GeolCarp_Vol55_No4_311

311

PALEOCEANOGRAPHY OF PARATETHYS DURING OLIGOCENE IN AUSTRIAN MOLASSE BASIN

GEOLOGICA CARPATHICA, 55, 4, BRATISLAVA, AUGUST 2004

311323

PALEOCEANOGRAPHY OF THE WESTERN CENTRAL PARATETHYS

DURING EARLY OLIGOCENE NANNOPLANKTON ZONE NP23 IN

THE AUSTRIAN MOLASSE BASIN

HANS-MARTIN SCHULZ

, ACHIM BECHTEL

, THOMAS RAINER

REINHARD F. SACHSENHOFER

and ULRICH STRUCK

Department of Petroleum Geology, Institute of Geology and Paleontology, Technical University of Clausthal, Leibnizstr. 10,

D-38678 Clausthal-Zellerfeld, Germany; schulz@geologie.tu-clausthal.de

Department of Geosciences, Montanuniversität Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria

*Present address: Institute of Mineralogy and Petrology, University of Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany

GeoBio-Center, Ludwig-Maximilians-University, Richard-Wagner-Str. 10, D-80333 München, Germany

(Manuscript received April 30, 2003; accepted in revised form October 2, 2003)

Abstract: The 5.5 m thick Dynow Marlstone in well Oberschauersberg 1 in the Upper Austrian Molasse Basin was

studied using mineral and maceral petrography, SEM, organic geochemistry, and C- and N-isotopy of the organic mate-

rial. The well is located on the former upper slope of the northern basin margin. The depositional period of the Dynow

Marlstone covers parts of the Early Oligocene nannoplankton Zone NP23, which corresponds to the culmination of the

first Paratethys isolation. The Dynow Marlstone represents a carbonate-rich, organic-poor interval (0.52 % TOC) inter-

calated between organic-rich formations. The onset of the deposition of the Dynow Marlstone was characterized by an

abrupt increase in primary carbonate productivity, but persisting photic zone anoxia. Both high organic carbon produc-

tivity and photic zone anoxia prevailed during deposition of the Dynow Marlstone. These constant conditions were

overprinted by cyclic increases in the trophic level favouring blooms of calcareous nannoplankton. Limestones with low

TOC contents were deposited during algal blooms, whereas organic-rich marls accumulated during periods with low

production of calcareous nannoplankton. Sulphate reduction extended into the water column. The intensive consumption

of labile organic material decreased the hydrogen index. Intensified photic zone anoxia and an increase in salinity

worsened the ecological environment for calcareous nannoplankton and led to deposition of the organic-rich marls of the

Eggerding Formation within a constantly eutrophic and normal-marine environment.

Key words: Kiscellian, Paratethys, Dynow Marlstone, paleoceanography, CN isotopes, biomarker, organic carbon.

Fig. 1. Paleogeography during nannoplankton Zone NP23 (modi-

fied after Rögl 1999) and study area.

Introduction

The separation of the Paratethys from the Tethys commenced

at the Eocene/Oligocene boundary and reached a maximum

during middle Kiscellian time in the nannoplankton Zone

NP23 of Martini 1971 (32.230 Ma; Rögl 1996) when the

Paratethys lost its connection to the World Ocean (Solenovian

Event, e.g. Popov et al. 1993; Fig. 1). Basin isolation was

coupled to the development of dysaerobic to anoxic bottom

water conditions from the Molasse Basin to the Caspian Sea

(Rögl 1999). These favoured the deposition of organic-rich

rocks acting as hydrocarbon source rocks in several

Paratethyan basins including the Molasse Basin (Wehner &

Kuckelkorn 1995; Schmidt & Erdogan 1996; Ziegler & Roure

1999).

Deposition of organic-rich sediments in the western Central

Paratethys (e.g. Upper Austrian Molasse Basin) commenced

during the latest Priabonian and continued till Early Miocene

(Egerian) times, but was progressively focussed to the north-

ern basin slope (Fig. 2). Different paleoceanographic models

have been proposed for the formation of the organic-rich

rocks. Many authors favour a stagnant basin model for the for-

mation of Kiscellian rocks (Gerhard 1982, 1988; Dohmann

1991; Schulz et al. 2002), whereas Wagner (1996, 1998) con-

sidering the asymmetric facies distribution during Egerian

times proposed an upwelling scenario.

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SCHULZ et al.

Accumulation of organic-rich rocks was only temporarily

interrupted during the middle part of NP23, when light

coloured marls and marly carbonates (Dynow Marlstone, for-

merly Heller Mergelkalk; Wagner 1998) representing short-

term paleoceanographic changes of less than 2 m.yr. (Krhovský

et al. 2001) were deposited.

The Dynow Marlstone was first described from the Polish

part of the Western Carpathians (Kotlarczyk 1979) and is

documented now in the German and Austrian Molasse Basin,

along the entire Carpathian Flysch Belt and in the Transylva-

nian Basin (Krhovský et al. 1991; Popov et al. 1993; Rögl et

al. 1997; Rusu et al. 1996). Nevertheless, a formal lithostrati-

graphic definition is missing.

