Active tectonics and geomorphology of the Gaenserndorf Terrace in the Central Vienna Basin (Austria)

Quaternary International xxx (2017) 1e14

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Active tectonics and geomorphology of the Gaenserndorf Terrace in the Central Vienna Basin (Austria) M. Weissl a, *, E. Hintersberger a, J. Lomax b, C. Lüthgens c, K. Decker a a

Department of Geodynamics and Sedimentology, University Vienna, Althanstrasse 14, A-1090 Vienna, Austria Institute of Geography, Justus-Liebig-University, Senckenbergstrasse 1, D-35390 Gießen, Germany c Institute of Applied Geology, University of Natural Resources and Life Sciences, Vienna, Peter-Jordan-Str. 70, A-1190 Vienna, Austria b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 April 2016 Received in revised form 10 November 2016 Accepted 14 November 2016 Available online xxx

In the central Vienna Basin, the area north of the River Danube is dominated by large river terraces consisting mainly of coarse sandy gravels and sand deposited by the Danube and the Morava River. IRSL dating for the terrace body yielded minimum ages from about 200 to 300 ka. The terrace deposits are locally covered with loess and aeolian sand of the last glacial period revealing OSL/IRSL ages of about 15 ka. The terraces are dissected by a system of normal faults. One of these faults, the Aderklaa-Bockfliess Fault, was investigated in 2014 by paleoseismological trenching. Eventually, the exact fault location and its vertical offset of 10 m were defined by combining electrical resistivity measurements and the analysis of remote sensing data. In addition, at the northern part of this so-called Gaenserndorf terrace, high-resolution digital terrain models based on LIDAR measurements show landforms comparable with relief features resulting from permafrost degradation. Large elongated and clam-shaped depressions are interpreted as basins of former thermokarst lakes. Current dry valleys are interpreted as the Pleistocene drainages of the terrace surface. The cryogenic morphology is preserved only in the elevated parts of the terrace and therefore in the footwall of the bounding normal faults. In contrast, Quaternary basins of the hanging wall are filled with up to 40 m thick Pleistocene and Holocene growth strata. Therefore, most characteristics of the recent geomorphology can be interpreted as a result of overlapping neotectonic processes and permafrost degeneration during the Pleistocene. © 2016 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The accumulation and erosion of fluvial gravel deposits in the Vienna Basin are linked to climatic changes and neotectonic processes during the Pleistocene (Gibbard and Lewin, 2009; Salcher and Wagreich, 2010; Homolov
a et al., 2012). Earlier studies have shown that some Miocene normal faults in the Vienna Basin were reactivated, and have vertically offset Pleistocene deposits (Stiny, 1932; Küpper, 1952; Fink, 1973; Decker et al., 2005; Hinsch et al., 2005b). Therefore, river terraces are useful marker horizons in defining Quaternary stratigraphy and fault systems (e.g., Decker et al., 2005). Active tectonics in the Vienna Basin has been studied during the last decade using subsurface seismic, shallow geophysical and geomorphological data as well as data derived * Corresponding author. E-mail address: [email protected] (M. Weissl).

from trenching surveys at particular sites (Decker et al., 2005; Hinsch et al., 2005a; Chwatal et al., 2005; Beidinger and Decker, €lzel et al., 2008; Hintersberger et al., 2014). However, the 2011; Ho interpretation of such data might be ambiguous because erosional, cryogenic and anthropogenic processes can produce similar morphological features as active faults (van Vliet-Lanoe et al., 2004). Therefore, fault scarp surveys using ground penetrating radar, reflection seismic, and resistivity profiles demonstrate that those methods are useful in this context to precisely localize faults (Chwatal et al., 2005; Beidinger and Decker, 2011). Optical stimulated luminescence was applied as appropriate dating method (Fattahi, 2009) to set Pleistocene terrace layers and basin sediments into a chronological relation. Here, we present a study where we investigate not only the younger tectonic processes affecting the river terraces, but also putting them in context to permafrost degradation during the Pleistocene in order to distinguish between both influences on present-day topography.

http://dx.doi.org/10.1016/j.quaint.2016.11.022 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.

