Integrated stratigraphy, conodont turnover and palaeoenvironments of ...

Newsletters on Stratigraphy, Vol. 49/2 (2016), 321–336 Published online January 2016; published in print April 2016

Article

Integrated stratigraphy, conodont turnover and palaeoenvironments of the upper Wenlock and Ludlow in the shallow marine succession of the Vilkaviškis-134 core (Lithuania) Sigitas Radzevičius1, Andrej Spiridonov1, Antanas Brazauskas1, Darja Dankina1, Algirdas Rimkus1, Giedrius Bičkauskas1, Donatas Kaminskas1, Tõnu Meidla2, and Leho Ainsaar2 With 9 figures Abstract. New data on the Upper Homerian and Ludlow (Silurian) stratigraphy of the shallow water sedimentary and biotic succession of the Lithuanian part of the Silurian Baltic basin are presented here based on detailed analysis of the Vilkaviškis-134 core. Four formations: the Riga, Gėluva and Širvinta formations of Wenlock age and the Neris Formation of Ludlow age were distinguished on the basis of detailed examination of polished rock slabs and natural gamma logs. According to the distribution of stratigraphically restricted conodonts and graptolites, as well as δ13C variations, four regional stages were established: the Jaagarahu, Gėluva, Dubysa and Pagėgiai. A heuristic “delay phase” evolutionary-palaeobiogeographic model for a global turnover pulse is proposed. The model is based on analysis of the temporal distribution of the conodont species. Palaeotemperatures and climate regimes were determined using δ18O and Al/Ti oxide ratio data. Key words. Silurian, Mulde event, graptolites, conodonts, δ13C, δ18O, Eastern Baltic Basin

1.

Introduction

Cramer et al. 2015). The tempo, duration and geographic and taxonomic impact of these events on the biota are still largely unknown in detail, mostly due to the lack of reliable stratigraphy independent of facies and of regional boundaries. The major focus of this study is the stratigraphy of the Mulde bioevent, which is also known as the lundgreni extinction (Koren’ 1987) and the Big Crisis (Jaeger 1991). The Mulde bioevent was proposed following detailed study of Gotland material, which showed that the conodont extinction occurred at the beginning of the

The later Wenlock Epoch and the Ludlow Epoch of the Silurian Period witnessed several geobiological events. The most prominent of these turnover episodes are the Late Homerian Mulde event and the Mid Ludfordian Lau event, which left a global stratigraphic record of extinctions and large positive stable carbon isotopic excursions, and also of profound changes in facies composition (Jeppsson et al. 1995, Jeppsson and Aldrigde 2000, Calner 2005, Eriksson et al. 2009,

Authors’ addresses: 1 Department of Geology and Mineralogy, Vilnius University, M. K. Čiurlionio 21/27, LT-03101 Vilnius, Lithuania; E-Mail (Corresponding author): [email protected] 2 Department of Geology, University of Tartu, Ravila 14a, 50411 Tartu, Estonia. © 2016 Gebrüder Borntraeger, Stuttgart, Germany DOI: 10.1127/nos/2016/0074

www.borntraeger-cramer.de 0078-0421/2016/0074 $ 4.00 eschweizerbart_xxx

322 S. Radzevičius et al.

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Here we provide data and interpretation of conodont and graptolite distributions in relation to changes in facies, natural gamma ray intensity and whole-rock δ13C trends as well as in geochemical palaeoenvironmental proxies in the shallow marine sedimentary succession of the Upper Homerian and Ludlow of the Vilkaviškis134 core. This study revealed a strong resemblance of the faunal succession to that found in other parts of the shallow marine facies belt of the Silurian Baltic Basin, namely those of eastern Lithuania (Radzevičius et al. 2014b) and eastern Poland (Jarochowska and Munnecke 2015). The data presented are also congruent with the conodont and graptolite succession in the “keystone” Viduklė-61 borehole section (Radzevičius et al. 2014a), which indicates the robustness of the preserved patterns.

Tallinn ESTONIA Gotland

LATVIA

Riga

East Baltic Basin LITHUANIA Vilkaviškis-134

Gdańsk

Vilnius Minsk

Grodno BELARUS

POLAND

Vis t

2.

ula Warsaw

Geological setting

Lithuania is located in the western part of the Baltica palaeocontinent or the East European Platform. During the Late Homerian and Ludlow, the Baltic Basin was located near the equator in the southern hemisphere (Cocks and Torsvik 2005). The Vilkaviškis-134 borehole is located in south Lithuania (φ 54° 38 48.00 N; λ 23° 19 45.00 E) in the shallow marine facial zone (Fig. 1). The interval studied includes the Jaagarahu (upper part), Gėluva, Dubysa and Pagėgiai regional stages (Fig. 2). The base of the Gėluva Regional Stage has been considered to be coincident with the lower boundary of the parvus biozone (Radzevičius 2013). The base of the Dubysa Regional Stage is within the global Gorstian Stage and coincides with lower boundary of the nilssoni Biozone (Paškevičius et al. 1994). The base of the Pagėgiai Regional Stage marks the lower boundary of the balticus Biozone (Paškevičius et al. 2012). The following brachiopod communities have been distinguished in the studied interval of the Vilkaviškis-134 core: Pentamerus gothlandicus (BA 3) in the