Detailed studies of the Dynow Marlstone were performed

in the Waschberg Zone (Lower Austria; Rögl et al. 2001) and

in the dánice Unit of the Western Carpathians (Krhovský &

Djurasoinovic 1993; Krhovský 1995; Krhovský et al. 2001).

There, typical features of the Dynow Marlstone are monospe-

cific, low-salinity tolerant nannoplankton (and diatoms) and

tiny endemic (brackish) bivalves. The lack of benthic organ-

isms indicates bottom water anoxia. Surface water salinities

below 27 have been referred to a higher runoff water sup-

ply (Budilova et al. 1992).

High-resolution data of the Dynow Marlstone from the

western Central Paratethys in Upper Austria are not available

yet. Therefore, in the present study vertical lithological and

sedimentological variations of a 5.5 m thick succession cored

by a borehole (Oberschauersberg 1) in the Upper Austrian

Molasse Basin are recorded together with organic geochemi-

cal proxies and CN isotopes of the organic material. Main

aims of the study are (1) to examine the paleoceanographic

changes in the western Central Paratethys, which resulted in

the accumulation of carbonate-rich, organic-poor rocks inter-

calated between prolific hydrocarbon source rocks, and (2) to

compare the factors controlling the deposition of the Dynow

Marlstone in the Upper Austrian Molasse Basin with those

prevailing in the Western Carpathians.

Fig. 2. Stratigraphic sketch of the Eocene/Oligocene transition from the Austrian Molasse Basin to the Carpathian Foredeep (modified after

Wagner 1988). The sections for the Waschberg Zone and the dánice Unit are from Krhovský et al. (2001).

Regional geology

The study area is located in the Austrian part of the Molasse

Basin (Fig. 3), an eastwest trending foreland trough, which

resulted from the subduction of the southern margin of the Eu-

ropean plate beneath the Adriatic plate (Ziegler 1987). The

basement is formed by crystalline rocks of the Bohemian Mas-

sif covered by autochthonous sediments of Jurassic and Creta-

ceous age. Sedimentation within the Molasse Basin lasted

from Late Eocene to Miocene times. The southern part of the

Molasse Basin was overridden by the Alpine nappes (Flysch

and Helvetic units, Calcareous Alps) and was included within

the overthrust system.

Sedimentation in the Molasse Basin commenced during the

Late Eocene in non-marine and shallow-marine environments,

which graded southwards into the deeper marine Helvetic

realm and the 3000 m deep Flysch Basin. At the Eocene/Oli-

gocene transition the Molasse Basin subsided rapidly to deep-

water conditions which resulted in a pronounced change in

depositional environments. The changes included the extinc-

tion of a carbonate platform with algal reefs (Bachmann et al.

1987), the beginning of slope currents and the deposition of

the organic matter-rich Schöneck Formation on the northern

basin slope (NP1920 to lower part of NP23; Schulz et al.

2002).

According to Schulz et al. (2002), deposition of the

Schöneck Formation terminated when decreasing surface wa-

ter salinity caused a break-down of water column stratification

and allowed oxygenation of the water body. The overlying or-

ganic-lean Dynow Marlstone (NP23; Rögl 1999) is typically

about 5 m thick (Fig. 3), but may reach a thickness of up to

15 m. In general, it is described as a light-coloured marlstone

originating from pure nannofossil chalk (the first evidence of

coccoliths was given by Müller & Blaschke 1971) deposited in

a basin with reduced salinity (Báldi 1984; Rögl 1999) and high

nutrient content (Rögl et al. 2001). After deposition of the

Dynow Marlstone, favourable conditions for the accumulation

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PALEOCEANOGRAPHY OF PARATETHYS DURING OLIGOCENE IN AUSTRIAN MOLASSE BASIN

of organic matter-rich sediments recurred during Late

Kiscellian to Early Miocene times resulting in the deposition

of the organic-rich Eggerding (formerly Bändermergel;

Wagner 1998) and Ebelsberg Formations (formerly Älterer

Schlier; Wagner & Wessely 1997; Wagner 1998; Fig. 2).

Materials and methods

The study is based on core material from the well

Oberschauersberg 1 (Osch1; Fig. 2) drilled in 1985 by Rohöl-

Aufsuchungs AG (RAG, Vienna), which recovered a complete

succession of the Dynow Marlstone including the Schöneck

Formation at the base and the Eggerding Formation at the top.

A previous study on the Schöneck Formation showed that the

Dynow Marlstone in this well is immature (R

<0.35 %;

Schulz et al. 2002). Additional core material from wells Fischl-

ham 1 (Fi1, 1970), Dietach 1 (Di1, 1972) and Rappersdorf 2

(Ra2, 1977; Fig. 3) representing the lowermost part of the

Dynow Marlstone was inspected. Sonic logs of these wells

and the wells Hochburg 1 (Hobg1, 1985) and Oberhofen 1

Fig. 3. Geological situation of the study area and site of wells Fischlham 1 (Fi1), Dietach 1 (Di1), Rappersdorf 2 (Ra2), Hochburg 1 (Hobg1),

Oberhofen 1 (Obhf1), and Oberschauersberg 1 (Osch1) which recovered a complete succession of the Dynow Marlstone. Below: Sonic log

patterns of the Dynow Marlstone.

(Obhf1, 1982) were provided by RAG for correlation pur-

poses (Fig. 3).

