Global extinctions and biotic overturns (bioevents) occur in striking abundance in the Devonian as compared to other periods of the Phanerozoic. At least ten remarkable bioevents (basal Pragian, Daleje, Choteč, Kačák... [ view full abstract ]
Global extinctions and biotic overturns (bioevents) occur in striking abundance in the Devonian as compared to other periods of the Phanerozoic. At least ten remarkable bioevents (basal Pragian, Daleje, Choteč, Kačák /otomari/, pumilio, Middlesex /punctata/, Lower and Upper Kellwasser, annulata, Dasberg and Hangenberg events) are faunistically well documented in the classical Devonian terrains of Europe, North Africa and North America. Being associated with faunal overturns in benthic and pelagic communities the bioevents are routinely studied using a biostratigraphic and lithofacies approach. However, the occurrence of many index taxa, in particular macrofossils, is primarily controlled by environmental, mainly depth-related sedimentological variables. The most readily dated fossiliferous strata and biohorizons are offshore, condensed facies and truncated zonal unconformities, which coincide with such keystones of the sequence-stratigraphic paradigm as the maximum flooding surfaces and sequence boundaries. Major extinctions in Earth´s history are commonly associated with habitat reduction during major regressions (lowstand) or with anoxic events during transgressions. Sequence stratigraphy thus provides a powerful predictive framework to examine macroevolutionary patterns and bioevents and their causes.
Four global, Lower to Middle Devonian bioevents, the basal Pragian event, Daleje, Choteč and Kačák events were first defined in predominantly carbonate-ramp facies of the Barrandian area, Czech Republic. All of them are associated with regionally significant facies shifts, which are traditionally interpreted as T-R trends. Literature search reveals that some bioevents (basal Pragian and Choteč event) are regressive whereas others are transgressive (Kačák event) and/or associated with very rapid R-T pulses (Daleje event). However, these interpretations are based on descriptive lithofacies observations in the field, supported by analysis of fossil assemblages.
In this study we present quantitative stratigraphic analysis of 18 Lower- to Middle Devonian sections of the Prague Basin (thickness ranging from 5 to 230 m) of the Barrandian area, based on field gamma-ray spectrometric logging, facies logging, quantitative allochem composition analysis, element geochemistry and laboratory petrophysical methods (magnetic susceptibility and visible-light spectral reflectance). The aim was to trace geophysical and geochemical trends across the whole, Lower to Middle Devonian stratigraphic succession, with focus on the bioevent horizons and interpret them in terms of relative sea-level changes. Results from this extensive field and laboratory dataset will be presented and discussed. This work was supported by the research project GAČR 14-18183S.
Topics: Biotic sedimentary processes , Topics: Carbonate platforms and reef , Topics: Sequence stratigraphy