Brest

UKRAINE Land areas Area of post-Silurian erosion Lagoon Barrier Inner shelf Outer shelf Tornquist-Teisseyre lineament Present erosional boundaries of Silurian deposits Reconstructed boundary of East Baltic Silurian Basin

Fig. 1. Facies map of the western margin of the East European Platform during Gothograptus nassa time (after Einasto et al. 1986) and location of the Vilkaviškis-134 borehole.

first Mulde stable carbon isotopic peak. It was followed by a long survival interval with depleted fauna, and later still by slow conodont species rediversification in the latest Homerian (Calner et al. 2012, Cramer et al. 2012). Studies of conodont distributions in other areas revealed varying patterns of conodont disappearance and originations in relation to the Mulde isotopic excursion and event (Radzevičius et al. 2014b, Jarochowska and Munnecke 2015). Some studies suggested earlier recovery of conodont species (Radzevičius et al. 2014a). Thus the inferred patterns of macroevolutionary change in the conodont clade in relation to this geobioevent probably reflect geographical, environmental and temporal sampling bias (Sadler 2012).

Fig. 2. Lithology, gamma log, distribution of graptolites and conodonts and carbon isotope (δ13C) trend in the interval of the Vilkaviškis-134 borehole investigated. Abbreviations: Reg. Stages, Regional stages (Paškevičius et al. 1994); GR, gamma ray; mcR/h, micro roentgens per hour; C. biozones, Conodont biozones. Legend: 1 – shale; 2 – marlstone; 3 – clayey limestone; 4 – clayey dolomite; 5 – limestone; 6 – red color; 7 – gypsum crystals.

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Jaagarahu Riga Gėluva 700

710

720

760

770

19

730

744.3

748.3 1

2

3

4

Graptolites

GR (mcR/h)

638

650 ?

659 ?

?

670

680

691.4

?

?

?

5

6

780

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first Mulde excursion

second Mulde excursion

crispa C. biozones Pseudooneotodus bicornis Dapsilodus obliquicostatus Ctenognathodus sp. S sensu Viira et Einasto Decoriconus fragilis Oulodus cf. ziegleri Panderodus recurvatus Ozarkodina sp. Walliserodus curvatus Ctenognathodus sp. Pseudooneotodus corniformis Ozarkodina bohemica bohemica Oulodus siluricus Ozarkodina cf. soegina Pseudooneotodus beckmanni Ozakodina snajdri Ozarkodina paraconfluens Ozarkodina crispa Ctenognathodus murchisoni Ozarkodina ambigua Kockelella ortus ortus Ozarkodina svetlanae Panderodus equicostatus Ozarkodina remscheidensis baccata Oulodus elegans Ozarkodina bohemica longa Wurmiella excavata Oulodus sp. Panderodus gracilis Ozarkodina confluens Panderodus unicostatus

Gothograptus nassa bohemica longa

Širvinta

System Series Ludlow Stages Gorstian Ludfordian Dubysa Pagėgiai Reg. Stages Formations Neris

grainstone packstone wackestone mudstone claystone 2

Pristiograptus pseudodubius

Gėluva

Silurian Wenlock Homerian

Integrated stratigraphy, conodont turnover and palaeoenvironments

Conodonts

-3.5 -2

0

323

δ13C[‰] 2 4

324 S. Radzevičius et al. Homerian (Musteikis 2005a), Atrypa reticularis (BA 3) in the Lower Gorstian (Musteikis 2005b) and Atrypa ludlowensis (BA 2) in the Upper Ludfordian (Musteikis 2005c).

3.

SEM microphotographs of conodonts coated in gold were taken in the Nature Research Centre (Vilnius).

3.3

Altogether 102 samples were analyzed for carbon and oxygen stable isotope composition in bulk carbonate. For the stable carbon and oxygen isotopic analysis, ca 2 g of each rock sample was selected, avoiding secondary veins and large crystals, and powdered. The powdered sample materials were analyzed by the mass spectrometer Delta V Advantage and (for preparation of gases) GasBench II by Thermo Scientific, using the international standards NBS 18, NBS 19, and LSVEC. The analytical work was conducted in the mass spectrometry laboratory in the Department of Geology, the University of Tartu, Estonia.

Material and methods

Samples for lithological, biostratigraphical and stable carbon isotopic analyses of the Vilkaviškis-134 core were taken from 780 to 630 m depth. The sampling was done every 1.5 m on average. All material is stored in the Geological museum of Vilnius University, Lithuania.

3.1

Lithology

General lithological description of the Vilkaviškis-134 core was made at the core store of the Lithuanian Geological Museum. For the purposes of detailed lithological description, polished rock slabs were prepared. The slabs were made by cutting core samples and polishing with fine-grained sandpaper. More detailed lithological description of rock slabs was performed under reflecting light binocular microscopes. Gamma log data was used as an additional source of palaeoenvironmental interpretation. The data was used to adjust formational boundaries and as an additional information source on the clay content of rocks. The gamma ray dataset was obtained from the Geological Survey of Lithuania.

