Biotic and environmental changes in the Permian Triassic boundary interval recorded on a western Tethyan ramp in the Bükk Mountains, Hungary - PDF

Global and Planetary Change 55 (2007) Biotic and environmental changes in the Permian Triassic boundary interval recorded on a western Tethyan ramp in the Bükk

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Global and Planetary Change 55 (2007) Biotic and environmental changes in the Permian Triassic boundary interval recorded on a western Tethyan ramp in the Bükk Mountains, Hungary János Haas a,, Attila Demény b, Kinga Hips a, Norbert Zajzon c, Tamás G. Weiszburg d, Milan Sudar e, József Pálfy f a Geological Research Group, Hungarian Academy of Sciences, Eötvös University, H-1117 Budapest, Pázmány s. 1/c, Hungary b Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaörsi út 45, Hungary c Department of Mineralogy and Petrology, University of Miskolc, H-3515 Miskolc, Egyetemváros, Hungary d Department of Mineralogy, Eötvös University, H-1117 Budapest, Pázmány s. 1/c, Hungary e Faculty of Mining and Geology, University of Belgrade, 1100 Belgrade, Kamenicka u. 6, Serbia and Montenegro f Research Group for Paleontology, Hungarian Academy of Sciences-Hungarian Natural History Museum, H-1083 Budapest, Ludovika tér 2, Hungary Received 19 June 2006; accepted 30 June 2006 Available online 25 September 2006 Abstract Complete, continuous marine Permian Triassic (P T) boundary sections in the Bükk Mountains, Northern Hungary, represent a ramp setting on the margin of the western Tethys. The Upper Permian succession comprises limestone rich in calcareous algae, foraminifera, and skeletal fragments of metazoans. A significant reduction of biogenic components occurs in the topmost limestone layers below the boundary shale bed (BSB). It coincides with the beginning of a gradual negative shift in δ 13 C carb values that continues into the BSB. The BSB consists predominantly of marly siltstones that are similar to the insoluble residue of the underlying limestone. A second biotic decline is recorded in the upper-third of the BSB, where the continuous negative shift in δ 13 C values is superimposed by a sharp and quasi-symmetric negative peak. The δ 13 C peak is confined to the shale bed and is not correlated with the lithological change, therefore diagenetic or other secondary effects are ruled out. The carbon isotope signal reflects primary processes related to significant changes in environmental conditions. Correlation and comparison of sedimentological, biotic, geochemical and mineralogical features of the studied sections in the Bükk Mountains with other Tethyan P T sections in the Southern Alps, Dinarides, Iran, Kasmir (India) and southern China are discussed. The continuous shift in δ 13 C values is most probably related to a decrease in bioproductivity, whereas the sharp peak is attributed to an addition of C strongly depleted in 13 C isotope to the ocean atmosphere system. The most plausible model is a massive release of methane from gas hydrate dissociation. This event led to the extinction of the already impoverished biota. Scarcity of metazoans and prolonged unfavourable environmental conditions gave rise to a bloom of microbial communities. Mineralogical and geochemical analyses failed to reveal any evidence for extraterrestrial effects or synchronous volcanism were found in the studied sections Elsevier B.V. All rights reserved. Keywords: Permian Triassic boundary; Bükk Mountains; microfacies; stable isotopes; detrital minerals Corresponding author. Fax: address: (J. Haas) /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.gloplacha J. Haas et al. / Global and Planetary Change 55 (2007) Introduction During the Phanerozoic, the greatest biotic extinction occurred at the Permian Triassic (P T) boundary (Erwin, 1993). The changes were similarly dramatic in the terrestrial as well as marine realms and affected both animals and plants. In the last decade a remarkable research effort has been made to understand the causes of the