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, AUGUST 2012, 63, 4, 295—305 doi: 10.2478/v10096-012-0023-5
Neogene uplift and erosion in the Alpine Foreland Basin
(Upper Austria and Salzburg)
JUERGEN GUSTERHUBER
1
, ISTVÁN DUNKL
2
, RALPH HINSCH
3
, HANS-GERT LINZER
3
and REINHARD F. SACHSENHOFER
1
1
Department of Applied Geosciences and Geophysics, Peter-Tunner-Strasse 5, A-8700 Leoben, Austria;
juergen.gusterhuber@gmail.com; reinhard.sachsenhofer@unileoben.ac.at
2
Sedimentology and Environmental Geology, Geoscience Center, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen,
Germany; istvan.dunkl@geo.uni-goettingen.de
3
RAG Rohöl-Aufsuchungs Aktiengesellschaft, Schwarzenbergplatz 16, A-1015 Vienna, Austria;
ralph.hinsch@rag-austria.at; hans-gert.linzer@rag-austria.at
(Manuscript received December 14, 2011; accepted in revised form March 13, 2012)
Abstract: In the present paper we apply a multi-technique approach (shale compaction data, seismic stratigraphy,
isopach maps, moisture content of lignite, fission track data) to assess timing and amount of uplift and erosion of the
Alpine Foreland Basin. The combination of the different techniques allows us to discriminate the effects of two differ-
ent erosion events during the Neogene: (1) Seismic stratigraphy and isopach maps indicate a Karpatian (Early Miocene)
regional tilting of the basin to the west (slope of about 0.5 %) and a minor erosion phase. (2) Moisture content of lignite
combined with fission track data provides evidence for extensive regional uplift after deposition of Late Miocene
fluvial deposits. It is estimated that sediments, 500 to 900 m thick, have been eroded. Shale compaction data derived
from sonic logs indicates additional uplift of the eastern part of the basin (near the river Enns). Here, 300 to 1000 m of
sediments were additionally eroded (giving a total erosion of about 1000 to 1900 m!), with a general increase of erosion
thickness towards the northeast. While the regional uplift is probably related to isostatic rebound of the Alps after
termination of thrusting, the local uplift in the east could be affected by Late Neogene E-W compressional events within
the Alpine-Pannonian system. Both, tilting and erosion influence the hydrocarbon habitat in the Molasse Basin (tilting
of oil—water contacts, PVT conditions, biodegradation).
Key words: Neogene, Alpine Foreland Basin, uplift, erosion, shale compaction, seismic stratigraphy, fission track data.
Introduction
Extensive oil and gas exploration activities in the Austrian and
German sectors of the Alpine Foreland Basin contributed
greatly to a detailed image of the subsurface. But while the
stratigraphy, architecture and evolution of the basin fill are
reasonably well understood (e.g. Nachtmann & Wagner 1987;
Bachmann et al. 1987; Wagner 1996, 1998; Kuhlemann &
Kempf 2002) knowledge on Neogene uplift and erosion pro-
cesses within the basin is largely missing (Genser et al. 2007).
This is in contrast to the Swiss sector of the Alpine Fore-
land Basin, where several authors, including Schegg & Leu
(1998) and Cederbom et al. (2011) investigated the amount
and timing of erosion. Beside this difference in knowledge a
significantly higher amount of erosion in the western part of
the Alpine Foreland Basin is proven.
The investigation of erosional events in the eastern part of
the Alpine Foreland has been initiated in the course of a ba-
sin and petroleum systems modelling study in Upper Austria
and Salzburg (Gusterhuber et al. 2011) which showed that
erosion and uplift may have strong effects on timing of gen-
eration, charging and preservation of hydrocarbons.
Different techniques, including shale compaction, lignite
compaction, and low temperature thermochronology are ap-
plied in the present paper with the objective of assessing the
timing and magnitude of Neogene uplift and erosion.