3.2

Isotope analyses

3.4

Climate reconstruction

Climate study based on the Al/Ti oxides ratio in 29 samples (2 of 31 samples were rejected because their Ti oxide value was below the analytical level of detection). The Al and Ti oxide data were obtained from the reports at the Geological Survey of Lithuania. According to (Akulshina 1976), arid climates correspond to Al/Ti oxide ratio values higher than 20; humid climates to values below 20; values close to 20 were linked to transitional climates. In addition, we calculated approximate temperatures of the water from which the calcite was precipitated according to Kim and O’Neil (1997): 1000 ln α = 18.03 (103 T –1) – 32.42, where T is temperature in degrees Kelvin, α – fractionation factor. The depth interval from 633.5 to 689 m is dominated by dolostone. For temperature calculations for the dolostones we used the Vasconcelos et al. (2005) formula: 1000ln α = 2.73 (103 T –1) + 0.26. According to recent (Cummins et al. 2014) estimates the tropical Silurian oceans had a temperature of 33 앐 7°C and a δ18OVSMOW of –1.1 앐 1.3‰. The latter value (–1.1‰) was used for the temperature calculations.

Palaeontology

The fossils analysed were collected in the Lithuanian Geological Museum’s core storage facility. Just four rock samples yielded graptolites in the lower part of the interval investigated. The graptolite samples were prepared using hydrofluoric acid (HF). Some graptolite rhabdosomes were recovered during conodont preparation. In order to reveal their internal structure, some rhabdosomes were bleached, using Berthollet’s Salt (potassium chlorate KClO3) and nitric (HNO3) or hydrochloric (HCl) acids. Photographs of graptolites were taken under a light microscope. Conodont samples were prepared using standard weak buffered organic acid solutions – acetic acid for limestones and formic acid for dolomitic rocks (Green 2001). Organic cement was dissolved using 5 % hydrogen peroxide solution. The size of each sample was 350 g on average. The rock digestion residua were exhaustively examined under the binocular microscope.

4.

Results

4.1

Lithostratigraphy and lithofacies

In the interval of the Vilkaviškis-134 borehole investigated, four formations are distinguished: the upper part of the Riga Formation in the Jaagarahu Regional Stage, the Gėluva and Širvinta formations in the Gėlu-

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va Regional Stage and the Neris Formation in the Dubysa and Pagėgiai regional stages (Fig. 2). The Riga Formation is distinguished in the 780– 744.3 m depth interval. The formation is mostly composed of marlstones (780–748.3 m) and shales at the top (748.3–744.3 m). The marlstone contains some detrital shelly fossils, including brachiopods. In some parts bioturbation is common as well as signs of pyritization (Fig. 3A). The sediments were deposited below the fair-weather wave-base in anoxic conditions on the outer ramp. The shale is represented by a 4 meters thick bedset which shows sparse bioturbation (Fig. 3D). In some parts of this bedset interlayering with calcareous marlstone was also distinguished. This lithofacies indicates a deep depositional environment; the sediments were deposited below the fair-weather wave-base in anoxic conditions on the outer ramp. The Gėluva Formation is recognised in the 744.3– 691.4 m depth interval. The formation is composed of dolomitic (Fig. 3B), calcareous mudstones (Fig. 3G) and calcareous wackestones (Fig. 2). Calcareous grainstone makes up the upper part (697.8–691.4 m interval) of the formation. The mudstone with rare brachiopods and sparse bioturbation indicates deposition below the fair-weather wave-base on the outer or mid ramp. The wackestone has nodular structure and is characterized by skeletal fragments of brachiopods and rare bryozoans and crinoids (Fig. 3C, N). This is an indication of a calm depositional environment below the fair-weather wave-base, but above storm wave-base. The calcareous grainstone is characterized by crinoids and rare corals and bryozoans. This lithofacies probably indicates a shallow and above the fairweather wave-base well oxidized depositional environment and most likely represents part of a calcareous sand paleobar (shoal). The Širvinta Formation is recognised in the 691.4– 659 m depth interval. The formation is composed of reddish dolomitic mudstone (Fig. 3 F, H) with gypsum crystal emplacement. The fine laminated mudstone indicates a shallow inner ramp tidal flat oxygenated depositional environment. The lamination could be caused by either cyclic tidal sedimentation (layers of carbonate and fine clastic sediment) or microbial mats such as stromatolites (Lasemi et al. 2012). Small-scale cracks in the fine laminated sediments clearly reveal post-sedimentation modification. Very rare or absent remains of fossil organisms in this facies indicate hypersaline conditions. Different sizes of the gypsum crystals show their synsedimentary origin and deposition in the supratidal environments.

325

The Neris Formation is distinguished in the 659– 632 m depth interval. The formation is composed of dolomitic mudstone (Fig. 3J) and calcareous packstone in its upper part. Different levels of bioturbation are common. Macropores and small caverns are also visible (Fig. 3K, O). The presence of angular and wellrounded clasts in the dolomudstones indicates that some parts of the sediments were reworked and transported. The clasts could also indicate sedimentological hiatuses. The vertical cracks filled with fine-grained material suggest the development of diastems. Sporadically mottled colours of the dolostones could be interpreted as an indication of different levels of oxidation and most likely were caused by bacterial activity. The calcareous packstone is confined to the uppermost part of the studied interval (Fig. 3N). The dominant component of the latter interval is brachiopod shell debris. The debris may be related to renewed transgression and a change from an inner ramp supratidal sedimentary environment to an inner ramp subtidal sedimentary environment, which was close to the fair-weather wave-base. Investigation of lithofacies from the study interval indicates general marine regression. The outer ramp (shale and calacareous marlstone) environments are replaced by inner ramp inter- to supratidal environments (dolomudstones).

