©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at JAHRBUCH DER GEOLOGISCHEN BUNDESANSTALT Jb. Geol. B.-A. ISSN 0016–7800 Band 141 Heft 4 S. 473–490 Wien, Dezember 1999 Lower Miocene Volcaniclastics in South Moravia and Lower Austria SLAVOMÍR NEHYBA & REINHARD ROETZEL*) 16 Text-Figures and 3 Tables Tschechische Republik Österreich Molasse Untermiozän Eggenburgium Ottnangium Vulkanismus Karte der Tschechischen Republik 1 : 50.000 Tuff Blätter 34-11, 34-12, 34-13, 24-34, 24-44 Petrologie Österreichische Karte 1 : 50.000 Zirkonstudien Blätter 8, 9, 21, 22, 25, 38 Geochemie Contents Zusammenfassung ...................................................................................................... 473 Abstract ................................................................................................................. 474 1. Introduction ............................................................................................................. 474 2. Geological Setting ....................................................................................................... 476 3. Volcanic Glasses ........................................................................................................ 477 4. Study of Pyrogenic Minerals .............................................................................................. 478 4.1. Zircon Studies ...................................................................................................... 479 4.1.1. Outer Morphology ............................................................................................ 479 4.1.2. Zircon Typology .............................................................................................. 479 4.1.3. Zircon Elongation ............................................................................................ 481 4.1.4. Zonality, Elder Cores and Inclusions in Zircons ................................................................. 482 4.1.5. Fission-Track-Dating ......................................................................................... 483 5. Bulk Rock Studies ....................................................................................................... 483 5.1. Geochemistry of Volcaniclastics ..................................................................................... 484 5.2. Grain Size Studies of Volcaniclastics ................................................................................. 485 6. Volcanic Source of Tephra ............................................................................................... 486 7. Conclusions ............................................................................................................. 487 Acknowledgements ...................................................................................................... 487 References .............................................................................................................. 487 Untermiozäne Vulkanoklastika in Südmähren und Niederösterreich Zusammenfassung Die vulkanoklastischen Ablagerungen aus untermiozänen Sedimenten der Karpatischen Vortiefe in Südmähren und der Molassezone in Niederösterreich entstammen einem sauren, rhyodazitischen Vulkanismus. Das vulkanische Material ist einer kalkalkalischen Provinz eines vulkanischen Bogens zuzuordnen. Die geschätzte Eruptionstemperatur liegt zwischen 720°C und 780 °C. Das Auswurfmaterial wurde ver- mutlich von dem neogenen, dazitischen und rhyolithischen Vulkanismus in der Karpato-Balkanischen Region, wahrscheinlich in Nordungarn und der Ostslowakei produziert. Die untersuchten Vulkanoklastika entstanden durch distale Ascheregen. Umlagerungsprozesse hatten z.T. großen Einfluss auf Zusam- mensetzung, Struktur und Textur der Vulkanoklastika. Aufgrund von Zirkonstudien können zwei Horizonte (Horizont I und II) unterschieden werden. Die Existenz von verschiedenen Horizonten wird durch mehrere Eruptionsphasen im gleichen Liefergebiet mit einer vertikal differenzierten, magmatischen Kammer erklärt. Der tiefere Horizont (Horizont I) wird in das Obere Eggenburgium gestellt. Das durchschnittliche absolute Fission-Track-Alter beträgt 20.3±2.4 Ma. Der höhere Horizont (Horizont II) kann mit vulkanischen Einschaltungen in der Zellerndorf-Formation und Langau-Formation des Ottnangium korreliert werden. Vulkanoklastika von Herrnbaumgarten im Wiener Becken, die bisher mit dem oben beschriebenen Auswurfmaterial in Verbindung gebracht wurden, zeigen unterschiedliche Charakteristika der vulkanischen Zirkone. Ein anderes Liefergebiet ist daher wahrscheinlich. *) Authors’ addresses: Dr. SLAVOMÍR NEHYBA: Department of Geology and Paleontology, Masaryk University, Fac. of Science, Kotláˇrská 2, CZ-611 37 Brno; Dr. REINHARD