Informace o publikaci
Lower Devonian pelagic red beds from the Barrandian area, Czech Republic
| Autoři | |
|---|---|
| Rok publikování | 2015 |
| Druh | Konferenční abstrakty |
| Citace | |
| Popis | We studied a carbonate succession of the Lochkov, Praha and Zlichov Formations, in a 120 m thick section of the Branžovy quarry, Barrandian area, Bohemian Massif. Bound to the underlying Lochkov Formation by a basal unconformity, the basal layers of the Praha Formation are composed of coarse-grained crinoidal limestone that pass upward into dark nodular calcisiltite and calcilutite with dacryoconarid tentaculites and abundant stromatactis structures (sheet cracks) forming an upward fining (transgressive) sequence. The upper parts of the Praha Fm. contain a marked 15 m thick layer of red limestone horizon, which can be correlated over distances of several tens of kilometers in the whole Barrandian area. The red pigment is dispersed in the micrite, at stylolites and inside the dacryoconarid shells. The section was logged for facies and field gamma-ray spectrometry with 0.25 to 0.5 m interval and sampled (477 samples) for magnetic susceptibility (MS), reflectance spectroscopy (DRS) and elemental geochemistry (EDXRF). Relatively high U concentrations and U/Th ratios are typical for the Lochkov Formation while both parameters rapidly decrease in the Praha Formation. The gamma-ray values are low at the base of the Praha Formation, but they markedly increase upwards consistently with the facies/microfacies trends and the fining-upward trend. Red limestones fall within the peak transgressive strata (as documented by CGR, XRF and microfacies). The first derivative of the reflectance curve identifies hematite as the main carrier of red colour in the red limestone samples. Their mass-specific MS values do not significantly differ from the remaining parts of the profile and they are correlated with iron concentrations. However, the red limestone colour is not a function of the Fe concentration. From the presence of Fe oxy-hydroxides inside dacryoconarid shells and no correlation between Fe and red colour, it can be inferred that the red colour carrier is not directly related to siliciclastic input from the continent and hence it is of diagenetic origin. Being associated with early diagenetic hematite, the red carbonates represent well-oxygenated sea-floor sediments (ocean red beds), which are similar in origin to the Cretaceous ocean red beds (CORBs), a significant feature of peak greenhouse climatic conditions. |
| Související projekty: |