*Addresses of the authors:PD Dr. Annette E. Götz, Institut für Angewandte Geowissenschaften, TU Darmstadt, Schnittspahnstraße 9, D-64287 Darmstadt, Germany ([email protected]);Dipl.-Geol. Sascha Gast, Martin-Luther-Universität Halle-Wittenberg; c/o Bundesanstalt für Geowissenschaften und Rohstoffe, Dienstbereich Berlin, Wilhelmstraße 25–30, D-13593 Berlin, Germany ([email protected]).
Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections (Central Europe)
Annette E. Götz & Sascha Gast*
Götz, A.E. & Gast, S. (2010): Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections (Central Europe). [Beckenentwicklung der Peri-Tethys im Anisium: Rückschlüsse ba-sierend auf Conodonten-Vergesellschaftungen in Schlüsselprofi len des Unteren Muschelkalks (Mitteleuropa).] – Z. dt. Ges. Geowiss., 161: 39–49, Stuttgart.
Abstract: Lower Muschelkalk carbonates of the Peri-Tethys Basin are considered as poor in age-diagnostic index fossils, thus unequivocal basin-wide correlations remain diffi cult. Additionally, the lack of a bed-by-bed analysis and documentation of the fossil content of complete key sections hinders high-resolution biostratigraphy and therefore the discussion on the stratigraphic use of different fossil groups is controversial.
The present study focuses on the microfossils of the German Lower Muschelkalk (Jena Formation) type section Jena-Steudnitz, representing deposits of the central part of the basin. The microfaunal assemblage is analysed with respect to eustatic signals as documented in the stratal and lateral facies architecture and distribution, and compared with recently obtained micropalaeontological data from other key sections of the Peri-Tethys gate areas, both in the SW (Switzerland) and SE (Poland). Finally, the particular signifi cance of conodonts to decipher the palaeogeographic and eustatic basin evolution is pointed out in the context of over-regional geodynamic processes, highlighting migration and distribution patterns during the Early Anisian.
Kurzfassung: Die Karbonate des Unteren Muschelkalks (Germanisches Becken, Peri-Tethys) werden als fossilarm angese-hen und eine beckenweite Korrelation basierend auf Leitfossilien daher als schwierig eingestuft. Ein wesentlicher Aspekt dabei ist jedoch die fehlende bankstratigrafi sche Analyse und Dokumentation von Fossilien in kompletten Referenzprofi len, welche letztendlich eine hochaufl ösende Biostratigrafi e verwehrt und damit die kontroverse Diskussion zur stratigrafi schen Verwertbarkeit von Fossilien im Wesentlichen die mangelnde Datenlage widerspiegelt.
In der vorliegenden Arbeit wird daher erstmals die Mikrofauna des Typusprofi ls des Unteren Muschelkalks Jena-Steud-nitz bankstratigrafi sch analysiert und hinsichtlich der Aussage zur Beckenentwicklung diskutiert sowie mit neuen mikropa-läontologischen Daten aus den Pfortenregionen der Peri-Tethys, im SW (heutige Nordschweiz) sowie im SE (heutiges Po-len, Oberschlesien), verglichen. Abschließend wird die Bedeutung der Conodontenfunde hinsichtlich der Entschlüsselung der paläogeografi schen und eustatischen Beckenentwicklung im überregionalen Kontext diskutiert und Migrations- und Verteilungsmuster des frühen Anisiums aufgezeigt.
Keywords: conodonts, Peri-Tethys Basin, Middle Triassic, Anisian, Muschelkalk, Central Europe
Lower Muschelkalk carbonates of the Triassic Peri-Tethys Basin cover large parts of Central Europe (Fig. 1). First studies on fossil content and marker beds of Muschelkalk series date back to the 19th century (e.g. Buch 1849, Bornemann 1885). These early works were followed by de-tailed palaeontological and sedimentological studies that
enabled the establishment of a bio- and lithostratigraphic framework as a basis for regional and global correlation. However, the evolution of the epeiric sea north of the Tethys Ocean, the interaction between a restricted, intracratonic basin and an open ocean and the faunal migration between these two different palaeogeographical settings are still un-der discussion (cf. Feist-Burkhardt et al. 2008a, Götz & Török 2008).