ROETZEL: Geologische Bundesanstalt, Rasumofskygasse 23 A-1031 Wien. 473 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at Abstract The Lower Miocene volcaniclastics from the Carpathian Foredeep in South Moravia and the Molasse Zone of Lower Austria have an acidic character (rhyodacites). The source of volcanic material was from a calc-alkaline volcanic suite of a volcanic arc. The estimated eruption temperature lies between 720 °C and 780 °C. Tephra was produced by the Neogene aerial type of dacite and rhyolite volcanic activity in the Carpatho-Pannonian region. The volcanic source is probably located in Northern Hungary and Eastern Slovakia. Studied volcaniclastics belong to the distal fallout tephra. Postdepositional processes partly highly influenced composition, texture and structure of the volcani- clastics. Based on zircon studies two horizons of volcaniclastics (horizon I and II) have been recognized. The existence of different horizons is explained by several eruptional phases in the same source area and a stratified magmatic chamber. The lower horizon (horizon I) is Upper Eggenburgian in age. Its average absolute age (Fission-track-dating) is 20.3±2.4 Ma. The upper horizon (horizon II) can be correlated with tephra layers of the Zellerndorf Formation and Langau Formation (Molasse Zone of Austria). These rocks are Ottnangian in age. Volcaniclastics from the locality Herrnbaumgarten (Vienna Basin) which were formerly assumed to correlate with the above mentioned tephra layers show different characteristics of volcanic zircons. For that reason a different source is most probable. 1. Introduction Tephra studies are often used to correlate marine and terrestrial en- glass shards and primary phenocrysts (zir- vironments. They can be an important tool for the definition of correla- con) together with bulk rock geochemistry tive surfaces, especially in basins without extensive outcrops. Occur- and grain size were studied. rences of volcaniclastics are reported from many Neogene basins in Various earlier data are available for the Central Europe. volcaniclastics of South Moravia and Lower In South Moravia (Czech Republic) and Lower Austria volcaniclastics Austria. Mainly geochemical data were pro- occur in Lower Miocene sediments along the Bohemian Massif. Volca- duced by different laboratories and incom- niclastics in South Moravia are connected with the Eggenburgian- patible methods. For that reason their value Ottnangian boundary (»TYROK›, 1982, 1991). Volcaniclastics reported for the tephrostratigraphy is limited. from Lower Austria are mainly part of the Weitersfeld Formation, Lan- Samples from South Moravia derive from gau Formation and Zellerndorf Formation (ROETZEL, 1991, 1993, 1994), outcrops at Znojmo-Prazská ˇ street, Chva- all of Ottnangian age. lovice, Pˇrímetice, ˇ Visˇ ˇ nové, Plavec, ˇ Horní In the studied area only distal fallout tephra was recognized. Such Dunajovice, Ivancice-Réna ˇ and Æerotice deposits can be easily reworked by various processes. Redeposition and from cores from the drillings Únanov V usually affects the actual thickness and composition of the layer 14, V 15, V 16, Znojmo-Hradist ˇ e ˇ H 32, H 36, (HUANG et al., 1973; RUDDIMAN & GLOVER, 1972). The studied volcani- Ivancice-Réna ˇ V 2, Strachotice V 1, V 2, cclasticlastic rocrocksks hhaveave very differentdifferent petrographicalpetrographical andand mineralogicalmineralogical compositions. Reliable tephrostratigraphic data from these rocks were missing so far, thus the opportunity for broader stratigraphic correla- tions has not been utilized. The classification of volcaniclastic rocks is a subject of discussion (ALLEN, 1991; CAS, 1991; ZUFFA, 1991). In the study area pyroclastics, tuffites or bentonites and smectitic clays have been found. A reliable correlation of volcaniclastics requires a multiple criteria ap- proach of tephra characterization (IZETT, 1981; WESTGATE & GORTON, 1980; WILCOX, 1965). This is especially important for distal tephra de- posits. In our Neogene deposits the physico-chemical properties of Text-Fig. 1. Location of the study area in South Moravia and Lower Austria. 474 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at Ædánice 66, PMK 1, PMK 2, PMK 3, PMK 6A, PMK 7, Zn 5, Berg and from the drilling Geras B-32. Samples from out- Zn 8 and HV 305. Geochemical data (bulk rock analyses) crops near Herrnbaumgarten (HBG-5, HBG 7) which were were produced by the chemical laboratory of the Czech also studied, are from the Vienna Basin. Geological Survey in Prague. Volcanic glasses were ana- Zircons from these samples were studied at the Depart- lysed by the chemical laboratory of the Institute of Mineral ment of Geology, Masaryk University Brno. Resources in Kutná Hora. Fission-track-datings