Biostratigraphy and paleoecology of benthic ...

N. Jb. Geol. Paläont. Abh. 2009, vol. 254/1-2, p. 41 – 61, Stuttgart, October/November 2009, published online 2009

Biostratigraphy and paleoecology of benthic foraminifera from the Eggenburgian “Ortenburger Meeressande” of southeastern Germany (Early Miocene, Paratethys) Martina Pippèrr, Munich and Bettina Reichenbacher, Munich With 8 figures and 2 tables

PIPPÈRR, M. & REICHENBACHER, B. (2009): Biostratigraphy and paleoecology of benthic foraminifera from the Eggenburgian “Ortenburger Meeressande” of southeastern Germany (Early Miocene, Paratethys). – N. Jb. Geol. Paläont. Abh., 254: 41– 61; Stuttgart. Abstract: A modern taxonomic and semi-quantitative assessment of the benthic foraminifera from the “Ortenburger Meeressande” is presented based on well-preserved material consisting of 5510 specimens and representing 77 different species from the Straß borehole and the Maierhof section. The overall composition of the foraminiferal assemblage along with the occurrence of certain index forms are indicative of an Eggenburgian age and can also be used to discriminate the middle and late Eggenburgian at Straß, whereas at Maierhof only the middle Eggenburgian is present. Lenticulina buergli (WENGER) is recorded for the first time for Eggenburgian sediments of the South German Molasse Basin. This species is interpreted as an index fossil for the Eggenburgian stage in the Molasse Basin, in Austria as well as in South Germany. We discuss the assemblages from Straß and Maierhof with regard to salinity, temperature, oxygenation, turbulence, and water depth. For the Straß succession we estimate a water depth of approximately 20-50 m, while a water depth lower than 20 m is suggested for the Maierhof section. The abundance of oxic foraminifera in the middle Eggenburgian suggests well-oxygenated conditions of bottom waters. The upper Eggenburgian is characterized by a shift in abundance of oxic and suboxic foraminifera. This change coincides with a shift towards less water turbulence and might be related to a slightly lower oxygenated environment because of an overall deepening. We do not recognize an erosional surface at the base of the upper Eggenburgian, as has been described from the Central Paratethys. Key words: Upper Marine Molasse, Eggenburgian, benthic foraminifera, biostratigraphy, paleoecology, paleogeography.

1. Introduction The Eggenburgian represents a regional chronostratigraphic stage of the Early Miocene in the Central Paratethys (Fig. 1). The lower and upper boundaries of this stage are traditionally based on the first appearance and last occurrence of a typical bivalve assem-

DOI: 10.1127/0077-7749/2009/0003

blage that is characterized by large-sized pectinids and cardiids (STEININGER & SENES 1971, MANDIC & STEININGER 2003). The Eggenburgian type section is located in the Austrian Molasse Basin, close to the city of Eggenburg. The deposits in the area of the type section have been subdivided into a lower, middle, and upper Eggenburgian based principally on differences

0077-7749/09/0003 $ 5.25 © 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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Fig. 1. Early Miocene geochronology and chronostratigraphy, geomagnetic polarity chrons (all after LOURENS et al. 2004) and correlation with Central Paratethys chronostratigraphy (after PILLER et al. 2007).

in the composition of the bivalve fauna (PILLER et al. 2007). Benthic foraminifera represent another group of fossils that have variously been shown to be of some significance with regard to the biostratigraphic circumscription of the Eggenburgian (WENGER 1987a, CICHA et al. 1998). In contrast to the bivalves, they have rarely been used concerning the biostratigraphic subdivision of the Eggenburgian stage. In addition, although it is well-known that benthic foraminifera represent excellent proxy indicators for certain paleoenvironmental conditions (e.g. ALTENBACH et al. 2003, HOLCOVÁ 2008), this group of organisms was rarely used to assess the paleoenvironment of Eggenburgian paleoecosystems. Apart from the Austrian Molasse Basin and the area of the type section, fossiliferous Eggenburgian sediments have been described from the eastern South German Molasse Basin. These deposits have been named the “Ortenburger Meeressande”, and consist of fine- to coarse-grained sands containing a marine bivalve fauna and a highly diverse foraminiferal community (HAGN et al. 1981). Historically, the “Ortenburger Meeressande” were exposed in several quarries and sand pits; the most famous of these is the Maierhof section in the vicinity of the town of Ortenburg (Fig. 2). This location is today referred to as the locus typicus for the “Ortenburger Meeressande”. Foraminifera from the Maierhof section were first described by EGGER (1857), and later re-visited by HAGN (1960) and HAGN et al. (1981). Based on the foraminiferal assemblages, the Maierhof section was interpreted as

a coastal environment of about 20 m water depth, and correlated with the middle Eggenburgian (WENGER 1987a). Outcrops exposing the “Ortenburger Meeressande” are no longer available for analysis. A drilling conducted in 2004 at the locality Straß near Ortenburg by the Geological Survey of Bavaria revealed “Ortenburger Meeressande” with a well preserved, rich and diverse benthic foraminiferal assemblage (4074 individuals assignable to 70 different species). In addition, a new material of benthic foraminifera (1436 individuals assignable to 47 different species) was gathered from sediment remains attached to the bivalve fossils from the former Maierhof section (kept in the collections of the Bavarian State Collection of Palaeontology and Geology). We here present (1) a modern taxonomic and semi-quantitative assessment of this new material from Straß and Maierhof, (2) a new biostratigraphic database for the Early Miocene of the Central and Western Paratethys based on the species-composition and proportional abundances of characteristic taxa, (3) assumptions on the paleoenvironmental conditions based on application of the concept of dominant and subsidiary foraminifera with > 10 % abundance, introduced by MURRAY (1991b), and (4) a comparison to contemporaneous sections located in the Central Paratethys. We demonstrate that benthic foraminifera are highly significant biostratigraphical markers that can be used for the definition and distinction of the lower, middle and upper Eggenburgian. The unusual richness and diversity of the assemblages from Straß are indicative of a marginal marine, sheltered habitat that provided optimal environmental conditions for these microorganisms. A slight change towards a less oxygenated environment occurs in the late Eggenburgian and is characterized by the abundance of suboxic foraminiferal species.

