Stratigraphic significance and resolution of spectral reflectance logs in Lower Devonian carbonates of the Barrandian area, Czech Republic; a correlation with magnetic susceptibility and gamma-ray logs

Abstract

Spectral reflectance (SR) is a relatively novel technique which is used as a proxy for organic carbon and siliciclastic input in carbonate sediments, in particular in core research. This study summarizes results from high-resolution outcrop SR logging of a Lower Devonian distal ramp carbonate section of the Barrandian area, Czech Republic. The SR data were correlated with parallel petrophysical and geochemical data with an aim to improving their cyclostratigraphic and event stratigraphic interpretation potential and the gaps SR data can fill. Transgressive–regressive trends, indicated by facies stacking patterns, CaCO 3 data and spectral gamma-ray (GRS) logs, are well correlated in the regional context and point to third-order eustatic sea-level fluctuations. Peak regressions of the TR cycles are sensitively marked by CIE L* a* b* parameters and percent reflectance in colour bands. The SR and magnetic susceptibility (MS) data are markedly cyclic on a more detailed scale. Their inferred periods vary between 49.2 and 205 kyr indicating that they may have been forced by orbital cycles in the Milankovitch band. The SR and MS cycles are either in phase or out of phase. The latter cycles point to a strong diagenetic forcing of the colour cyclicity. Hematite concentration cycles, inferred from SR signal in the Pragian red hemipelagic carbonates, are interpreted as reflecting changing bottom oxygenation and subsurface redox gradients during early diagenesis. The CIE L* parameter was found to be related to the presence of diagenetic hematite, which indicates that cyclostratigraphic analysis of simple greyscale data can be affected by considerable error due to chromacity. Although the CIE L* parameter is partly correlated with CaCO 3 concentrations, the MS data are not, which suggests that the MS signal can be significantly influenced by diagenetic minerals and its cyclostratigraphic interpretation can be potentially misleading. The SR data, backed up by GRS, MS and CaCO 3 data indicate that the red colour in the Pragian hemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of an oxygenated ocean floor. This suggests that deep-sea bottom oxygentation may have been in operation during the early Devonian, which is traditionally perceived as a typical greenhouse period. The SR data can be obscured in several m thick zones of elevated CIE L* and other colour changes (“leaching zones”) related to postdepositional alteration along subvertical faults, however, this fault-related telogenic alteration has little influence on the MS signal. The combined use of SR and MS has a synergic effect and can significantly improve the interpretation of their respective cyclic signals, particularly in relation to the diagenetic overprint and possible carriers of the MS.