Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Relics of the Cenomanian in the Blansko Graben, locality U tetřeva spring


Jiří Otava, Helena Gilíková

Geoscience Research Reports 50, 2017, pages 289–292
Map sheets: Brno (24-32)

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Published online: 18 December 2017

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Several denudation relics of the Cretaceous, Cenomanian sandstone, conglomerate (Fig. 3) and locally breccia were mapped and documented west of the Olomučany village (Figs 1and 2) in the Blansko district, some 15 km NNE of the Brno City. Older geological maps mostly ommited the sediments laying directly on granitoids of the Neoproterozoic Brno massif. On geological map of the Brno City and its surroundings (scale 1 : 50 000, Hanžl et al.1999) the sediments are considered as Miocene, Lower Badenian basal clastics. The current geological map of 1 : 25 000 scale, map sheet 24-322 Blansko (Hanžl et al. 2007) shows the relics as Lower Miocene (continental Ottnangian - equivalent of the Middle Burdigalian). New geological survey accompanied with mineralogical and petrological (Fig. 4) evaluation proved analogy with the freshwater Cenomanian (Peruc Member of the Peruc-Korycany Formation) at the southeastern margin of the Bohemian Cretaceous Basin. The analyses of translucent heavy minerals from two sandstone samples of the investigated locality U tetřeva and its surroundings (Figs 2, 5 and 6) show great affinity to other Cenomanian samples from the Blansko Graben and the Rudice Plateau (Figs 1 and 6). Most important and typomorphic minerals of the freshwater Cenomanian clastics are kyanite and staurolite, accompanied with sillimanite and andalusite. Among ultrastable minerals rutile and tourmaline are frequent and in some samples also zircon. On the other hand, there is no similarity of the assemblage with Miocene, either Badenian (garnetiferous), or Ottnangian (staurolite-garnet rich) clastics of the area. The relics are undoubtedly preserved due to the upper Cretaceous dips and faults running in the NNW-SSE direction. Later the Cretaceous sediments were uplifted and the only zone where they have been preserved is the structure of the Blansko Graben. Recently, the Cenomanian sediments are scattered up to an elevation of 500 m a. s. l., nevertheless the altitude of the described locality is around 420 m a. s. l. We can conclude, that the width of the Blansko Graben should be enlarged (widened) in the western vicinity of the Olomučany village, and/or the marginal fault of the Graben is running some hundreds of meters more to the SW.