Contribution of hydrogeological core drilling to stratigraphy, sedimentology and tectonics of the Upper Cretaceous deposits in the NW-part of the Bohemian Cretaceous Basin


Roland Nádaskay, Jaroslav Valečka, Stanislav Čech

Geoscience Research Reports 50, 2017, pages 129–136
Map sheets: Děčín (02-23)

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Published online: 10 August 2017

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Extensive core drilling sponsored by hydrogeological project “Re-assessment of groundwater resources” was carried out in the NW-part of the Bohemian Cretaceous Basin (BCB) between June 2014 and December 2015. In total, 13 boreholes (including wells) were drilled in the region extending from Ústí nad Labem to Varnsdorf (northern Bohemia). Their aim was to enhance dataset for creating a geological model of groundwater aquifers in areas with sparse borehole coverage. As a result, the new lithostratigraphic and biostratigraphic data enabled to resolve uncertainties in stratigraphic division and its implication for hydrogeology. In addition, geophysical well logs facilitated borehole correlation on local and regional scale to investigate the areal extent of a particular formation/aquifer, and to understand the structure and tectonics of the NW-part of the BCB. This report presents the most remarkable outputs from selected boreholes, categorized by their geographic position and geological setting. In particular, boreholes 4650_A and 4650_F contributed to understanding the tectonic setting of the area in the vicinity of Nový Bor, located in the NE flank of the Ohře Rift; both of them have shed light on depositional facies of Coniacian sandstones (Březno Fm.) and their spatial correlation as well. The so-called flyschoid facies that has only rare outcrop analogues was observed in boreholes 4650_A and 4650_X. Sedimentological analysis of drill cores provided by both boreholes will allow a revision of depositional processes during the Coniacian, as interpreted by Čech et al. (1987) and Nádaskay and Uličný (2014). Similar appears to be borehole 4640_C that penetrated an unusual development of the Upper Turonian in the proximal zone adjacent to the basin margin. It has also helped to determine the exact stratigraphic position of the adjacent Coniacian outcrops near Sonnenberg in Zittau Mts. (Germany). Lithofacies development and stratigraphy of the so-called Kozly Sandstone has been explored by borehole 4730_B. The Kozly Sandstone is separated from sandstones of the Jizera Fm. by ca 19 m thick sequence of mudstones, comprising Didymotis 0 event, occurence of the bivalve Mytiloides scupini (Heinz) as well as ammonite Prionocyclus germari (Reuss). As evidenced by the fossil finds, most of the Kozly Sandstone belongs to the middle part of the Teplice Fm. (Upper Turonian), except for the upper part that is likely to be Coniacian. Although hydrogeological boreholes (wells) did not yield any drill core, they provided valuable indirect data on stratigraphy, mainly due to geophysical well logs. For instance, well logs serving as lithological proxy controlled by mud log, revealed presence of silicified limestones (Rohatce Mb.), an important basin wide correlation horizon, in borehole 4612_1C. All the boreholes provided valuable data that will in the future serve as a dataset for either detailed sedimentological and high-resolution stratigraphic research or basin modeling.