integrated stratigraphy and ar/ ar chronology of the early to middle miocene upper freshwater

ORIGINAL PAPER

Integrated stratigraphy and 40Ar/39Ar chronologyof the Early to Middle Miocene Upper Freshwater Molassein eastern Bavaria (Germany)

H. Abdul Aziz Æ M. Bohme Æ A. Rocholl ÆA. Zwing Æ J. Prieto Æ J. R. Wijbrans ÆK. Heissig Æ V. Bachtadse

Received: 11 August 2006 / Accepted: 17 December 2006 / Published online: 25 January 2007� Springer-Verlag 2007

Abstract A detailed integrated stratigraphic studywas carried out on middle Miocene ßuvial successionsof the Upper Freshwater Molasse (OSM) from theNorth Alpine Foreland Basin, in eastern Bavaria,Germany. The biostratigraphic investigations yieldedsix new localities thereby reÞning the OSM biostra-

tigraphy for units C to E (sensu; Heissig, Actes duCongres BiochroMÕ97. Mem Trav EPHE, Inst Mont-pellier 21, 1997) and further improving biostratigraphiccorrelations between the different sections throughouteastern Bavaria. Radioisotopic ages of 14.55 ± 0.19and 14.88 ± 0.11 Ma have been obtained for glassshards from the main bentonite horizon and the Riesimpactite: two important stratigraphic marker bedsused for conÞrming our magnetostratigraphic calibra-tion to the Astronomical Tuned Neogene Time Scale(ATNTS04; Lourens et al. in Geologic Time Scale2004, Cambridge University Press,2004). Paleomag-netic analysis was performed using alternating Þeld(AF) and thermal (TH) demagnetization methods. TheAF method revealed both normal and reverse polari-ties but proofs to yield unreliable ChRM directions forthe Puttenhausen section. Using the biostratigraphicinformation and radioisotopic ages, the magneto-stratigraphic records of the different sections are ten-tatively correlated to the Astronomical TunedNeogene Time Scale (ATNTS04; Lourens et al. inGeologic Time Scale 2004, Cambridge UniversityPress, 2004). This correlation implies that the mainbentonite horizon coincides to chron C5ADn, which iscorroborated by its radioisotopic age of 14.55 Ma,whereas the new fossil locality Furth 460, belonging toOSM unit E, probably correlates to chron C5Bn.1r.The latter correlation agrees well with the Swiss Mo-lasse locality Frohberg. Correlations of the older sec-tions are not straightforward. The Brock horizon,which comprises limestone ejecta from the Ries im-pact, possibly correlates to C5ADr (14.581Ð14.784 Ma), implying that, although within error, theradioisotopic age of 14.88 ± 0.11 Ma is somewhat tooold. The fossil localities in Puttenhausen, belonging to

Electronic supplementary material The online version of thisarticle (doi: 10.1007/s00531-006-0166-7) contains supplementarymaterial, which is available to authorized users.

H. Abdul Aziz ( &) � V. BachtadseDepartment for Earth and Environmental Sciences,Section Geophysics, Ludwig-Maximilians-UniversityMunich, Theresienstrasse 41, 80333 Munich, Germanye-mail: [email protected]

M. Bo¬hme � J. PrietoDepartment for Earth and Environmental Sciences,Section Palaeontology, Ludwig-Maximilians-UniversityMunich, Richard-Wagner-Str. 10, 80333 Munich, Germany

A. RochollDepartment for Earth and Environmental Sciences,Section Mineralogy, Ludwig-Maximilians-Universita¬tMu¬nchen, Theresienstrasse 41, 80333 Munich, Germany

K. HeissigBavarian State Collection for Palaeontologyand Geology Munich, Richard-Wagner-Str. 10,80333 Munich, Germany

J. R. WijbransDepartment of Isotope Geochemistry,Vrije Universiteit Amsterdam, De Boelelaan 1085,1081 HV Amsterdam, The Netherlands

A. ZwingLudwig-Maximilians-University Munich,Leopoldstrasse 3, 80802 Munich, Germany

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Int J Earth Sci (Geol Rundsch) (2008) 97:115Ð134DOI 10.1007/s00531-006-0166-7

the older part of OSM unit C, probably coincide withchron C5Cn.2n or older, which is older than the cor-relations established for the Swiss Molasse.

Keywords Biostratigraphy � Magnetostratigraphy �40Ar/ 39Ar dating � Middle Miocene � Molasse

Introduction

The Molasse Basin is a classical foreland basin situatedat the northern margin of the Alps and has been thesubject of numerous studies focusing on facies distri-bution, stratigraphic, sedimentological and structuralevolution of the basin and its response to the tectonichistory of the orogenic wedge (Schlunegger et al.2002;Kuhlemann et al. 2002; Kuhlemann and Kempf 2002;and references therein). Although a detailed (micro)-mammal biostratigraphy has been established forMiocene deposits of the Molasse Basin (Heissig1997;Bo¬hme et al. 2002) it does not provide a precise tem-poral resolution necessary for establishing a causal linkbetween the stratigraphic record of the Molasse Basinand the tectonic evolution of the Alps as well as globalclimate change. Recent studies in the Swiss part of theMolasse Basin have successfully established a detailedmagnetostratigraphic and biostratigraphic frameworkfor Oligocene to Middle Miocene sequences (Schlu-negger et al. 1996; Kempf et al. 1997, 1999). Con-versely, the central and eastern part of the MolasseBasin in southern Germany, well known for its vastamount of sedimentological and paleontological aswell as paleoclimatological information, lacks a reli-able time-stratigraphic framework. This especiallyholds for the Early to Late Miocene Upper FreshwaterMolasse of the Bavarian part of the Molasse Basin,which is one of the richest and most densely sampledNeogene basins of Europe with over more than 200vertebrate and paleobotanical localities.

In this paper, we present a chronostratigraphicframework for different surface outcrops in the easternpart of Bavaria using an integrated biostratigraphic,magnetostratigraphic and radiometric approach. Weinclude results of the 2004 paleomagnetic and bio-stratigraphic Þeldwork season, advances in the bio-stratigraphic subdivision, and results of radioisotopicdating of the two important stratigraphic tie-points: theRies event and the main bentonite horizon. Our inte-grated stratigraphic approach is a Þrst attempt to cali-brate the existing biostratigraphic record to thenew Astronomically Tuned Neogene Time Scale(ATNTS04, Lourens et al. 2004). This approach willallow establishing correlations between continental

sequences throughout the Molasse Basin, creating alink to global climate change and the tectonic evolutionof the Alps, and contribute to improved biostrati-graphic correlations across European Neogene basins.Moreover, it will provide the chronologic frameworknecessary to address key-issues related to mammalevolution and migration rates for Central and WesternEurope.

Geological setting

The Molasse Basin, also known as the Northern AlpineForeland Basin (NAFB), extends about 1,000 kmalong the Alpine front from Lake Geneva in the westto the eastern termination of the Alps in Austria(Fig. 1). During the Cenozoic, the NAFB was formedas a mechanical response to the tectonic load of theevolving Alps (e.g., Homewood et al. 1986; Schluneg-ger et al. 1997) causing a ßexural bulge in the Euro-pean lithosphere, which acted as a sediment sink forthe erosional debris of the uplifting Alps. The sedi-mentary facies of the Molasse is characterized by apronounced temporal and lateral variability with radialalluvial fan sedimentation in the southern part of thebasin and E-W ßuvial alluvial sedimentation morenorthward along the basin axis. Two long-term sedi-mentary cycles, reßected by a repetitive change frommarine to terrestrial sedimentary environments, dom-inate the Molasse sequences and are, accordingly,subdivided into Lower Marine/Freshwater Molasse(UMM/USM) and Upper Marine/Freshwater Molasse(OMM/OSM) (Bachmann and Mu¬ller 1991; Rei-chenbacher et al.1998).

