Early Middle Frasnian platform reef strata in the Moravian Karst interpreted as recording the atmospheric dust changes: the key to understanding perturbations in the punctata conodont zone

Abstract

Development of new stratigraphic techniques has led to better understanding of the compositional variability and quantity of dust-related impurity components in pure marine limestones, with a considerable impact on stratigraphic resolution and explanation of causality. The early Middle Frasnian (E-MF, punctata Zone) stratigraphic interval was selected for this study in order to assess the contribution of atmospheric dust and to distinguish between the robust climatically driven anomalies and the potential distant effect of the Alamo impact. The location of the study in the Moravian Karst area has two advantages: there are no mixed carbonate-siliciclastic sediments, only the pure limestone of a platform reef complex, and the authors have access to voluminous survey and drilling reports. The methodology employed for undertaking the most recent research was based on succession and combination of the following steps: biostratigraphy and facies analysis, magnetic susceptibility (MS), gamma-ray spectrometry (GRS), instrumental neutron activation analysis (INAA), and finally, separation and assessment of rare non-carbonate particles. The natural atmospheric dust burden, deposition, and embedding in pure carbonate traps were, most likely, threefold higher during the Frasnian in the Upper Devonian in comparison with Quaternary platform reef counterparts. Variation of 10-100 ka averages is medium in terms of the Frasnian ranges of MS-GRS values, and the base and top of the E-MF interval are manifested by robust elevations of these values. Forced anomalies in MS, GRS and geochemical signals that defy the normal sequence of rhythms were found: two anomalies disturb the broad middle part of the E-MF interval and one is superimposed on the upper part of the punctata-hassi zones strata. The most significant disturbance was found near the mid-punctata Zone level. In spite of the relatively low magnitude, it shows features that are usually related to major environmental crises which occured in the Devonian, such as the Choteč, Kačák or Kellwasser events. These include a large depression in MS and dust-particle concentration values coupled with a period of sea level lowstand and calm atmospheric conditions (stage A), and an abrupt shift to high MS that has a comb-like pattern that gradually fades upward, developed together with a forced flooding surface, increased detritism, and stormy conditions (stage B). It is a reverse of the normal cyclicity in pure limestone when the high impurity corresponds to lowstands. A very small amount of particulate material of an exotic nature was found at the A-B stage interface and assessed: it contained iron-rich silicate microspherules and drops, devitrified glasses, tiny mineral/rock clasts of hyperbasite compositions, as well as pellets and clumps of glasses and phlogopites. The silt-sized particles show ablated and striated surfaces, flow deformation of devitrified glasses, and layered onion-like structures. Iron-rich lamellae with meshwork crystal patterns occur among decrepitated crystalli...

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

“…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.…”

“…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.…”

The Paleozoic record of Thaumatoporella PIA, 1927?

“…A cross-reference is hereby given to the "out-of-balance" facies rich in microbial ("bacinellid") crusts and other structures assumed to be coeval to the Early Aptian OAE 1a (e.g., IMMENHAUSER et al, 2005;RAM-EIL et al, 2010). Also the increased abundance of Devonian thaumatoporallecean-like forms corresponds to dysoxic episodes and events, e.g., in the Upper Givetian, basal Frasnian or with the Middle Frasnian, e.g., in the Pa. punctata Zone (HLADIL et al, 2009). It is very likely that both the major and medium-magnitude Palaeozoic events which caused pro-liferation of microbially dominated benthic communities and organomineralization of sediment in the post-crisis period (BOURQUE et al, 2004) led to the right setting for production and release of the cysts.…”

Magnetic susceptibility application: a window onto ancient environments and climatic variations: foreword

“…A positive correlation between MS and independent palaeo-environmental proxies is the first indication that the primary signal is preserved. These non-magnetic palaeo-environmental proxies include the following: † Clay or quartz content or insoluble residues in carbonate rocks are typically of detrital origin and are classical parameters to be compared with MS in order to infer a primary, detrital origin of the MS signal (Ellwood et al 1999;English 1999;Hladil et al 2009). For example, showed that the MS increased with increasing quartz content on two Middle Devonian sections from Belgium (Fig.…”