2. Geological setting The Molasse Basins of Switzerland, South Germany and Austria together form the Northalpine Foreland Basin, which developed as a result of the tectonic loading of the Alpine orogeny. They form part of the Paratethys realm. The basin infilling, i.e. the Molasse sediments, were deposited from the Late Eocene until the Late Miocene. In the Swiss and South German Molasse Basin, the Molasse sediments contain two transgressive-regressive megacycles, each characterized by deposits of marine sediments at the base and terrestrial sediments or an erosion phase above

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

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Fig. 2. (A.) Schematic map showing a part of the South German and Austrian Molasse Basin. Important sections and lithostratigraphic formations are indicated. (B.) Geographic location of the Straß and Maierhof sections.

(e.g. HOMEWOOD et al. 1986, LEMCKE 1988, SCHWERD et al. 1996). The “Ortenburger Meeressande” belong to the transgressive part of the second megacycle and represent the basal deposits of the Upper Marine Molasse. The whole succession of the Upper Marine Molasse is Eggenburgian and Ottnangian in age (= late Aquitanian and Burdigalian of the Mediterranean chronostratigraphy; see Fig. 1); the “Ortenburger Meeressande” are Eggenburgian in age. The Eggenburgian sea flooded the Austrian and South German Molasse Basin from the Eastern Paratethys and the western Mediterranean Sea (RÖGL & STEININGER 1983). However, the seaway to the West probably did not exist before the middle Eggenburgian (WENGER 1987 b, PILLER et al. 2007). Eggenburgian deposits are widespread in the eastern part of the South German Molasse Basin and in the Austrian Molasse Basin, whereas they are limited to a narrow

zone proximal to the Alps in the western South German Molasse Basin (LEMCKE 1973, WENGER 1987b, KUHLEMANN & KEMPF 2002). Conversely, Ottnangian sediments are widespread throughout the Austrian and South German Molasse Basin and indicate the existence of seaways to the West and East, as well as a progradation to the North (LEMCKE 1973, WENGER 1987b, KUHLEMANN & KEMPF 2002). Due to the existence of regional differences in facies composition, different informal and formal names have been given for the Upper Marine Molasse succession based on individual local exposures. Examples are the “Eggenburg-Sand-Mergel-Folge” (e.g. in the Traunand Sur section) and the “Ortenburger Meeressande” in the South German Molasse Basin, or the Hall Formation in Austria (FAUPL & ROETZEL 1987, WAGNER 1998).

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Fig. 3. Profiles of the drilling Straß (by courtesy of D. Jung and B. Schäfer, Bavarian Geological Survey) and Maierhof (modified after HAGN et al. 1981), with sample numbers and macrofossils.

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

3. Geographic location and description of the Straß and Maierhof sections The Straß and the Maierhof sections are situated in the eastern part of the South German Molasse Basin (Lower Bavaria), close to the southwestern margin of the Bohemian Massif, near the small town of Ortenburg (Fig. 2). The Straß borehole was drilled in a gravel pit located approximately 6 km northeast of Ortenburg (R 45 94 800, H 5383017); the Maierhof quarry was located approximately 3 km to the north of Ortenburg (R 45 89 570, H 53 82 420). The Straß borehole reached a maximum depth of 24 m (Fig. 3). The “Ortenburger Meeressande” are 10.3 m thick and characterized by fine shelly sands in the lower part, followed by marls and fine sands. The core has yielded a diverse bivalve assemblage recently described by SCHNEIDER (2008). The “Ortenburger Meeressande” are overlain by alternating Ottnangian sands and marls (“Glaukonitsande und Blättermergel”) without a visible unconformity. At Maierhof, the “Ortenburger Meeressande” reach a thickness of approximately 7 m, and are represented by medium- to coarse-grained sands containing bioclastic shelly layers and abundant bivalves, including the remains of the character fossil Oopecten gigas gigas (SCHLOTHEIM) (Fig. 3; see also HAGN et al. 1981, UNGER 1984). The sands are cut by an erosional unconformity that is marked by a decimeter-thick residual soil. Marine deposits of the Ottnangian “Glaukonitsande und Blättermergel” follow above.