In eastern Bavaria (Fig. 1), the Upper FreshwaterMolasse (Obere Su¬§wasser Molasse, OSM) can be di-vided lithostratgraphically into limnische Sußwassers-

chichten, fluviatile Sußwasserschichten, Nordlicher

Vollschotter (NV) and Hangendserie (Wurm 1937;Batsche 1957; Hofmann 1973; Doppler et al. 2000).Our studied sections in the Mainburg and Landshutarea (Fig. 1) all belong to the NV lithostratigraphicunit (Fig. 2). This NV is subdivided into two partsseparated by the Sußwasserkalk (SK) (Batsche 1957)and typically consists of coarse grained and poorlysorted gravels. The lower part of the NV is generallycoarser (maximum gravel size 22Ð25 cm) and is over-lain by the SK unit, which comprises an up to 10 mthick, typically strongly calcareous paleosol horizon.The upper part of the NV comprises in the Landshutarea several paleosol horizons, which show a relativelycontinuous distribution over several tens of kilometers(Batsche 1957; Herold 1970; Hofmann 1973). The

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stratigraphically uppermost paleosol corresponds tothe ~7 m thick Zwischenmergel (ZM, Hofmann 1973).The base of the gravel unit above the ZM containsJurassic limestone boulders of the Ries impact (Brockhorizon) and can be found in several sections in thestudy area, (e.g., Kreut section, Enghausen, Wach-elkofen; Ulbig 1994). Finally, the uppermost gravel-horizon of the NV, usually deeply weathered (Herold1970), is overlain by 5Ð7 m thick Þne-grained, marlysediments of the Sand-Mergel-Decke (SMD). In thisunit, an up to 3 m thick tuff horizon occurs: the main-bentonite layer of eastern Bavaria.

Biozonations

The Þrst biozonation of the OSM deposits was given byDehm (1955), who introduced the terms Older, Middleand Younger Series, deÞned respectively by the ab-sence of deinotherid proboscidians, the presence of thesmall sized Prodeinotherium bavaricum and the pres-ence of the large sizedDeinotherium giganteum. Later,Fahlbusch (1964), Heissig (1990) and Boon (1991)proposed a biozonation for the Older and MiddleSeries based on cricetid (hamsters) evolutionary stagesand successions of small mammal communities. For the

Older and Middle Series, Heissig (1997) introduced sixfaunal units (OSM AÐOSM F) and a correspondingcyclostratigraphy of 11 Þning upward cyclothems(OSM 0ÐOSM 10). Each cyclothem is deÞned (fol-lowing Scholz 1986) by a course, gravely and sandyßuvial unit at the base and a Þne-grained ßoodplain orpedogenic unit at the top. The faunal units OSM A toOSM D correspond to the Older series, whereas theunits OSM E to OSM F correspond to the Middleseries. Both series are separated by an erosional hiatus,the pre-Riesian hiatus of Birzer (1969). The most re-cent stratigraphical update was given by Bo¬hme et al.(2002) who deÞned a new faunal unit OSM EÕ betweenOSM E and OSM F.

Sections and correlations

Relative lithostratigraphic correlations between thestudied sections (Fig.2) are established using one ormore of the following important stratigraphic markers:vertebrate fossil localities, the main bentonite layerand the Brock horizon of the Ries impact.

The Puttenhausen section comprises an 18 m thicksuccession of Þne-grained alluvial deposits (Fig.2).

Fig. 1 Location maps ofstudy area and sections, andsketch map of Molasse Basin

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The sediments show a cyclic alternation of blueÐgreyand darkÐred colors, which are indicative of intensepedogenetic processes (rubiÞed calcic pseudogleysolafter Schmid 2002). At least six red paleosols rich infossil vertebrates (our new Puttenhausen localities Ato E) are recognized. The lowermost horizon containsthe classic Puttenhausen fossil level of Fahlbusch andWu (1981). Laterally, a thick ßuvial channel cuts intothe cyclic paleosol proÞle.

The famous Sandelzhausen fossil locality (Fahlbuschet al. 1972), a gravel pit of the Karger Co. (acronymSZA) is abandoned since 2004. The Sandelzhausen(SZ) section sampled for this study corresponds to thefossil-bearing layers of SZA (Fig. 2). The section issituated on top of gravels of the NV and contains 2.5 mthick moderate to weakly pedogenized gravelly marls

(base), marls and clays (top) with well developed cal-crete horizons. Several hundreds of meters from San-delzhausen, two other outcrops have also been studied:one to the east (Sandelzhausen gravel pit, Beck Co.,acronym SZB) and the other to the west (Sandelz-hausen former gravel pit Mitterfeld, acronym SZM).The lithologic characteristics and relative topographicposition of these sections enable a correlation to theSZA section.

Only the uppermost part of the Unterempfenbachgravel pit [448Ð465 m above mean sea level (amsl)] wastemporarily exposed in 2004 (Fig.2). The lower part ofthe section, currently inaccessible, contains gravels ofthe NV (Gregor 1969; Heissig personal observations).The vertebrate fauna of Unterempfenbach 1c wasfound in a gastropod rich, pedogenic horizon at around

Fig. 2 Lithostratigraphic subdivision (Hofmann 1973) of theeastern Bavarian Molasse and correlation between studiedsections. Fossil sites with their corresponding OSM unit are

indicated. In the Puttehausen section,R indicates red paleosol.The sections are positioned according to their topographic height


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438 m amsl. This horizon is followed by limnic marlsand a paleosol containing the vertebrate faunas ofUnterempfenbach 1b and 1d. The upper part of thesection consists of intensively pedogenized Þne-grainedalluvial sediments, intercalated with calcrete horizons.The top of this pseudogleysol is slightly rubiÞed and iserosively overlain by sandy gravels.

The 42 m thick Furth section (Fig. 2) consists of Þvesuccessive Þning-up cyclothems each beginning, exceptthe last one, with coarse, grey colored carbonate richgravels (gravel diameter up to 20 cm, typical for theNV) and ending with Þne grained ßoodplain sediments.The three lower Þne-grained cyclothems are weaklypedogenized. The fourth one (~10 m thick) is inten-sively pedogenized with a partly rubiÞed calcic pseu-dogleysol on the top. This paleosol, labeled Furth-2section in this study, correlates to the ZM and in thetop part contains a new fossil locality (Furth 460 m).Laterally, the paleosol is eroded by a steep E-W ori-ented channel consisting of sandy sediments (Furth-1section) with numerous reworked carbonate concre-tions at the base. Hence, Furth 1 lies stratigraphicallyabove Furth 2. The uppermost gravel of the last cy-clothem is yellowish and rusty colored. The gravelsshow similar lithological characteristics as the gravelsin the Kreut section (2 km southwest of Furth section;see below), which contains at the base Ries impactboulders (Brock horizon).

The sand and gravel pit at Kreut is situated 2 kmsouthwest of the Furth section. The base of the sectionconsists of pedogenically overprinted (rhizocrete),laminated lacustrine marls (Fig. 2). These marls areerosively overlain by 3 to 4 m thick yellowish tobrownish carbonate-free coarse gravels and 1.5 m sand.Locally, the base of these gravels contains up to 10 cmlarge angular Late Jurassic limestone boulders, origi-nating from the Ries impact (Brock horizon, Bo¬hmeet al. 2002). These sediments are subsequently ero-sively overlain by several meters of coarse gravelscontaining reworked calcretes and up to 10 m of large-scale cross-bedded sands. One kilometer south ofKreut, the main bentonite layer was historically ex-posed at a height of 475Ð480 m NN, which is strati-graphically above the Kreut section (Ulbig 1994).