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2. Tectonics of the Vienna Basin Transfer Fault System (VBTF) The Vienna Basin is a SSW-NNE oriented Neogene basin of about 200 km length and 55 km width. It extends from the eastern Alps in Austria to the western Carpathians in the Czech Republic (Fig. 1a and b). The crustal-scale pull-apart basin evolved between two leftstepping segments of a large sinistral transform fault system with basin subsidence starting during the Middle Miocene (Royden, 1985; Decker and Peresson, 1998). Former studies and remote sensing data show that a number of faults offset not only the Miocene sediments, but also Pleistocene deposits (Decker et al., 2005; Peresson, 2006; Wessely, 2006). These faults are consequently regarded as active and capable of generating severe earthquakes with magnitudes up to M ~7 (Hinsch and Decker, 2011). Active tectonics in the Vienna Basin is dominated by a seismically active sinistral strike-slip fault system, the so-called Vienna Basin Transfer Fault System (VBTF), which is located at the SE margin of the Miocene basin. Towards the central part of the basin,

the VBTF system splits up into several normal fault splays crossing the basin (Beidinger and Decker, 2011). Even though those splay faults do not show any historical or instrumental seismicity, geological, geophysical, and morphological data prove that they moved at very slow velocities of <0.1mm/a during the Quaternary (Decker et al., 2005). One of the major indications for Quaternary activity along those faults is the observed displacement of a Pleistocene river terrace extending from Vienna to the Carpathian Mountains in Slovakia (Fig. 2a and b). Here, we present data from the western margin of this so-called Gaenserndorf Terrace (GDT), where the NW-dipping Aderklaa-Bockfliess normal fault (ABF), forms a distinct fault scarp with heights up to 5 m. 2.1. Geomorphology: Floodplains and terraces 2.1.1. Marchfeld area The Marchfeld area roughly coincides with the Quaternary floodplain in the central part of the Vienna basin, between Vienna and the Carpathians (Fig. 1b). The greater part of the floodplain is

Fig. 1. a) Active faults in the Vienna Basin. Red lines indicate the left lateral Vienna Basin Transfer Fault (VBTF) and its splay normal faults dissecting the Gaenserndorf Terrace (GDT) in the central basin. The dashed yellow line marks the outlines of the Vienna Basin. MFD: Marchfeld area; SW: Seewinkel Area. Coordinates: Gauss Kruger, MGI M34. b) Inset shows the location of the Vienna Basin within Central Europe. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. a) DEM of the Marchfeld area, important geomorphologic features, and faults. GDT: Gaenserndorf Terrace; TWS: Terrace west of Seyring; SHT: Schlosshof Terrace; AB: Aderklaa Basin; OB: Obersiebenbrunn Basin; LB: Lassee Basin; ABF: Aderklaa-Bockfliess Fault; MNF: Markgrafneusiedl Fault; VBTF: flower structure of the Vienna Basin Transfer Fault. b) DEM based cross-section of the Vienna Basin and thickness of the Quaternary sediments based on drilling data.

covered by sediments of the Danube River, whereas the area in the northeast is dominated by sediments transported from the north by the Morava River. Towards the north, the floodplain is limited by the slopes of a hilly landscape, consisting mostly of Miocene sediments with a Quaternary loess cover (Grill, 1968; Wessely and Draxler, 2006). Between the recent flood plain in the south and the hillsides in the north, several Pleistocene river terraces, among them the Gaenserndorf Terrace (GDT) as the most prominent, cover the plain between the basin margin in the west and the Morava river floodplain in the east (Fink, 1955) (Fig. 2a). Tectonic subsidence along the splay faults during the Quaternary resulted in fragmentation of the river terraces and the generation of small Quaternary basins, such as the Aderklaa and Obersiebenbrunn basins (Fig. 2b). Earlier efforts to correlate river terraces via their surface elevations lack precise knowledge on the tectonic processes (Fink and Majdan, 1954; Grill, 1968; Wessely and Draxler, 2006). At the northern parts, alluvial silt and sand were washed down from the hillsides and redeposited as alluvium within the