4.2

Biostratigraphy

4.2.1 Graptolites There are few graptolites in the investigated interval of the Vilkaviškis-134 core (Fig. 2). Pristiograptus pseudodubius was found in the upper part of the Riga Formation. This species ranges from the Upper Sheinwoodian (belophorus Biozone) up to the Middle Homerian (parvus Biozone) at the lundgreni graptolite event (Radzevičius and Paskevičius 2005). The species P. pseudodubius is a typical member of the Pristiograptus dubius species group. The Sheinwoodian and Lower Homerian pristiograptids of dubius type do not have sicular rings (Urbanek et al. 2012). Sicular rings appear in the Upper Homerian (nassa Biozone) pristiograptids. The specimens of P. pseudodubius from the Riga Formation do not have sicular rings (Fig. 4B). Consequently P. pseudodubius is coming from the lundgreni pre-extinction interval of the Vilkaviškis-134 well. Some fragments of Gothograptus nassa rhabdosomes are found in 736.8–712.8 m interval (Fig. 4C, D). G. nassa is a typical graptolite of the Gėluva Re-

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326 S. Radzevičius et al. open ocean and deep water conodont species, similar to those present in the nearby Ledai-179 section (Radzevičius et al. 2014b). At the base of the studied interval the local disappearance of Pseudooneotodus bicornis, Dapsilodus obliquicostatus, and higher up Panderodus recurvatus, Walliserodus curvatus, Pseudooneotodus

gional Stage and is common in the parvus-praedeubeli biozones in Lithuania (Radzevičius 2006). 4.2.2 Conodonts The lower part of the section corresponding to the Jaagarahu Regional Stage is characterized by a suite of

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corniformis as well as Pseudooneotodus beckmanni was accompanied by the appearance of nearshore shallowwater species which belong to Kockellella, Ozarkodina, Oulodus and Ctenognathodus (Fig. 5, 6). The environmentally broadly tolerant species Wurmiella excavata ranged through the most of Wenlock without significant interruptions in occurrences up to the uppermost Gėluva Regional Stage, where restricted intertidal to supratidal environments (typical for sabkhas (Lasemi et al. 2012)) precluded its local existence (Fig. 2). The influence of increasing environmental restrictions on the distribution of conodonts is also recorded by the disappearance of the Kockellella ortus ortus lineage from the section in the lower part of the Gėluva Regional Stage. The conodont taxonomic study revealed interesting results – as it was unambiguously determined that the rare shallow-water species Ctenognathodus murchisoni was found slightly before the first Mulde excursion episode, at a depth of 750 m in the Jaagarahu Regional Stage, and also at 727 m in the middle of the Gėluva Regional Stage. This finding concurs with the conclusions of an earlier paper that this species had a significantly longer range (Radzevičius et al. 2014a). The biozonal species Ozarkodina bohemica longa, based on the extent of its observed stratigraphic distribution (744.3–684 m) and the density of its occurrences (Fig. 2) most possibly approximates its true stratigraphic duration. The uppermost part of this species’ stratigraphic distribution is almost certainly truncated to some degree due to the onset of unfavourable conditions in the upper Gėluva. This interval (685–638 m) of uppermost Homerian and lower-

327

most Gorstian strata can be correlated with the analogous interval of the Ledai-179 section where there is an intercalation of sedimants from marginal and strongly restricted environments (Radzevičius et al. 2014b). In the Gėluva Regional Stage there are two other species of possible stratigraphic significance. One of these is Oulodus siluricus at depth 736.8 m in the interval of the first Mulde peak, and is found at a similar stratigraphic level as in the Vidukė-61 core (Radzevičius et al. 2014a). The other species is the uppermost Wenlock Stage to Lower Ludlow Oulodus cf. ziegleri at a depth interval of 693.3 to 686 m at the end of the second Mulde peak (Fig. 2). Though, since the specimens of later form cannot be unambiguously assigned to Oulodus ziegleri their range should be treated as of secondary stratigraphic importance. The distribution of conodont species (Fig. 2) in the uppermost part of the section reveals a significant stratigraphic gap which spans most of the Ludlow, except the lowermost and uppermost parts of it. Ctenognathodus sp. S. (sensu (Viira and Einasto 2003)) (synonymous with Ctenognathodus sp. E. (sensu (Strömberg 1997)), Ozarkodina bohemica bohemica and Ozarkodina soegina – conodont species which are found exclusively in the uppermost Wenlock and lowermost Ludlow (Viira and Einasto 2003, Radzevičius et al. 2014b) – at depth of 638 m are suddenly replaced by a species complex of the uppermost Ludlow (Ozarkodina snajdri, Ozarkodina remscheidensis baccata) and also species typical of the upper Ludlow and the lowest Pridoli (Ozarkodina crispa, Ozarkodina ambigua and Oulodus elegans) (Miller and Aldridge 1997, Viira and Aldridge 1998,