end- Permian events. A large number of hypotheses were presented and various scenarios were elaborated (White, 2002). However, many questions remained open as to the succession and timing of the series of events, the nature and relative importance of controlling factors, and the ultimate causes of the ecological disaster. There are only a few areas in the world where the P T boundary is exposed in an undisturbed, continuous marine succession, providing an opportunity to learn more about this globally important event. The Bükk Mountains in Hungary is one such area where detailed geological mapping discovered important P T boundary sections. In the last decades, numerous studies were carried out on palaeontology, stratigraphy, lithofacies, mineralogy, geochemistry and stable isotopes of the boundary interval (Haas et al., 1988; Kozur, 1988; Fülöp, 1994; Haas et al., 2004; Posenato et al., 2005). These studies documented the existence of complete, continuous marine boundary sections, which contain macro- and microfossils of critical importance (Haas et al., 2004). A significant negative carbon isotope peak, considered as the chemostratigraphical marker of the boundary, was found within the boundary shale bed (BSB) (Haas et al., 2006). Elsewhere in Europe, classic marine P T boundary successions are known in the Dolomites (Italy) and in the Carnic Alps (Austria). Those sequences were deposited in shallow inner ramp setting, whereas the boundary sections in the Bükk Mountains represent a deeper ramp that is unique in Europe. This palaeogeographic setting provides a good potential for comparison of the boundary sections of the Bükk Mountains with those in the Tethyan realm. The main aim of this paper is to summarize the changes in the lithoand biofacies characteristics, mineralogical and geochemical properties of the biostratigraphically constrained boundary interval in the best key sections in the Bükk Mountains and to draw conclusions for the causes of these changes. Comparative evaluation of these sections with other Tethyan boundary sections may contribute to a better understanding of the end-permian global environmental changes and the related extinction phenomena. 2. Geologic and stratigraphic setting The Bükk Mountains is located in northern Hungary, south of the Inner Western Carpathians (Fig. 1). According to recent palaeogeographic reconstructions, in the Late Permian the Bükk area was a segment of the western Tethyan ramp (Fig. 2), situated in the neighbourhood of the depositional areas of the Carnic Alps, southern Karavanke Mountains, and the Jadar block in the Dinarides (Protić et al., 2000; Filipović et al., 2003). The Bükk Mountains is made up of anchi-metamorphosed Palaeozoic Mesozoic formations that were subjected to intense deformation, and are overlain by Fig. 1. Schematic geological map of the Pannonian basin and its surroundings. The location of the study area in the Bükk Mountains is marked by a triangle. Abbreviations: DO Dolomites; CA Carnic Alps; KV Karavanks; SL Slovenian area; J Jadar block. 138 J. Haas et al. / Global and Planetary Change 55 (2007) Fig. 2. Position of the continents in the Late Permian (after Erwin, 1994) and setting of the sections referred in the present paper. B: Bükk Mountains; TR: Transdanubian Range; 1: Southern Alps; 2: Jadar block; 3: NW Iran; 4: Central Iran; 5: Kashmir; 6: South China. non-metamorphic Palaeogene Neogene formations. Palaeozoic and Lower Triassic formations, and consequently the P T boundary sections, are known only from the northern part of the mountains. The Upper Permian is represented by the Nagyvisnyó Limestone Formation ( m in thickness). It is made up of black and dark grey, thick-bedded limestone with dolomite in the lower part and marlstone, nodular marlstone interlayers mainly in the uppermost m (Fig. 3). The Nagyvisnyó Limestone contains a rich biota that consists of calcareous algae, foraminifera, sponges, anthozoans, bivalves, gastropods, nautiloids, ostracods, trilobites, brachiopods, bryozoans, echinoderms, scolecodonts, and