Flow pattern and residence time of conduit flow and diffuse flow in calcareous sandstones (Bohemian Cretaceous Basin, Czech Republic)

<p>Localities containing karst features were studied in the northwestern part of Bohemian Cretaceous Basin. Namely Turnov area in facies transition between coarse-delta sandstones and marlstones (Jizera Formation, Turonian) and Miskovice area in limestones and sandy limestones - sandstones (Peruc-Korycany Formation, Cenomanian). Evolution of karst conduits is discussed elsewhere (Kůrková et al. 2019).</p><p>In both localities, disappearing streams, caves and karst springs with maximum discharge up to 100 L/s were documented. Geology and hydrogeology of this area was studied from many points of view to describe formation of karst conduits and characterize groundwater flow. Tracer tests were performed using NaCl and Na-fluoresceine between sinkholes and springs under various flow rates to evaluate residence times of water in conduits and to describe geometry of conduits. Breatkthrough curves of tracer tests were evaluated by means of Qtracer2 program (Field 2002). Groundwater flow velocity in channels starts at 0.6 km/day during low water levels up to 15 km/day during maximum water levels, the velocity increases logarithmically as a function of discharge. Similar karst conduits probably occur in other parts of Bohemian Cretaceous Basin where lot of large springs can be found.</p><p>Mean residence time of difussed flow based on tritium, CFC and SF<sub>6</sub> sampled at karst springs is 20 years for 75% of water and 100 years for remaining 25%, based on binary mixing dispersion model. This shows that most of the water drained by karst conduits is infiltrated through the soil and fractured environment with relatively high residence time. Residence times in different types of wells and springs were also measured in whole north-western part of Bohemian Cretaceous Basin. Results indicate long residence times in semi-stagnant zones represented by monitoring wells and short residence times in preferential zones represented by springs and water-supply wells.</p><p> </p><p>Research was funded by the Czech Science Foundation (GA CR No. 19-14082S), Czech Geological Survey – internal project 310250</p><p> </p><p>Field M. (2002): The QTRACER2 program for Tracer Breakthrough Curve Analysis for Tracer Tests in Karstic Aquifers and Other hydrologic Systems. – U.S. Environmental protection agency hypertext multimedia publication in the Internet at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=54930.</p><p>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</p>

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

<p>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.</p><p>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.</p><p>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.</p><p>In sediments with detrital quartz admixture in central or western parts of the BCB the total disintegration mostly occurs between 35-50% CaCO<sub>3</sub> 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.  </p><p>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.</p><p> </p><p>The research was financially supported by the GA ČR 19-14082S.</p><p> </p><p>References:</p><p>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.</p>

REE-Rich Turonian Phosphates in the Bohemian Cretaceous Basin, Czech Republic: Assessment as Source of Critical Elements and Implications for Future Exploration

Numerous phosphate occurrences are located in the Bohemian Cretaceous Basin (BCB) of the Czech Republic, within the Cenomanian–Turonian sequences. Small phosphate occurrences have been reported in the Upper Cenomanian, Lower Turonian, and Upper Turonian marine glauconitic siliciclasts. The phosphates are generally <1 m thick, present as phosphatized hardgrounds, nodules, coprolites, skeletal remains, phosphatized shells, peloids, sponges, and tube-fills, associated with black mudstone and other siliciclasts. Only recently the critical elements have been highlighted in these phosphates. The present study covers eight of these occurrences and provides information on petrography, mineralogy, and chemical composition of major elements, trace elements, and stable isotopes. The phosphate mineralogy is comprised of carbonate-fluorapatite, associated with quartz, glauconite, smectite, kaolinite, and pyrite. Most of the phosphates are rich in organic matter. The phosphate chemistry is dominated by P2O5, CaO, F, Na2O, SO3, and CO2. Minor amounts of SiO2, Al2O3, K2O, and MgO are found, related to quartz and alumino-silicate impurities. Evidence of fossil microbial structures is revealed. The indices derived from rare earth elements (REE) indicate phosphogenesis at various redox conditions, ranging from anoxic to oxic, whereas the carbon stable isotopes of the apatite suggest generally reducing conditions. The critical and other valuable elements found in these Mid-Cretaceous phosphates include P2O5 (18.9–26.76 wt. %), F (1.67–3.25 wt. %), REE (325–1338 ppm), Y (74–368 ppm), and U (10.4–37.9 ppm). The investigation of the Turonian phosphate occurrences show that those located at the base of the Bílá Hora Formation (earliest Turonian) are the most persistent in the southern margins of the BCB, and found in localities extending for about 200 km. They were developed at the onset of the Early Turonian global transgression and are strata-bound to the base of the Bílá Hora Formation. Future exploration for marine sedimentary phosphorites should focus on thicker and better developed deposits at the base of the Turonian sediments as the main target.

A new specimen of Araucaria fricii from the early Coniacian of the Bohemian Massif, Central Europe

A specimen of Araucaria fricii is described from the upper part of the Teplice Formation in the Bohemian Cretaceous Basin. It extends the first occurrence of A. fricii from the mid-Coniacian back to the early Coniacian. Found in the Radovesice locality near Kučlín in the northern part of the Czech Republic, it is characterised by a deltoid cone scale complex with a centrally placed seed. It is compared to the type material of A. fricii from the mid-Coniacian Březno Formation and other European Cretaceous species of Araucaria. The taphonomy and palaeoecology of A. fricii is briefly discussed.

Cirripedes from hemipelagic deposits of the Bohemian Cretaceous Basin (Czech Republic), with remarks on an exceptionally well-preserved capitulum of Diotascalpellum angustatum (GEINITZ, 1843)

An updated overview of cirripede taxa (Thoracica, Pedunculata) from hemipelagic facies of the Bohemian Cretaceous Basin (Czech Republic) is presented. In addition to previously recorded data, recently collected capitular plates of Cretiscalpellum glabrum, Diotascalpellum angustatum, semi-articulated Stramentum pulchellum and unpublished specimens in the Josef Soukup Collection are described and illustrated herein. Soukup's material comprises a near-omplete capitulum of Diota scalpellum angustatum; this represents the best-preserved individual of that species known to date. A revised picture of the stratigraphical distribution of cirripedes from the Bohemian Cretaceous Basin, and a discussion of habitat preferences and distributional patterns of each taxon within this area are added.