Fig. 3. Microfacies of the interval investigated in the Vilkaviškis-134 core. A, D: Riga Formation; A: depth 756.3 m, marlstone bioturbated with very few detrital remains; D: depth 744.3 m, shale with subhorizontal and diagonal small cracks. B, C, E, G, L, M, N: Gėluva Formation; B: depth 742.8 m dolomitic mudstone mottled, bioturbated, quite porous with vertical crack filled by carbonate; C: depth 738.8 m calcareous wackestone with debris of brachiopod and trilobite fossils, intensely bioturbated; E: 697.8 m calcareous grainstone with debris of crinoids and rugose corals (and algae?), porous, and with subvertical crack filled by carbonate crystals; G: 711.3 m calcareous mudstone and packstone with brachiopod shells. Shells are different in size with infiltrated carbonate mud, some of them with geopetal infills, partly filled by calcite cement; L: depth 693.3 m calcareous grainstone, with bryozoans; M: depth 694.8 m calcareous grainstone with crinoid detritus; N: depth 726.3 m calcareous wackestone, bioturbated with sub-vertical ʻcracksʼ or lamination. F, H: Širvinta Formation; F: depth 674.6 m dolomitic mudstone mottled with so-called breccia structure, which is sub-vertically cut and filled with clay size material in post-depositional cracks; H: depth 663.7 m dolomitic mudstone, very thinly laminated, mottled, with small postdepositional cracks, with very finegrained pyrite-rich intercalations; J, K, O: Lower Neris Formation; J: depth 646.4 m dolomitic mudstone with very thin lamination (may be stromatolitic) and breaks of lamination; K: depth 650.7 m dolomitic mudstone clearly divided into two parts: one consists of pure dolostone and the second with significant admixture of clay size fraction material, porous. Vertical post-sedimentary filled crack also present. The presence of angular different size clasts in the upper part indicates erosion and short distance transportation of paleo hardground. Stylolitization is present also; O: depth 656.2 m dolomitic mudstone, very thinly and unclearly laminated, bioturbated, with abundant pores of different size. I: Upper Neris Formation, depth 631.9 m calcareous packstone with abundant detritus and brachiopod shell fragments. Nodular, very fine grained pyrite is present. The scale bar for all pictures is 1 cm.

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Fig. 4. The Upper Homerian graptolites from Vilkaviškis-134 borehole. A, B: Pristiograptus pseudodubius (Bouček), depth 748.9 m, Riga Formation, Jaagarahu Regional Stage; A: no. VU-VIL-134-02, A1: general view; A2: the view of third theca (tal – the end of thecal apertural lip); B: an isolated and bleached rhabdosome, no. VU-S.V134-395 (s – sicula without sicular rings). C, D: Gothograptus nassa (Holm), Gėluva Formation, Gėluva Regional Stage; C: medial part of rhabdosome, no. VU-S.V134-R-2a, depth 712.8 m; D: fragment of rhabdosome, no. VU-S.V134-R-1, depth 736.8 m.

like Panderodus unicostatus and Panderodus gracilis are also present in the uppermost part of the section. In this case the clustering of apparent extirpation and origination (or invasion) events of conodonts may be a result of an unconformity (Patzkowsky and Holland 2012) formed due to widespread marine regression in this case, related to the profound Lau global cooling event (Jeppsson et al. 1995, Lehnert et al. 2007a, Lehnert et al. 2007b, Cramer et al. 2015).

Märss and Miller 2004, Kaljo et al. 2014). The taxon Ozarkodina svetlanae (Mashkova 1972), which was first described in the shallow water (reef) facies of the Timan-Pechora province (Russia), and later in the Volyno-Podolia (Ukraine) (Drygant 1984), apparently also has a restricted stratigraphical distribution (upper Wenlock to Pridoli), though similar forms can be found in the uppermost Wenlock of Lithuania (Šileikytė and Brazauskas 2008). Additionally, some long-ranging taxa

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4.3

δ13C chemostratigraphy

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idly to + 3.67 ‰ at the depth of 741.3 m. The latter value is the maximum of δ13C in the interval of Vilkaviškis-134 well studied. The δ13C values stabilize near + 2 ‰ and gradually fall from + 1.94 ‰ at depth 732.3 m to – 0.04 ‰ at depth 717.3 m, having a single negative value at the depth of 727.8 m (– 1.13 ‰). The values of δ13C rise rapidly with minor fluctuations (– 1.39 ‰ at depth 702.3 m probably represents an outlier) in the upper part of the Gėluva Formation and reach their maximum in the lower part of the Širvinta Formation at the depth 696.3 m (+ 2.47 ‰). In the

The results of stable carbon isotopic study of the core were briefly described by Radzevičius et al. (2014c). In this study we expand and stratigraphically further constrain the stable carbon isotopic trend. The results have shown that in the upper part of the Riga Formation the δ13C values were close to zero with some minor fluctuations (Fig. 2). The values of δ13C varied from – 0.01 ‰ to + 0.72 ‰. In the lower part of the Gėluva Formation the δ13C values rose rap-

