jihlava Massif (gneisses, mica-schistes, granitoids, less importantly pegmatites and phyllites). High concentrations of Th and U have been found in the studied deposits. These high concentrations can be associated with the source rocks. The values of Th /U and Th /K ratios are signifi cantly higher in the studied deposits than in eluvium. This indicates conditions of weathering and sedimentation, and also points to input of the material from several sources. The model of crystalline basement shows that there were probably at least two spatially separated fluvial river systems. One river system was situated in the area of Velký Beranov and Měšín. The preserved remains of the second system are visible in the area of Jihlava city and its peripheral parts (Bedřichov and Pávov).