The cores were described, photographed, and sampled in

detail. Samples were analysed by means of thin sections and

scanning electron microscopy (SEM). Maceral analysis was

performed by incident and blue light excitation and point

counting transects (400 points per sample).

Powdered samples were analysed for total sulphur (S), total

carbon (TC), and organic carbon contents (TOC, after acidifi-

cation of samples to remove carbonate) using a Leco CS-225

analyser. The difference between TC and TOC is the total in-

organic carbon (TIC). Calcite is the only carbonate mineral

present. Therefore, calcite contents were calculated using the

formula TIC*8.33. Pyrolysis measurements were performed

using a Rock-Eval 5 instrument.

After removing carbonates by 2 N HCl, powdered samples

were analysed simultaneously for δ

TOC and δ

tot

with a

Thermo/Finnigan MAT Delta plus isotope ratio mass spec-

trometer, coupled to a Thermo NA 2500CN elemental

analyser via a Thermo/Finnigan Conflo II interface. The refer-

ence gas was pure N

and CO

from a cylinder calibrated

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SCHULZ et al.

against IAEA standards N-1 and N-2 and carbonate (NBS-18,

NBS-19), respectively. The isotopic results are expressed in

the usual delta notation δ

TOC

or δ

tot

. The standard de-

viation of the isotope analyses was better than 0.15%. Bulk

rock nitrogen isotopes δ

tot

may closely represent those val-

ues expected from organic nitrogen in organic-rich sediments

(Calvert et al. 1996; Caplan & Bustin 1998). Most of the ni-

trogen resides in organic matter according to the very good

correlation between TOC and N

tot

= 0.91).

As for organic geochemical analyses, portions of the pul-

verized samples were extracted for approximately 1 h using

dichloromethane in a Dionex ASE 200 accelerated solvent ex-

tractor at 75 °C and 5 MPa. After evaporation of the solvent to

0.5 ml total solution in a Zymark Turbo Vap 500 closed cell

concentrator, asphaltenes were precipitated from a hexane-

dichloromethane solution (80 : 1) and separated by centrifuga-

tion. The fractions of the hexane-soluble organic matter were

separated into saturated hydrocarbons, aromatic hydrocarbons

and NSO compounds by medium-pressure liquid chromatog-

raphy using a Köhnen-Willsch MPLY instrument (Radke et

al. 1980).

The saturated and aromatic hydrocarbon fractions were

analysed by a gas chromatograph equipped with a 25 m DB-1

fused silica capillary column (i.d. 0.25 mm) and coupled to a

Finnigan MAT GCQ ion trap mass spectrometer. The oven

temperature was programmed from 70 to 300 °C at a rate of

4 °C min

followed by an isothermal period of 15 min. He-

lium was used as carrier gas. The mass spectrometer was oper-

ated in the EI (electron impact) mode and a scan range from

50 to 650 daltons (0.7 s total scan time). Data were processed

Fig. 4. Core intervals of the Dynow Marlstone with Schöneck Formation at the base and Ebelsberg Formation on the top. a Transition of

Schöneck Formation to Dynow Marlstone, b Transition of Dynow Marlstone to Eggerding Formation. 1, 2, 3 Top of cycles in the

Dynow Marlstone. Note difference between upper cycle and boundary between Dynow Marlstone and Eggerding Formation (see also text).

with a Finnigan data system. Identification of individual com-

pounds was accomplished by retention time in the total ion

current (TIC) chromatogram and by comparison of the mass

spectra with published data. Absolute biomarker concentra-

tions in the saturated and aromatic hydrocarbon fractions were

calculated using peak areas from the gas chromatograms in re-

lation to that of internal standards. The concentrations were

normalized to the TOC content.

Sedimentology and diagenesis

The Dynow Marlstone in well Osch1 represents a heteroge-

neous sedimentary unit with a sharp lower boundary towards

the Schöneck Formation and a gradual transition into the

Eggerding Formation at the top (Figs. 3, 4, 5). The lower

boundary is developed as a 2 cm thick interval with rapidly

upward increasing carbonate contents, which grades into a

massive whitish mudstone (according to DUNHAMs carbon-

ate classification) about 35 cm thick (Fig. 5). This type of

massive whitish mudstone recurs one meter above. Apart

from the mudstone layers, the Dynow Marlstone is predomi-

nantly composed of laminated to wavy bedded white limy

marlstones to dark grey silty marlstones (Fig. 4). The upper

boundary of the Dynow Marlstone is poorly defined, because

an increasing portion of laminated, dark grey marlstones

forms a transitional interval to the Eggerding Formation. In

Figs. 4 and 5 the upper boundary of the Dynow Marlstone is

drawn at the top of the uppermost relatively bright, carbonate-

rich marlstone.

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SCHULZ et al.

characterized by mean sizes between 3 and 4 µm and a stan-

dard deviation (σ) of less than 3 µm (Fig. 7). Size distribu-

tions of framboidal pyrite have been applied to reconstruct the

oxygenation state of depositional environments (Wilkin et al.

1996). Framboid nucleation and growth of pyrite within an

anoxic water column in euxinic environments are generally

shorter than in sediments with anoxic pore waters. Thus,

unimodal-distributed small pyrite framboids in the Dynow

Marlstone and the transition to the Eggerding Formation re-

flect crystallization within an anoxic bottom water column.

