A first report of the Ludlow Lau event from the Prague Basin (Barrandian, Czech Republic)
Oliver Lehnert1, Jirí Frýda2, Werner Buggisch1 and Štepán Manda3
1Institut für Geologie
und Mineralogie, Universität Erlangen, Erlangen, Germany. E–mail: firstname.lastname@example.org–erlangen.de
2 Czech Geological Survey, Klárov 3/131, 118 21 Prague 1, Czech Republic.
Key words: Lau event. Ludlow. Silurian. Barrandean area. Czech Republic.
The goal of this paper is to document the late Ludlow Lau event for the first time from the larger peri–Gondwana area. In Bohemia, we recognize the start of the d13C excursion within a characteristic brachiopod bed dominated by Dayia minor in the upper part of Neocucullograptus kozlowskii graptolite zone (late Polygnathoides siluricus conodont zone). In the upper Kopanina Formation, this level is just below a sequence boundary underneath grey crinoidal limestones bearing the Ananaspis fecunda – Cyrtia postera Community (= Monograptus latilobus graptolite assemblage zone). The d13C values reach a maximum (+4,6 ‰) within the lower part of these strata. Underneath this sequence boundary several extinctions and faunal changes have been observed in different groups. The faunal turnovers presumably have been caused by a combination of changes in climate, sea level and sedimentation.
The Lau event is named after the locality of Lau and was reported from several palaeocontinents (Australia, Laurentia, Baltica) (for references see Munecke et al., in press). The record from Bohemia (Perunica Terrane) represents a first report of this global event from a palaeogeographic position in higher latitudes between Baltica and northern Gondwana. The history of Prague Basin during Silurian times was summarized by Kríz (1998a). Here, synsedimentary faults caused the developement of different palaeoenvironments and fast lateral facies changes over short distances. In general, there is a shift in the Silurian from siliciclastic to carbonate sedimentation. During the late Llandovery and from mid Wenlock through mid Ludlow sedimentation was influenced by volcanic activity (Kríz, 1998a). The deposition of the Kopanina Fm. started during the earliest Gorstian and especially during the Ludfordian it is characterized by a wide distribution of brachiopod, crinoidal, and cephalopod limestones (Kríz, 1998a,b). In Mušlovka Quarry near Reporyje, a location 700 m northeast of the famous GSSP of the Prídolí stratotype at Pozary Quarry (Figure 1), we sampled a section of the upper Kopanina Fm. (Ludfordian) for carbon isotopes (Figure 2).
The dataset from the Prague Basin fits well with changes in sedimentology, faunal changes and isotope composition in other areas. Such fluctuations in the d13C record are interpreted to reflect changes in palaeoclimates by several Silurian workers (e.g., Jeppsson 1990, 1998, Munnecke et al., in press) independently from different models for oceanic circulations.
Figure 1. A. Geologic scetch map of the Prague Basin with the position of the Mušlovka section. B. detailed location map. C. Record of the Lau event (for locations see Munnecke et al., in press) plotted on the paleogeographic reconstruction (Scotese and McKerrow, 1990); note: one black star may represent several reports of the Lau event.
Carbon isotope record
The Lau event is displayed by one of the strongest d13C excursions during the entire Phanerozoic and its record was recently compiled by Munnecke et al. (in press). It reaches even values up to 12 ‰ (V–PDB) in Australia (Andrew et al., 1994), in the Baltic up to 11,2 ‰ in the shallow water sediments of southern Sweden (Wigforss–Lange 1999) and 5 ‰ for the eastern Baltic (Kaljo et al., 1997). In North America the maximum is at about 4 ‰ (Saltzman, 2001). In Bohemia, the d13C values reach a maximum of +4,6‰. Therefore, the excursion itself with respect to the "normal" values spans about 4 ‰ (Figure 2). Isotope values in the lower part of the section exposed at Mušlovka are between –0,4 and +1,4 ‰ (in an average around 0,5 ‰) and represent the regular isotope "background" data for Ludfordian in this area. The typical d13C excursions start with values of +2,08 and +2,01 ‰ in bed 16 (dots in the section indicate conodont sample horizons of Kríz and Schönlaub 1980) characterized by Dayia minor. There is a slight decrease at the base of bed 17, just above the black micritic layer (+1,4 ‰) and then it constantly increases and the peak reaches its maximum at the level of Schönlaub’s sample Nş 18 (+4,6 ‰), drops back to values around +2,5 ‰ in level 19, to +1,9 ‰ just at the level of conodont sample 20, and then goes back to normal data around +1 ‰ within massive bed 21 (Figure 2).
This characteristic d13C shift correlates especially well to the faunal and isotope data reported from the Eke beds in the Gotland. There, the increase of d13C values correspond of the Lau Primo–Secundo Event (Jeppsson and Aldridge, 2000). It triggered several extinction events affecting different groups like conodonts, chitinozoans, and fishes. Unfortunately, because of lack of detailed data we cannot present the same resolution to distinguish a sequence of 5 extinction events in this interval like in Gotland (Jeppsson, 1993), but the initial increase of d13C values starts here also within the Dayia beds and the shift to higher values coincides with the top of the P. siluricus conodont zone (Kríz and Schönlaub in Chlupác et al., 1980, Figure 6). Data treating the Lau event and several other events in the Silurian of Gotland have been compiled in detail by Samtleben et al. (2000), and Munnecke et al. (in press) compared this dataset with all published records on a global scale.
