Explosive breccias in boreholes in the NW-part of the Bohemian Cretaceous Basin


Roland Nádaskay, Jaroslav Valečka

Geoscience Research Reports 50, 2017, pages 181–188

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Published online: 31 October 2017

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Extensive drilling in the NW part of the Bohemian Cretaceous Basin carried out within groundwater exploration provided core material from boreholes 4630_B and 4650_A. Both locations (Fig. 1) are situated in the Lužice sub-basin, dominated by coarse-grained siliciclastic deposits, mainly quartzose sandstones. In both cores, loose rusty-stained sandstones were described (Fig. 4). An apparent contrast to the wallrock (compact quartzose and argillaceous sandstones) allowed their distinction as a new facies that was tentatively labelled as a dyke fill. In thin sections, besides ubiquitous rounded quartz grains, several types of “exotic” fragments were identified. These include rare lithic grains of volcanic origin, namely scoria fragments (Fig. 8) and porphyric basaltic rocks (Fig. 9), wood fragments up to 3.5 mm long (Figs 10, 12), slightly altered clasts of mudstones (Fig. 11), and fine accumulations of iron hydroxides up to 2 mm in size. Furthermore, some quartz grains exhibit obvious sharp pressure cracking. This probably results from explosive brecciation of Cretaceous (Cenomanian-Turonian) sandstones, their contamination with “exotic” material transported from underlying strata and subsequent redeposition as an explosive pipe fill. In case of volcanic rock fragments, it is not clear whether they originated as juvenile material or by destruction of the older volcanic structures. The facies is genetically attributed to either phreatic or phreatomagmatic process. The absence of silica cement precipitated from hydrothermal fluids indicates that explosive process was driven strictly by hydrothermal gases. Explosions were triggered either directly by the intrusion of hot magma into porous, meteoric water-saturated Cretaceous sandstones, or indirectly by the magmatic heat released into the aquifer. Since it is impossible to determine the exact nature of the process (phreatic/phreatomagmatic), we have decided to use the term “explosive breccias” for this facies. The depth of formation of both pipe fills is inferred to ca. 200?300 m under the palaeosurface. This is supported by the presence of unusual coniferous wood fragments that may have been transported from the surface by early post-explosive circulation of suspended brecciated material, as long as similar plant fossils are not known from underlying Cretaceous strata. Such process was proved in similar structure in the town of Jáchymov in the Krušné hory Mts. Based on borehole data, the spatial extent and dimensions of the explosive pipes cannot be estimated. Although in previous years this facies was described neither in outcrop (sensitive to weathering) nor in core (prone to core loss), we assume that it could be more frequent than expected. Thus, its identification requires more attention.