Similar pyrite framboid distributions have been found in the

middle and upper part of the Schöneck Formation (e.g. I in

Fig. 7; approx. 1.5 m below the top of the Schöneck Forma-

Fig. 6. Sedimentary development of cycle 2 in Dynow Marlstone in

well Oberschauersberg 1. a Transition from well-bedded bitumi-

nous marlstone to well-bedded white to light grey marlstone with

distinct bituminous silty marlstone layers. Core 2, Box 7, 3254.

b Intercalation of grey and dark grey bituminous silty marl-

stones with slightly wavy bedding. Core 2, Box 8, 1032. c Tran-

sition from well-bedded grey marlstone to well-bedded dark grey

marlstone. Core 2, Box 8, 4060.

Fig. 7. Mean framboid diameter vs. standard deviation (σ) of the py-

rite diameter (according to Wilkin et al. 1996) in Dynow Marlstone

and transitions to the top and bottom (well Oberschauersberg 1). See

Fig. 8 for sampling.

tion). During deposition of the uppermost part of the

Schöneck Formation, brackish water conditions resulted in a

break-down of water stratification (Schulz et al. 2002). This is

reflected by larger framboidal pyrites and by distributions

with a higher standard deviation (II; III). Sample IV in

Fig. 7 marks the re-establishment of an anoxic bottom water

body, which prevailed during deposition of the Dynow Marl-

stone and the Eggerding Formation.

Log correlation

The insert in Fig. 5 shows that in the case of the Dynow

Marlstone and the lower Eggerding Formation, the interval

transit time recorded by the sonic log is mainly a function of

carbonate contents. Therefore, the sonic log can be used to

correlate limestone layers. The (marly) limestone beds form-

ing the base of cycles 1, 2 and 4 are clearly visible in the logs

(Fig. 3), but a separation between cycles 3 and 4 is barely pos-

sible. In some logs the upper limestone layer in cycle 1 can be

distinguished.

In well Obhf1 the Dynow Marlstone is missing in the au-

tochthonous Molasse section. However, Dynow Marlstone

occurs in this well at a depth of about 2730 m in

allochthonous Molasse imbricates (Wagner et al. 1986;

Fig. 3). According to palinspastic reconstructions by Wagner

(1998), these sediments were deposited at least 30 to 65 km

south of their present-day position.

The sonic logs presented in Fig. 3 document that the

Dynow Marlstone continues laterally. The major cycles 1, 2

and 4 can be traced over roughly 100 km in an EW direction

(Di1Hobg1) and at least 50 kilometers in a NS direction.

This clearly proves that the mechanisms controlling the cyclic

structure of the Dynow Marlstone and the lower Eggerding

Formation were effective on a basin-wide scale.

Organic petrography and proxies of the organic

material

The TOC contents in the Dynow Marlstone range from 0.5

to 2 % (Fig. 8), while within the studied interval of the

Eggerding Formation they are up to 3.5 %. The TOC contents

are closely related to cycles 1 to 4 with upward increasing

TOC contents within each cycle. There is a strong negative

correlation between TOC and calcite (correlation coefficient

= 0.79; Fig. 9) which, according to Ricken (1991), indicates

roughly constant production of organic matter and dilution by

varying amounts of calcite. Calcite in the studied section is

mainly derived from calcareous nannoplankton, suggesting a

negative correlation between TOC contents and algal blooms.

The organic petrographic composition of the Dynow Marl-

stone is related to the lithology of the host rock. The massive

mudstones at the base of the first two cycles contain exclu-

sively bituminite (petrographic association I in Fig. 8). Grey

marlstones are also dominated by bituminite, but include mi-

nor amounts (<5 vol. % of total visible organic matter) of

small alginite and detrital liptinite, huminite and inertinite,

(association III). Alginite and humodetrinite percentages

are above 5 vol. % in association II, which occurs in dark

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PALEOCEANOGRAPHY OF PARATETHYS DURING OLIGOCENE IN AUSTRIAN MOLASSE BASIN

grey marlstones in the upper part of the Dynow Marlstone and

in the Eggerding Formation. Thus, a slight but progressive in-

put of terrestrial organic material into the depositional setting

can be recorded.

Most hydrogen index values (HI) fall in the range between

500 and 600 mg

TOC

. TOC/S ratios vary from 1 to 3

(Fig. 8). The trend lines for both proxies highlight a continu-

ous decrease within the lower part of the Dynow Marlstone

and a continuous increase within the uppermost part and

within the Eggerding Formation.

Because there is no indication for massive changes in or-

ganic matter input, the slightly reduced HI values in the

middle part of the section may be due to intensified bacterial

overprint of the organic material during deposition of the

Dynow Marlstone. Suppressed TOC/S ratios point to more ef-

fective sulphate reduction and support this hypothesis. The

observed TOC/S ratios <2.8 furthermore indicate anoxic bot-

tom water conditions (Berner 1984; Berner & Raiswell 1983).