Z. dt. Ges. Geowiss., 161/1, p. 39–49, 6 figs. ArticleStuttgart, March 2010
Due to the lack of a bed-by-bed analysis and documenta-tion of the fossil content of Lower Muschelkalk key sections, the stratigraphic use of different fossil groups for basin-wide correlation has not been proved yet. Therefore, the present study focuses on the microfossils of the type section Jena-Steudnitz of the German Lower Muschelkalk (Jena Forma-tion), conodont assemblages of type sections in North Swit-zerland and South Poland (Upper Silesia) and their signifi -cance to decipher the palaeogeographic and eustatic basin evolution.
2. Geological setting
During Triassic times, the Germanic Basin was a peripheral basin of the western Tethys Ocean, the so-called northern Peri-Tethys (Szulc 2000). The basin was bordered by land-masses and open to the Tethyan shelf by three tectonically controlled gates in the South and Southeast, known as the
East Carpathian, Silesian-Moravian and Western Gates (Fig. 1). The East Carpathian Gate was already active in the Late Induan, the Silesian-Moravian Gate opened in the Olenekian (Szulc 2000) and the westernmost communica-tion to the Tethys developed during the Anisian (Feist-Burkhardt et al. 2008a). The semi-closed situation of the basin and the diachronous communication with the Tethys Ocean resulted in a distinctive facies differentiation be-tween the western and eastern parts of the basin. While in the Silesian and Carpathian domains the Early Anisian is already represented by carbonates, the central and western areas were still dominated by siliciclastic red beds (Röt facies). In the eastern subbasin, open marine sedimentation continued during almost the entire Anisian, while the west-ern part experienced restricted circulation during the Early and Late Anisian.
Up to now, the biostratigraphic framework of the Ger-manic Middle Triassic is based mainly on the work of Kozur (1974) focussing on conodonts. The well-established litho-
Fig. 1: Palaeogeography of the Peri-Tethys Basin during Pelsonian times (modifi ed after Götz et al. 2005) and location of key sections studied in Germany (1), Poland (2) and Switzerland (3). RM = Rhenish Massif, MM = Małopolska Massif.
Abb. 1: Paläogeografi e des Peri-Tethys-Beckens während des Pelsoniums (verändert nach Götz et al. 2005) mit Lage der bearbeiteten Schlüsselprofi le in Deutschland (1), Polen (2) und der Schweiz (3). RM = Rheinische Masse, MM = Małopolska Masse.
41Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections
Fig. 2: Steudnitz section (Thuringia, Germany): stratigraphy, distribution of conodont marker species, stratigraphic horizons of maximum abundance and species diversity of marine vertebrates (marked by stars), depositional se-quences. Sequence stratigraphic inter-pretation from Rameil et al. (2000).
Abb. 2: Profi l Steudnitz (Thüringen): Stratigrafi e, Vorkommen stratigrafi sch leitender Conodonten, Horizonte mit der maximalen Häufi gkeit und Artendiversi-tät mariner Vertebraten (Sternsignatur), Ablagerungssequenzen. Sequenzstrati-grafi sche Interpretation aus Rameil et al. (2000).
stratigraphic framework of the 2–3 Ma lasting carbonate ramp system uses characteristic marker beds for basin-wide correlation (e.g. Hagdorn et al. 1987). Facies diachroneity and the scarcity of age-diagnostic index fossils (conodonts, ammonoids), however, make unequivocal basin-wide corre-l ations diffi cult (cf. Feist-Burkhardt et al. 2008b). Recently, sequence stratigraphy has been employed to approach the problem of regional litho facies variations and provided a framework of principal stages in basin evolution during Mid-dle Triassic times (Szulc 2000).