Geological setting
The Alpine Foreland Basin (Molasse Basin) of Salzburg
and Upper Austria represents a part of the Alpine-Carpathian
Foredeep (Fig. 1). It was formed due to the collision of the
Alpine orogenic system with the southern margin of the
European platform in the Middle Paleogene. The basin dis-
plays a typical asymmetrical peripheral foreland basin in
terms of an increasing basin depth towards the Alpine thrust
front in the south and a gradual shallowing and narrowing
trend from west to east towards the spur of the Bohemian
Massif (Malzer et al. 1993).
The crystalline basement of the Molasse Basin is overlain
by an incomplete cover of Late Paleozoic and Mesozoic
rocks (Fig. 2). Erosion during the latest Cretaceous left a
peneplain on which the Tethyan Sea progressively trans-
gressed during the latest Eocene and earliest Oligocene
times. At this stage the Molasse Basin was formed and be-
came the pelagic Alpine Foredeep. The area rapidly subsided
to deep-water conditions accompanied by the development
of an E-W trending fault network due to downward bending
of the European Plate (Wagner 1996, 1998).
Approximately at the Eocene-Oligocene boundary, strong
tectonic activities changed the Eurasian configuration and sep-
arated the Tethyan Sea into the Paratethys in the north and the
Mediterranean in the south. The closure of the Indo-Pacific
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connection caused the first isolation of the Paratethys. Deep
basins with reduced circulation and oxygen deficient bottom
conditions led to the deposition of marine organic matter-
rich rocks, which are the source for thermogenic hydrocar-
bons in the Molasse Basin (Schoeneck, Dynow, Eggerding
Formations; Schulz et al. 2002, 2005; Sachsenhofer et al.
2010). Thereafter new seaways opened from the Mediterra-
nean and the Indian Ocean to the Paratethys causing normal
marine oxygenated bottom conditions (Rögl 1999). In the
Late Oligocene and earliest Miocene, uplift of the Alps
caused increased sediment discharge from the south correlat-
ing with a distinct eustatic sea-level fall and the incision of a
slope-parallel (E—W) trough by strong bottom currents
(Krenmayer 1999) and corresponding widespread deposition
of deep-water channels (Linzer 2002; De Ruig & Hubbard
2006). Contemporaneous sediments of the deep-water Lower
and Upper Puchkirchen Formation (Egerian) are represented
by coal-bearing continental to brackish clays and sands
along the northern margin of the study area (Krenmayer
1999). In the Early Burdigalian (Eggenburgian), deep-water
conditions persisted during deposition of the Hall Formation
when a gradual transition to shallow-water sedimentation oc-
curred with continental slope-delta progradation across the
area (Hinsch 2008; Hubbard et al. 2009).
Marine conditions continued during the Ottnangian with the
deposition of the Innviertel Group (e.g. Faupl & Roetzel 1987;
Grunert et al. 2010). While fully-marine, tidal dominated silts
and sands represent Early and Middle Ottnangian transgressive
and highstand phases, brackish-fluvial sediments of the Onco-
phora Beds were deposited during a Late Ottnangian regressive
phase (Rögl 1998; Grunert et al. 2012). As a result of their
brackish character, a separation of the Oncophora Beds from the
Innviertel Group is under discussion (Rupp et al. 2008).
Following a major hiatus, a thick succession of coal-bear-
ing clays, sands and fluviatile gravels was deposited (Upper
Freshwater Molasse). In Upper Austria, freshwater deposi-
tion commenced in the Early Badenian in the western
Trimmelkam area (Rein in Weber & Weiss 1983; see Fig. 1
for location) and became gradually younger towards the east.
In the Hausruck area Badenian and Sarmatian deposits are
missing and Pannonian coal measures directly overlie Ott-
nangian deposits (Czurda 1978). This indicates that sedi-
mentation proceeded eastwards on a tilted surface and agrees
with the observation that coal-bearing beds in the Hausruck
area were deposited in erosional depressions within a gener-
ally southwestward dipping peneplain (Pohl 1968; Groiss
1989). The youngest preserved deposits in the Upper Austrian
part of the Molasse Basin are the fluviatile Hausruck Gravels.