Sedimentologic evolution and biostratigraphy across the Lau event
The typical shift in C isotopes is recorded in the shallow–water carbonate succession at Mušlovka Quarry. Before we can observe an increase of d13C values, a 3 m interval of grey, bioclastic cephalopod limestones bearing the Cardiola alata Community (Kríz, 1998b) was deposited. This community corresponds to the N. kozlowskii graptolite zone (late P. siluricus conodont zone). The shift in C isotopes starts within amassive, approx. 0,7 m thick bed of a grey bioclastic brachiopod limestone with abundant Dayia minor, which is only exposed in this section. The occurrence of the trilobite Balizoma transiens within this bed (Boucek, 1937) also correlates to the N. kozlowskii graptolite zone. The subsequent distinct layer of a dark clayey limestone contains almost no fossils and separates different faunal assemblages underneath and above this 25 cm thick horizon. Recent research by Manda (2003) documents that this interval corresponds to a sequence boundary related to erosion and/or hiatus in the eastern part of the central segment of the Prague Basin. The micritic bed was deposited in paleogeographic depression while erosion took place in surrounding areas. There, the shallowing and regression is documented by a characteristic succession of dark grey micritic cephalopod limestones with a erosional surface at the top which is covered by accumulations of the trilobite species Ananaspis fecunda (occurring at this level for the first time) and then overlain by bioclastic pack– to grainstones with new brachiopod and trilobite taxa.
The regression in the Prague Basin compares well with the situation documented in the sedimentary succession of Gotland, where near the base of the event and just above the Dayia flags a hiatus reflecting the time interval of the regression is observed in the shallow shelf areas (for references see Samtleben et al., 2000). There, high concentrations of Dayia navicula occur already in a facies with an extreme input of windblown siliciclastic material and a high content of dolomite on Gotland during falling sea level (Samtleben et al., 2000).
In Mušlovka, the overlying five beds of light grey to pinkish grey crinoidal limestones (approx. 6.5 m) are characterized by abundant crinoid remains, and by trilobites and brachiopods belonging to the A. fecunda – C. postera Community (Havlícek and Štorch, 1990). The d13C values reach a maximum value in the lower part of these limestones, which corresponds to M. latilobus graptolite assemblage zone (Kríz, 1998b).
Figure 2. Mušlovka section with conodont zones modified from Kríz and Schönlaub (in Chlupác et al., 1980). Numbers on the left side of the column correspond to the levels of Schönlaub’s conodont samples as indicated by Kríz and Schönlaub; the graptolite subzonation is based on Štorch (1995) and zones are characterized by Kríz (1998b); black dots in the carbon isotope curve correspond exactly to the sampled levels.
Faunal response to changes in sedimentology and climate
The most detailed studies on faunal assemblages within certain limestone horizons in the Mušlovka section were carried out by Boucek (1937), Kríz (1992, 1998a,b), and Manda (2003, and unpubl. data). Several faunal changes and extinctions have been observed, namely at the base of N. inexpectatus, N. kozlowskii, and Monograptus latilobus graptolite zones (Boucek, 1937; Kríz, 1992; Manda, 2003; Figure 2). After the latter extinction, the almost unfossiliferous, dark clayish limestone bed covered the bed with Dayia minor like a shroud. Faunas of the overlying limestone beds with the A. fecunda – C. postera Community (Havlícek and Štorch, 1990) are characterized by an immigration of new elements (trilobites, cephalopods, and brachiopods) to the Prague Basin. This faunal overturn fits with the peak in the d13C record in Mušlovka.
Based on cyclic changes in Silurian conodont faunas, Jeppsson (1990, 1998) proposed in his model that Silurian oceanographic cycles are triggered by climatic changes. According Munnecke et al. (in press) the relatively short transitional intervals (104 to 105 years) between humid H– and arid A– periods (P– and S–states of Jeppsson, 1990) represent times of climate oscillations between the two states. In their detailed example of the Ireviken Event it is also the case that the first extinctions occur well below the d13C excursion. It seems that these Silurian excursions are responding to changes in climatic, and in turn changes in organic productivity in general succeed the extinctions which are coupled with fluctuations in sea–water temperatures and sea–level. In the Prague Basin we observe the shift in d13C in a level above the extinctions and the peak of lowest faunal diversity.
We want to thank Jiri Kríz (Czech Geol. Surv.) for many informations and extremely useful discussions. The reviews by Lennart Jeppsson and Matt Saltzman improved our manuscript. Field work and research of O. Lehnert in the Prague Basin was carried out during a stay at the Charles University in Prague (Czech Republic) supported by the Alexander von Humboldt Foundation (Bonn, Germany) with a Feodor Lynen Fellowship. Jirí Frýda thanks the Humboldt Foundation for providing a possibility for this common project and its continuation.
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Received: February 15, 2003
Accepted: June 15, 2003