In addition to the presence of marker beds, the vertical facies succession of the Lower Muschelkalk deposits is char-acterised by a hierarchically organised cyclic sedi mentation pattern. Small-scale depositional sequences build up medium- and large-scale composite sequences. Geochemical and paly-nofacies signatures also show stacked cyclic patterns that con-fi rm the high-resolution sequence stratigraphic interpretation (Rameil et al. 2000, Conradi et al. 2007). The stratal architec-ture of small-scale depositional sequ ences systematically changes in relation to their relative proximal-distal position on the Muschelkalk ramp system. Conceptual correlation of small-scale depositional sequences within a cyclo- and se-quence stratigraphic framework improved time resolution and the understanding of basin evolution (Götz 1996, Kedzierski 2000, Pöppelreiter 2002, Borkhataria et al. 2006). Further-more, the application of palynofacies analysis to high-resolu-tion sequence strati graphy of carbonate series of the Peri-Tethys Basin and the northern Tethys shelf area proved to be a powerful correlation tool (Götz & Feist-Burkhardt 1999, Ra-meil et al. 2000, Götz et al. 2003, Götz et al. 2005, Feist-Burk-hardt et al. 2008a, Götz & Török 2008). On a regional scale, debris fl ows, seismites and tsunamites are used to defi ne time-lines from short-term events (Föhlisch & Voigt 2001).
Up to now, detailed investigations of conodont assem-blages of type sections of the entire Peri-Tethys Basin are missing. Thus, in contrast to studies in the Tethyan realm (Orchard 2005, 2007), an attempt at a multielement recon-struction (cf. Orchard & Rieber 1999, Purnell et al. 2000) has not been made yet.
Recently, Narkiewicz & Szulc (2004) studied conodont assemblages from selected well sections in Poland and dis-cussed the eustatic control of migration as documented from conodont data of the eastern part of the basin. Based on this fi rst bed-by-bed analysis of conodonts, the present study focuses on the entire basin and especially the gate areas, representing the key areas with respect to basin evo-lution.
3. Central basin
The Steudnitz quarry (East Thuringia, Germany) exposes a complete section from top Röt (uppermost Bunt sand stein) to the basal Middle Muschelkalk (Rameil et al. 2000) with a total thickness of a little more than 100 m (Fig. 2). The car-bonate series unequivocally displays a hierarchical stacking of high-frequency cyclic sediment packages (Rameil et al. 2000, Kedzierski 2000). Overall facies evolution and the stratal architecture of small-scale depositional sequ ences re-fl ect deposition in middle to distal parts of the Muschelkalk carbonate ramp system (Götz 2004).
The lower part of the quarry exposes the Grenzgelbkalk (“yellow boundary limestone”), defi ning the base of the Lower Muschelkalk, and most of the Wellenkalk-1 Member. The basal 10 m of the Wellenkalk-1 Member are character-ised by numerous thin bioclastic beds. Higher up in the sec-tion, this member mainly consists of nodular, bioturbated mud- and wackestones that were deposited in subtidal, low-energy environments. This facies is also typical for the other two Wellenkalk members.
The Oolithbank Member is characterised by well-devel-oped hardgrounds defi ning the Oolithbänke α, β1 and β2. Moreover, several mass-fl ow horizons are concentrated in and around this stratigraphic interval. Götz (1996) observed a regressive trend within this member (prograding of facies belts) and thus the Oolithbänke are interpreted as large-scale (“third-order”) late highstand deposits (Szulc 2000, Götz 2004, Götz & Török 2008).
Fig. 3: Conodonts of the type section Jena-Steudnitz (Thuringia) of the German Lower Muschelkalk (Anisian). Scale bar 50 μm.1 = Nicoraella germanica (Kozur) (Oolithbank Member, sample C6, HLMD-WT-607); 2 = Nicoraella kockeli (Tatge) (Oolithbank Mem-ber, sample C7, HLMD-WT-608); 3 = Neogondolella bulgarica (Budurov and Stefanov) (Terebratelbank Member, sample C11, HLMD-WT-612); 4 = Cornudina breviramulis breviramulis (Tatge) (uppermost Wellenkalk-1 Member, sample C5, HLMD-WT-606); 5 = Cornu-dina latidentata (Kozur and Mostler) (Oolithbank Member, sample C7, HLMD-WT-608); 6 = Cornudina latidentata (Kozur and Mostler) (Terebratelbank Member, sample C11, HLMD-WT-612); 7 = Cornudina tortilis (Kozur and Mostler) (Schaumkalkbank Member, sample C15, HLMD-WT-616); 8 = Diplododella lanceata (Götz) (uppermost Wellenkalk-1 Member, sample C5, HLMD-WT-606); 9 = Metalon-chodina magnidentata (Götz) (uppermost Wellenkalk-1 Member, sample C5, HLMD-WT-606); 10 = Neohindeodella excurvata (Götz) (uppermost Wellenkalk-1 Member, sample C5, HLMD-WT-606); 11 = Neohindeodella germanica (Götz) (uppermost Wellenkalk-1 Mem-ber, sample C5, HLMD-WT-606); 12 = Neohindeodella nevadensis (Müller) (Schaumkalkbank Member, sample C15, HLMD-WT-616); 13 = Neohindeodella triassica riegeli (Mosher) (Schaumkalkbank Member, sample C15, HLMD-WT-616); 14 = Neohindeodella triassica triassica (Müller) (Schaumkalkbank Member, sample C15, HLMD-WT-616); 15 = Neoplectospathodus muelleri (Kozur and Mostler) (Schaumkalkbank Member, sample C15, HLMD-WT-616).Figured specimens are housed in the Collection of Vertebrate Fossils at the Hessian State Museum (Hessisches Landesmuseum Darmstadt, Germany), curation repository numbers HLMD-WT-606-616. Supplementary material is housed in the collections of the Institute of Ap-plied Geosciences (TU Darmstadt, Germany).