4. Material and methods The material from Maierhof and Straß is kept in the Bavarian State Collection for Palaeontology and Geology, Munich and the Bavarian Geological Survey. Photographs were taken at the Zoological State Collection at Munich, using a scanning electron microscope, model LEO Gemini 1430 VP. Straß borehole – Samples are preceded by SP; the numbering of the sampled layers in the profile starts from top (SP-1) to bottom (SP-13, see Fig. 3). We focus on samples SP-13 and SP-12, of which a large amount of sediment was available. SP-13 and SP-12A represent the fine shelly sands in the lower part of the “Ortenburger Meeressande”, and originate from a core section of approximately 7.5 m length. Sample SP-12B consists of about 1.5 m of marls. The sediments were wet-sieved down to a mesh size of 150 µm. Residues down to a grain size of 500 µm were

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picked completely; smaller grain-sizes were picked from representative splits of the samples using a stereoscope. Consequently, the species diversity of the fractions smaller and larger than 500 µm are mentioned separately. Foraminifera from samples SP-11 to SP-1 were provided by the Bavarian Geological Survey (LfU). This material is not quantified because the number of specimens is too low. SP-11 through SP-1 are included in our taxonomic and biostratigraphic considerations, but these data cannot be used for a detailed palaeoecological evaluation. Maierhof – All samples come from sediment attached to oysters and other bivalves (e.g. Oopecten gigas gigas) from the Maierhof section (collection material). Sample ELW represents a composite sample from the oyster horizon at the section base, while samples 07 and 08 correspond to the shelly horizons in the upper third of the profile (Fig. 3). Sediments were washed under running water with 63 µm mesh sieves and dried; microfossils were hand-picked under a stereomicroscope. Methods – Foraminifera from Straß and Maierhof were identified to species-level (as far as possible), sorted and counted. The widely used Fisher’s α index (FISHER et al. 1943) was calculated to assess species diversity. Biostratigraphic considerations are based on all data sources available about the stratigraphic distribution of Oligocene and Early Miocene foraminifera (WENGER 1987a, 1987b, CICHA et al. 1998, RUPP & HAUNOLD-JENKE 2003, PIPPÈRR et al. 2007a, b). In order to reconstruct the paleoecological conditions, we have calculated the relative abundance (in percentages) of the individual benthic foraminiferal genera in each of the samples (for the Straß material, the size-fractions 500 µm have been calculated separately). We use the actualistic abundance-concept of MURRAY (1991b, 2006) for a paleoecological interpretation. Due to this concept, all species should occur in low abundances in a wide range of environmental conditions, but in higher abundances towards (near-) optimum conditions. As a result, our suggestions with regard to the paleoenvironmental conditions are primarily based on the dominant genera and taxa displaying > 10 % abundance, while the subsidiary taxa are less regarded. Data concerning the environmental ranges of modern foraminiferal genera and species were obtained from MURRAY (1973, 1991 a-b, 2006), KAIHO (1994, 1999), SEN GUPTA (1999), and RÖGL & SPEZZAFERRI (2003).

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Fig. 4 (Legend see p. 47)

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

5. Results 5.1. Benthic foraminiferal assemblages from Straß

The foraminiferal assemblages gathered from samples SP-13 and SP-12 are rich in species and specimens, and generally excellently preserved and large-sized. A total of 4074 individuals assignable to 70 different species have been identified (Table 1, Figs. 4-6). Almost all species belong to the hyaline group; two species (Textularia gramen, Spirorutilus pectinatus) represent the agglutinated taxa. SP-13 has yielded the most diverse association, containing 1914 individuals assignable to 60 species, but also the assemblages obtained from SP-12A and -B are relatively rich, containing 981 individuals assignable to 46 species and 1179 individuals assignable to 41 species, respectively. Fisher’s α index for these samples ranges between 8 and 12 (see Table 1). Compared with the benthic species, the abundance and diversity of planktonic foraminifera is extremely low (see end of Table 1). Globigerina praebulloides BLOW is the most abundant taxon; followed by Globigerina ottnangiensis RÖGL. In addition to foraminifera, all three samples contain a rich and large-sized ostracod fauna, numerous echinoid spines, several shark and sparid fish teeth, and a few otoliths. In addition, sample SP-12B yielded abundant and well preserved radiolarians. Figure 7 shows the distribution of the most important genera of benthic foraminifera in the three samples. In SP-13 and SP-12A, Lenticulina dominates the large fraction (>500 µm), in which also Elphidium and Porosononion granosum occur, while the small fractions (< 500 µm) contain abundant Elphidium, Elphidiella, Porosononion granosum, Nonion commune, and Ammonia. A marked decrease in abundance of Nonion commune and simultaneous increase of Elphidium and Ammonia occurs from SP-13 to SP-12A. Furthermore, Reussella spinulosa, Virgulopsis tuberculatus and cibicidids are more numerous in SP-13 than in SP-12A. The foraminiferal assemblage of

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SP-12B, however, is distinctly different from that of the other samples: Lenticulina is the only taxon in the size fraction >500 µm, and Nonion commune is dominant in the small fraction. Compared with the other samples, the proportional abundance of Elphidium is distinctly lower and Porosononion granosum is missing. Relatively common genera include Ammonia, Elphidiella and Bulimina; the latter taxon has only occasionally been recorded for the underlying samples. Samples SP-11 and SP-9, which belong to the uppermost part of the “Ortenburger Meeressande”, are dominated by large and well preserved tests of elphidiids, cibicidids, Ammonia, Nonion, Bulimina, Nodogenerina? perscripta, Siphonodosaria? consobrina, and Bolivina antiqua; subordinate taxa include polymorphinids, Laevidentalina communis, Lenticulina inornata, Fursenkoina acuta, Asterigerinata planorbis, and Pararotalia batavensis. SP-8 has only yielded a few foraminiferal species and individuals, which mostly belong to Ammonia, elphidiids and cibicidids. Foraminifera are absent in the overlying “Glaukonitsande und Blättermergel”; only SP-1 contains many well preserved Ammonia tests. 5.2. Benthic foraminiferal assemblages from Maierhof