These are poorly dated, but generally attributed to the
Pannonian (Rupp et al. 2008). Obviously, today the Molasse
Basin is an erosional domain.
Badenian lignite seams have been mined in the western
Trimmelkam district, whereas Pannonian lignite was exploited
in the Hausruck mining district (Weber & Weiss 1983).
Data and methods
The study is based on seismic data, well log data, moisture
content of lignite and fission track data.
Fig. 1. Simplified geological map of the study area superposed on shaded relief digital elevation model created from SRTM data (SRTM
2004). The positions of data discussed in the paper are given: The regional seismic section (Fig. 5), the well position of the Fission Track
samples (Per-001) and the outlines of coal mining areas (samples for Lignite diagenesis analysis; Fig. 6). The inset map (top left) shows the
position of the study area in the frame of the Northern Alpine Foreland.
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Seismic data
Large parts of the Upper Austrian part of the Molasse Ba-
sin are covered by high quality 3D seismic data. These data
have been used to outline progradational patterns within the
Hall Formation. In addition, stratigraphic information from
the wells was used to map the base of the Innviertel Group
(representing a significant part of the Upper Marine Molasse)
and the base of the Upper Freshwater Molasse, as well as
thickness of sediments between both. These shallow strati-
graphic markers are often picked from mud logging or well
log response and not always confirmed by micropaleontology.
Thus, some uncertainties in the order of 10th of meters might
be regarded to the individual selection. To moderate uncer-
tainties, the created surfaces have been smoothed on a
1 1 km grid. The created surface therefore reflects the
trends of the stratigraphic surfaces.
Shale compaction/Log data
Shale compaction is irreversible and directly related to over-
burden stress (burial depth) if pore pressure is hydrostatic. Ob-
viously, deeply buried shales which reached a given depth
will be more strongly compacted after uplift
and erosion than shales at the same depth in an
area without erosion. Thus, in an area with hy-
drostatic pressure shale compaction trends can
be used to estimate the thickness of eroded
rocks (Magara 1976, 1980). The amount of
compaction can be quantified from the sonic
log because sonic transit time is a result of in-
teraction between porosity and the rock matrix
in a uniform lithology like shales.
In the present study sonic logs from 80
boreholes have been used to quantify erosion.
Moisture content of lignite
As in the case of shale, the compaction of
low-rank coal is mainly controlled by burial
depth and increasing overburden pressure.
The moisture content of lignite (on an ash
free basis, af) provides a great tool to monitor
this process. A data set from (as received) lig-
nite in the Lower Rhine Embayment (Kothen
& Reichenbach 1981) confirms this relation
(Fig. 6). Because the ash yield of lignite from
the Lower Rhine Embayment is typically only
1 to 2 %, the difference between moisture con-
tents on an ash received and an ash free basis
is negligible. Thus, in the present paper the
moisture depth trend in Fig. 6 is used to esti-
mate the thickness of overburden rocks in the
Hausruck and Trimmelkam areas. Analytical
data from 31 pillar samples from the Pannon-
ian-age Hausruck lignite have been reported
by Pohl (1968). Data from the Badenian-age
Trimmelkam lignite have been provided by
Weber & Weiss (1983). Both data sets have
been used to calculate moisture contents (af).
Low temperature thermochronology
In order to detect the magnitude and timing of the post-dep-
ositional burial temperature we have performed low tempera-
ture thermochronology using apatite fission track and apatite
(U-Th)/He methods (AFT and AHe, respectively). Several
samples from different stratigraphic horizons have been inves-
tigated, but only one sample from well Per-001 (see Fig. 1;
1600 m depth) yielded suitable contents of apatite.