conodonts (Schréter, 1963; Balogh, 1964; Schréter, 1974; Kozur, 1985; Pešić et al., 1988; Fülöp, 1994; Bérczi-Makk et al., 1995). The age of the formation is Capitanian to Changhsingian (Kozur, 1988, 1989). The Nagyvisnyó Limestone is overlain by nearly 1 m of marly siltstone, referred here as the BSB. It is in turn overlain by dark grey, thin-bedded, mostly stromatolitic limestones, 8.5 m in thickness that represent the lowermost part of the m thick Gerennavár Limestone Formation. The next 17.5 m of this formation comprises thick-bedded massive mudstones. Bioclastic grainstone interlayers in gradually increasing thickness appear upsection, followed by ooidal limestone that makes up the bulk of the Gerennavár Limestone (Hips and Pelikán, 2002). In the layers overlying the stromatolitic interval, the ostracods Hollinella tingi (Patte) and Langdaia sp. were found (Kozur in Fülöp, 1994). An advanced form of Hindeodus parvus (Kozur et Pjatakova) was reported by Kozur from 14 m above the BSB. From among several P T boundary sections identified during geological mapping in the northern part of the Bükk Mountains (Haas et al., 2004), the two most complete sections were selected for detailed studies. These are located on the northern slope of Mount Bálvány and referred to as Bálvány-North and Bálvány-East (Fig. 4). The lower part of the 3-m thick Bálvány-North section (Fig. 5) is composed predominantly of dark grey, thin-bedded limestone of the Nagyvisnyó Formation. It is overlain by the BSB that, in turn, is overlain by laminated limestone of the Gerennavár Formation. Fig. 3. Stratigraphical scheme of the Upper Permian Lower Triassic interval in the Bükk Mountains. J. Haas et al. / Global and Planetary Change 55 (2007) Fig. 4. Geological map on the northern part of the Bükk Mountains (after Less et al., 2002) showing location of the most important P T boundary sections. The Bálvány-East section is located some 500 m from the Bálvány-North section and exposes the same interval but is structurally more disturbed. However, it is a useful complement to the Bálvány-North section as it exposes a thicker part of the overlying stromatolitic beds. A composite section of the two closely spaced exposures is shown on Fig. 5 that also displays the characteristic microfacies of the main lithofacies types. 3. Methods The Bálvány-North and Bálvány-East sections were sampled in detail for sedimentological, macro- and micropalaeontological, mineralogical and geochemical (stable isotope and trace elements) investigations. The microfacies and diagenetic history was determined using conventional petrographical methods on all samples analysed for their carbon and oxygen isotope composition. Carbon and oxygen isotope compositions of bulk rock carbonate samples were determined using the conventional H 3 PO 4 digestion method (McCrea, 1950). 13 C/ 12 C and 18 O/ 16 OratiosofCO 2 generated by the acid reaction were measured on a Finnigan MAT delta S mass spectrometer at the Institute for Geochemical Research in Budapest. The results are expressed in the δ notation [δ=(r 1 / R 2 1) 1000, where R 1 is the 13 C/ 12 C or 18 O/ 16 O ratio in the sample and R 2 the corresponding ratio of the standard V-PDB, in parts per thousand ( )]. Duplicates of standards and samples reproduced to better than ± samples, each of 1 2 kg weight, were collected for mineralogical studies from the Bálvány-North section, 13 samples represent the BSB, 9 were taken from the underlying Nagyvisnyó Formation and 12 from the overlying Gerennavár Formation. For clay mineral studies, carbonate was removed by diluted acetic acid and the residual phases were separated by grain size at 10 μm steps.the separation process was monitored by routine X-ray powder diffractometry (XPD). The clay size fraction was analysed by XPD in oriented and oriented+ ethylene glycol-treated samples. The sulphur isotope composition of pyrite was determined both on hand-picked individual grains of 1 2 mmindiameter( large crystal generation ) and on mm grains from the residual heavy mineral fraction ( small crystal generation ) by mass spectrometry (Zajzon and Vető, submitted for publication). The iridium content was measured after chemical pre-concentration, by INAA (Zajzon et al., submitted for publication-a). 