The base of the 85 m thick Unterwattenbach sectioncontains gravels rich in carbonate pebbles (maximumdiameter 22 cm), which are characteristic for the top ofthe lower part of the NV (Fig. 2). Above these sedi-ments, between 409 and 419 m, a heavily pedogenizedwhite to bluish marl is exposed, comprising three tofour pedogenic cycles. This marl could correlate to theSK. In the subsequent gravel interval (maximum peb-ble diameter 15 cm), Þne-grained sediments containing

partly eroded sands and clays and pedogenized greenmarls occur. The gravel interval is followed by 2.5 m ofÞne sands and 4.5 m of a green, pedogenized marls,which correlate to the ZM. The transition to theoverlying gravels is not well exposed. The top part ofthe Unterwattenbach section comprises a 6 m thickÞning up (from middle sand to silty clay) unit, whichcorrelates to the SMD and the (reworked) main ben-tonite horizon. This interval is Þnally erosively overlainby gravels.

The only section studied in the Augsburg area is thegravel and sand pit of Pfaffenzell. In Pfaffenzell, post-Riesian sediments, including the Brock horizon at thebase of the proÞle (about 30 m below out measuredsection), are exposed. At the northern end of thegravel pit, near the village Pfaffenzell-Weiler, the topof the proÞle crops out. This proÞle comprises 10 mthick Þne to coarse gravels (Gallenbacher Schotter ofFiest 1989), followed by ~7 m of Þne-sandy silts (Sand-Mergel-Abfolge of Fiest 1989). In our studied Pfaf-fenzell-Weiler section, an erosional depression is foundon the top of this silt unit and comprises 6 m thicklaminated, dark bituminous and blue grey lacustrinemarls and about 2 m of yellowish marls and silt with aweak pedogenic overprint. About 150 m east of oursection, the main bentonit layer is found at a topo-graphic height of 515 m.

The bentonite pits of the Bruckberg-Ried and Eg-gersdorf sections belong to the SMD and are similar tothe sedimentary cycle of Pfaffenzell-Weiler. Both sec-tions contain the main bentonite layer with theiroverlying Þne-grained sediments. The base of thesesections contains an up to 3 m thick consolidated, rel-atively fresh whitish tuff, which is subsequently fol-lowed by 2 m of industrially excavated high-qualitybentonite and up to 3 m of reworked bentonite orsandy clays.

Biostratigraphy

In the NAFB the biostratigraphy is based on evolu-tionary succession of small-mammal assemblages(Fig. 3). Following Heissig (1997) and Bo¬hme et al.(2002), the regional faunal unit OSM A is character-ized by the presence of the generaLigerimys florancei

and the very small sizedMegacricetodon aff. collong-

ensis. Faunal unit OSM B contains no Ligerimys but isdeÞned by the First Appearance Date (FAD) of Ker-

amidomys and Megacricetodon bavaricus, the latterevolving from M. aff. collongensis. The younger faunalunit OSM C + D is characterized by a further size in-crease of Megacricetodon (M. aff. bavaricus) and the


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FAD of a small size M. cf. minor (in the later part ofthis unit). In contrast to Heissig ( 1997) and Bo¬hmeet al. (2002), we uniÞed faunal unit OSM C and D. Thecharacteristic species of OSM D was deÞned asAnomalomys minutus. This very rare species is knownfrom only two localities in the NAFB, Gisseltshausen1a (Lower Bavaria, see Fig.2) and Tobel-Hom-brechtikon (Switzerland, type locality, Bolliger 1992).Gisseltshausen 1a is superposed by Gisseltshausen 1b,which contains a M. aff. bavaricus in the size-range ofPuttenhausen classic (see ÔÔAppendixÕÕ). In contrast,Puttenhausen classic contains Anomalomys minor

(Table 1). At this moment we assume that A. minutus

(which differ from A. minor only in 10% reduced size,Bolliger 1992) lived contemporary with A. minor in theNAFB. Faunal unit OSM E is deÞned by the FAD andLAD (Last Appearance Date) of Megacricetodon lappi

and Cricetodon meini and a M. minor of typical sizewhereas in OSM EÕM. aff. lappi is replaced by theimmigrant M. aff. gersi. Finally, OSM F is character-ized by the FAD of Cricetodon aff. aureus andAnomalomys gaudryi.

The oldest studied lithostratigraphic unit, the lowerpart of NV in the Unterwattenbach section, contains nofossils. The only biostratigraphic useful fauna for thelower part of NV comes from Niederaichbach 4.5 kmSE of Unterwattenbach section. The fauna contains atypical sized Megacricetodon bavaricus and could thuscorrelate to the fauna of Langenmoosen and Bellenberg(Scho¬tz 1993), corresponding to faunal unit OSM B.

Puttenhausen section

The six red soil horizons have produced rich micro-vertebrate assemblages (PUT A to E). The lowermostsample PUT A correlates to the classical Puttenhausenfauna (Fahlbusch and Wu1981; Wu 1982, 1990; Zieglerand Fahlbusch1986; Ziegler 2000, 2005). The presenceof the larger sized Megacricetodon aff. bavaricus sug-gests that both samples belong to faunal unit OSMC + D. Because of the evolutionary stage of Megac-

ricetodon and Miodyromys, these samples are consid-ered to be older than the Sandelzhausen assemblage(Fig. 3).

The faunal assemblages found in PUT B to PUT Dshow no signiÞcant difference to PUT A or the classicallevel (Fig. 3, Table 1), although the biostratigraphicimportant large Megacricetodon species is rare in allsamples. However, PUT E (445.5Ð448 m) yielded twom/2 of a small Megacricetodon cf. minor, which aresmaller than the rare small Megacricetodon from PUTclassic (Wu 1982) and similar in size to M. cf. minor

found in Sandelzhausen [Fahlbusch1964; Wessels andReumer 2006 (personal communication)]. Also thepresence ofGalerix aurelianensis-stehlini in PUT E aswell as in Sandelzhausen supports a younger levelwithin OSM C + D unit. This species is absent in olderlevels but present in Sandelzhausen (Ziegler2000) andin Ma§endorf (Scho¬tz 1988; the Ma§endorf fossil levelsituated in the upper part of NV, Fig. 2), bothbelonging to a later part of OSM C. In addition,

Fig. 3 Size increase ofMegacricetodon m1 molar from OSM Ato OSM E. Large grey dots represent samples from the easternBavarian Molasse (SAND Sandelzhausen,PUT Puttenhausen,GIS1b Gisseltshausen 1b andNAI Niederaichbach), black dots(white triangles) from the German (Swiss) Molasse and opencircles are samples from Vieux Collonges (France). The scatter

range is indicated for the type localities Vieux Collonges(M. collongensis, M. lappi) and Langenmoosen (M. bavaricus,grey dot). Arrows indicate the position of the two Þne-grainedhorizons Su¬§wasserkalk and Zwischenmergel. Measurements forall samples are listed in ÔÔAppendixÕÕ


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Table 1 Herpeto- and small mammal fauna of the localities Puttenhausen (PUT cl.-classic, PUT A to E), Sandelzhausen (SAND), andFurth 460 m (FU 460)

PUT cl. PUT A PUT B PUT C PUT D PUT E SAND FU460Cricetidae Megacricetodon aff. bavaricus

Megacricetodon lappiMegacricetodon cf. minorDemocricetodon gracilis Democricetodon mutilus Democricetodon sp.Eumyarion bifidusEumyarion cf. weinfurteriNeocometes sp.