Quaternary basins. Several small creeks drain the hills towards the south. One of them, the Russbach Creek, crosses the western part of the GDT complex and the Aderklaa Basin (Fig. 2a). Another creek, the Weidenbach, drains the hills towards the east to the Morava River. During the Pleistocene and Holocene, this creek eroded the terrace gravels and Miocene sediments forming the northern margin of the GDT (Fink, 1955). Before damming and draining facilities were established during the 19th century, particularly the Russbach frequently produced heavy floods, which transported large amounts of sediment from the hills to the floodplain (Wiesbauer and Denner, 2013; Loderer, 1982). Before these creeks had formed their recent streambeds, wide areas in the north of the Gaenserndorf Terrace were covered by the overbank sediments. 2.1.2. Gaenserndorf Terrace and Aderklaa Basin The GDT is dissected by several normal faults into several segments which are named GDT1-GDT3 in the following for further reference (see Fig. 2a). The western and eastern margins of its central segment, GDT2, are tectonically controlled by the NNE-SSW

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trending fault scarp of the WNW-dipping Aderklaa Bockfliess Fault (ABF) and by the SE-dipping Markgrafneusiedl Fault (Fig. 2a and b). The terrace body consists of sandy coarse gravel and sand. From former investigations, drillings, and gravel pit exposures, it is known that the Quaternary river sediments reach a thickness of about 10 m at the terrace (Grill, 1968). Digital elevation models show that the terrace surface between the Aderklaa Basin and Obersiebenbrunn Basin is gently dipping towards the west, contrary to the eastward flowing direction of the Danube (e.g. Decker et al., 2005). Therefore, the terrace body must have been tilted after its deposition. Based on this evidence the Quaternary basins north of the Danube are interpreted to overly a major active SEdipping listric normal fault located west of the Aderklaa Basin (Hinsch et al., 2005b). Towards the west, the GDT2 segment is bounded by the Aderklaa Basin (Fig. 2a). Between the town of Deutsch Wagram and the village of Bockfliess, the fault scarp of the ABF forms the distinct eastern margin of this shallow basin (Fig. 3a). The basin has a width of about 2 km in E-W direction extending about 6 km in NNE-SSW direction. Today this basin is drained by two dammed channels of the Russbach Creek. 3. Methods 3.1. Remote sensing High-resolution (1
1 m LIDAR) digital elevation models provided by the province government of Lower Austria were used to

investigate the terrace surface, particularly that of GDT2, in order to localize the fault scarps. Beside fault scarps and erosional landforms, the digital elevation model show spacious, largely elongated, sometimes clam-shaped, depressions of different size, especially in the northern part of the GDT surface (Fig. 2a). In addition, satellite images of different dates (©Google Earth) were analyzed because relict landforms as ice wedges and levees of former streams and ponds can be identified from crop marks. 3.2. Trenching and geophysical investigations at the AderklaaBockfliess fault scarp The scarp of the Aderklaa-Bockfliess Fault (ABF) dips very gently with a dip of about 1 towards the Aderklaa Basin so that precise fault localization is not easy to determine according to topography. Therefore, we applied geophysical methods and excavated trenches in order to locate the fault plane near the surface. A geophysical survey using 40 MHz, 200 MHz and 400 MHz ground penetrating radar (GPR) antennas was applied to precisely define the fault position. However, and in contrast to good results from other studies in the area (Chwatal et al., 2005; Beidinger and Decker, 2011), the GPR data at our study site was ambiguous, and it was not possible to localize the fault based solely on GPR data. Therefore, two electrical resistivity tomography sections with a length of 200 m and third one with a length of 400 m were measured using GEOTOM4MK100 equipment manufactured by GEOLOG2000. For the two shorter sections, a Wenner array with electrode space of 2 m was chosen to achieve sufficient subsurface

