Preliminary conodont biostratigraphy and gamma-ray spetrometry of the Famennian calciturbidites of the Líšeň Fm, Brno, Czechia
| Autoři | |
|---|---|
| Rok publikování | 2025 |
| Druh | Konferenční abstrakty |
| Fakulta / Pracoviště MU | |
| Citace | |
| Popis | The Hády-Říčka Limestone of the Líšeň Formation represents deposits of a carbonate turbiditic system that developed on the slopes of the Moravosilesian Basin during the Famennian. This study examines conodont biostratigraphy of the “lower” Hády-Říčka Limestone exposed in the Lesní and Hády quarries in Brno, Czechia, as well as their facies associations, gamma-ray spectrometry, and provides relative sea-level change, paleoenvironmental and biofacies interpretations. The Hády quarry section features a rich palmatolepid and polygnathid conodont fauna spanning from the Palmatolepis minuta minuta Zone (lower Famennian) to the Bispathodus costatus (upper Famennian) Zone. The main section of the Lesní quarry is characterised by rich bispathodid, palmatolepid and polygnathid conodont fauna. The stratigraphic range of this section spans from Palmatolepis gracilis expansa Zone (upper Famennian) to Bispathodus ultimus Zone (uppermost Famennian). Measured concentrations of potassium (K), thorium (Th), and parameter of computed (or “clay”) gamma-ray (CGR) reveal an initial increase in content of detrital material and decrease in carbonate input in the lower part of the Hády section in Palmatolepis minuta minuta to Palmatolepis marginifera marginifera zones. This is followed by a continuous decrease in CGR from Palmatolepis marginifera marginifera to Bispathodus costatus zones, interpreted as decrease in detrital input and increase in input of carbonate material. In the Lesní quarry section, the decrease in CGR continues up to the Bispathodus ultimus Zone but shifts to the increase in the upper part of the measured section higher up in the same zone. Furthermore, the ratio between uranium and thorium (U/Th) was used as a proxy for environmental redox conditions (Nath et al. 1997; Wignall and Twitchett 1996). The lowest part of the Hády section indicates a dysoxic to anoxic interval, while the rest of the lower Famennian features predominantly oxygenated environment (Palmatolepis minuta minuta to Palmatolepis marginifera utahensis zones). The upper part of the Hády section is characterized by mostly oxic to dysoxic environment with two brief anoxic intervals (Palamtolepis rugosa trachytera to Bispathodus costatus zones). The Lesní quarry section is primarily dysoxic to anoxic in nature from the Palmatolepis gracilis expansa Zone to the Bispathodus ultimus Zone. Based on the facies associations, gamma-ray spectrometry, conodont biozonation and biofacies, changes in depositional environment were interpreted as follows: During the earliest Famennian the deposition was situated in the middle carbonate ramp. The ramp was drowned during Palmatolepis minuta minuta and Palmatolepis crepida zones, as the depositional environment shifted to deep-marine setting of the lower slope or possibly even the base of the slope. Deep depositional environment lasted until Palmatolepis marginifera marginifera Zone, correlating well with the lower Famennian global transgression (Becker et al. 2020). During the Palmatolepis marginifera marginifera to Palmatolepis gracilis manca zones, the depositional environment shifted to the middle and/or upper part of the carbonate slope, represented by progradation of the carbonate turbiditic system. Progradation peaked between Palmatolepis gracilis expansa and Bispathodus ultimus zones, when the most proximal calciturbidites were deposited. When compared to the global T-R cycles the correlation between described development and global regression during middle to upper Famennian becomes apparent. Correlation of the two short anoxic intervals in the Palmatolepis rugosa trachytera to Bispathodus costatus zones with the global events needs additional higher resolution biostratigraphic analysis which will be carried out in the following research. |
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