Fig. 5. SEM photographs of stratigraphically significant conodonts from the interval of the Vilkaviškis-134 borehole studied. A: Ctenognathodus murchisoni (Pander), Pa element, no. VU-CON-V134-001, depth 750.3 m, Riga Formation, Jaagarahu Regional Stage, B: Kockelella ortus ortus, Sc element, no. VU-CON-V134-002, depth 750.3 m, Riga Formation, Jaagarahu Regional Stage. C, D: Oulodus siluricus, depth 736.8 m, Nevėžis Formation, Gėluva Regional Stage; C: Pa element, no. VU-CON-V134-003; D: Sb element, no. VU-CON-V134-004. E, F: Ozarkodina bohemica longa, Pa elements, depth 723.3 m, Gėluva Formation, Gėluva Regional Stage; E: no. VU-CON-V134-005; F: no. VU-CON-V134-006. G: Ozarkodina crispa, Pa element, no. VU-CON-V134-007, depth 631 m, Neris Formation, Pagėgiai Regional Stage; H, I: Ozarkodina remscheidensis baccata, Pa elements, depth 631 m, Neris Formation, Pagėgiai Regional Stage; H: no. VU-CON-V134-008; I: no. VU-CON-V134-009. J, K: Ozarkodina svetlanae, depth 631 m, Neris Formation, Pagégiai Regional Stage; J: Sc element, no. VU-CON-V134-010; K: Pa element, no. VU-CON-V134-011. L: Oulodus elegans, Pa element, no. VU-CON-V134-012, depth 631 m, Neris Formation, Pagėgiai Regional Stage.

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Fig. 6. SEM photographs Ctenognathodus sp. S. sensu Viira and Einasto, from the studied interval of the Vilkaviškis-134 core, depth 659.1 m, Neris Formation, Dubysa Regional Stage; A: Sa element, no. VU-CON-V134-013; B, F–H: Pa elements; B: no. VU-CON-V134-014; F: no. VU-CON-V134-015, G: no. VU-CON-V134-016, H: no. VU-CON-V134-017. C, E: Sb elements; C: no. VU-CON-V134-018; E: no. VU-CON-V134-019. D: Sc element, no. VU-CON-V134-020. I: M element, no. VU-CON-V134-021.

696.3–665.9 m interval (the Širvinta Formation) δ13C values fall from + 2.47 ‰ to – 0.59 ‰. The 665.9– 659.2 m interval (the uppermost part of the Širvinta Formation) has higher δ13C values (from – 0.59 ‰ to –3.08‰). In the lower part of Neris Formation the δ13C values rise rapidly with minor fluctuations from –3.08‰ at the depth of 659.2 m to 0.55 ‰ at the depth of 647.8 m. Passing this point the δ13C values fall to –2.25‰ at the depth of 631.9 m. In the uppermost part of the investigated interval at 639.6–630.4 m depth (the Pagėgiai Regional Stage), the δ13C values are constantly negative and vary from – 2.25 ‰ to – 1.1 ‰.

4.4

Palaeotemperature and Al/Ti oxide ratio trends

As calculated from the stable oxygen isotopic data, palaeotemperatures within the studied section vary from 19 to 53°C (Fig. 7). The upper palaeotemperature estimate is very high. However, it has been determined that during the Holocene, dolomites which formed in sabkhas of the Persian Gulf developed in temperatures ranging from 34 up to 49°C (McKenzie 1981). We do not exclude, however, that some of the early precipitated material was recrystallized (Kaminskas et al. 2010). Thus, the palaeotemperature curve may be used for general temperature trend estimation. The smoothed

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-7

Temperature(°C) δ18O [‰] -4 -1 15 35 55 0

Al2O3/TiO2 20

40

A

H

Širvinta 670

Series Ludlow Stages Gorstian Ludfordian Dubysa Pagėgiai Reg. Stages Formations Neris 659 638

trend of water temperatures obtained shows significant long-term temperature fluctuations (Fig. 8). In the later Jaagarahu age, temperatures started to drop (from a long-term value of 32.4°C) to the minimal values of 30.4°C in the early Gėluva age. Later, there was steady rise of temperatures, to the maximum average values of 33.6°C at the beginning of Širvinta age. It is possible that inferred water temperatures experienced constant decline up to the latest Ludlow. Interestingly, the average palaeotemperature value obtained in the studied section is 32 앐 4.7°C and close to a recent (Cummins et al. 2014) estimate for the tropical Silurian ocean temperature of 33 앐 7°C. As stated above, the Al/Ti oxide ratio may be linked to climate. According to this ratio in the lower part of our studied section (Riga and Early Gėluva times) an arid type of climate prevailed. In the uppermost part of Gėluva time and during Dubysa time (earliest Ludlow) a humid climate regime prevailed. During early Širvinta time an arid type of climate is suggested by an increased Al/Ti ratio. In the uppermost part of the Vilkaviškis-134 section, which corresponds to the upper part of the Ludfordian Stage (Pagėgiai time), climate fluctuations from humid to arid may be pinpointed (Fig. 7).

331

691.4 700

H

5.