Biomarkers and CN isotopes of the organic

material

After a significant drop across the base of the Dynow Marl-

stone, pristane/phytane ratios increase slightly upwards from

about 2 to 3 (Fig. 10). Pristane/phytane ratios are known to be

affected by maturation (Tissot & Welte 1984) and by differ-

ences in the precursors for acyclic isoprenoids (i.e. bacterial

origin; Volkman & Maxwell 1986; ten Haven et al. 1987). An

influence of different maturity on pristane/phytane ratios can

Fig. 8. Organic geochemical proxies, organic petrography and C-N isotopes in Dynow Marlstone and transitions to the top and bottom (well

Oberschauersberg 1). 13 cycles within Dynow Marlstone, 4 cycle leading to permanent depositional conditions of the Eggerding For-

mation. Roman numbers I, III and II in the field for organic petrography are explained in the text. Sampling and sample numbers are indi-

cated in the field for TOC.

Fig. 9. TOC vs. calcite content in Dynow Marlstone and Eggerding

Formation in well Oberschauersberg 1. Insert in the upper right cor-

ner shows type of deposition reflected by different relationships be-

tween TOC and calcite (simplified after Ricken 1991). Note insert

in the lower left corner: TOC-Calcite plots for single cycles (14)

yield similar regression lines and prove decreasing carbonate input

from cycle 1 to cycle 4.

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SCHULZ et al.

be ruled out, due to the low vertical distance of the samples

within the investigation profile in well Osch1 (about 6 m).

The low maturity at Osch1 (R

<0.35 %) further argues

against the formation of pristane from tocopherols (vitamin-E)

or chromanes (Goossens et al. 1984). However, a bacterial

origin of phytane from phytanyl ether lipids found in

archaebacteria cannot be excluded (Volkman & Maxwell

1986; ten Haven et al. 1987). According to previous studies

(Didyk et al. 1978), an increase in pristane/phytane ratios

would indicate the establishment of more oxic conditions in

the bottom water during sedimentation. This interpretation

contradicts the increase in C

-arylisoprenoid concentrations

(Fig. 10; see below).

Aryl isoprenoids are thought to be derived from the caro-

tenoid isorenieratene, which is specific for the photosynthetic

green sulphur bacteria Chlorobiaceae and purple sulfur bacte-

ria Chromatiaceae (Summons & Powell 1987). These organ-

isms are phototrophic anaerobes and, thus, require both light

and H

S for growth. In modern environments they appear in

sulphate-containing water bodies that are sufficiently quies-

cent and organic-rich to enable sulphide production close to

Fig. 10. Carbon preference index (CPI), pristane/phytane and steranes/hopanes ratios, and concentrations of 4-methylsteranes, tri-MTTC

and C

-arylisoprenoid in the Dynow Marlstone and transitions to the top and bottom (well Oberschauersberg 1).

the photic zone (Summons 1993). Euxinic conditions in the

deep water zone are required, and the intensity of the green

spectral component of the light used for photosynthesis

should be reduced by particulate organic matter or vegetation

in the water column (Pfennig 1977). Besides the lowermost

sample, the concentration of aryl isoprenoids remains rather

constant within the Dynow Marlstone and increases in the

Eggerding Formation. Increasing concentrations may also be

due to a rising chemocline (Repeta 1993; Sinninghe-Damsté

et al. 1987b).

Constant values of the carbon preference index (CPI, calcu-

lated after Bray & Evans 1961) throughout the Dynow Marl-

stone and along the transition to the Eggerding Formation

(Fig. 10) point to similar sources of humic material, which is

abundant in different concentrations (see organic petrogra-

phy). Furthermore, the percentages of saturated and aromatic

hydrocarbons, NSO compounds and asphaltenes remain con-

stant throughout the investigated profile (18:15:50:17) and

indicate a rather similar organic matter composition.

A trimethylated 2-methyl-2-trimethyl-tridecylchroman

-chroman; tri-MTTC) occurs in significant amounts and

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PALEOCEANOGRAPHY OF PARATETHYS DURING OLIGOCENE IN AUSTRIAN MOLASSE BASIN

has been identified by comparison of the mass spectrum with

published data (Sinninghe Damsté et al. 1987a; Schwark &

Püttmann 1990; for a review about chroman geochemistry see

Schwark et al. 1998). The corresponding dimethylated com-

pound (di-MTTC) has also been found in very low intensities

in the aromatic hydrocarbon fractions of the sediment

samples. The predominance of tri-MTTC over its dimethyl-

ated counterpart is indicative for mesohaline to euhaline (30

40 ) conditions. Chroman assemblages dominated by di-

MTTC were found in sediments deposited under hypersaline

conditions (Sinninghe Damsté et al. 1987a; Schwark &

Püttmann 1990). Except for one outlier at the base, the ratio of

both biomarkers (di-/tri-MTTC) remains on a fairly constant

level of less than 0.1 throughout the Dynow Marlstone

(Fig. 10). A value of more than 0.2 recorded in the lower

Eggerding Formation suggests an important increase in salinity.

Other constituents of the saturated hydrocarbons are αβ-

and βα-hopanes from C

to C

, but the C

hopanes are ab-

sent. Hopanoids have been identified as membrane constitu-

ents in many procaryotes (e.g. bacteria) including some grow-

ing anaerobically (Ourisson et al. 1979).

