Publication details
Spectral reflectance outcrop logging of Lower Devonian ramp carbonates, Barrandian area, Czech Republic; correlation with magnetic susceptibility and gamma-ray logs
| Authors | |
|---|---|
| Year of publication | 2009 |
| Type | Article in Proceedings |
| Conference | 27th Meeting of IAS 2009, Alghero Italy, Book of Abstracts |
| MU Faculty or unit | |
| Citation | |
| Field | Geology and mineralogy |
| Keywords | spectral reflectance; magnetic susceptibility; gamma-ray spectrometry; devonian; carbonates; pelagic red beds |
| Description | Spectral reflectance (SR) is a relatively novel technique, which is used as a proxy for organic carbon and siliciclastic input in carbonate sediments especially in core research. This paper summarizes results from high-resolution outcrop SR logging of a Lower Devonian ramp slope carbonate section of the Barrandian area, Czech Republic. The SR data are correlated with parallel gamma-ray spectrometry and magnetic susceptibility logs with an aim to improve their cyclostratigraphic and event stratigraphic interpretation potential. Two transgressive-regressive cycles can be identified: an early to late Lochkovian cycle and a Pragian to early Emsian cycle. The cycles are identified with increasing and decreasing concentrations of Th and K. The maximum regressions in the early Lochkovian, earliest Pragian (the Basal Pragian Event) and the earliest Emsian intervals, which correlate with the Euramerican eustatic sea-level curve, are marked with peaks on the brightness (CIE L*) curve. Brightness cycles in the grey-scaled Lochkovian-earliest Pragian carbonates, which correlate with the MS cycles are interpreted as carbonate productivity/siliciclastic supply cycles. Their wavelength varies between 1 and 3.5 m but their periodicity is uncertain. In the Pragian hemipelagic red carbonates, the brightness cycles are coupled with the cyclicity of hematite content but they are not in harmony with the Pragian MS cyclicity. The MS cycles, the most prominent petrophysical signal in this lithology, with average wavelength of 37.5 cm and inferred periodicity of 37.0 ka are interpreted as obliquity cycles within the Milankovitch band. The hematite cycles are interpreted as diagenetic in origin, reflecting changing bottom oxygenation and subsurface redox gradient during early diagenesis. The red colour in the Pragian hemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of oxygenated ocean floor, which is supported by relatively high Th/U ratios (4.9 in average) and low TOC values (0.24 in average). It is suggested that the brightness (CIE L*) signal is capable of reflecting changes in carbonate/siliciclastic (or TOC) input ratio in grey-scaled, two-component colour systems but it can be obscured by additional colour components, e.g. hematite, in multi-component colour systems. The SR data can be obscured several m thick zones of faded colour, elevated brightness values, and/or hematite to goethite transformation in the red carbonate ("leaching zones"). These colour changes are associated with postdepositional sediment alteration along subvertical faults, which results in disappearance of the cyclic signal on the SR log. |