Anomalomyidae Anomalomys minorEomyidae Keramidomys thaleri ? sp.Gliridae Microdyromys complicatus

Microdyromys koenigswaldiGlirulus diremptusProdryomys satusMiodyromys aegercii aff. aff. aff. aff. aff. aff.Bransatoglis astaracensis ( after Bruijn & Petruso 2005 ) = B. cadeoti ( after Wu 1990: 89 )Glirudinus cf. undosusGlirudinus sp."Eomuscardinus" aff. sansaniensis

Sciuridae Spermophilinus besanusPaleosciurius sutteriHeteroxerus aff. rubricatiBlackia miocaenicaForsythia cf. gaudryiMiopetaurista dehmi sp. sp.

Castoridae cf. Steneofiber deperetiDidelphidae Amphiperatherium frequens n. ssp.Soricidae Dinosorex zapfei aff. ?aff. ?aff. aff. ?aff.

Florinia aff. stehliniMiosorex sp. 1Miosorex sp. 2Lartetium dehmi ?Limnoecus n. sp.Soricidae indet. ( several species )

Dimylidae Plesiodimylus chantreiPlesiodimylus n. sp.Metacordylodon aff. schlosseri

Talpidae Talpidae indet. ( several species )Talpidae indet. ( sensu ZIEGLER 2000: 118, Plate 6, fig. 75 )Prosacapanus sansaniensisProscapanus sp.( small )Myxomygale hutchisoniMygaela jaegeri?Urotrichus cf. dolichochirTalpa minuta cf.

Erinaceidae Galerix aff. exilisGalerix aurelianensis-stehliniLanthanotherium aff. sansaniensis sp.Mioechinus sp. sp.

Vespertilionidae Eptesicus sp.Myotis aff. murinodes

Lagomorpha Prolagus oeningensis ?Prolagus crusafonti-like form ??Amphilagus sp.Lagopsis verusLagopsis cf. penai sp.

Allocaudata Albanerpeton inexpectatumUrodela Salamandra sansaniensis sp.

Chelotriton paradoxus sp. sp. sp.Triturus (vulgaris) sp.Triturus cf. marmoratus

Anura Latonia gigantea sp. sp. sp.Eopelobates sp.Pelobates nov.sp. sp. sp.Pelobatinae indet.Rana (ridibunda) sp.Bufo cf. viridis

Crocodylia Diplocynodon styriacusChelonia Trionyx (triunguis) sp.

Clemmydopsis turnauensisMauremys sophiae aff. sp. aff. sp.Testudo rectogularis sp. sp. sp. sp.Ergilemys cf. perpinianaChelonia indet.

Iguania Chamaeleo caroliquartiChamaeleo bavaricus sp.

Scincomorpha Lacerta sp. 1Lacerta sp. 2Miolacerta tenuisAmblyolacerta dolnicensisScincidae indet. 1Scincidae indet. 2Cordylidae indet. ? ?Scincomorpha indet. 1Scincomorpha indet. 2

Anguimorpha Ophisaurus cf. spinari sp. sp.Pseudopus moguntinus sp.Anguis sp.Anguidae indet.? Varanidae indet.

Amphisbaenidae Palaeoblanus cf. tobieniAmphisbaenidae indet.

PUT cl. and SAND after Bo¬hme (1999, 2003), Fahlbusch (1964, 2003), Fahlbusch and Wu (1981), Wu (1982, 1990), Ziegler andFahlbusch (1986) and Ziegler ( 2000, 2005). PUT A to E and FU460 this paper. The presence of taxa is indicated in black


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Ziegler ( 2000) observed that the Dinosorex from PUTclassic is somewhat smaller than from Sandelzhausen,an observation we also recognize in the size increase ofD. zapfei from PUT C to PUT E.

Hence we conclude that the biostratigraphic infor-mation indicates that PUT E is more closely related toSandelzhausen than to PUT classic.

Furth section

The biostratigraphic position of the Furth 460 mlocality can be Þxed to the faunal unit OSM E becauseof the presence of Megacricetodon lappi (Fig. 3). It istherefore similar in age to Ebershausen and Mohren-hausen (Boon 1991) in the western part of the Bavar-ian Molasse Basin and could be correlated to the baseof the Middle Series. In the Swiss part of the MolasseBasin, the Furth locality shows faunal characteristicssimilar to Frohberg and Aspitobel (Bolliger 1992, 1997;Ka¬lin 1997; Ka¬lin and Kempf 2002).

Localities between Brock horizon

and main bentonite layer

Within our studied section we have no fossil localitiesbelonging to OSM EÕ or F. To date, OSM EÕ is docu-mented in one locality only (Derching 1b, Augsburgarea), found in a sandy unit 2 m below the Brockhorizon (Bo¬hme et al. 2002). All localities belonging toOSM F are found between the Brock horizon and themain bentonite layer, both in the Landshut (e.g., Sall-mannsberg) and the Augsburg area (e.g., Laimering 3)(Heissig 1997).

40Ar/39Ar chronology

Samples

The Miocene main bentonite in eastern Bavaria hasbeen sampled in a commercially exploited quarry atHachelstuhl, about 5 km south of Landshut. At thislocation, the bentonite reveals a trisection typical formost bentonites in the MainburgÐLandshut area. Itscentral part, a hard bluish-grey, granular to Þbrouslayer divides the bentonite into a high-quality lowerclay and an upper clay of lower montmorillonite con-tent (Ulbig 1994). This hard layer represents the leastaltered part of the former tuff horizon and contains upto 20% of residual glass fragments.

Similar to the age of the Bavarian main bentonite,and also exposed in the eastern Bavarian Molasse, is

the so-called ÔÔBrock horizonÕÕ. This central horizonconsists of isolated boulders or layers of Upper Jurassiccarbonate rocks, which are considered as distal ejectaof the Ries impact at No¬rdlingen. While radioisotopicages are ambiguous with respect to the age relationshipbetween the main bentonite and Ries event (Storzerand Gentner 1970), Þeld evidence indicates that Riesejecta are stratigraphically older (e.g., Unger et al.1990) and about 30 m below the bentonite (Heissig1989). For this reason we also analyzed Ries suevite, animpact melt derived from the Phanerozoic basement,as an external control for the bentonite data. Thesample was collected at the O¬ttingen quarry and haskindly been provided by P. Horn (Munich).

Analytical details

The glass-rich central horizon at Hachelstuhl bentonitewas previously sampled for chemical analyses by Ulbig(1994; sample Ha-2a). To avoid destroying the fragileglass particles, the rock was treated by thermal disin-tegration by repeatedly freezing at Ð25�C and defrost-ing until the rock disintegrated into smaller parts, whichwere subsequently dispersed ultrasonically in water andsieved. The grain size fraction (63lm was treatedultrasonically and the ßoating clay fraction wasrepeatedly decanted. In this way, all clay minerals couldbe effectively removed from the glass particle surfaces.

To obtain clean glass fragments for40Ar/ 39Ar dating,associated mineral grains were largely sorted out byFranz magnetic separation and then sieved at differentmesh sizes. The maximum-size fraction (125Ð150lm)was mildly leached for 5 min in cold 0.2 N hydroßuoricacid in an ultrasonic bath, to remove possible clay-intergrowth and hydrated surfaces, and then ultrasoni-cally treated in deionised water. Clean, fresh andtranslucent glass shards were then hand-picked under abinocular microscope. Shards showing alteration fea-tures, such as yellowish-milky surfaces, clay-inter-growth, mineral inclusions, and gas bubbles, wereexcluded. The hand-picked glass fraction included23 mg of overall translucent, elongated, high-porosityglass shards of rhyolitic composition (Ulbig 1999), withshapes characteristic for Plinian-type eruptions.