Abb. 3: Conodonten des Typ-Profi ls Jena-Steudnitz (Thüringen), Unterer Muschelkalk (Anisium). Maßstab 50 μm.
43Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections
44 Annette E. Götz & Sascha Gast
was terminated by basin-wide shallowing and emersion, pro-ducing post-evaporitic vuggy limestones and dolomites (Zel-lenkalk Horizon). The next marine incursion resulted in a gradual deepening and the sedimentation of fi ne-grained, slightly dysoxic limestones and marls of the Upper Gogolin Beds. Subsequent deposition of coarser grained limestones of the Gorazdze Beds refl ects gradual shallowing of the ba-sin up to emersion. The next transgression progressed very quickly and the exposed shoals were directly covered by partly dysoxic fi ne-grained limestones, the Terebratula Beds, which represent the most pronounced Anisian transgressive event recognised over the whole Peri-Tethys Basin (Szulc 1999, Götz et al. 2005). After this drowning, the basin under-went progradational fi lling and shallowing that led to the on-set of bioclastic shoals and sponge-coral reefs of the Karcho-wice Beds. As the shallowing progressed, the reefs were re-placed by oncolites and oolitic bars of the Diplopora Beds at the top of the Lower Muschelkalk succession in the Upper Silesian gate area.
Studies on the conodont assemblages of Poland (Holy Cross Mountains and Upper Silesia) were fi rst carried out by Trammer (1971, 1972) and Zawidzka (1975). Recently, a de-tailed documentation of the conodont assemblages from Anisian well sections of central Poland was provided by Narkiewicz & Szulc (2004). Here, the stratigraphic distribu-tion of conodont marker species of the Upper Silesian Strzelce Opolskie section is published for the fi rst time.
Conodont appearances are recognised during transgres-sive phases. Nicoraella germanica and Neogondolella bul-garica occur from the lower part of the Upper Gogolin Beds, Nicoraella kockeli is fi rst recognised within the middle part of the Upper Gogolin Beds (Fig. 4), marking the maximum fl ooding phase within the Bithynian. Neogondolella hanbu-logi appears in the lower part of the Terebratula Beds, N. bi-furcata in the uppermost part of the Terebratula Beds. The phases of maximum fl ooding and early highstand are marked by the highest abundance and species diversity.
5. Western gate area
The drilling programme of the NAGRA (National Coopera-tive for the Disposal of Radioactive Waste) provided numer-ous cores in the northern part of Switzerland including sedi-ments of Anisian age. These Muschelkalk series represent key sections for the interpretation of the palaeogeographical evolution of the northern Peri-Tethyan realm during Middle Triassic times (Feist-Burkhardt et al. 2008b).