Discussion

5.1

Remarks on integrated stratigraphy

744.3

730

Gėluva 720 710

Wenlock Homerian

Gėluva

A

780

Jaagarahu Riga 770 760

The pattern of stable carbon isotopic variation and the distribution of conodonts and graptolites in the Vilkaviškis-134 core allowed more precise correlation to the other parts of the Baltic basin as well as to the global geological time scale (Fig. 8) than was previously possible. All studied stratigraphic data sets confirmed that the lower part of the core section studied (the Riga Formation) belongs to the Lower Homerian (Cramer et al. 2011). The maximum of the δ13C excursion at depth 741.3 m may be correlated with the lower boundary of the Gėluva Regional Stage (Kaljo et al. 1998, Radzevičius et al. 2014a) and further with the end of the lundgreni graptolite extinction event (Porębska et al. 2004). The positive excursion in δ13C (744.3–720.3 m depth) is the first Mulde excursion peak (Calner et al. 2012). The interval from 720.3 to – 672.9 m spans the second Mulde δ13C excursion (Fig. 2). Thus, the geological section of the Upper Homerian in the Vilkaviškis-134 well should be relatively complete. Our new stable carbon isotopic data suggests that Širvinta

A

Fig. 7. The distribution of δ18O [‰], calculated temperature and Al/Ti trend in the interval of the Vilkaviškis-134 borehole investigated. A – arid and H – humid climate.

Formation should be referred to the Upper Homerian. In previous studies this formation was assigned to the Ludlow (Paškevičius 1997, Lazauskienė et al. 2003). We should state, however, that the boundary between the Wenlock and Ludlow strata in the Vilkaviškis-134 core, using even our new data, cannot be established precisely. On the other hand at the boundary interval between the Wenlock and Ludlow there is a similar

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Fig. 8. Silurian time scale with graptolite and conodont biozonation (Melchin at al. 2012), Lithuanian regional stages, lithostratigraphical interpretation of Vilkaviškis-134 borehole, and smoothed trend of sea water temperatures derived from the stable oxygen isotopic data.

pattern of δ13C fluctuations in the West Midlands, England (Cramer et al. 2012, Marshall et al. 2012). According to conodont data the lower part of the Neris Formation represents the lowermost Gorstian and the upper part of the Neris Formation belongs to the uppermost Ludfordian. It is evident that in the Vilkaviškis-134 section just a very small part of the Ludlow is preserved. The profound gap described here is most probably due to the global regression caused by the Lau climatic event (Cramer et al. 2015).

5.2

The material from the Lithuanian sections indicates that the recovery of conodont faunas could have been much quicker than previously thought, since the latest Homerian species Ctenognathodus murchisoni was found in the nassa graptolite biozone (Radzevičius et al. 2014a). The present study has supported the previous suggestion of a significantly earlier appearance of this species, because it was observed before the beginning of the Mulde stable carbon isotopic event (lundgreni biozone). Ozarkodina bohemica longa appears slightly later in the section studied. Therefore it is possible that it also originated at the onset of the perturbation, during the Mulde extinction event (Gotland material concurs with this suggestion (Calner et al. 2012)). A possibly significantly earlier origin of the zonal subspecies Kockellela ortus absidata is supported by data from other regions. In Bohemian sections it coappears with the significantly earlier form Ozarkodina sagita sagita (Slavík 2014). In the Baltica, however, it appears just during the second Mulde excursion (Calner and Jeppsson 2003, Loydell et al. 2010, Cramer et al. 2012) or in the middle of the Gėluva Regional Stage (Radzevičius et al. 2014a).

Conodont turnover

The distribution of conodonts in relation to the Mulde carbon isotopic event suggests synchronicity of conodont extinctions and re-diversifications. Detailed studies of conodont extirpations in the Gotland sections revealed that conodont disappearances occurred at the beginning of or slightly before the first Mulde carbon isotopic excursion (Calner et al. 2012, Cramer et al. 2012). The present material suggests that the extirpation of conodont lineages occurred slightly before the first Mulde carbon isotopic excursion at the onset of climatic instabilities (Radzevičius et al. 2014b).

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Integrated stratigraphy, conodont turnover and palaeoenvironments high

The temporal distribution of the Later Homerian conodonts described reveals a punctuational pattern (Eldredge and Gould 1972) of coordinated evolutionary response (synchronous extinctions and originations) of conodont communities due to the strong physical perturbation. This may be envisioned as an example of coordinated stasis (Brett and Baird 1995). As described in earlier studies there is direct causal connection linking the spread of adverse environmental conditions to increased risk of extinctions and extirpations through disruption of species metapopulational structure and also paradoxically with the increased probability of speciation due to the formation of populational isolates (Stanley 1990, Vrba 1993, McKinney and Allmon 1995, Allmon et al. 1998, Gavrilets 2004). It is expected that new species appear from small populations (Vrba and DeGusta 2004) which later expand to their maximal range, before ultimately succumbing to changing environmental conditions: they form a “hat-like” occupancy/time relation (Foote et al. 2007, Liow et al. 2010). Thus it is highly probable that initial synchronous radiation of new species during the turnover pulse would be missed in most of the records. This scenario would imply high initial beta (interregional) diversity and low alpha (local) diversity, with following reversal of relations in these metrics due to dispersal. This is expected even if global diversity is left unchanged. If this point of view is correct, then the so-called “survival interval” of the Mulde event (sensu Cramer et al. 2012) would be rather the “delay phase” in species dispersal and subsequent regional community assembly. The greatest apparent delay in recovery of the fauna would be exemplified by environmental specialists (stenotopes) which would experience strong adverse influence of paleogeographic barriers to migration. The tools of integrated stratigraphy assembled evidence strongly indicating that paleobiogeographic intercontinental diachroneity of conodont species is not an exception to the rule but is rather the expected pattern (Cramer et al. 2010). The inter-regional diachroneity of first appearances is also expected. This was shown even in vagile groups (such as mammals) occupying single continents (Alroy 1998). Moreover the same rarefying process of regional extirpations during a turnover event will produce Lazarus taxa (which are far from uncommon in the conodont clade after strong Silurian bioevents (Jeppsson 1997, Jeppsson and Aldrigde 2000)). This fact should strengthen the likelihood of observing this pattern of delayed re-