Steroids are dominated by 5α-steranes from C

to C

and

minor amounts of 5β-steranes, as well as 4α-methylsteranes.

The predominant primary producers of sterols are phytoplank-

ton and photosynthetic bacteria living in the photic zone of

the water column (Volkman 1986). Sterane concentrations in-

crease in cycle 1 of the Dynow Marlstone indicating increased

sterol productivity and remain on a rather constant level in the

upper part of the section.

The steranes/hopanes ratio increases upwards slightly but

constantly from about 1 to 2 (Fig. 10). High steranes/hopanes

ratios (up to 3) in the lower part of the Schöneck Formation

were interpreted as consistent with full-marine conditions

(Schulz et al. 2002). Furthermore, increasing steranes/hopanes

ratios in the German Kupferschiefer reflect an increasing ma-

rine influence during the initial Zechstein transgression

(Bechtel & Püttmann 1997). Additionally, variations of the

steranes/hopanes ratio can be attributed to fluctuations of the

trophic level during deposition of the Schöneck Formation

(Schulz et al. 2002). Thus, an increase of this ratio in the in-

vestigated profile points either to increasing nutrient contents

or to the establishment of normal marine conditions.

C values in the studied section range from 30 to 23

and show an upward trend towards heavier values (Fig. 8).

Because organic carbon of terrigenous material is typically

isotopically lighter (~ 27 ) than marine organic material

(~ 21 ; Meyers 1994), the measured δ

C values may re-

flect a mixed organic matter source. The trend towards

heavier values parallels the change from petrographic associa-

tion I dominating in the lower part of the Dynow Marlstone to

petrograpic association II prevailing in the Eggerding Forma-

tion (Fig. 8) and may be triggered by the change from brack-

ish to fully marine conditions in the Eggerding Formation (see

di-/tri-MTTC ratio in Fig. 10). According to Schulz et al.

(2002), the very light δ

C values at the top of the Schöneck

Formation result from CO

recycling.

Small-scale cyclic variations, each with a tendency to

higher values at the top, exist within the general tendency to

heavier δ

C values. As the sterane concentrations data for the

first cycle indicate an increase in primary sterol productivity

(Fig. 10), heavier δ

C signals may reflect CO

limitation and

less fractionation during CO

uptake. This phenomenon can

be explained by the reduced buffering capacity of carbonate

systems with low salinity water, which leads to an increase in

pH during high primary productivity periods (Voß & Struck

1997). However, the sterane concentrations remain on a fairly

constant level throughout the following cycles 24 (Fig. 10).

Thus, intra-cycle variations to heavier δ

C values may be con-

tingent on an increased amount of marine organic material.

N data in the Dynow Marlstone cycle 1 scatter widely

(Fig. 8). In contrast, the cycles 2 and 3 reveal clear trends with

upward increasing values, whereas cycle 4 exhibits a (weak)

trend towards lighter values. δ

N values of sedimentary or-

ganic matter reflect integrated signals of various factors. First,

they can be used to distinguish between organic matter de-

rived from algae and land-plants (Meyers 1997). Atmospheric

(δ

N about 0 ) is the nitrogen source for terrestrial

plants, whereas dissolved nitrate with heavier δ

N values is

the nitrogen source for plankton. Land-plants, therefore, are

characterized by lower δ

N values. Second, high nutrient

concentrations in surficial waters lead to the production of or-

ganic matter with low δ

N values, because faster uptake ki-

netics cause preferential assimilation of

N (relative to

when nutrients are abundant. Third, the totally different inter-

pretation invoking diagenetic alteration of nitrogen isotopic

ratios in the presence of oxygen (cf. Sachs & Repeta 1999 and

references herein) can be excluded, because C

-aryl-

isoprenoids (Fig. 10) indicate permanent bottom water anoxia

throughout deposition of the studied profile. Thus, the ob-

served trends in cycles 2 and 3 reflect either increasing con-

tents of plankton, or more likely decreasing N-isotope

fractionation because of decreasing nutrient availability. Isoto-

pic fractionation due to partial utilization of dissolved inorganic

nitrogen may account for the tendency towards lighter δ

values within cycle 4 (according to Altabet & Francois 1994).

Paleoceanographic implications

In the Upper Austrian Molasse Basin, the Dynow Marl-

stone, about 5 m thick, is interbedded between two organic-

rich formations (Schöneck and Eggerding Formation). The

rock unit, relatively poor in organic matter, represents a major

break in the evolution of the Paratethys. In some wells the

Dynow Marlstone is missing (e.g. the autochthonous part of

Obhf1), perhaps because of submarine erosion or slumping.

Furthermore, this sedimentary unit is not described from mar-

ginal basin connections (Upper Rhine Valley, Lower Inn De-

pression, Slovenian Corridor; Ortner & Sachsenhofer 1996;

Schmiedl et al. 2002; Doebl 1970). Nevertheless, the lateral

continuity of this unit is remarkably high, arguing for tectoni-

cally stable conditions.

Nannoplankton of three samples from the second cycle in

the Dynow Marlstone have been described by Báldi-Beke

(2003; see Fig. 5 for position of samples). In general, the nan-

noplankton diversity is poor, but indicative of the nanno-

plankton Zone NP23. Reticulofenestra ornata predominates.