AFT
The apatite crystals were embedded in epoxy resin and the
crystal mounts were polished by diamond using a five-step
procedure. In order to reveal the spontaneous tracks the apa-
tite mounts were etched by 5.5 N nitric acid at 21 °C for
20 seconds (Donelick et al. 1999). Neutron irradiations were
performed at the nuclear reactor of Oregon State University,
USA. The external detector method was used (Gleadow
1981); after irradiation the induced fission tracks in the mica
detectors were revealed by etching in 40% HF for 35 min.
Fig. 2. Stratigraphy of the Austrian part of
the Alpine Foreland Basin (modified after
Wagner 1998).
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Track counts were made with a Zeiss-Axioskop microscope
– computer-controlled stage system (Dumitru 1993), with a
magnification of 1000 . The FT ages were determined by
the zeta method (Hurford & Green 1983) using the age stan-
dards listed in Hurford (1998).
AHe
Only single crystal aliquots were dated; and only inclusion
and fissure-free specimens with a well-defined external mor-
phology were used. The shape parameters were determined
and archived by multiple digital microphotographs. The
ejection correction factor (Ft) was determined for the single
crystals by the method of Farley (2002). The crystals were
wrapped in ca. 1 1 mm sized platinum capsules and de-
gassed by heating an infrared diode laser. The extracted gas
was purified using a SAES Ti-Zr getter at 450 °C. The chem-
ically inert noble gases and a minor amount of other gases
due to lithology variations, only the shaly upper part of the
Eggerding Formation, which is typically characterized by
uniform transit times, is considered (see inset in Fig. 3). The
data, which are from wells located in different parts of the
study area (Fig. 4a), follow a well-defined exponential trend,
which reflects increasing shale compaction with burial
depth. Actually exponential relations are the most widely ac-
cepted equation for describing shale compaction (Issler
1992), although linear shale compaction trends have also
been reported (e.g. Wells 1990).
It is important to note, that the Eggerding Formation
shows relatively low transit times and significant deviations
from the “normal” exponential compaction curve in some
shallow wells. All “abnormal” wells are located in the east-
ern part of the study area. A comparison of the gamma ray
logs from the “abnormal” Steyr W 1 and the “normal”
Kemating 1 wells (inset in Fig. 3) suggests that this is not due
to an eastward increase in sand content, but due to overcom-
Fig. 3. Transit-time of the shaly interval of the Oligocene Eggerding Formation vs. meters above sea-
level for several wells.
were expanded into a Hiden
triple-filter quadrupole mass
spectrometer equipped with a
positive ion counting detector.
No analysed crystal exhibited
residual gas > 1 % after the
first extraction. Following de-
gassing, samples were re-
trieved from the gas extraction
line, spiked with calibrated
230 Th and 233 U solutions
and dissolved in a 2%
HNO
3
+ 0.05% HF acid mix-
ture in teflon vials. Each sam-
ple batch was prepared with a
series of procedural blanks (in-
cluding Pt tube blanks) and
spiked normals to check the
purity and calibration of the re-
agents and spikes. Spiked so-
lutions were analysed as 0.5 or
0.8 ml of ~ 0.5 ppb U-Th solu-
tions by isotope dilution on a
Perkin Elmer Elan DRC II
ICP-MS with a APEX micro-
flow nebulizer. Procedural U
and Th blanks by this method
are usually very stable and be-
low 1.5 pg. Sm, Pt and Ca
were determined by external
calibration.
Results
Shale compaction
Figure 3 shows the depth
trend of the sonic transit times
for the Eggerding Formation.
In order to minimize effects
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paction. Consequently, we conclude that the easternmost
part of the study area experienced more uplift and erosion
than its main part. The vertical deviation of the measured
data points from the normal compaction trend allows a quan-
tification of the additional amount of erosion. These data
range from 300 to 1000 m and are mapped in Fig. 4a.
Seismic stratigraphic aspects of the Hall Formation
Eggenburgian sediments of the Hall Formation exhibit
progradational patterns, which indicate eastward sediment
transport (Fig. 5). It is reasonable to assume that during pro-
gradation, the top of the sedimentary package (toplap sur-
face) was horizontal. Thus, the observed present-day
inclination of the toplap surface was horizontal during its
formation. Consequently, the observed pattern implies a dis-
tinct eastward tilting of the basin since the Eggenburgian.
