Stratigraphic correlation potential of magnetic susceptibility and gamma-ray spectrometric variations in calciturbiditic facies (Silurian-Devonian boundary, Prague Synclinorium, Czech Republic)

Abstract

Magnetic susceptibility (MS) and gamma-ray spectrometry (GRS) stratigraphy were used for correlation and characterization of eight Silurian-Devonian (S-D) sections in the Prague Synclinorium (Czech Republic). They represent two different facies developments: lower subtidal to upper slope deposits and slope-to-basin-floor distal calciturbidites. Sections from relatively shallow-and deep-water sections are easy to compare and correlate separately, although the detailed relationship between these two facies is still not entirely clear and correlations between the two settings are difficult. This may be due to sharp facies transitions and presence of stratigraphic gaps. The MS and GRS stratigraphic variations combined with sedimentologic data have been also used for reconstruction of the evolution of the sedimentary environment. The beds close above the S-D boundary show noticeably enhanced MS magnitudes but weak natural gamma-ray emissions. It may correspond to an increased amount of terrigenous magnetic material occurring with short-term shallowing (sedimentological evidence). In deep-water sections the uppermost Silurian is characterized by high MS and GRS values. It corresponds to a supply of recycled sediment to the lower wedge which occurred during the late Pridoli regression phase. The basal Devonian beds correspond to gradual deepening, but the overlying sequences reflect other shallowing episodes which are expressed in increasing MS and gamma ray activity of rocks. The MS and GRS fluctuations are interpreted as a result of local subsidence of the sea bottom along synsedimentary growthfaults and/or a biotic event rather than of eustatic sea-level changes.

Palaeoclimatic event at the Lochkovian-Pragian boundary recorded in magnetic susceptibility and gamma-ray spectrometry (Prague Synclinorium, Czech Republic)

“…The MS signal may be strongly modified by secondary processes such as magnetite neomorphism during diagenesis or secondary remagnetization (see e.g., Riquier et al 2010), which certainly devalues interpretations that the primary MS record reflects depositional conditions and processes. Vacek et al (2010) and Koptíková et al (2010b), who studied analogous facies from this stratigraphic level in other sections in the Prague Synclinorium, ascribed the major effect on the MS signal to changing amounts of various paramagnetic minerals (pyroxenes, amphiboles, micas, chlorite, a.o.) whereas ferromagnetic components (mainly magnetite, hematite, and goethite) have been present only subordinately and have had only limited influence on the bulk MS.…”

“…However, these changes need not be a result of a significant sea-level fluctuation, but rather result from a climatic event. As described by Vacek et al (2010), synsedimentary tectonics also might have caused short-term facies changes, which might be confused with effects of minor eustatic sea-level changes. The widespread distribution of shallow-water facies (including reef facies in the Koněprusy area, SW part of the Prague Synclinorium) in the lower part of the Praha Fm.…”

“…These interpretations come from the general assumption that there is a relationship between the amount of terrigeneous supply responsible for increased MS values and eustasy (Ellwood et al 2000). However, there are also various factors which play against this generalization (e.g., Hladil et al 2006Hladil et al , 2009Da Silva et al 2009;Vacek et al 2010). The MS signal may be strongly modified by secondary processes such as magnetite neomorphism during diagenesis or secondary remagnetization (see e.g., Riquier et al 2010), which certainly devalues interpretations that the primary MS record reflects depositional conditions and processes.…”

“…4). As these MS peaks mostly correspond to limestone beds, this pattern is related to increased delivery of magnetic particles by calciturbidites to the carbonate slope environment with prevailing hemipelagic sedimentation (see Vacek et al 2010).…”

“…In the Czech Republic this approach has been used mainly in the Devonian of the Prague Synclinorium (central Bohemia; e.g., Slavík et al 2000;Crick et al 1997Crick et al , 2001Ellwood et al 2006;Hladil et al 2010b;Koptíková 2010;Koptíková et al 2010a, b;Vacek 2010;Vacek et al 2010) and Moravosilezian Zone (Moravia; e.g., Hladil 2002;Hladil et al 2000Hladil et al , 2003aHladil et al , b, 2006Hladil et al , 2009Geršl & Hladil 2004;Bábek et al 2007;Boulvain et al 2010). Physical stratigraphic methods have been used for intrabasinal or interregional correlations as well as auxiliary data for characteristics of depositional environments and their evolution, especially in cooperation with sedimentologists and biostratigraphers.…”