Jindřich Hladil

Carbon and oxygen isotope record across Pridoli to Givetian stage boundaries in the Barrandian basin (Czech Republic)

Abstract Carbon and oxygen isotope composition and the chemistry of limestones, clayey limestones and shales were studied across stage boundaries from Pridoli to Givetian in the Barrandian basin. No substantial isotopic shifts were found at the Pragian–Emsian and Emsian–Eifelian boundaries. A gradual shift in δ13C values from −2.6 to +3.6‰ was observed in Pridoli and Lochkovian limestones. This shift is inferred to have been related to a combination of higher productivity, increased deposition of organic matter and shallowing of the basin. The global Kacak–otomari event just below the Eifelian–Givetian boundary is marked by an abrupt change in carbon isotopic values from about 1 to 3‰ in pelagic carbonates. No δ13C shift was found in shallow-water carbonates at the Eifelian–Givetian boundary. High δ18O values up to −1.3‰ in Pragian, Emsian and early Eifelian limestones indicate that the Barrandian basin was relatively isolated from the world ocean during those intervals. Lower δ18O values up to −7.5‰ in late Eifelian limestones may indicate the end of isolation.

Seawater strontium isotope curve at the Silurian/Devonian boundary: a study of the global Silurian/Devonian boundary stratotype

Abstract Present application of 87Sr/86Sr chemostratigraphy to detailed stratigraphical tasks is limited by inaccurate calibration of the general seawater strontium curve to absolute as well as to relative time scales. For this reason, refinement of the general seawater strontium curve has been suggested, using mainly clearly defined global boundary stratotype sections. This study reports the first 87Sr/86Sr data from the global Silurian/Devonian boundary stratotype section and fills an existing 1-Ma gap in available data. Generally, the data from the stratotype fit the range interpolated from published 87Sr/86Sr data of the general curve, but the slight differences may suggest an existence of a high-order oscillation near the Silurian/Devonian boundary. Higher 87Sr/86Sr values in the Devonian part of boundary bed 20 (20-beta) may indicate an exotic material influx of recycled sediment.

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

Stratigraphic correlation potential of magnetic susceptibility and gamma-ray spectrometric variations in calciturbiditic facies (Silurian-Devonian boundary, Prague Synclinorium, Czech Republic) 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 growth-faults and/or a biotic event rather than of eustatic sea-level changes.

Searching for extinction/recovery gradients: the Frasnian-Famennian interval, Mokrá Section, Moravia, central Europe

Abstract A series of ancient seafloors colonized by diverse organisms has been documented from the Upper Devonian rocks of the Western Mokrá Quarry. Situated in the southern tectonic closure of the Moravian Karst, the Frasnian-Famennian shallow carbonate ramps exhibit both Rhenish and Ukrainian affinities. Reconstruction of palaeo-sea floor horizons results in a series of 28 quadrats sufficient for further evaluation. Eighty-five taxa involved were scrutinized for abundance, occupied area, skeletal mass production and biomass production. The aim of the study was to determine whether the observed sequence of quadrats can be distinguished from a random one, and to discover any possible unidimensional gradient as a latent control. Monte Carlo simulations and a graph theoretical approach were utilized. Although the raw data seemed chaotic, the simulations demonstrated the observed sequence is not random. A significant influence of a hidden control is thus suggested. Fifteen characteristics of quadrats (e.g. diversity, number of taxa, vertical stratification of community, number of patches) were utilized for final interpretation. The gradient reconstructed by TSP algorithm reveals a significant crisis within the uppermost part of the Amphipora-bearing limestone.

The Odivelas Limestone: evidence for a Middle Devonian reef system in western Ossa-Morena Zone (Portugal)

The Odivelas Limestone: evidence for a Middle Devonian reef system in western Ossa-Morena Zone (Portugal) The Odivelas Limestone constitutes one of the few records of Middle Devonian sedimentation in the western Ossa-Morena Zone. Although deformed and metamorphosed the limestones have an abundant fossil content which allows their positioning as late Eifelian/early Givetian in age and to relate the reef fauna with the typical Rhenish facies for the same time period. Magnetic susceptibility analysis was attempted and is in agreement with the biostratigraphy, but the limited extent of sections and the metamorphism precludes firm correlations. The field evidence, petrographic and geochemical analysis point to a close paleogeographical relation and dependence of the reef system on volcanic structures which are included in the Beja Igneous Complex. The age of part of the volcanic and sub-volcanic suite of this complex is thus constrained.

A Large Trace Fossil in Nodular Limestones (Lower Devonian, Czech Republic): Sedimentological Consequences, Ethology and Taphonomy

Large star-like trace fossil was found on the upper bedding plane of nodular limestone of the Praha Formation (Pragian, Devonian) at Praha, Czech Republic. It was tentatively placed to the ichnogenus Capodistria. It is interpreted as a feeding trace that originated on the seafloor surface. The structure consists of 10–11 radial rays and its longer axis is 60 cm. The rays are preserved as concave epireliefs, up to 10 mm deep and 25 mm wide. The trace fossil partly intersects nodules that cover surfaces of most bedding planes of the Praha Formation, demonstrating that the nodules formed during the earliest stages of diagenesis.