The seismic section flattened to the base of the Innviertel
Group ( = top Hall Formation) displays the original geometry
before the tilting event (Fig. 5).
Elevation of base Innviertel Group and base Upper Fresh-
water Molasse
On the basis of 3D seismic data and information from 698
wells, depth-maps of base Innviertel Group ( = top Hall For-
mation), base Upper Freshwater Molasse ( = top Innviertel
Group) and a thickness-contour-map of the Innviertel Group
have been compiled (Fig. 4b—d).
Figure 4b shows the elevation of the smoothed base of the
Innviertel Group. The general trend of this surface reflects the
morphological evolution reasonably. The elevation increases
gradually from west to east from —300 m to + 300 m a.s.l.
(above sea-level). The Upper Freshwater Molasse is only
preserved in the eastern part of the study area. There the ele-
vation of its base shows a similar trend to the base of the
Innviertel Group and increases eastwards from + 300 m to
+ 600 m a.s.l. (Fig. 4c). The intersection of both horizon
maps with the seismic line is also displayed in Fig. 5.
The thickness-contour-map of the Innviertel Group shows
a rather uniform thickness of 550 to 600 m along the basin
axis (W-E direction; Fig. 4d).
This suggests that (1) the effect of erosion during Karpatian
time before deposition of the Upper Freshwater Molasse was
minor and (2) that the Ottnangian Innviertel Group was tilted
together with the Hall Formation.
Moisture content of Hausruck and Trimmelkam lignite
The average moisture content of Pannonian-age Hausruck
lignite is 44.2 % (af) (standard deviation: 2.05 %; Pohl
1968). Considering the moisture depth trend for the Lower
Rhine Embayment (Fig. 6), this value suggests burial of the
Hausruck lignite beneath an overburden about 650 m thick.
The present-day elevation of the Hausruck lignite seams var-
ies between 580 and 650 m a.s.l. This suggests a Late Mio-
cene paleo-land-surface at about 1250 m a.s.l., which is in
contrast to the highest present-day elevation in the Hausruck
Fig. 4. Results from different analysis, displayed on a study area map (for background geology see Figure 1). a – Estimated amount of
erosion from shale compaction analysis of the Eggerding Formation in several wells (small diamonds). Hand-contoured isolines mark
500 m (green) and 1000 m (red). b – Interpolated present day depth map (meters above sea-level – m a.s.l.) of the base of Innviertel
Group from well data (small dots). c – Interpolated present day depth map of the base of Upper Freshwater Molasse from well data (small
dots). d – Interpolated thickness of the Innviertel Group from well data (small dots). In order to determine the thickness of the Innviertel
Group before deposition of the Upper Freshwater Molasse, only areas where the Innviertel Group is overlain by the latter are considered.
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Fig. 5. Regional W-E 3D-reflection-seismic section in depth (vertical exaggeration 20 times, for position see Figure 1) showing the present
day geometry (upper section) and the geometry “flattened” on the interpreted base of the Innviertel Group (point 1) in the lower section.
The morphology (point 2) is created from SRTM 2004 data. Small squares represent projected well-tops of Base of Upper Freshwater
Molasse and Base of Innviertel Group (points 3 and 4 respectively) as well as the interpolated surfaces (dotted lines, points 5 and 6, cf.
Figure 4b,c). The prograding system in the Hall Formation is indicated (point 7). Additional regional horizons marked are Base of Hall
unconformity (point 8), Top of Eocene (point 9) and Top of Crystalline Basement (point 10).
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Fig. 6. Moisture depth trend of lignite from the Lower
Rhine Embayment (after Kothen & Reichenbach 1981).
