The correlation potential of magnetic susceptibility and outcrop gamma-ray logs at Tournaisian-Viséan boundary sections in western Europe
Babek, O.; Kalvoda, J.; Aretz, M.; Cossey, P.J.; Devuyst, F.-X.; Herbig, H.-G.; Sevastopulo, G. (2010). The correlation potential of magnetic susceptibility and outcrop gamma-ray logs at Tournaisian-Viséan boundary sections in western Europe. Geol. Belg. 13(4): 291-307
In: Geologica Belgica. Geologica Belgica: Brussels . ISSN 1374-8505; e-ISSN 2034-1954, more
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| Keyword |
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| Author keywords |
Outcrop logging techniques; foraminifer biostratigraphy; Carboniferous;deep-marine sediments; sea-level changes |
| Authors | | Top |
- Babek, O.
- Kalvoda, J.
- Aretz, M.
- Cossey, P.J.
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- Devuyst, F.-X.
- Herbig, H.-G.
- Sevastopulo, G.
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| Abstract |
We have measured five deep-water carbonate and carbonate-siliciclastic sections at the Tournaisian-Visean (Tn/V) boundary in western Europe, using petrophysical outcrop logging techniques (gamma-ray spectrometry /GRS/ and magnetic susceptibility /MS/). The aim was to trace correlatable log patterns across the flanks of the London-Brabant Massif from eastern Ireland to western Germany. Both GRS and MS logging proved useful for long-distance (up to similar to 1000 km) correlation. The log patterns can be interpreted in terms of sea-level fluctuations. A late Tournaisian regression, a sequence boundary at the Tn/V boundary, early Visean lowstand systems tract and an overlying transgressive to regressive succession can be identified from the GRS and MS logs. The Tn/V sequence boundary can be correlated with exposure features and karstic surfaces in the up-dip shallow-water settings at the boundary between sequence 4 and 5 of Hance et al. (2001, 2002). This indicates that sea-level fluctuations around the Tn/V boundary were synchronous and traceable on the flanks of the London-Brabant Massif. The GRS-based logging has a greater correlation potential than MS as it can be applied in a broad spectrum of facies and depositional settings. In certain sections, the MS signal shows an increasing trend during transgression and a decreasing during regression, which is opposite to the MS paradigm from shallow-water carbonate platform settings. These trends are assumed to result from landward/basinward facies shifts of low-productivity carbonate ramp systems. Lowstand shedding of carbonate tempestites and turbidites results in low MS values while during sea-level rise the ramp systems backstep, developing retrograding facies successions in their distal parts, which are associated with upward-increasing MS values. |
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