The samples from Maierhof contain a benthic foraminiferal assemblage that is again rich in species and specimens, and characterized by large and well preserved tests, but Fisher’s α index only ranges between 6 and 8. A total of 1436 individuals, probably assignable to 47 different species, have been counted (Table 1). Planktonic foraminifera are extremely rare; only sample 08 yielded two individuals of Globigerina praebulloides. Moreover, samples 07 and 08 (from the upper third of the section) contain abundant and largesized ostracods, as well as balanid plates, echinoid spines and, less frequently, brachiopod fragments. The oyster horizon and the sands at the base of the “Ortenburger Meeressande” (sample ELW) yielded

Fig. 4. (1) Elphidium felsense PAPP; Straß SP-12A. (2) Elphidium crispum (LINNÉ); Straß SP-13. (3a) Elphidium ortenburgense (EGGER); Maierhof 08. (3b) Detail of 3a. (4) Elphidium ortenburgense (EGGER); Maierhof 08. (5) Elphidium cf. ortenburgense (EGGER) – similar to Elphidium fichtelianum praeforme PAPP, from which it differs by a less laterally compressed test; Straß SP-12B. (6) Elphidium subtypicum PAPP; Straß SP-12A. (7) Elphidium cf. flexuosum (D’ORBIGNY); Straß SP-12A. (8) Elphidiella dollfusi (CUSHMAN); Straß SP-12B. (9) Elphidiella cf. cryptostoma (EGGER); Straß SP-13. (10-11) Elphidiella cryptostoma (EGGER); Maierhof 08. (SEM).

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Table 1. Distribution and abundance of benthic foraminifera in Straß and Maierhof. Ecological data and preferred depth range for selected genera (and species) are indicated according to MURRAY (1973, 1991a, 1991b, 2006) and RÖGL & SPEZZAFERRI (2003). The foraminifera from the Straß samples were subdivided into tests smaller and larger 500 µm. Lenticulina spp. = L. calcar (LINNÉ), L. cultrata (MONTFORT) sensu WENGER (1987a), L. cultrata group sensu CICHA et al. (1971), L. limbosa (REUSS), and less frequent L. inornata (D’ORB.) and L. paupercula (REUSS).

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

Table 1 cont.

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Fig. 5 (Legend see p. 51)

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

210 individuals assignable to 24 species; the most frequent taxa are Bolivina trunensis, Elphidium, Elphidiella, and Asterigerinata planorbis (Table 1 and Fig. 7). Samples 07 and 08 contain 328 individuals assignable to 25 species, and 898 individuals assignable to 37 benthic foraminiferal species, respectively. The proportional abundance of Asterigerinata and Elphidium is distinctly higher in samples 07 and 08 (Fig. 7), where these taxa represent the dominant forms. Other relatively frequent taxa are cibicidids, and in sample 08 also Reussella spinulosa is quite abundant. In contrast to Straß, however, Lenticulina, Nonion and Ammonia are rare in all samples from Maierhof (Fig. 7). 5.3. Biostratigraphy

Basics. – The Eggenburgian in the South German Molasse Basin coincides with a marked change in the composition of foraminiferal assemblages (see WENGER 1987a) that has parallels with the change in the composition of foraminiferal assemblages in the Central Paratethys (see CICHA et al. 1998, HOLCOVÁ 2008). One of the most important Eggenburgian index fossils is Elphidium felsense, which is restricted to that stage (WENGER 1987a, CICHA et al. 1998). Elphidium ortenburgense was historically considered a species restricted to the Eggenburgian (WENGER 1987a), but this form has more recently also been recorded as a rare element (and often displaying abnormal test morphologies) in Ottnangian strata (PIPPÈRR unpublished data). A frequent occurrence of E. ortenburgense, however, can still be considered a characteristic feature of the Eggenburgian (see Table 2). Moreover, a subdivision of the Eggenburgian into a lower, middle and upper segment was proposed based on foraminiferal assemblages from the ~700 m thick Eggenburgian succession within the Traun section (WENGER 1987a). According to WENGER’s analysis, the middle Eggenburgian is characterized by the first appearance of Elphidium felsense and E. ortenburgense, and the last occurrence of both Elphidiella cryptostoma and Bolivina fastigia. In the upper Eggenburgian, Elphidium felsense is absent or rare, and Elphidiella semiin-

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cisa and Bolivina concinna substitute E. cryptostoma and B. fastigia. Usually, E. semiincisa and E. cryptostoma are easily recognized and distinguished from each other, but sometimes morphologically intermediate tests may occur that are more difficult to assign (RUPP & HAUNOLD-JENKE 2003). 5.3.1. Biostratigraphic correlation of the Straß and Maierhof sections

Table 2 summarizes the biostratigraphically most significant benthic foraminiferal species from the “Ortenburger Meeressande” at Straß and Maierhof. Based on the presence of several index fossils, samples SP-13 and -12 (Straß), and ELW, 07 and 08 (Maierhof) can be dated as Eggenburgian with confidence. This is additionally supported by typical assemblages and characteristic abundances of certain species such as Rosalina uhligi austriaca (sensu WENGER 1987a 1) and R. semiporata, which represent frequent species in many Eggenburgian strata, but have rarely been recorded for the Ottnangian and Egerian (Table 2). Moreover, Virgulopsis tuberculatus, Reussella spinulosa and Asterigerinata planorbis are abundant in the Eggenburgian, but rare in Ottnangian and Egerian strata of the South German Molasse Basin (WENGER 1987a, PIPPÈRR et al. 2007a). Also samples Straß SP-11 and SP-9 are dated as Eggenburgian. Despite the fact that some of the index fossils are missing, the occurrence of Elphidium ortenburgense, Elphidiella dollfusi, E. cryptostoma,