Table 1:
Apatite
fission
track
and
apatite
(U-Th)/He
(AFT
and
AHe,
resp
ectively)
results
from
the
Per-001
borehole
(measured
depth
160
0 m;
position
in
Figure 1).
area of 801 m a.s.l. (Goebelberg). Uncertainties of
the erosion estimate are related to the influence of
coal facies on moisture depth trends and the role of
erosion in the Lower Rhine Embayment. In any
case, the moisture content in the Hausruck lignite
implies extensive uplift and erosion.
Lignite in the Trimmelkam area is characterized by
even lower moisture contents of ( ~ 35 % af; Weber
& Weiss 1983). Unfortunately, Fig. 6 cannot be
used to estimate the thickness of the original over-
burden. However, the lower moisture content well
agrees with the lower elevation of the Trimmelkam
lignite (300—350 m a.s.l.) and might be a result of
burial beneath nearly 1000 m of overburden. Note,
that the low moisture content of the Trimmelkam lig-
nite has been previously explained by the pressure ef-
fect of the Pleistocene Salzach Glacier. However, the
Salzach Glacier only reached a maximum thickness
of 600 m in the Trimmelkam area (van Husen 1987).
Fission track data (U-Th/He; AFT, AHe)
The AFT and AHe data of sample Per-001
(1600 m; Upper Puchkirchen Formation) are rather
consistent (Table 1). The apparent AFT age is older
than the age of deposition (55 Ma vs. 22 Ma), but
the mean track length shows significant shortening
(12.8 µm). This is an indication of a young thermal
overprint. The AHe ages also prove thermal reset;
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they are clearly younger than the age of deposition (single-
grain ages are between 4.6 and 7.9 Ma).
For the proper interpretation of the detected rejuvenations
in the thermochronometers several modelling runs of the
thermal history were performed. For the modelling we have
used the computer program HeFTy (Ketcham 2005). This
forward modelling algorithm considers the apparent AFT
age, track length distribution (Fig. 7), the angle of confined
tracks relative to the crystallographic C-axis, kinetic parame-
ters (Dpar), AHe ages and the geometry and actinide concen-
trations of the dated apatite crystals. For the modelling we
only considered the age of sedimentation (22 Ma at 16 °C)
and the current borehole temperature (ca. 55 °C) as invari-
able time-temperature points. The pre-depositional cooling
age of the apatite grains was assumed to be between 100 and
60 Ma, because it is a dominant cooling age phase in the
Eastern Alps, which is the major source area of the sediment.
The thermal modelling indicates a remarkable turn in the
post-depositional thermal history. The rather rapid increase
of the burial temperature was terminated around 10 Ma ago
and since ca. 8 Ma the temperature decreases. Another less
pronounced turn can also be observed at ca. 5 Ma. Since that
time the temperature conditions are rather stable. However
we should consider that this last turn is already in the less
sensitive time-temperature range of the thermochronometers,
thus this turn is much less constrained than the turn in the
trend at ca. 10 Ma ago.
The enveloping of the acceptable tT paths (goodness of fit:
GOF > 0.05) is represented by a light grey field (Fig. 7) and
the envelope of ‘good fits’ (GOF > 0.5) by a black belt.
These areas do not include those individual paths that offer
numerically correct solutions, but tT paths, which are geo-
Fig. 7. a – Confined horizontal track lengths in the apatite crystals
from the Per-001 borehole, measured depth 1600 m (position in
Fig. 1). b – Time-temperature plot showing the results of the ther-
mal modelling performed with HeFTy software (Ketcham 2005).
logically meaningless. For example, paths showing extreme-
ly quick rate of post-depositional warming or indicating
sharp turns (running in zigzags) were not considered.