The tuff and Ries suevite glasses were wrapped inaluminum foil and loaded in 6 mm ID quartz vials.Together with the Fish Canyon Tuff (FCT) sanidine(assumed age 28.02 Ma) and Drachenfels DRA1 (as-sumed age 25.26 Ma) monitor standards, the sampleswere irradiated for 1 h with fast neutrons in the Cd-lined RODEO facility of the EU/Petten HFR Reactor(The Netherlands). Argon isotope analysis was carriedout at the Department of Isotope Geochemistry, Vrije


123

Universiteit Amsterdam (The Netherlands), using aMAP 215-50 noble gas mass spectrometer. Eleven and15 total fusion replicate analyses were carried out forthe Hachelstuhl and Ries samples, respectively. Thereplicate experiments for Ries suevite were conductedin a single session, while those for Hachelstuhl glasseswere performed in two sessions. Instrumental massfractionation was corrected for by repeated analysis of40Ar/ 38Ar reference gas and 40Ar/ 36Ar air pipettealiquots. Data processing included corrections forblank contribution, mass interferences, regression ofmass intensities and mass fractionation using ArArCalc software (Koppers 2002). Analytical errors referto 1r conÞdence level. Details on the data evaluationand processing can be found in Kuiper et al. (2004).

Results

40Ar/ 39Ar ages were calculated using the decay con-stants of Steiger and Ja¬ger (1977), assuming an age of28.02 Ma for the FCT monitor standard according to

the Renne et al. (1998) convention. The data are listedin Tables 2 and 3. Plateau ages in Table2 refer toweighted means over all accepted total-fusion data.One Ries data and two Hachelstuhl data were con-sidered as outliers and have, therefore, been excludedfrom the age calculations (Table 2).

Eleven total fusion experiments were performed forHachelstuhl tuff glasses. The K/Ca ratios (Table2),which were derived from 39Ar/ 37Ar, are consistent withthe degree of variation in the chemical composition ofthe glasses, as observed by electron microprobe anal-ysis, and are not correlated with age (not shown). Thisindicates that possible preferential loss of K during latealteration did not affect the glasses and, hence, the40Ar/ 39Ar ages. Nine out of the eleven fusion data wereused for the age calculation, yielding a weighted pla-teau age of 14.68 ± 0.11 Ma. Within error, this age isidentical to the normal isochron (14.56 ± 0.19 Ma) andinverse isochron ages (14.54 ± 0.19 Ma). Plateau agesof individual total fusion experiments and the runschosen to calculate the weighted plateau ages are listedin Table 2 and in Fig. 4.

There is an apparent 100 ka offset between plateauand isochron ages, which may be due to some excessargon present in some of the Hachelstuhl samples.Hence, the three lowest plateau ages would best rep-resent the ÔÔtrueÕÕ age. The isochron excess argonintercept supports this interpretation and the lowerisochron age is consistent with the ages obtained forthe three youngest steps. Based on these arguments weestimate an age of 14.55 ± 0.19 Ma for the Hachelstuhltuff. This age compares well with Þssion track data ofmain bentonite glasses from Mainburg and Unter-Haarland/Malgersdorf, yielding ages of 14.6 ± 0.8 and14.4 ± 0.8 Ma, respectively (Storzer and Gentner

Table 2 40Ar/ 39Ar plateau ages and 39Ar/ 37Ar derived K/Caratios of rhyolitic glasses from Hachelstuhl bentonite and sueviteimpact glasses from O¬ttingen (Ries)

Plateauage (Ma)

±2r K/Ca ±2r

HachelstuhlHa-2a-1a 15.21 ±0.10 5.079 ±0.625Ha-2a-2a 14.91 ±0.11 5.179 ±0.293Ha-2a-3 14.80 ±0.09 4.890 ±0.563Ha-2a-4 14.79 ±0.22 3.305 ±0.456Ha-2a-5 14.79 ±0.18 5.172 ±0.325Ha-2a-6 14.74 ±0.25 2.915 ±0.432Ha-2a-7 14.71 ±0.13 3.692 ±0.470Ha-2a-8 14.66 ±0.11 4.686 ±0.626Ha-2a-9 14.59 ±0.11 5.389 ±0.352Ha-2a-10 14.58 ±0.22 3.239 ±0.487Ha-2a-11 14.58 ±0.10 5.370 ±0.384

RiesRies-1a 17.44 ±0.26 1.214 ±0.064Ries-2 15.03 ±0.21 1.163 ±0.060Ries-3 15.01 ±0.12 1.165 ±0.059Ries-4 14.99 ±0.19 1.163 ±0.061Ries-5 14.98 ±0.12 1.180 ±0.062Ries-6 14.95 ±0.32 1.175 ±0.062Ries-7 14.95 ±0.17 1.171 ±0.061Ries-8 14.87 ±0.22 1.176 ±0.061Ries-9 14.87 ±0.13 1.165 ±0.060Ries-10 14.86 ±0.14 1.174 ±0.061Ries-11 14.86 ±0.15 1.153 ±0.059Ries-12 14.84 ±0.14 1.197 ±0.062Ries-13 14.83 ±0.13 1.156 ±0.059Ries-14 14.82 ±0.16 1.167 ±0.060Ries-15 14.79 ±0.10 1.162 ±0.059

a Rejected

Table 3 Summary of 40Ar/ 39Ar plateau and isochron ages ofrhyolitic glasses from Hachelstuhl bentonite and suevite impactglasses from O¬ttingen (Ries)

Hachelstuhl n Ries n

Weighted plateau (Ma) 14.68 ± 0.11 9 14.89 ± 0.10 14MSWD 2.33 1.2639Ar K used in plateau calc. 71% 96.3%Total fusion (Ma) 14.82 ± 0.10 11 14.99 ± 0.10 15Normal isochrone (Ma) 14.56 ± 0.19 9 14.88 ± 0.11 14Non-radiogenic

40Ar/ 36Ar intercept313.8 ± 23.5 273.2 ± 85.1

MSWD 1.73 2.66Inverse isochrone (Ma) 14.54 ± 0.19 9 14.84 ± 0.11 14Non-radiogenic 40Ar/ 36Ar

intercept317.6 ± 24.4 360.8 ± 67.1

MSWD 1.75 0.92Suggested age (Ma) 14.55 ± 0.19 14.88 ± 0.11

n Number of analyses


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1970). We are not aware of any other age determina-tions for the main bentonite.

For the Ries suevite glass we obtain a weightedplateau age (14 out of 15 total fusion experiments) of14.89 ± 0.10 Ma, which is identical to the normal(14.88 ± 0.11 Ma) and inverse isochron ages(14.84 ± 0.11 Ma). Again, K/Ca ratios reßect thechemical composition of the suevite and do not cor-relate with age (Table 2). Plateau ages of individualtotal fusion experiments and the runs chosen to cal-culate the weighted plateau ages are listed in Table2and shown in Fig. 4. Except for one, all analyses yieldvery consistent results. Based on these results and the

consistency of plateau and isochron ages, we suggest anage of 14.88 ± 0.11 Ma for the Ries impact glasses.This age is identical to the mean (14.87 ± 0.36 Ma) of51 published KÐAr, 40Ar/ 39Ar, and Þssion track agescompiled by Storzer et al. (1995), but up to 0.5 Maolder than the more recent data on moldavites andRies glasses (Schwarz and Lippolt2002; Laurenzi et al.2003; Buchner et al. 2003).

