Formation of high-transmisivity zones in the facies transition in the Bohemian Cretaceous Basin (Czech Republic)

The Bohemian Cretaceous Basin (BCB) is the most important hydrogeological structure in the Czech Republic, with large sources of groundwater. The origin of high-transmissivity zones is poorly understood in many BCB areas. The doyen of Czech hydrogeology prof. Hynie described some of the largest springs to be of karst origin and he attributed the most permeable areas to facies transition between shallow-water sandstones and deep-water marlstones. In many BCB areas with large springs we can find thin sandstones and siltstones layers with high carbonate content even in stratigraphical levels corresponding with aquifers.

Research is focused on Vysoké Mýto and Ustí synclines in BCB, 125 km east of Prague in the Czech Republic. Overall 167 rock samples were taken from borehole cores and rock outcrops in this area, the most from Jizera and Bílá Hora formations. Cores were taken from intervals where: (i) high carbonate content was expected, (ii) conduits and enlarged fractures were observed at outcrops and in wells, (iii) inflows to boreholes were determined by well logging. Calcium carbonate content was determined by calcimetry in cores. Cores were leached in 10 % hydrochloric acid to observe the degree of subsequent disintegration. Polished sections were prepared from selected cores and Ca, Si, Na, K, Al content was automatically mapped by SEM-EDS to visualize the calcium, silica, feldspar and clay mineral distribution in cores.

Leaching in hydrochloric acid is an accelerated simulation of natural processes of dissolution by acidic solutions (Kůrková et al. 2019). In many aquifers in BCB there are thin calcite-rich layers with quartz sand which disintegrates after leaching calcite. Leaching of the samples in acid results in the decrease of sample strength, sometimes to their disintegration. Leaching experiments showed that the carbonate content is not the only controlling factor in the karstification process.

In sediments with detrital quartz admixture in central or western parts of the BCB the total disintegration mostly occurs between 35-50% CaCO₃ content depending on insoluble material content (Kůrková et al. 2020). In contrast, in the eastern part of the BCB, a degree of disintegration above 10% is documented in only 7% of the studied samples. In sediments with diagenetically precipitated cement from marine sponges even calcite content as high as 80% may not be sufficient for material to disintegrate after leaching. Disintegration occurs mainly along fractured zones where rock is heavily fractured.  

It seems that the increased content of microcrystalline silica cementy in sandy limestones and calcareous sandstones (spongolites) of the studied area has a fundamental influence on the higher cohesion and resistance of rocks to dissolution. Cause for increased cohesion is the specific spatial distribution of  microcrystalline silica, which bound the quartz grains together or formed a foam-like supporting structure in fine calcite-rich deposits.

 

The research was financially supported by the GA ČR 19-14082S.

 

References:

Kůrková I., Bruthans J., Balák F., Slavík M., Schweigstillová J., Bruthansová J., Mikuš P., Grundloch J. (2019): Factors controlling evolution of karst conduits in sandy limestone and calcareous sandstone (Turnov area, Czech Republic). Journal of Hydrology: 574: 1062-1073.