Discussion
The integration of the different data sets allows the separa-
tion of at least two tectonic events during Neogene times:
Karpatian/Badenian westward tilting
A regional Miocene westward tilting event affecting the Aus-
trian part of the Molasse Basin is strongly indicated by the tilt-
ing of progradational patterns within the Hall Formation
(Eggenburgian). The uniform thickness of Ottnangian sedi-
ments of the Innviertel Group along the basin axis suggests that
the tilting occurred after Ottnangian time. The onlap relation of
Lower Badenian to Pannonian rocks of the Upper Freshwater
Molasse and the top of the Innviertel Group suggests that tilting
occurred before the Badenian. Thus, the tilt of about 0.5 % can
be attributed to the Karpatian. There is no regular change in the
amount of Karpatian erosion along the E-W trending basin axis.
This suggests that tilting postdates erosion.
Late Miocene regional uplift
Moisture contents of lignite in the Hausruck mining dis-
trict suggest that the hypothetical Late Miocene paleo-land-
surface is located at about 1250 m a.s.l. This implies major
erosion after deposition of the Upper Freshwater Molasse in
post-Early Pannonian times. Erosion removed 450 (Goebel-
berg area) to 800 m (valley areas between Trimmelkam and
Hausruck) thick sediments.
Late Miocene to Pliocene cooling, as indicated by thermo-
chronological data, supports regional exhumation after 10
million years before present. Although the amount of cool-
ing is poorly constrained, its most likely value of about
20 °C (Fig. 7) fits well with an erosion in the order of 500 to
700 m (in consideration of typical geothermal gradients for
this part of the basin of about 3.0 °C/100 m; Kamyar 2000).
Moreover, the modelling results suggest that cooling reached
a peak before 5 Ma.
Local uplift in the eastern Molasse Basin
Shale compaction data suggest that erosion in the eastern
part of the study area removed sediments, which are up to
1000 m thicker than in the main part. Thus sediments in the
order of 1500 to 1800 m thick were probably eroded. Al-
though thermochronological data are missing, we assume
that uplift was contemporaneous with regional uplift in the
Late Miocene.
Comparison with Neogene uplift in the Swiss part of the
Molasse Basin
For the Swiss part of the Alpine Foreland Basin eroded
rocks at least 2 km thick related to a Late Miocene uplift
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event have been proved (Kuhlemann & Kempf 2002).
Schegg & Leu (1998) clarify the thicknesses of eroded Neo-
gene sediments with values of 700 m in the northeast and
1500—3000 m near the Alpine front to the south. Numerous
analyses on mineral cooling ages demonstrate that the basin
underwent the erosion during Pliocene times. Concerning
the cause of the uplift it is proposed that accelerated erosional
unroofing of the Swiss Alps triggered isostatic rebound and
erosion of the foreland basin after 5 Ma. Additionally a
change in climate and/or drainage reorganization is suggested
(Cederbom et al. 2004, 2011). Andeweg & Cloetingh (1998)
propose that the Late Cenozoic uplift, which superimposes
the former flexural process due to loading, might have been
caused by breaking up or delamination of part of the subsided
European crust.
Consequences for hydrocarbon systems in the Molasse Basin
Changes in basin geometry have an influence on both ther-
mogenic and biogenic hydrocarbon systems. Tilting of the
basin shifts oil—water contacts and controls migration paths.
Basin uplift and erosion may cause termination of hydrocar-
bon generation if the system leaves the oil window (due to
temperature decrease) along the way. Cooling favours bio-
degradation processes and biogenic gas generation. Decreas-
ing pressure changes the gas—oil ratio. Upcoming basin and
petroleum systems modelling based on structural forward
modelled sections will provide information about these issues.
Possible mechanism for Neogene tectonic activity
A main extensional phase captured the region during Late
Ottnangian or Karpatian to Early Badenian times. East of the
Tauern window a number of intramontane pull-apart basins
were formed along major stike-slip fault zones (e.g. Mur-Mur-
Mürz-valley, Enns-valley). Due to these tectonic movements,
stress regimes and sedimentation patterns significantly
changed in the central and eastern part of the Eastern Alps
(Frisch et al. 1998) and may also have affected the foreland
basin even though in a minor way.