Magnetostratigraphy

Samples and methods

The nine sections sampled for paleomagnetic analysiscoincide, in many cases, with important vertebratefossil localities. Our aim was to sample at a resolution of5Ð10 cm; however, due to variable lithologies the sam-ples are unevenly spaced throughout the sections. Allsamples were hand-cored inÑoften unconsoli-datedÑÞne sands and silts or in clays and orientedusing a standard compass. Since the samples werefragile, they were subsequently pushed into plasticcylinders. The initial magnetic susceptibility of thesamples was measured on a Kappa bridge KLY-2. Thecharacteristic remanent magnetization (ChRM) wasdetermined by both thermal (TH) and alternating Þeld(AF) demagnetization using in a laboratory-builtshielded furnace and a single axis AF-demagnetizer(ambient Þeld (5 nT), respectively. During thermaldemagnetization, the plastic cylinder of each samplewas carefully removed and replaced by a thermallydemagnetized (at 700�C) ceramic cylinder. The sampleswere thermally demagnetized using incremental heat-ing steps of 20, 40 and 50�C. AF demagnetization wascarried out in small steps of 5 mT up to 30 mT, followedby steps of 10 mT up to a maximum Þeld of 200 mT.The natural remanent magnetization (NRM) wasmeasured on a vertically oriented 2G Enterprises DCSQUID cryogenic magnetometer (noise level 10Ð7 A/m) in a magnetically shielded room at the Niederlipp-ach paleomagnetic laboratory of Ludwig-Maximilians-University Munich, Germany. Demagnetization resultswere plotted on orthogonal vector diagrams (Zijder-veld 1967) and ChRM directions were calculated usingprinciple component analysis (PCA, Kirschvink 1980).

Results

The TH and AF demagnetization diagrams of theapproximately 450 measured samples are of variablequality (Fig. 5). NRM intensities are low, varying

(a)

(b)

Fig. 4 Weighted mean plateau ages of individual total fusionexperiments and runs for Hachelstuhl (a) and Ries glasses (b).Plateau steps and their 1r error are shown. (Total procedureblanks ranged from 1.2· 10Ð3 to 3.8 · 10Ð3 for mass 36,1.0 · 10Ð3 to 3.2 · 10Ð3 V for mass 37, 0.2· 10Ð4 to 13 · 10Ð4 Vfor mass 38, 0.6· 10Ð4to 3.1 · 10Ð4 V for mass 39, and 2.3· 10Ð2

to 4.8 · 10Ð2 V for mass 40, based on a system sensitivity of 1.8.10Ð17mol/V)


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between 0.05 and 100 mA/m (average 0.8 mA/m) withonly a few values reaching up to 2,450 mA/m for redpaleosols in the Puttenhausen section. TH and AFdemagnetization show that most of the samples carrythree components (Fig. 5). The Þrst component shows

a random oriented, viscous direction, which is removedat temperatures of 100�C and at Þelds of 2 mT. Asecond component is removed between 100 and 200�Cand at 15 mT, and is interpreted as the present-dayÞeld overprint. In Pfaffenzell and Puttenhausen

Fig. 5 Thermal (TH) andalternating Þeld (AF)demagnetization diagrams forselected samples of thestudied eastern Bavariansections.Black rectangles(white triangles) denotes theprojection on the vertical(horizontal) scale. Valuesalong demagnetizationtrajectories indicatetemperature and applied Þeldsteps in �C and mT,respectively. Q1 and Q2denote quality 1 and 2 type ofdemagnetization, respectively(see text for details)


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sections, a low temperature component is removed at360�C.

The third component is removed at higher temper-atures, ranging between 480 and 580�C suggesting thatthe magnetic signal is carried by magnetite. Higherunblocking temperatures between 600 and 680�C arefound for few samples from Puttenhausen, Pfaffenzelland Unterempfenbach sections and are usually asso-ciated with paleosol horizons, suggesting the presenceof (Þne-grained) hematite. As for the AF demagne-tized samples, the magnetic remanence is fullydemagnetized between 60 and 80 mT. Although a fewsamples are also demagnetized at Þelds up to 120 mT,a signiÞcant progressive increase in intensity is oftenobserved when demagnetizing at Þelds higher than80 mT.

Because of the variable quality of the Zijdervelddiagrams and different temperature and AF ranges ofdemagnetization we used the following criteria todetermine the ChRM direction in the third component:(1) reliable ChRM direction must be observed in thetemperature range 240/360Ð480�C or higher and theAF Þeld range 20Ð50 mT or higher, and (2) withinthese windows at least four stable end points must beused for PCA calculation of the ChRM direction.Based on these criteria, three ChRM demagnetizationqualities are distinguished. (1) About 37% of both THand AF demagnetized samples belong to the quality 1type of demagnetization, characterized by a clear andstable demagnetization showing a more or less lineardecay towards the origin. Except for the Pfaffenzellsection, dual polarities are recorded in all sections. (2)Samples with quality 2 type of demagnetization showfair demagnetization behavior, however, no clear de-cay to the origin can be observed. Rather, the ChRMend-points show a cluster in either the normal or re-versed quadrants. About 21% of thermally and 16% ofthe AF demagnetized samples belong to this qualitygroup. (3) The majority of the demagnetized samples,which comprise 54% thermal and 57% AF treatedsamples, are assigned quality 3 type of demagnetiza-tion. The Zijderveld diagrams of these samples do notshow any clear demagnetization path nor do they meetthe two criteria; they are therefore non-interpretable.

Since the sections are more or less in the vicinity ofeach other, we have calculated the average direction offor all quality 1 samples with a MAD smaller than 15(Fig. 5). The average declination/inclination is 358/56for the normal samples (n = 83) and 183/Ð44 for thereverse samples (n = 27). Notably, the number of re-verse samples is limited and the scatter is rather large(a95 = 10.8). In addition, the inclination of the reversedsamples is shallower by 10� compared to the normal

values, which could be explained by a possible sec-ondary normal overprint making it impossible tocompletely isolate the primary ChRM component.Mean directions of the ChRM should therefore betreated with caution when interpreted in terms of tec-tonic rotations.

Reliability of the polarity signal

For each section, all quality 1 declination and inclina-tion results have been plotted in stratigraphic order(Figs. 6, 7). Since the studied sections are horizontallybedded, no bedding-tilt correction could be appliedand therefore it is impossible to distinguish primaryfrom secondary components. Most of the magneto-stratigraphic records, however, show dual polaritiesand/or similar polarity directions were obtained for AFas well as thermally demagnetized samples. Therefore,we consider that most of the studied records are reli-able. An exception is the Puttenhausen section, whichreveals several polarity reversals but most polaritydirections of the AF demagnetized samples do notagree with the directions obtained for the thermalsamples. Moreover, the quality 1 AF demagnetizedsamples are, except for one sample, all normal. Basedon the unblocking temperatures (~630�) we assumethat the ChRM of the Puttenhausen samples is carriedby Þne-grained hematite having a low temperaturenormal overprint, which cannot be resolved using theAF demagnetization method. Hence, the polarity re-cord of Puttenhausen is based on ChRM directionsfrom thermally demagnetized samples only.

Magnetostratigraphic calibration

The polarity records of the studied sections comprisenone or only a few reversal levels and are insufÞcientfor a direct and unambiguous magnetostratigraphiccorrelation to the ATNTS04 (Lourens et al. 2004).Hence, to establish a chronostratigraphic frameworkfor the eastern Bavarian Upper Freshwater Molasse,the magnetostratigraphic results are combined withbiostratigraphic and lithostratigraphic correlationsestablished between sections and with40Ar/ 39Ar ages(Fig. 8).