140 J. Haas et al. / Global and Planetary Change 55 (2007) Fig. 5. Bálvány-North section. Lithology, conodont biostratigraphy, biofacies, quantity of bioclasts, carbonate content, carbon and oxygen isotope composition. Abbreviations: bd biotic decline; EB event boundary. For heavy mineral study, 500 g of each sample was processed. Carbonate was removed by 10% hydrochloric acid. The insoluble part was separated into sizefractions by grinding and wet sieving. The largest size fraction ( μm) was further separated by heavy liquid (2.78 g/cm 3 ) and all separated fractions were studied under a binocular microscope. Approximately 600 representative individual grains from the heavy J. Haas et al. / Global and Planetary Change 55 (2007) mineral fractions were selected for detailed instrumental mineralogical analysis (optical, SEM + EDX, WDX, LA-ICP-MS, limited number of single crystal XRD) (Zajzon and Weiszburg, submitted for publication; Zajzon et al., submitted for publication-b). 4. Results 4.1. Litho- and biofacies, biostratigraphy The lowermost part of the Bálvány-North section (Beds 1 3, Fig. 5)ismadeupofdarkgreytoblacklimestoneof bioclastic wackestone texture (Fig. 7a). Crinoid detritus is predominant, fragments of brachiopod shells and spines, bivalves, gastropods, foraminifera, ostracods, and calcareous algae also occur, locally in large numbers. Both Bed 1 and 3 yielded conodonts and holothurians. Significantly, from Bed 3 the conodonts Hindeodus praeparvus Kozur (Fig. 6 a d) and Isarcicella cf. prisca Kozur and the holothurians Theelia dzhulfensis Mostler and Rahimi- Yazd, Th. multiradiata Kozur and Th. mostleri Kozur were identified. Based on the co-occurrence of H. praeparvus and Isarcicella cf. prisca this bed probably corresponds to the upper praeparvus Zone of Perri and Farabegoli (2003). The next bed (Bed 4) is composed of alternating dark grey limestone (bioclastic wackestone) and purple to reddish-brownish calcareous marlstone layers that contain limestone nodules of cm in diameter. The bioclasts are usually smaller than in the underlying beds, typically in the medium to fine sand to silt size range. There is no noticeable change in the composition of bioclasts. Bed 5 is a dark grey limestone of patchy, bioturbated bioclastic wackestone texture. Small gastropods are common, fragments of thin-shelled bivalves, echinoderms, spines of brachiopods, ostracods and a few foraminifera also occur. Along with marine acritarchs, striate bisaccate pollen grains (Lueckisporites virkkiae Potonié and Klaus, emend. Clarke, Lunatisporites sp., Striatoabieites sp.) were found in this bed suggesting that it is still Permian in age (det. Götz, in Haas et al., 2004). The next layer (6.1) is made up of grey, platy, argillaceous limestone and calcareous marlstone of bioclastic Fig. 6. Biostratigraphically important conodonts from the studied sections. a d: Hindeodus praeparvus Kozur, Nagyvisnyó Limestone Fm., Bálvány- North section Bed 3; e h: Hindeodus parvus (Kozur and Pjatakova), Gerennavár Limestone Fm., Bálvány-East section Bed 4.3. Lateral view: a, c, e, g; upper view: b, d, f, h. 142 J. Haas et al. / Global and Planetary Change 55 (2007) wackestone packstone texture with relatively coarse grains. Fragments of molluscs and brachiopods are predominant, echinoderm detritus and ostracods are common. Abundance of the foraminifera Hemigordius is the key biofacies feature of this layer (Fig. 5). There is a considerable decrease in the amount of biogenic components in the next two thin layers (6.2 and 6.3, Figs. 5 and 7b), although no prominent lithological change is observed. These grey argillaceous and silty limestone (mudstone) layers contain only a small amount of fine sand- to silt-sized bioclasts, fragments of