Peresson & Decker (1997) proposed that a Late Miocene
(after 9 Ma and prior to 5.3 Ma) compressional event along
the entire Alpine-Pannonian system terminated the former
eastward lateral extrusion and reversed the stress regime.
The cause for prominent Neogene uplift in the Alpine re-
gion is still intensively discussed. It is probably related to
isostatic rebound of the Alps after termination of thrusting.
Cederbom et al. (2004) suggest that erosional unroofing of
the Swiss part of the North Alpine Foreland Basin occurred
due to isostatic rebound of the mountain belt in response to a
wetter climate (driven by increased precipitation) post 5 Ma.
The strong local uplift in the eastern Molasse Basin paral-
lels the southwestern margin of the Bohemian Spur. This
spur exhibiting thickened crystalline crust extends some
100 km to the southeast below the Eastern Alps (see inset in
Fig. 1 for location of the Bohemian Spur) (e.g. Tari 2005).
Thus, isostatic uplift of the Bohemian Spur might have
caused the observed differential uplift. Within this context,
uplift data from the eastern margin of the Bohemian Spur
would be interesting. According to our knowledge such data
are not available.
Conclusion
The combination of geophysical, petrophysical and thermo-
chronological techniques provides new insights into the tim-
ing and dimensions of uplift and erosion events in the
Austrian part of the Northern Alpine Foreland Basin. The
most important conclusions drawn from the present study in-
clude:
An inclined toplap surface within the Hall Formation and
the geometry of the Innviertel Group provide evidence for re-
gional westward tilting of the study area. The gradient of the
slope is about 5 m per kilometer (0.5 %). Tilting occurred dur-
ing Karpatian time ( ~ 17 Ma before present) and postdates the
Late Ottnangian filling of the marine basin. Tilting might be
related to the contemporaneous onset of continental escape
within the Eastern Alps. As a result of tilting, Badenian to
Pannonian sediments of the Upper Freshwater Molasse onlap
onto the inclined top Innviertel unconformity.
Moisture contents of lignite in the Hausruck mining dis-
trict and thermochronological (apatite fission track and
(U-Th)/He) data from a sample in the western part of the
study area indicate uplift and erosion after deposition of the
Upper Freshwater Molasse. Erosion estimates are in the order
of 500 to 900 m. Major erosion probably commenced at
about 8 Ma and slowed down about 4 Ma before present. Al-
though poorly constrained, this time interval excellently fits
with the time constraints of Cederbom et al. (2004) and
Genser et al. (2007) although the thicknesses of eroded rocks
reach greater dimensions in the Swiss part of the basin.
Shale compaction data have been derived from sonic
logs of the Eggerding Formation (Lower Oligocene). Most
data follow a regular depth trend. Deviations from this trend
indicate that uplift along a narrow zone in the easternmost
part of the study area was significantly higher (up to 1000 m)
than in the rest of the study area. Considering the regional
uplift of 500 to 900 m, it is concluded that sediments, up to
1500 or even 1900 m thick, have been eroded along the nar-
row eastern zone. It is reasonable to assume that eastern up-
lift was contemporaneous with regional uplift (i.e. Late
Miocene). Uplift in the east is probably related to differential
isostatic rebound of the crystalline rocks along the Bohemian
Spur compared to clastic basin fill or to a compressional
event (Peresson & Decker 1997).
We can expect that both Karpatian tilting and Late Mio-
cene erosion have major influence on the petroleum systems.
For example, tilting probably influenced oil—water contacts
in E-W-elongated hydrocarbon deposits. Moreover uplift
and erosion probably stopped hydrocarbon generation and
influenced gas to oil ratios in existing accumulations.
Acknowledgments: We thank RAG AG for their permission
to publish the data. Special thanks go to RAG geological
personnel for constructive discussion inputs. For helpful
comments concerning the moisture content of coal we are
notably thankful to Bernhard Salcher (ETH Zürich). The
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manuscript benefited from the useful comments of the re-
viewers, Gabor Tari (OMV) and Nestor Oszczypko (Jagiel-
lonian University Krakow).
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