The Lochkovian-Pragian boundary interval of the Barrandian area - discussion of oxygen and carbon isotope data

0554007 - GLÚ 2022 RIV CZ eng A - Abstrakt
Weinerová, Hedvika - Bábek, O. - Slavík, Ladislav - Vonhof, H. - Joachimski, M. M. - Hladil, Jindřich
The Lochkovian-Pragian boundary interval of the Barrandian area - discussion of oxygen and carbon isotope data.
35th IAS Meeting of Sedimentology: virtual meeting. Prague, Czech Republic 21-25 June 2021. Book of Abstracts. Olomouc: Palacký University Olomouc, 2021 - (Bábek, O.; Vodrážková, S.). s. 461-462. ISBN 978-80-244-5929-5.
[IAS Meeting of Sedimentology, Virtual Meeting /35./. 21.06.2021-25.06.2021, Prague]
Grant CEP: GA ČR(CZ) GA14-18183S; GA ČR(CZ) GA17-06700S
Institucionální podpora: RVO:67985831
Klíčová slova: Prague Basin * Devonian * oxygen isotopes * carbon isotopes * calcite * apatite
Obor OECD: Paleontology
https://www.iasprague2021.com/book-of-abstracts/

The Barrandian represents the type/classical area for the Lochkovian and Pragian stages with the Lochkovian-Pragian boundary Event recognized here. The Lochkovian-Pragian boundary coincides with a sea-level fall, changes in facies, faunal content, organic productivity and bottom water redox conditions. In this contribution, new oxygen and carbon stable isotope data from middle to distal carbonate ramp environments of the Prague Basin are presented. In the studied Na Branžovech section, the upper part of the Lochkov Formation (Lochkovian) is represented mainly by crinoidal calcarenites revealing a coarsening-upward trend. Instead, the overlying Praha Formation (Pragian–lower Emsian) shows fining-upward from crinoidal calcarenites/ calcirudites to calcisiltites (dacryoconarid-rich wacke – /packstones) with calcareous shale/marlstone intercalations. The Požáry section shows a similar facies development. Carbonate δ13C and δ18O data from the Na Branžovech section (109.5m, upper middle Lochkovian–lower Emsian) come from 50 polished thin-sections examined for cathdoluminescence. In total, 91 samples of powdered brachiopod calcite, fine-grained carbonate matrix and sparite obtained by microdrilling were analysed. Carbonate δ13C values range from -0.9 to 4‰ VPDB. Positive shifts in δ13C often coincide with decreasing computed gamma-ray (CGR) values interpreted as indicating regressions, whereas negative δ13C shifts are observed together with increasing CGR values interpreted as transgressions. The upper Lochkovian–lower Pragian positive δ13C excursion reported by previous authors was recorded. Carbonate δ18O values are between -6.6 and -1.3‰ VPDB and show a trend to higher values across the Lochkovian-Pragian boundary. δ18O of the fine-grained matrix and of non-luminescent/slightly luminescent brachiopod shells show comparable trends. Generally, non-luminescent/slightly luminescent brachiopod shells show higher δ18O values than luminescent brachiopod shells, fine-grained matrix and sparite in fine-grained limestones and lower values in coarse-grained crinoidal limestones. Conodont apatite δ18O values from the Požáry section (137m, upper Silurian–lower Emsian, N= 45) reach 16.5–20.8‰ VSMOW. An increase in δ18O is documented for the Lochkovian-Pragian boundary interval. This trend starts in the middle Lochkovian (≈17‰ VSMOW, boundary of transitans – trigonicus and trigonicus – kutscheri zones) and continues to the lower Emsian (≈21‰ VSMOW, gracilis/excavatus – gronbergi Zone). The Lochkovian-Pragian boundary interval is connected with cooling. The temperature difference between the upper middle Lochkovian and lower Emsian calculated from both, δ18O of non-luminescent/slightly luminescent brachiopod calcite as well as δ18O of conodont apatite is relatively large (14 °C). Relatively high Pragian/lower Emsian carbonate δ18O values from some Barrandian sections were ascribed to elevated salinity during temporal partial isolation of the Prague Basin by previous authors. Likewise, conodont apatite δ18O values from the Prague Basin are higher than values from Australia. Our data indicate that the complexity of the carbon and oxygen stable isotope record is a result of an interplay between temperature changes, facies variability due to relative sea-level changes, facies-dependent, marine and deep-burial diagenesis, and variation in salinity supposed by previous authors.
Trvalý link: http://hdl.handle.net/11104/0330284