The Lower Muschelkalk of North Switzerland represents Anisian shallow-water deposits of the SW part of the Peri-Tethys Basin. The relatively monotonous succession of marly and dolomitic limestones reaches a thickness of up to 50 m and is subdivided into three lithostratigraphic units: Wellendolomit, Wellenmergel and Orbicularis-Mergel (Fig. 5). Dolomitic limestones of the lower Wellendolomit are overlain by silty marlstones and siltstones with intercalated bioclastic beds of the upper Wellendolomit and Wellen-mergel. An erosional surface in the middle part of the Wellen-
The Terebratelbänke 1 and 2 (Tere bratel bank Member) consist of bioclastic limestones that show an increased abun-dance of crinoid debris. They are interpreted to represent large-scale maximum fl ooding deposits. Towards the top of the section, the Schaumkalkbänke are characterised by deci-metre-scale cross-bedding. They are interpreted as prograd-ing carbonate sandbars that formed in a shallow, high-energy setting, most likely infl uenced by storms, and represent the late highstand deposits of the overall Lower Muschelkalk transgressive/regressive sequen ce. Horizons of leached evaporite nodules in the upper part of the Schaumkalkbank Member and the orbicularis Member mark the onset of the Middle Muschelkalk evaporite sequence.
A fi rst study on the conodont assemblage of the Steudnitz section was carried out by Götz (1995) focussing on the lower part of the section (Oolithbank Member) to determine the Bithynian/Pelsonian boundary. Recently, a bed-by-bed documentation of the conodonts of the entire Lower Muschel-kalk section was provided by Gast (2008). Here, this mate-rial is published for the fi rst time (Fig. 3).
Conodont appearances are recognised during transgres-sive and maximum fl ooding phases. First conodonts are de-tected in the upper part of the Wellenkalk-1 Member within the fi rst maximum fl ooding interval (Fig. 2). Nicoraella ger-manica occurs up to the Oolithbank Member, Nicoraella kockeli is fi rst recognised within the upper Oolithbank Mem-ber (Oolithbank β1) and Neogondolella bulgarica appears within the lowermost Wellenkalk-2 Member, marking the most prominent transgressive phase within the Lower Muschelkalk (Pelsonian) with maximum fl ooding docu-mented by the Terebratelbank Member. Within this member, Neogondolella hanbulogi appears for the fi rst time in the central basin. Early highstand deposits are marked by the highest abundance and species diversity.
Findings of numerous vertebrate remains, such as shark teeth (Hybodus sp., Acrodus sp., Polyacrodus sp., Lissodus sp., Synechodus sp.), teeth of ray-fi nned fi shes (Saurich-thys sp., Gyrolepis sp., Birgeria sp., Colobodus sp.), jaw fragments and scales document the high diversity of fi shes and marine reptiles of the Peri-Tethys Basin. Maximum abundance and species diversity are recognised within the Late Bithynian and Middle Pelsonian, representing phases of maximum fl ooding (Fig. 2). The stratigraphic use of these fossils will be analysed elsewhere (Gast et al., in prep.).
4. Eastern gate area
The Strzelce Opolskie quarry (Upper Silesia, South Poland) exposes a nearly complete Anisian (Lower Muschelkalk) succession with a total thickness of around 100 m (Fig. 4). Overall facies evolution refl ects deposition within an outer ramp setting (Szulc 1991, 2000). In Upper Silesia, open ma-rine carbonate sedimentation started at the beginning of the Anisian and is recorded by bioclastic limestones (which in-clude stenohaline crinoids such as Dadocrinus gracilis) of the Lower Gogolin Beds. The open marine sedimentation
45Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections
Fig. 4: Strzelce Opolskie (Upper Silesia, Poland): stratigraphy, distribution of conodont marker species, depositional sequences. Sequence strati-graphic interpretation from Szulc (2000) and Götz et al. (2005). Legend see fi gure 2.
Abb. 4: Profi l Strzelce Opolskie (Oberschlesien, Polen): Stratigrafi e, Vorkommen stratigrafi sch leitender Conodonten, Ablagerungssequenzen. Sequenzstratigrafi sche Interpretation aus Szulc (2000) und Götz et al. (2005). Legende siehe Ab-bildung 2.
dolomit Unit is most prominently developed in the Leuggern well. Crinoidal limestone beds were recognised in the lower and middle Wellenmergel in the Leuggern and Weiach wells. The Spiriferina Bed, a marker bed in the Anisian of South-west Germany, was also identifi ed in these two wells. This marker bed represents a stratigraphical equivalent of the Pel-sonian brachiopod shell beds of Germany (Terebratelbank Member) and Poland (Hauptcrinoidenbank of the Terebrat-ula Beds). A bioclastic siltstone bed in the uppermost Wellen-
mergel of the Leuggern and Weiach wells is characterised by an erosional base. Upsection a ca. 2 m thick evaporitic inter-val marks the lower part of the Orbicularis-Mergel, overlain by marlstones. The next dolomitic limestones represent the basal part of the Middle Muschelkalk (Matter et al. 1988, Peters et al. 1989, NAGRA 2001).