Fig. 3. a) DEM of the western margin of the GDT2 with the trench site of Deutsch Wagram (DW). Former thaw lakes (TL1-3), dunes (D), and wind parallel ridges similar to yardangs (Y), are situated at the northern part of the GDT2. Dells (d) corrugating the fault scarp indicate advanced subsidence before the last glacial period. The dotted line is a gas pipeline crossing the Aderklaa Basin Fault (ABF). GP: gravel pits. Streambeds of the Russbach Creek show situation prior stream regulation in the 19th century. b) Inset figure shows the DEM between Aderklaa Fault System (AFS) and Matzen Fault System (MFS). Coordinates: Gauss Kruger, MGI M34.

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penetration and resolution. The third section had a length of 400 m with an electrode space of 4 m, overlapping partly the two former measurements. The data was filtered and inverted using the Res2dinv© software. Two trench segments having a maximum depth of 3 m and a length of 78 m (DW 1) and 65 m (DW 2) respectively were excavated in the north of the town of Deutsch Wagram in 2014 (Fig. 3a). After cleaning the sections and fixing a coordinate grid (1 m grid distance), the stratigraphic sequence of each trench segment were documented both graphically and photographically. 3.3. Luminescence dating 3.3.1. Basics Luminescence dating techniques enable the reconstruction of the point in time, when luminescent minerals were last exposed to daylight, and therefore the determination of burial ages of sediments. Naturally occurring ionizing radiation (cosmic radiation and primarily the radiation from the 238U and 232Th decay series, and 40 K) leads to the build-up of a latent luminescence signal in quartz and potassium-rich feldspar minerals, as long as these are shielded from daylight. Once these minerals are exposed to daylight during transport, the luminescence signal is erased (optically bleached) and reset to zero. Only after subsequent deposition, the signal starts to build up again. Regarding the physical background and the basics of luminescence dating methods we refer to previously published review papers of Preusser et al. (2008), Rhodes (2011), and Wintle (2008). 3.3.2. Sampling and sample preparation Samples were collected in the field by driving an opaque steel cylinder into the freshly cleaned sediment surface and transferring the material into light tight plastic bags. All subsequent sample preparation steps were conducted under subdued red light conditions in the Vienna laboratory for luminescence dating (VLL). Samples were first dried and dry sieved. The grain size fraction of 100e200 mm was used for further preparation steps. The material was etched in 15% HCl to remove carbonates, treated with Na2C2O4, (0.01 N) to disperse clay particles, and with 10% H2O2 to dissolve organic components. Quartz and feldspar separates were obtained by density separation using LST Fastfloat. The quartz separates were treated in addition with 40% HF to remove the outer 10 mm affected by alpha radiation.

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dried, homogenised and stored in sealed Marinelli beakers (500 ml, about 1 kg dry weight) for at least a month to establish secondary secular radon equilibrium. Measurements were conducted at the VLL using a Canberra HPGe detector (40% n-type).