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TOLLMANN (1957) introduced Rotorbinella uhligi austriaca as a new subspecies, but this taxon is a synonym of Biapertorbis biaperturatus POKORNY 1956 (see RUPP & HAUNOLD-JENKE 2003). However, it is likely that the foraminiferal species described as Discorbis uhligi austriacus by WENGER (1987a) is not identical with Biapertorbis biaperturatus. It may represent a new species, but it could also be a morphotype of Ammonia discigera (EGGER) (RUPP & HAUNOLD-JENKE 2003), which we cannot decide at present. With regard to the taxonomy we follow CICHA et al. (1998), according to which the specimens described by WENGER (1987a) have to be classified as Rosalina uhligi austriaca.

Fig. 5. (1-2) Elphidiella subcarinata (EGGER); Straß SP-13. (3-4) Porosononion granosum (D’ORBIGNY); Straß SP-13. (5) Protelphidium roemeri (CUSHMAN); Straß SP-13. (6-7) Virgulopsis tuberculatus (EGGER); Straß SP-13. (8) Amphicoryna danuviensis (WENGER); Straß SP-12A. (9) Reussella spinulosa (REUSS); Straß SP-13. (10) Cibicidoides cf. tenellus (REUSS) – umbilical view – transitional form to Cibicides lobatulus (WALKER & JACOB); Straß SP-12B. (11-12) Pararotalia batavensis WENGER – (11) umbilical view, (12) spiral view; Straß SP-13. (SEM).

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Fig. 6. (1) Marginulina hirsuta D’ORBIGNY; Straß SP-12A; LM. (2-4) Lenticulina buergli (WENGER); Straß SP-12B; (2-3) = LM, (4) = SEM. (5) Lenticulina cf. buergli (WENGER); Straß SP-12B; LM. (6) Lenticulina calcar (LINNÉ) – keeled, with distinct spines (some are broken); Straß SP-13; SEM. (7-8a) Lenticulina cultrata (MONTFORT); Straß SP-13; SEM. (8 b) Detail of 8a – aperture with radial slits. (SEM = Scanning Electron Microscope, LM = Light Microscope).

and E. subcarinata provides sufficient evidence in support of Eggenburgian age. Part of the data from Straß indicate a middle Eggenburgian age because SP-13 and SP-12A include

relatively frequent Elphidium felsense. In the overlying layers SP-12B to SP-9, E. felsense is absent, and Lenticulina buergli occurs for the first time in abundance. Moreover, certain species are rare or absent

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

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Fig. 7. Relative abundances (%) of the most important benthic foraminiferal genera in the samples from Straß and Maierhof.

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Table 2. Stratigraphic distribution of benthic foraminiferal species according to WENGER (1987) and CICHA et al. (1998), and abundance of species in Straß and Maierhof (• = 1-4 tests; x = 5-20 tests; ο = > 20 tests;  = further stratigraphic distribution, light grey = infrequent occurrence).

in these samples, while they are abundant in SP-13, SP-12A, and Maierhof (e. g. Pararotalia batavensis, Elphidiella subcarinata, Reussella spinulosa, Virgulopsis tuberculatus). Furthermore, samples SP-12B to SP-9 contain E. cryptostoma in its characteristic form, as well as transitional forms to E. semiincisa. In conclusion, SP-12B most probably correlates with the upper Eggenburgian, which is consistent with an upper Eggenburgian age of L. buergli in the Traunand Sur sections (see below). Above layer SP-9, a distinctly different foraminiferal assemblage (SP-8) has been recovered that contains a low number of species and individuals (predominantly Ammonia, elphidiids and cibicidids). Index fossils are absent, but similar assemblages are well documented from middle Ottnangian strata (WENGER 1987a, PIPPÈRR et al. 2007a, 2007b) and marginal marine environments of the lower Ottnangian elsewhere in the South German Molasse Basin (PIPPÈRR

unpublished data). As a result, we assign the upper part of the Straß section (SP-8 to SP-1) to the Ottnangian without further subdivision (see also Fig. 3). WENGER (1987a) correlated the “Ortenburger Meeressande” of the Maierhof section with the middle Eggenburgian. Our data support this assignment, especially because we found abundant Elphidium felsense in the upper third of the Maierhof section (07, 08). 5.3.2. The index fossil Lenticulina buergli

Lenticulina buergli (= Planularia willingensis sensu BÜRGL 1946, Planularia buergli sensu WENGER 1987a) is regarded as an index fossil for the Eggenburgian stage in the Austrian Molasse Basin because it occurs abundantly in the Hall Formation (“Haller Schlier”) but is absent in older and younger sediments (CICHA et al. 1998, RUPP & HAUNOLD-JENKE 2003).