Up to now, studies on conodont assemblages of northern Switzerland are missing. Recently, a fi rst documentation of the conodont assemblages from Anisian well sections of
46 Annette E. Götz & Sascha Gast
Fig. 5: Benken well (North Switzerland): stratigra-phy, distribution of conodont marker species, deposi-tional sequences. Sequence stratigraphic interpreta-tion from Feist-Burkhardt et al. (2008a).
Abb. 5: Bohrung Benken (Nordschweiz): Stratigra-fi e, Vorkommen stratigrafi sch leitender Conodonten, Ablagerungssequenzen. Sequenzstratigrafi sche In-terpretation aus Feist-Burkhardt et al. (2008a).
47Basin evolution of the Anisian Peri-Tethys: implications from conodont assemblages of Lower Muschelkalk key sections
northern Switzerland was provided by Götz & Gast (2007). Here, the stratigraphic distribution of conodont marker spe-cies of the Benken well section is published for the fi rst time.
Conodont appearances are recognised during transgres-sive and maximum fl ooding phases. First conodonts appear in the lower Wellendolomit (Fig. 5). Nicoraella germanica occurs up to the middle Wellendolomit, Nicoraella kockeli is fi rst recognised in the upper part of the Wellendolomit and Neogondolella bulgarica appears within the middle Wellen-dolomit, marking the fi rst prominent transgressive phase within the Lower Muschelkalk (Bithynian). N. hanbulogi ap-pears within the maximum fl ooding phase of the Bithynian. N. bifurcata was found in early highstand deposits of the Pel-sonian. These sediments are characterised by the highest abundance and species diversity.
6. Discussion
The detailed bed-by-bed analysis of conodont assemblages of different parts of the Peri-Tethys Basin raises again the question on the stratigraphic use of this fossil group, since we see characteristic patterns linked to the eustatic evolution of the basin.
In the eastern part of the basin, in Upper Silesia, open marine carbonate sedimentation started at the beginning of the Anisian, thus the fi rst Middle Triassic conodonts of the epeiric Muschelkalk basin appear at that time, slightly earlier compared to the central part of the basin. Also conodont data from central Poland (Narkiewicz & Szulc 2004), represent-ing the eastern basin, confi rm this facies dependent migra-tion. Surprisingly, also in the western part of the basin, in Northern Switzerland, conodonts appear coeval to the Sile-
Fig. 6: Migration phases of conodonts from the Tethys Ocean into the northern Peri-Tethys Basin. The East Carpathian Gate was already active in the Late Induan, the Silesian-Moravian Gate opened in the Olenekian and the westernmost communication to the Tethys devel-oped during the Early Anisian (Bithynian) via a central alpine gate continuing northwards into the Hessian Depression. Coeval conodont migration from the Tethys shelf via the eastern (Silesian) and western (Swiss) gates into the Peri-Tethys Basin took place in the Bithynian and Pelsonian, representing two immigration pulses linked to major transgression phases during the Anisian. Later, during the Ladinian, the Burgundy Gate opened. RM = Rhenish Massif, MM = Małopolska Massif. Legend see fi gure 1.