echinoderms, ostracods, and the foraminifera Hemigordius. The uppermost limestone layer (Bed 6.3) is directly overlain by marly siltstone of the BSB (Bed 7) that is 94 cm in thickness. Based on lithological properties, Bed 7 is subdivided here into six parts. In the lowermost brownish grey silty marlstone layer (7.1), well-preserved bivalves (Bakevellia cf. ceratophaga (Schlotheim), Eumorphotis lorigae Posenato, Entolium piriformis (Liu), Pernopecten latangulatus Yin), and brachiopods (Sinomarginifera sp., Orthothetina ladina (Stache), Ombonia tirolensis (Stache), Orbicoelia tschsernyschewi (Likharew), Posenato et al., 2005) were found. This part also contains 1 3-mm thick graded quartz mica siltstone laminae. The amount of bioclasts observed in thin sections is low, less than 1% (Figs. 5 and 7c). Among them fragments of echinoderms predominate, a few ostracods and brachiopod fragments also occur. Pyrite is abundant in stripes, patches, and mould fillings. A 2-cm thick, light grey argillaceous silty limestone interlayer that contains a 4-mm thick, graded crinoid coquina lamina (7.2) occurs within the BSB. It is overlain by grey marly siltstone that becomes more limy and silty upsection (7.3). In the lower part of this interval, bioclasts including fine echinoderm detritus (5 10/cm 2 ), fragments of foraminifera, molluscs, and ostracods still occur but in strongly reduced number. The last determinable bivalves and brachiopods were collected from the lower part of this interval. Alternating calcareous marlstone and siltstone laminae occur in the next 5-cm thick interval that almost entirely lacks bioclasts (7.4). Only 1 2 fine sand-sized echinoderm detritus were counted per cm 2, along with ostracods and fragments of foraminifera. A 4-cm thick, light grey sandy marlstone layer occurs in the upper part of the BSB (7.5). It consists of sand-sized grains of detrital quartz and subordinate mica, and microsparitic carbonate cement. It is overlain by a 2-cm thick marly siltstone layer (7.6) that forms the uppermost part of the BSB. Numerous small cavate spores (cf. Endosporites papillatus Jansonius, Krauselisporites sp.) were found in a sample taken from the topmost 30 cm of Bed 7, suggesting that this level is already Triassic in age (det. Götz, in Haas et al., 2004). The BSB is overlain by 45 cm of platy limestone (plate thickness is 1 3 cm) with thin shale laminae (8.1). The conodonts H. parvus Kozur and Pjatakova and H. praeparvus Kozur were extracted from this bed, allowing to assign it to the parvus Zone of Perri and Farabegoli (2003) and to draw the conodont-based P T boundary at the base of this bed (Fig. 5). The limestone layers are made up of micrite with small amounts of bioclasts and silt-sized siliciclasts (quartz and mica). Millimetre- to centimetre-sized patches, probably caused by bioturbation, are characteristic. Clay-sized films and solution seams are also typical. Fragments of brachiopod spines are relatively common; ostracods and foraminifera (Nodosariids and Earlandia) also occur. A horizon rich in fine sand-sized siliciclasts (quartz and mica) was found 25 cm above the base of the bed. In the Bálvány-East section, the BSB is also overlain by grey platy limestone, in a thickness of 0.5 m (Bed E4, Fig. 7) that corresponds to bed 8.1 in the Bálvány-North section and contains the same conodont fauna (Fig. 6e h). Bioturbated mudstone texture with a few ostracods and foraminifers characterizes the limestone layers. The next 5-m thick stromatolite succession is divided into two parts (Fig. 7). The lower part is thin-bedded and evenly laminated, whereas the upper part is thick-bedded and its crinkle lamination (Fig. 7d) gradually disappears upward (Hips and Haas, 2006) Diagenesis and metamorphic overprint The Palaeozoic and Mesozoic formations of the Bükk Mountains were affected by regional metamorphism in the Cretaceous. However, the intensity of the deformation and metamorphism was variable depending on the structural positio
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