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

Conversely, L. buergli was previously known only from three locations in the South German Molasse Basin. The layer containing this species in Germany was termed “Planularia buergli horizon”, and was correlated with the lowermost Ottnangian (WENGER 1987a). This interpretation, however, does not concur with the stratigraphic concept of CICHA et al. (1998) and RUPP & HAUNOLD-JENKE (2003), and also stands in contrast to the results presented here (L. buergli occurs in SP-13 and SP-12, both samples are indisputably Eggenburgian in age, see above). WENGER (1987a) noticed difficulties while trying to biostratigraphically classify the “Planularia buergli horizon”, due to the absence or scarcity of index fossils and certain similarities with the Eggenburgian foraminiferal assemblage from the Hall Formation. Despite these discrepancies, WENGER concluded that the deposits are Ottnangian in age based mainly on the presence of the Ottnangian index fossils Bolivina scitula and Amphicoryna ottnangensis. Re-analysis of WENGER’s (1987a) original material yielded a single fragment of Amphicoryna in the “Planularia buergli horizon” from the “Sur-Profil” (sample 5569). The specimen, however, is poorly preserved, and thus we cannot decide as to whether it belongs to Amphicoryna ottnangensis or A. danuviensis. Although the two species are very similar, the latter form differs from A. ottnangensis in having fewer longitudinal ribs and a less well-differentiated aperture. Specimens of A. danuviensis from Straß and Maierhof (see Fig. 5.8) differ markedly from A. ottnangensis, with the exception of a single, unusually large individual that displays a higher number of longitudinal ribs. The fragment in sample 5569 of the “Sur-Profil” obviously belongs to a similar, unusually large specimen, and this could explain the increased number of ribs. Nevertheless, poor preservation (corrosion and missing aperture) and the here recorded intraspecific variability of A. danuviensis render determining this fossil problematic. Also the second index fossil, B. scitula, could not be identified with certainty from the “Planularia buergli horizon” (drilling Ortenburg 1003, sample 220.0 -222.1 m). As a result, our analysis does not support a correlation of the “Planularia buergli horizon” with the Ottnangian. We conclude that Lenticulina buergli can be used as an Eggenburgian index fossil also in the South German Molasse Basin. This corresponds to the data for the Eggenburgian of the Central Paratethys (CICHA et al. 1998, RUPP & HAUNOLD-JENKE 2003). The “Planularia buergli horizon” of the Traun- and

55

Sur sections cannot be considered older than upper Eggenburgian because the underlying strata are evidently upper Eggenburgian in age (WENGER 1987a). Conversely, L. buergli is not restricted to the upper Eggenburgian in the Hall Formation in Austria (CICHA et al. 1998).

6. Discussion 6.1. Paleoecology of the Eggenburgian

The benthic foraminiferal assemblages in the Eggenburgian sediments from Straß and Maierhof are well preserved and not size-sorted, and thus they can largely be viewed as autochthonous. They offer an excellent basis for considerations concerning the palaeoenvironment. We have interpreted the assemblages with regard to salinity, temperature, oxygenation, turbulence, and water depth. These factors, however, are often interrelated, and might not represent all environmental influences (cf. LECKIE & OLOSN 2003). In the Conclusion chapter we provide a suggestion as to how the individual data sets might be used to depict a coherent reconstruction of the palaeoenvironment at Straß and Maierhof. 6.1.1. Salinity

The highest diversity of foraminifera is considered to occur in normal marine conditions (32-37 ‰). Progressively fewer species are observed towards extreme salinity values, and Ammonia and certain species of the genus Elphidium are usually considered as tolerant to salinity fluctuations (SEN GUPTA 1999, MURRAY 2006). Elphidium is abundant in the “Ortenburger Meeressande” at Straß and Maierhof, and Ammonia has frequently been recorded for the samples from Straß (Fig. 7). The Straß and Maierhof samples, however, predominantly contain keeled Elphidium species (e. g. E. crispum, E. felsense, E. cf. flexuosum, E. ortenburgense, E. subtypicum, E. macellum), which prefer normal-marine salinity (MURRAY 1991b, 2006). This fact, along with the high species diversity expressed in Fisher’s α indices of between 6 and 12, suggest normal-marine salinity for the “Ortenburger Meeressande” at Straß and Maierhof. Brackish or hypersaline marginal marine environments should range at Fisher’s α indices of < 6 (MURRAY 2006). The abundance of well preserved radiolarians in sample Straß SP-12B also supports marine conditions.

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The remarkable low number of planktonic foraminifera probably was not caused by salinity fluctuations. A poor planktonic foraminiferal assemblage is also known from the Eggenburgian sediments in the Central Paratethys (WENGER 1987a, CICHA et al. 1998). Therefore, it is likely that the isolated position of the Northalpine Foreland Basin led to a generally restricted planktonic foraminiferal assemblage. 6.1.2. Temperature

Distributional patterns of shallow-water benthic foraminifera (marginal marine and inner shelf) are controlled by water temperature, among other factors; many taxa are known to prefer cold, temperate or warm bottom water. Bottom water temperature is interrelated with water depth, i. e. temperate bottom waters will even occur in tropical areas if the water depth exceeds ~100 m (e. g. MURRAY 2006). The following data on the preferred temperature range of benthic foraminifera is compiled from SEN GUPTA (1999) and MURRAY (2006). The assemblages from Straß and Maierhof are partially dominated by Elphidium species (see Fig. 7); Elphidium assemblages, with or without co-dominance of Ammonia, are typical of cold-temperate to tropical inner shelf environments. Nonion, which is the dominant species in Straß SP-12B, is common in cold to warm bottom waters, and Lenticulina and Elphidiella, which are also abundant in the Straß samples, are known to prefer cold bottom water (cf. MURRAY 2006). In the upper section of Maierhof, however, these latter taxa are relatively rare (Fig. 7); here, the dominant genus is Asterigerinata, which prefers temperate water (MURRAY 2006). This may indicate slightly warmer bottom water temperatures at Maierhof in comparison with Straß, which could have been a result of the lower water depth at Maierhof (see below). 6.1.3. Oxygen