333

delay phase

global diversity

low

number of species

turnover event

beta diversity alpha diversity

species turnover time

Fig. 9. Heuristic evolutionary and paleobiogeographic model explaining effects of global turnover pulse on global, inter-regional (beta) and local (alpha) species diversity. In this case an example of a symmetrical turnover event (net originations = net extinctions) is illustrated. However, the new equilibrium levels of diversity metrics and length of the delay phase could differ depending on the severity of disturbance and also on macroevolutionary and dispersal properties of surviving taxa (see for example Alroy 2010). Here disturbance has no long-term effect on global diversity, though short-term fluctuations are possible. Extinction of old species, restriction of ranges of established species and origination of new restricted species will initially decrease alpha diversity and in proportion increase between-regional (beta) diversity. Following alleviation of the disturbance, alpha diversity will experience logistic growth to previous level and beta diversity will experience proportional logistic decay to the pre-disturbance values.

covery of faunas (see Fig. 9 for the explanation of the proposed heuristic “delay phase” model).

5.3

Paleoclimate

Estimated variation in climate type and palaeontemperatures in the Vilkaviškis-134 section broadly agree with the results of previous studies. The Al/Ti oxide ratio revealed that during the studied Jaagarahu and most of the Gėluva age intervals an arid climate prevailed. This suggestion is concordant with the inferences that recognized the Mulde event as a kind of secundo-secundo (arid-arid) climate transition (Jeppsson et al. 1995, Calner and Jeppsson 2003, Lehnert et al. 2007a). The smoothed trend of estimated ancient water temperatures seems to be realistic since long-term palaeotemperature estimates fall within 30.4 to 33.6°C (Fig. 8). Those temperatures are similar to the estimates obtained from unaltered skeletal calcareous remains from Gotland, Sweden (Cummins et al. 2014). This suggests the possibility that bulk carbonate sam-

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334 S. Radzevičius et al. network initiative. We thank L. Šiliauskas for assistance in taking SEM micrographs. We would like to thank Jan Zalasiewicz (University of Leicester) for language corrections and Peter Sadler (University of California Riverside), Mark Kleffner (Ohio State University) and also an anonymous reviewer for thorough review and linguistic advice.

ples from the portion of the section studied have low diagenetic impact and preserve a primary climatic signal. The long-term temperature decline during the mid-part of the Wenlock is congruent with earlier suggestions which demonstrated global sea level fall and tied it to the cooling event (Calner and Jeppsson 2003). It should also be mentioned that the present smoothed seawater temperature curve is remarkably similar to the global sea level curve obtained by Loydell (1998) and has similarities to the sea level curve of Ray and Butcher (2010) for the same time slice. This suggests possible causal relations between eustatic sea level change and the obtained palaeotemperatures. Nevertheless, in order to draw final conclusions comparison of the pattern obtained with other proxies is needed.

6.

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Conclusions

All the data obtained indicate that the Upper Homerian geological section is relatively complete in the Vilkaviškis-134 well. The two δ13C peaks correlate with global Mulde carbon isotope excursion peaks. The interval investigated shows a marine regression trend. Additionally, new data revealed a significant stratigraphic gap within the Neris Formation which spans the entire middle part of the Ludlow. It was determined that the conodont species C. murchisoni first appeared at least in the uppermost Jaagarahu Regional Stage. This finding, as well as other findings of earlier appearances of O. bohemica longa and K. ortus absidata which are described in the literature, imply that Mulde event was more of a turnover episode for conodont fauna than a pure minor mass extinction. Analysis of geochemical proxies revealed that the later Jaagarahu and Gėluva ages (later Homerian) were characterized by decreased temperatures and an arid climate. A humid climate prevailed in the later Gėluva and early Dubysa ages (the end of the Homerian and Early Gorstian). At the end of the Ludfordian (Pagėgiai Age) an arid climate dominated. Acknowledgements. This study is part of the project “Event Stratigraphy in Silurian Sedimentary Basin of Lithuania” and also a contribution to the International Geoscience Programme (IGCP)-591 (The Early to Middle Palaeozoic Revolution). The research on this project was sponsored by the Lithuanian Academy of Sciences grant for young scientists (A. S.) and by the Estonian Research Council grant IUT20-34 (T. M., L.A.). This research was also supported by the Open Access to research infrastructure of the Nature Research Centre (Vilnius) under the Lithuanian open access

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Manuscript received: July 24, 2015; rev. version accepted: October 14, 2015.

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