This species formed blooms during NP23 and NP24, but has a

range from NP22 to NP25. At the top of cycle 2 Transverso-

pontis fibula has been identified, which is characteristic for

320

SCHULZ et al.

Fig. 11. Depositional model for the Dynow Marlstone (a) and the

upper part of the Dynow Marlstone cycles and the lowermost Egg-

erding Formation (b) in well Oberschauersberg 1. The accumulation

of the Dynow Marlstone was controlled by periodical blooms of cal-

careous nannoplankton (a). The produced carbonate dilutes the or-

ganic matter in the sediments and results in varying and relatively

low TOC contents. Nannoplankton blooms were caused by high

trophic levels probably related to fresh water ingressions. Contem-

poraneously, bottom currents reworked the sediment and led to fluc-

tuations of sulphate reduction intensity due to limited sulphate

availability. The primary productivity of organic carbon remained

roughly constant and anoxic bottom water conditions favoured or-

ganic matter preservation during the deposition of the Dynow Marl-

stone and Eggerding Formation. Changes at the transition from the

Dynow Marlstone to the Eggerding Formation include intensified

photic zone anoxia and an increase in salinity (b).

the lowermost NP23. The Transversopontis fibula-Reticulo-

fenestra ornata assemblage (olbinian-type nannoflora) has

been described as characteristic for the Central and Eastern

Paratethys and may indicate brackish water conditions

(Nagymarosy & Voronina 1992).

Transversopontis fibula is often related to a level to en-

demic bivalves (Cardium lipoldi-fauna) and ostracods

(Cyprididae), a fact which enables correlations throughout the

entire Paratethys (Popov et al. 1993). This marker horizon has

been described from the Dynow Marlstone in the Waschberg

Unit (Rögl et al. 2001) and the Carpathians (Krhovský et al.

2001), but is not present in well Osch1.

Reticulofenestra ornata also predominates in the lower part

of the Eggerding Formation, whereas Transversopontis fibula

was not found. This phenomenon has been referred to a strong

salinity decrease (Nagymarosy & Voronina 1992). However,

this interpretation is in conflict with increasing di-/tri-MTTC

ratios (Fig. 10), which point to increasing salinities.

According to our results, oligotrophic conditions prevailed

and surface water salinities decreased during the final stages of

the deposition of the Schöneck Formation. Decreasing salinities

were referred to increasing fresh water ingressions. Further-

more, the end of CO

recycling was referred to a break-down of

water stratification (Schulz et al. 2002). Pyrite framboid diam-

eters suggest a short time interval with dysoxic (to oxic?) condi-

tions (Fig. 7). However, detectable C

-arylisoprenoids across

the transition and within the complete Dynow Marlstone advo-

cate for persistent photic zone anoxia (Fig. 10).

Cartoons illustrating different processes, active during

deposition of the Dynow Marlstone and the lowermost

Eggerding Formation are presented in Figs. 11a and 11b, re-

spectively. With the onset of Dynow Marlstone sedimenta-

tion, an abrupt increase in nutrients favoured an abrupt return

to the eutrophic conditions that earlier had occurred during

deposition of the lower Schöneck Formation. Eutrophic con-

ditions continued until the deposition of the Eggerding For-

mation. The high primary production level during deposition

of the Dynow Marlstone promoted cyclic blooms of calcare-

ous nannoplankton.

This scenario favoured nitrogen fixation (δ

N = 1 to +4;

Fig. 8). Variations of the δ

N signals to heavier values within

the single cycles are referred to diminished N-isotope frac-

tionation probably due to decreasing nutrient supply. How-

ever, the primary organic carbon productivity remained fairly

constant during deposition of the Dynow Marlstone (sterane

concentrations in Fig. 10). The different δ

N values are not

useful source indicators (land plant vs. plankton) in the case

of the Dynow Marlstone. Questions remain regarding the ex-

tent of denitrification and anaerobic oxidation of ammonia in

the anoxic water column during deposition of the Dynow

Marlstone (anammox reaction; Kuypers et al. 2003;

Dalsgaard et al. 2003).

A shift to heavier carbon isotope values within the single

cycles indicates increasing portions of marine organic mate-

rial. Thus, stronger CO

reduction during enhanced primary

production resulting from eutrophication and high primary

production in low salinity water can be excluded (Voß &

Struck 1997). Preservation of the organic material was en-

hanced by contemporaneously establishing anoxic bottom

water. On the other hand, intensive sulphate reduction low-

ered the hydrogen index in the upper part of the Dynow Marl-

stone. Due to the lack of reactive iron within the pore water,

pyrite formation was depressed and hydrogen sulphide es-

caped into the bottom water.

The organic geochemical data indicate euhaline to

mesohaline (3040 ) surface water conditions during deposi-

tion of the Dynow Marlstone as tri-MTTC predominates by far

over its dimethylated counterpart (Sinninghe Damsté et al.

1987; Schwark & Püttmann 1990). On the other hand, the ini-

tial deposition of the Eggerding Formation was coupled to a

progressively intensified water stratification and a salinity in-

crease (increasing C

-arylisoprenoid concentrations and di-/tri-

MTTC ratios; Fig. 10). The salinity proxy (di-/tri-MTTC ratio)

indicates a gradual return to normal-marine conditions, but con-

tradicts the nannoplankton findings (see previous chapter).