Several benthic foraminifera are known to tolerate low oxygen, but species that are restricted to low-oxygen conditions have not been described to date (MURRAY 2006). In general, calcareous perforate species dominate low-oxygen assemblages, but agglutinated forms may also occur in considerable abundance (BERNHARD & SEN GUPTA 1999). The relationship between calcareous benthic foraminifera and dissolved-oxygen levels in oceanic bottom waters has been defined by the “benthic foraminiferal oxygen index” (BFOI),

subgrouping oxic, suboxic and dysoxic conditions (KAIHO 1994, 1999). Indicators of the oxic group are characterized by planoconvex, biconvex, rounded trochospiral and spherical tests with thick walls and large test sizes (>350 µm) (epifauna in high-oxygen bottom waters). Common representatives include Cibicides, Cibicidoides (cf. KAIHO 1994, 1999), and keeled Elphidium species (cf. RÖGL & SPEZZAFERRI 2003). We suggest that the planoconvex epifaunal Asterigerinata may also belong to the oxic group (see also MARTINS et al. 2007). “Suboxic A” indicators are small-sized specimens of oxic taxa. “Suboxic B” indicators include Lenticulina and Nonion (cf. KAIHO 1994, 1999), and probably also Bulimina elongata/B. schischkinskaye, which are common in low-oxygen environments (cf. RÖGL & SPEZZAFERRI 2003). Dysoxic indicators are thin-walled, small, elongate and flattened tests (infauna in high-oxygen bottom waters); common representatives include Bolivina, but not all species of this genus may indicate oxygen depletion (BERNHARD & SEN GUPTA 1999). The proportional abundance of oxic and suboxic taxa in the “Ortenburger Meeressande” from Straß depicts a slight decrease in oxygenation of the bottom waters section-upwards (see Fig. 7). At base (SP-12A), Elphidium (oxic) is dominant (34 %), whereas Nonion (suboxic B) is comparatively rare (7 %). Above (SP12B), an inverse pattern is recognizable: Nonion is dominant (35 %), Elphidium less frequent (10 %) in the small fraction and absent in the large fraction, and Bulimina elongata (suboxic B) occurs in abundance for the first time. Such a composition of oxic and suboxic taxa is typical for bottom water with a slightly reduced oxygen content (= low oxic, 1.5-3.0 ml/l, according to KAIHO 1994). This interpretation is additionally supported by the species diversity, which is slightly reduced in SP-12B compared with SP-12A (Table 1). Possible reasons for the oxygen depletion may have been intensified flux of organic debris as a result from increased freshwater input or the appearance of a sill that inhibited recharge of aerated water and thus led to stratification (cf. BERNHARD & SEN GUPTA 1999). Furthermore, open continental shelves of large enclosed or semi-enclosed basins with a total depth of 20 m) to bathyal environment was suggested (RÖGL & SPEZZAFERRI 2003). We conclude that the Eggenburgian sediments at Straß and Maierhof were deposited in a shallow marine environment. An infralitoral environment with water depths between 20 and 50 m is likely for Straß, whereas the foraminiferal assemblage from Maierhof is probably indicative of a lower water depth (< 20 m). It is known from modern environments that a distinct decrease in species diversity occurs in nearshore waters with a depth shallower than 15 m (SEN GUPTA 1999). Since the recorded species diversity at Maierhof is comparable to that at Straß, a water depth 100 m (MURRAY 2006); this species is a common constituent of the foraminiferal faunas from the Hall Formation and the Traun- and Sur sections. Further strengthening the existence of a uniform foraminiferal bioprovince is the fact that the Eggenburgian foraminiferal assemblages from Maierhof and Straß correspond well with those from the shallow water deposits of the Eggenburg Group (Fels-, Loibersdorf-, Burgschleinitz-and Gauderndorf-Formation) in Austria (see TOLLMANN 1957, PAPP 1963, STEININGER 1971, CICHA et al. 1971). Species that occur in both regions include, among others, Elphidium felsense, E. ortenburgense, Elphidiella cryptostoma, E. dollfusi, E. subcarinata, Porosononion granosum, and Reussella spinulosa.

7. Conclusion

burger Meeressande”. This age interpretation is also supported by the overall structure of the foraminiferal community, which differs distinctly from Egerian and Ottnangian foraminiferal assemblages with regard to species composition and proportional abundances. Furthermore, we are able to discriminate the middle from the upper Eggenburgian at Straß, whereas at Maierhof only the middle Eggenburgian is present. The absence of upper Eggenburgian deposits at Maierhof may be due to the prominent erosional unconformity (see Fig. 3), which is also reflected in the lower thickness (7 m) of the Eggenburgian deposits at Maierhof in comparison to those at Straß (10,3 m). The subdivision of the Eggenburgian into three foraminifera-based sub-stages, which was first proposed by WENGER (1987a), probably correlates with the bivalve-based Eggenburgian sub-stages in Austria sensu PILLER et al. (2007). The Eggenburgian benthic foraminiferal assemblages from Straß and Maierhof document (1) paleoecological differences between the two sections (water depth, temperature of bottom water), and (2) a change in the environmental conditions during the Eggenburgian (oxygenation, turbulence).