The youngest interval: Sand-Mergel-Decke (SMD)and main bentonite horizon

The youngest sections belonging to the main bentonitelayer and the SMD include Pfaffenzell, Bruckberg andthe uppermost part of Unterwattenbach. The main


123

bentonite, sampled at Hachelstuhl, is found in all (or inthe vicinity of) sections. Using the all the time-strati-graphic constraints, we correlate the short reversedpolarity in Bruckberg-Ried to C5ACr, the normalpolarity of Pfaffenzell to C5ADn and the reversedpolarity R1 in Unterwattenbach to C5ADr. Thesecorrelations imply that the Hachelstuhl bentonite oc-curs in the lower part of chron C5ADn, which closelycorresponds to our 40Ar/ 39Ar estimated age of 14.55(±0.19) Ma.

The middle interval: the Brock horizon

The sections in the middle part belong to the upperpart of the NV (Hangender Teil) and the Zwischen-mergel (ZM) and include Unterempfenbach, Kreut,Furth 1 and 2 and the upper part of Unterwattenbach(R1 to R2). We correlate the long upper normalpolarity of Unterempfenbach to C5Bn.1n and thelowermost to C5Bn.2n. Since Furth 1 represents achannel Þll postdating the deposits of Furth 2 section,

Fig. 6 Lithology columns, paleomagnetic results and polaritycolumns for the eastern Bavarian Molasse sections. Theblackand grey dots in the declination and inclination records representquality 1 ChRM directions for alternating Þeld (AF) and thermal(TH) demagnetized samples, respectively. The white dotsrepresent quality 2 type of demagnetizations for both AF and

TH treated samples. In the polarity columns, black (white) zonesindicate normal (reversed) polarity and grey shaded zonesuncertain polarity. The stratigraphic position of Furth 1 withrespect to Furth 2 section is indicated. For explanation oflithology columns, see legend in Fig.2


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the reversed polarity in this section is therefore cor-related to chron C5ADr. The upper reversed polarityof Furth 2 correlates to CBn.1r and R2 in Unterwat-

tenbach to the top of C5Br. This implies that theBrock horizon, which is only found in Kreut section ina gravel interval (incising the underlying ZM) where

Fig. 7 Lithology columns, paleomagnetic results and polaritycolumns for the eastern Bavarian Molasse sections (see captionto Fig. 6). Inset: equal area projection of quality 1 ChRMdirections (with MAD values <15) for all sections. The 95%conÞdence ellipse for the normal and reverse directions is

indicated. Statistical information includes number of samples(No.), declination (Dec.), inclination ( Inc.), FisherÕs parameter(k) and radius of the 95% conÞdence cone (a95). For explanationof lithology columns, see legend in Fig.2


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no polarity direction could be obtained, either occursin chron C5Bn.1n or in C5ADr. Past paelomagneticstudies on Ries suevites have indicated a reversedpolarity signal (Pohl 1965; Petersen et al.1965) hencesuggesting that the Brock horizon falls within chronC5ADr, yielding an age between 14.581 and14.784 Ma (average 14.68 ± 0.1 Ma). This age more orless coincides within error to our estimated 40Ar/ 39Ar

age of 14.88 ± 0.11 Ma for the Ries suevites fromO¬ttingen.

The oldest interval: OSM C biozonation

The sections in this interval belong to the lower part ofthe NV and Su¬§wasser Kalk (SK) and include Furth,Sandelzhausen, Puttenhausen and the middle and

Fig. 8 Calibration of eastern Bavarian magnetostratigraphic records to the Astronomical Tuned Neogene Time Scale (ATNTS04) ofLourens et al. 2004. The 40Ar/ 39Ar ages of the Ries and Hachelstuhl glasses are displayed with their 1r analytical error


123

lower part of Unterwattenbach (R3 to R5). The un-clear magnetic polarity records hamper a straightfor-ward correlation to the ATNTS04. Nevertheless, usingthe biostratigraphy (OSM C + D fauna) of the sec-tions, the uppermost normal level in the long Furthsection is correlated to C5Cn.1n. The small-mammalfauna of Sandelzhausen and uppermost part of Put-tenhausen show similar characteristics and all coincidewith a normal polarity interval, which possibly corre-lates to C5Cn.2n. The reversed zone R4 in Unterwat-tenbach correlates to C5Cn.2r. The unclear polarity inthe lower part of Puttenhausen prohibits any reliablecorrelation to the ATNTS04.

Discussion

Pre-Riesian hiatus

It must be noted that our magnetostratigraphic cor-relations to the ATNTS04 represent best-Þt correla-tions and could be interpreted differently. Thecorrelations established for the youngest and middleintervals seem rather robust and are supported bylithostratigraphic Þeld observations as well as40Ar/ 39Ar ages for the main-bentonite sampled atHachelstuhl.

The most obvious feature of this correlation, how-ever, is the absence of the long reversed interval ofC5Br in the sections of eastern Bavaria. This absencecould be explained by the presence of a sedimentarygap, the so-called pre-Riesian hiatus. Indications of ahiatus have been found in the sedimentary record atthe northern margin of the Bavarian part of the Mo-lasse Basin with up to 150 m deep erosional channels(Birzer 1969) and as a turnover in the mammal recordthroughout the basin (Bo¬hme et al. 2002). This turn-over is contemporaneous with a stratigraphic gap be-tween biostratigraphic units OSM C + D and OSM E(Fig. 3). Similarly, several sedimentary gaps, whichmay be associated with the hiatus, are present in sec-tions in the Swiss Molasse (Kempf et al.1997). In theeastern Alpine area several hiatuses have been recog-nized in the Austrian part of the Molasse Basin (Ro¬glet al. 2002) and in other Austrian Neogene basins(Ro¬gl et al. 2006) attributed to the Styrian phase ofStille (1924). Nevertheless, since the magnetostrati-graphic records of our sections in this time-intervalare not well-constrainedÑespecially the Puttenhau-sen section, which we aim to re-sample and extendupward in the near futureÑwe do not exclude thatinconsistencies may be present in the calibration to theATNTS04.

Biostratigraphic implications

Our best-Þt magnetostratigraphic correlation allowscalibration of the micro-mammal faunaÕs of the easternBavarian Molasse with the ATNTS04 enabling acomparison with the faunal records from the SwissMolasse Basin. We refrain from using MN ÔÔzonesÕÕ forcomparisons because these ÔÔzonesÕÕ are deÞned byevolutionary levels, faunal distribution patterns andpresence of important taxa (Bolliger 1997; Heissig1997) of which the latter two may vary geographicallydue to ecological differences and migration events.Therefore, no reliable comparisons can be madeespecially when aiming at understanding detailed fau-nal changes in a short time interval, such as the case inthis study. Hence, only the local OSM zones are usedfor comparison.

The OSM F faunal unit in eastern Bavaria corre-sponds to reference level Ru¬mikon in the Swiss Mo-lasse Basin, which correlate to C5ADn (Ka¬lin andKempf 2002). This agrees with our magnetostrati-graphic calibration of the sections between the Brockhorizon and the main bentonite layer. The fauna ofOSM EÕ, stratigraphically below the Brock horizon, issimilar to the Chatzloch fauna which is correlated toC5ADr (Ka¬lin and Kempf 2002) and also agrees withour calibration of the Furth 1 section.