Abb. 6: Migrationsphasen der Conodonten von der Tethys ins nördlich gelegene Peri-Tethys-Becken. Die Ostkarpaten-Pforte war bereits im späten Indusium, die Schlesisch-Mährische Pforte ab dem Olenekium geöffnet und eine Verbindung über eine westliche, zentralalpine Pforte bestand ab dem frühen Anisium (Bithynium) nach Norden bis in den Bereich der Hessischen Senke. Im Bithynium und Pelsonium, während der Haupttransgressionsphasen innerhalb des Anisiums, erfolgte eine zeitgleiche Migration über die östliche (Schlesische) und westliche (Schweizer) Pforte. Ab dem Ladinium war auch die Burgundische Pforte geöffnet. RM = Rheinische Masse, MM = Małopolska Masse. Legende siehe Abbildung 1.
sian assemblage. These data support the hypothesis of a western communication between the Tethys Ocean and its northern Peri-Tethys Basin from the Early Anisian (Götz & Gast 2007, Feist-Burkhardt et al. 2008b) via a central alpine gate (Western Gate of Szulc 2000, Alpenrhein-Depression of Einsele & Schönenberg 1964) and continuing northwards into the Hessian Depression (Götz & Gast 2007) and is also corroborated by Gisler et al. (2007) who investigated Early Triassic deposits from the central part of Switzerland. Coeval conodont migration from the Tethys shelf via the eastern (Si-lesian) and western (Swiss) gates into the Peri-Tethys Basin (Fig. 6) took place in the Early Ansian (Bithynian) and Pel-sonian, representing two immigration pulses linked to major transgression phases. This eustatic signal is also documented in sections from the central part of the basin (Germany, Po-land) and conodont assemblages from the northern Tethyan shelf region, e.g. in Southern Hungary (Götz 2005, Kovács & Rálisch-Felgenhauer 2005, Kovács et al. 2005). Thus, two aspects have to be taken into account with respect to basin-wide correlation: (1) conodont immigration is facies depend-ent, i.e. eustatically controlled by major fl ooding phases; (2) conodont immigration is gate controlled, i.e. tectonically controlled. Thus, the described conodont assemblages refl ect the Early Anisian basin evolution within a high-resolution time framework. Based on the high-resolution cyclostratig-raphy of the Peri-Tethys Basin (Rameil et al. 2000, Götz 2004, Götz & Török 2008), we are able to detect migration patterns within a 100 Ka framework, lasting in total approxi-mately less than 1 Ma.
1 Ma refl ects the time span of maximal biostratigraphic resolution on species level due to species longevity in terms of morphological stasis (cf. Lawton & May1995, Jackson & Cheetham 1999). Thus, we can prove that the migration took place below that time span which serves to establish a higher resolution by ecologically controlled migration patterns than by biostratigraphic (species) resolution. Identifying conodonts as ecological markers of epeiric seas does not contradict to their use as stratigraphical correlation tool. This knowledge enables us to use conodonts for basin-wide correlation in a more precise way, though we showed they are facies depend-ent. In the context of over-regional geodynamic processes, migration and distribution patterns of conodonts during the Early Anisian seem to have a high potential in deciphering the palaeogeographic and eustatic basin evolution on high time resolution.
7. Conclusions
Conodont assemblages of the Lower Muschelkalk (Anisian) Peri-Tethys Basin clearly document the facies dependence by migration and distribution patterns. Immigration from the Tethys shelf is linked to two major transgressive and fl ood-ing phases within the Bithynian and Pelsonian. These inter-vals are also marked by the highest abundances and species diversity of the entire microfaunal assemblage. The described conodont assemblages and the well-established cyclostrati-graphic framework of the basin serve as a basis for high-
resolution basin-wide correlation. From the yet available data it seems that migration took place within only a short time span of less than 1 Ma. Ongoing studies on continuous reference sections of the entire Muschelkalk basin are very promising and will complete our knowledge on the evolution of epeiric seas in Earth’s history.
8. AcknowledgementsThis study is part of a project on Middle Triassic microfaunal assemblages of the Peri-Tethys Basin and the northern Tethys shelf region. Access to the quarries in Thuringia (Dornburger Zement GmbH & Co. KG, Dorndorf-Steudnitz) and Strzelce Opolskie (Heidelberg Cement) is gratefully acknowledged. NAGRA, the National Cooperative for the Disposal of Ra-dioactive Waste, Wettingen, Switzerland, is also acknow-ledged for giving us the opportunity to sample the Leuggern, Weiach and Benken wells. We thank Michael J. Orchard (Vancouver) and Joachim Szulc (Cracow) for their careful reviews and thoughtful comments.
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Manuscript received: 30.07.2009Accepted for publication: 19.12.2009