Similar environments occurred during deposition of the

Dynow Marlstone in the Western Carpathians in the dánice

Unit (Czechia; Krhovský 1995). There, brackish surface water

321

PALEOCEANOGRAPHY OF PARATETHYS DURING OLIGOCENE IN AUSTRIAN MOLASSE BASIN

and anoxic bottom water conditions were inferred from the

paleontological record. In contrast to the Molasse Basin, the

Dynow Marlstone in the dánice Unit contains silica from

diatom frustules. According to Krhovský (1995), the silicified

marlstones were deposited during relatively dry climatic peri-

ods characterized by high seasonality (hot and dry summers,

cool and wet winters) linked to short-term, orbitally forced

changes of seasonality within a long-eccentricity orbital

cycle. Sandy layers within the upper part of the Dynow Marl-

stone should indicate a following wetter period, which caused

intensified weathering on the Bohemain Massif and stimu-

lated the input of detrital material.

The Dynow Marlstone in the dánice Unit is overlain by

slumps and pebbles that transfer to the deposition of pelitic

rocks (itboøice Event; Krhovský 1995). This regional event

is considered a consequence of a sea-level fall during eustatic

cycle TA 4.5 according to Haq et al. (1987) or of tectonic ac-

tivity (Krhovský & Djurasoinovic 1993).

Considering the results from the dánice Unit, there are

three factors that may have influenced the sedimentary change

from the Dynow Marlstone to the Eggerding Formation in the

Upper Austrian Molasse Basin:

1 Intensified photic zone anoxia and increasing salinities

may have changed the ecological conditions for calcareous

nannoplankton and limited the carbonate production. Increas-

ing salinities and a return to full-marine conditions are prob-

ably related to the reactivation of the connection of the

Paratethys with the open sea (Popov et al. 1993; Popov &

Stolyarov 1996).

2 Climatic changes postulated by Krhovský (1995)

caused increased weathering and run-off from the Bohemian

Massif and provided enhanced amounts of detrital material.

However, this interpretation may be in conflict with the ob-

served rise in salinity during deposition of the Eggerding For-

mation.

3 The transition to the Eggerding Formation may be re-

lated to the itboøice Event (Krhovský 1995). However, this

event was not described until now in the Upper Austrian

Molasse Basin, and no evidence for slumping or sedimenta-

tion in lowstand fans was found in the present study.

Therefore, a basin-wide decline in carbonate production

due to ecological changes is the most probable explanation for

the observed change from the deposition of bright-coloured

calcareous muds to the deposition of dark-coloured (calcare-

ous) shales. This is also in accordance with the observed

negative correlation between carbonate and TOC (Fig. 9)

showing that organic matter deposition was controlled by the

amount of carbonate rather than by the amount of detrital ma-

terial and explains why this change extended across the whole

outer Paratethyan shelf during the second half of NP23.

Conclusions

Massive organic carbon accumulation in the Upper Aus-

trian Molasse Basin was interrupted during NP23 by deposi-

tion of the organic-poor intervals of the approximately 5 m

thick Dynow Marlstone.

During deposition of the uppermost part of the organic-rich

Schöneck Formation, oligotrophic conditions prevailed and

surface water salinities decreased. The base of the Dynow

Marlstone is characterized by an abrupt increase in primary

organic productivity, but persisting photic zone anoxia.

This stable depositional scenario of the Dynow Marlstone

was overprinted by cyclic increases in the trophic level

favouring blooms of calcareous nannoplankton. Subse-

quently, trophic levels and the production of calcareous nan-

noplankton decreased gradually. Within each cycle, the per-

centage of marine organic material increases. Limestones with

low TOC contents, a consequence of the dilution of organic

matter by the calcareous nannoplankton, were deposited dur-

ing algal blooms, whereas organic-rich marls accumulated

during periods with low production of calcareous nanno-

plankton. Sulphate reduction occurred in the sediment as well

as in the anoxic bottom water. Hydrogen sulphide generated

in the sediment escaped due to the lack of reactive iron. The

intensive consumption of labile organic material decreased

the hydrogen index.

The cyclic physicochemical conditions were modified after

the last major bloom of calcareous nannoplankton (top of

Dynow Marlstone). The biomarker data suggest intensified

photic zone anoxia (increase of concentrations of C

arylisoprenoids) coupled to a marked increase in salinity (in-

crease of di-/tri-MTTC; both proxies in Fig. 10). These

changes resulted in the deposition of the marls of the

Eggerding Formation, characterized by upward increasing

TOC contents. Sulphate reduction in impermeable pelites of

the Eggerding Formation was limited by sulphate availability.

Acknowledgments: The authors thank Rohölaufsuchungs

AG (Vienna) for providing core material and well logs. Tech-

nical assistance was given by colleagues at the Geological De-

partments in Clausthal (Germany) and Leoben (Austria). Spe-

cial thanks to Fred Rögl, Maria Báldi-Beke and András

Nagymarosy, whose help on various aspects of the Dynow

enhanced our interpretations. Furthermore, the paper ben-

efited greatly from the critical remarks of Alessandra Negri,

Ján Soták, and Phil Meyers.

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