Benthic index fossils in the Maierhof and Straß sections indicate an Eggenburgian age for the “Orten-

(1) Maierhof was located in a marginal-marine depositional area (see Fig. 8), for which we suppose a

Benthic foraminifera from the Eggenburgian “Ortenburger Meeressande”

water depth below 20 m based on the composition of the foraminiferal fauna. The foraminiferal fauna from the Straß section, located to the east of Maierhof (Fig. 2B), is indicative of a slightly deeper and less turbulent environment than at Maierhof (between 20 and 50 m). This corresponds well with the more finegrained sediments at Straß (Fig. 3), and is most likely explained by the presence of a heterogeneous coastline and the position of Straß in the centre of a coastal bay (see Fig. 8). (2) The environmental change towards a situation with lower oxygenation and lower turbulence during the Eggenburgian at Straß may have been caused by an overall deepening. Likewise, a deepening-upward trend was reported for the Eggenburgian succession at Maierhof (WENGER 1987a). Our data indicate that the deepened environment persisted at Straß throughout layers SP-11 and SP-9, and thus must have persisted into the late Eggenburgian. This means that, in this succession, we do not recognize an erosional surface at the base of the upper Eggenburgian as described from the Central Paratethys (PILLER et al. 2007). It is possible that no regression occurred due to synsedimentary local tectonics, because the here considered sites are located in the fault zone of the “Ortenburger Senkungsfeld” (see UNGER 1984). We, however, cannot preclude the possibility of a sampling bias because we did not have the opportunity for high resolution sampling. In all, the Eggenburgian strata yielded approximately 77 benthic foraminiferal species. Both Maierhof and Straß contain extraordinarily species-diverse assemblages with high abundances. This is rather uncommon for marginal-marine environments. The majority of such environments have values of Fisher’s α of < 6 (MURRAY 2006), whereas the Fisher’s α-index of our samples ranges between 6 and 12. Similarly (or even more) diverse assemblages were described from approximately time-equivalent infralittoral to near-shore deposits in the Aquitaine Basin (CAHUZAC & POIGNANT 2005). The species composition is different, but about 16 species in these deposits also occur in Straß and Maierhof, e. g. Reussella spinulosa, Virgulopsis tuberculatus, Elphidium crispum, and Elphidiella dollfusi. These species, however, have long stratigraphic ranges, and thus cannot be used in tracing the Eggenburgian seaway connections. In the Central Paratethys, the top of the siliciclastic upper Eggenburgian sediments is characterized by a prominent erosional surface that probably correlates with the eustatic Bur 3 lowstand and the Eggenbur-

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gian-Ottnangian boundary (KOVÁC et al. 2004, PILLER et al. 2007). The foraminifera from the lowermost Ottnangian sediments in Straß indicate a shallowing of the environment. In Maierhof, strong erosion separates the Ottnangian from the Eggenburgian part of the section. Therefore, a relative sea level drop and erosion period, influenced by global eustasy, can be proposed also for the here studied area at the Eggenburgian-Ottnangian boundary.

Acknowledgements This study is part of the research project “Vertiefte Untersuchungen der Schichtenfolge des Eggenburg und Ottnang in der Region 12” (Bavarian Geological Survey, LfU). We are grateful to GERHARD DOPPLER, DIETMAR JUNG, KLAUS SCHWERD and WALTER WENGER (LfU) for discussion, information and providing samples of the core Straß. SIMON SCHNEIDER (Munich) provided the Maierhof samples and information about macrofossils. ALEXANDER ALTENBACH and MICHAEL KRINGS (both Munich) are acknowledged for constructive comments, TANJA SCHULZ-MIRBACH (Munich) for technical assistance at the SEM. In addition, we thank the reviewers GERHARD SCHMIEDL (Hamburg) and CHRISTIAN RUPP (Wien) for their fruitful comments.

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TYSON, R. V. & PEARSON, T. H. (1991): Modern and ancient continental shelf anoxia: an overview. – Geological Society Special Publication, 58: 1-24. UNGER, H. J. (1984): Geologische Karte von Bayern 1:50 000. Erläuterungen zum Blatt Nr. L 7544 Griesbach im Rottal. – 245 pp.; München (Bayerisches Geologisches Landesamt). WAGNER, L. R. (1998): Tectono-stratigraphy and hydrocarbons in the Molasse Foredeep of Salzburg, Upper and Lower Austria. – In: MASCLE, A., PUIGDEFABREGAS, C., LUTERBACHER, H. P. & FERNANDEZ, M (Eds.): Cenozoic Foreland Basins of Western Europe. – Geological Society Special Publication, 134: 339-369. WENGER, W. F. (1987a): Die Foraminiferen des Miozäns der bayerischen Molasse und ihre stratigraphische sowie paläogeographische Auswertung. – Zitteliana, 16: 173340. WENGER, W. F. (1987b): Die Basis der Oberen Meeresmolasse im westlichen Oberbayern, am Überlinger See, in Vorarlberg und St. Gallen. – Mitteilungen der Bayerischen Staatssammlung für Paläontologie und historische Geologie, 27: 159-174. Addresses of the authors: MARTINA PIPPÈRR, BETTINA REICHENBACHER, Department of Earth and Environmental Sciences, Section Palaeontology, Ludwig-Maximilians-University, Richard-Wagner Str. 10, D-80333 Munich, Germany. e-mails: [email protected]; [email protected]