The micro-mammal fauna unit OSM E, found in ourstudied Furth 2 section, has similar faunal character-istics as Frohberg locality in the Swiss Molasse (Bol-liger 1992). The magnetostratigraphic calibration tochron C5Bn.1r agrees well with Frohberg, which cor-relates to the same chron (Kempf et al. 1997; Schlu-negger et al.1996). Based on the evolutionary stage ofMegacricetedon aff. bavaricus, the younger part offaunal unit OSM C + D deÞned in localities Sandelz-hausen and upper part of Puttenhausen (PUT E) isvery similar to the Swiss faunaÕs from Tobelholz andVermes 2 (Reichenbacher et al. 2005). However, therelationship of Toblholz and Vermes 2 to the TobelHombrechtikon fauna, which is magnetostratigraphi-cally correlated to chron C5Bn.2 (Ka¬lin and Kempf2002), is unclear because of the lackM. aff bavaricus

(Ka¬lin 1997). Therefore, the magnetostratigraphiccalibration of the Swiss localities is not reliable.

The older OSM C + D fauna is found in the Put-tenhausen section (PUT classic), and corresponds tothe Swiss reference fauna of Vermes 1, also deÞned inthe magnetostratigraphic calibrated locality of Mar-tinsbru¬nneli (Kempf et al. 1997; Heissig 1997). Mar-tinsbru¬nneli and the fauna of Hu¬llistein are situatedbelow the Hu¬llistein marker bed, which coincides withthe boundary of C5Br/C5Cn.1n (Kempf et al. 1997).


123

Based on the evolutionary stage ofM. aff. bavaricus,however, Hu¬llistein is clearly older than Sandelzhausenand all Puttenhausen levels (Fig.3). At this momentwe correlate the younger faunas of OSM C + D toC5Cn.1r/C5Cn.1n yielding an age discrepancy of about500 ka with respect to the Swiss calibration. This largediscrepancy can be related to the poor quality of thePuttenhausen polarity record and hence an incorrectcalibration to the ATNTS04.

Comparison with 40Ar/ 39Ar ages

The magnetostratigraphic calibrated age for the Ha-chelstuhl bentonite to the lower part of chron C5ADn(~14.5 Ma) of the ATNTS04 closely coincides with our40Ar/ 39Ar estimated age of 14.55 (±0.19) Ma. So far,the main bentonite horizon in eastern Bavaria has onlybeen dated by Þssion track analysis. Storzer andGentner (1970) sampled the main bentonite at Main-burg and Unter-Haarland and found Þssion track agesof 14.6 ± 0.8 and 14.4 ± 0.8 Ma, respectively. Thesedata are very similar to our estimated age of14.55 ± 0.19 Ma. Since Þssion track and40Ar/ 39Ardating are two independent radiometric dating meth-ods, which not only differ in analytical approach butalso in decay systematics and constants, the consistencybetween both data sets suggests that the Hachelstuhlbentonite age is very robust.

Although within error, the estimated 40Ar/ 39Ar agefor the Ries glass is about 200 kyr older than themagnetostratigraphic calibrated ATNTS04 age of~14.68 ± 0.10 Ma for the Brock horizon. However, themagnetostratigraphic calibration of the Ries impactsupports earlier calibration efforts established in theSwiss Molasse (Schlunegger et al.1996). Similar to themain bentonite, our ATNTS04 age for the Ries event isidentical to Þssion track ages: Storzer et al. (1995)analyzed 15 Ries glasses, yielding a mean age of14.68 ± 0.25 Ma. Hence, the magnetostratigraphiccalibrated age for the Brock horizon seems convincing.

Our 40Ar/ 39Ar age estimate of 14.88 ± 0.11 Macompares well with mean argon ages determined byStorzer et al. (1995). They evaluated 15 Þssion trackages (mean 14.68 ± 0.25 Ma) and 12 argon ages (mean15.14 ± 0.51 Ma) of Ries suevite and 7 Þssion trackages (mean 14.82 ± 0.18 Ma) and 17 argon ages (mean14.82 ± 0.32 Ma) of moldavites. All weighted meansfor both the different impact glasses and methods are,within error, identical to our results.

In contrast, recently published 40Ar/ 39Ar ages ofRies ejecta are up to 550 ka younger. Schwarz andLippolt ( 2002) report plateau ages of 14.50 ± 0.16 and14.38 ± 0.26 Ma for tectites from Bohemia and Lute-

tia, respectively. Laurenzi et al. (2003) obtained anaverage laser total fusion age of 14.34 ± 0.08 Ma forBohemian and Moravian moldavites while Buchneret al. (2003) found a 40Ar/ 39Ar laser total fusion valueof 14.32 ± 0.28 Ma for a Ries suevite. The large agedifference may partly be explained by the differentmonitor standards and/or their assumed ages used inthese studies. For example, Laurenzi et al. (2003) usean age of 27.95 Ma for the FCT standard while Sch-warz and Lippolt ( 2002) use a HD-B1 bt standard thatis not cross-calibrated to the Fish Canyon standard(Laurenzi et al. 2003). Similarly, Buchner et al. (2003)used a third standard (TCs-1) with an age of27.92 Ma.

Buchner et al. (2003), however, explain that thepublished suevite ages older than 14.3 Ma could be dueto the presence of relict minerals in the Ries glass,derived from the Paleozoic basement. Although thisexplanation could account for the 200 kyr offset be-tween our 40Ar/ 39Ar and ATNTS04 ages, severalobservations argue against it. Relict minerals have, toour knowledge, not been reported from moldavites andtheir absence is convincingly demonstrated in theextensive data set of Laurenzi et al. (2003) and Sch-warz and Lippolt ( 2002). Laurenzi et al. (2003) ob-tained 76 plateau ages from analysis of seven differentmoldavites, of which the ages varied by 400 ka only. Itis therefore highly unlikely that such age uniformitycan be obtained with relict minerals. Moreover, theSchwarz and Lippolt step-heating plateaus are veryuniform and the high-temperature steps do not indicatethe presence of relict minerals.

Finally, moldavites and Ries glasses originated fromdifferent source rocks, namely Miocene sedimentarycover and Paleozoic basement, respectively (Graupet al. 1981). In the case of inherited argon, Ries glassesshould yield different and characteristic age spectra forthe two rock types, which this is not the case (Storzeret al. 1995, see above). We thus conclude that althoughthe different monitor standards could partially accountfor the inconsistency between the different ages ob-tained for Ries ejecta, a satisfactory explanation forthis inconsistency remains unresolved.

Conclusions

Paleomagnetic analysis was performed using two dif-ferent demagnetization methods: AF and TH demag-netization. The AF method revealed both normal andreverse polarities but proofs not to be reliable forsamples with the primary ChRM carried by Þne-grained hematite.


123

The magnetostratigraphic calibration of the studiedsections to the ATNTS04 gives reliable results for theyoungest and middle intervals. The three biostrati-graphic units OSM E, EÕ and F are correlated to chronsC5Bn.1r, C5ADn and C5ADn, respectively. This isconÞrmed by radioisotopic dating of the Brock horizon(14.88 ± 0.11 Ma) and the main bentonite layer(14.55 ± 0.19 Ma).

So far, the results for the older interval (OSMC + D) are ambiguous. A long hiatus (pre-Riesianhiatus) of several hundred thousand years separatingOSM C + D from OSM E may be present within chronC5Br. However, we do not exclude that the poorquality of the Puttenhausen polarity record resulted inan incorrect calibration to the ATNTS04.

Acknowledgments We thank Daniel Ka¬lin and Johann Hohe-negger for critically reviewing the manuscript. Georg Bauer(Ziegelwerke LeipÞnger-Bader, Puttenhausen), Franz LuderÞn-ger (Isarkies GmBH, Unterwattenbach) and Helmut Eichstetter(Eichstetter GmBH, Furth) are thanked for working permissionsand technical help in the gravel and clay pits. Albert Ulbig isthanked for providing the glass samples from Hachelstuhl. Thisproject was supported by DFG grant BO 1550/7Ð1.

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