4. Results and discussion 4.1. Remote sensing and field observations High resolution digital terrain models (LIDAR) show spacious depressions especially in the northern parts of the GDT (Fig. 2a). Diameters can vary from hundred meters up to 2 km (e.g. Fig. 3a). Many of them have rounded rims towards the SE rising 1e3 m above the centre of the depression. The rims of the depressions are also visible in satellite images as bright crescent stripes because these margins dry out faster than the surrounding areas. Comparable landforms are the characteristic features of the Seewinkel area, east of Lake Neusiedl (Fig. 1b), but contrary to the GDT, some of the Seewinkel depressions are still filled with water today.
kely Those depressions were assumed to be relict thaw lakes (Sze et al., 2009; Draganits et al., 2016) and similarities to recent thermokarst lakes in arctic regions support this hypothesis (Bird, 1967; Jorgenson and Shur, 2007; French, 2007, pp. 201e206; Morgenstern, 2012; Grosse et al., 2013). Some of the well-defined depressions on the GDT shown by the LIDAR data were crossed and exposed by a pipeline trench (Fig. 3a and 8). At exposed profiles, intense soil formation is distinct mainly in the centre of these depressions which can be interpreted as former lake basins. In the deepest parts of the largest basins with diameters of up to 2 km the cover of haugh and sand layers is partly missing, probably indicating a redistribution of the uppermost layer during permafrost degradation (e.g. Fig. 4). The central part of GDT2 is characterized by an aeolian sand cover with wind parallel dunes (Fink and Majdan, 1954; Fink, 1955; Wiesbauer et al., 1997). Wide parts of the Marchfeld area and the river terraces north of Vienna were originally covered by aeolian sediments (Fig. 2a). Earlier studies (Fink, 1955; Grill, 1968) distinguished three NW-SE oriented zones based on different characteristics of the sediment cover. The middle zone of the GDT is dominated by a cover of aeolian sand called the “Older Windblown

3.3.3. Experimental setup In this study, we used both quartz and potassium-rich feldspar as luminescence dosimeters for age determination. Both fractions were measured with small aliquots of 1 mm diameter mask size using a grain size fraction of 100e200 mm. All measurements for determination of the equivalent dose were conducted in the VLL on RISØ TL-OSL DA 20 automated luminescence reader systems (Bøtter-Jensen et al., 2000, 2003). For the De determination of the quartz fraction, a single aliquot regenerative dose protocol (Murray and Wintle, 2000, 2003) was applied. For the protocol, a preheatcutheat temperature combination of 230 and 200  C was used, and stimulation was carried out with blue LEDs at 125  C for 40 s. For De determination of the feldspar fraction, a conventional SAR IRSL protocol was applied (Wallinga et al., 2000; Blair et al., 2005), using a preheat of 250  C for 20 s and a stimulation at 50  C for 300 s. 3.3.4. Determination of the dose rate Separate samples from the direct surroundings of the luminescence samples were taken for radionuclide determination using high resolution, low level gamma spectrometry. Samples were first

Fig. 4. North section of the gas pipeline (location F2 in Fig. 3a), viewpoint towards the west. Alluvial sediments in the lower part are overprinted by cryoturbation. Overlying sand layers and loamy haugh have been transformed to dark soil within a former basin. On the right a distinct offset (F2 in Fig. 3a) is marked with arrows.

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€ Sand” (Alterer Flugsand). This sand cover reaches only few decimetres in thickness, and therefore, no thermokarst depressions could develop in this zone. The northern parts of the terrace are covered by up to 5 m of loess, haugh and colluvial sandy sediments. Large depressions, probably former thaw lakes, are scattered over this zone. In contrast, the southern terrace margins show a zone of thick “Rim Loess” (Randloess) without characteristic periglacial landforms. The former sand dunes of the lower Holocene Danube floodplain are almost completely levelled out today. They differ in lithologic composition from the sand on the higher Gaenserndorf Terrace (Frasl, 1955). Therefore this sediment is referred to as “Younger Windblown Sand” (Jüngerer Flugsand). Several ridges oriented in WNW-ESE direction extend at the northern margins of the GDT, and are most likely parallel to the assumed wind direction of the latest Würm stage (MIS 2) (Sebe et al., 2015) (Fig. 3a). Such large-scale ventifacts differ from the lower and rather erratic shaped dunes which cover the central parts of the terrace and its southern margins. The elongated windparallel ridges in the north of the GDT may be interpreted as eroded sediment cover and therefore as yardang like forms which are very distinct landforms further east in Hungary (Sebe et al., 2011). During the uppermost Pleistocene, it is stated that steady winds accumulated windblown sand and loess in the form of parallel dunes running WNW to ESE (Sebe et al., 2015). Sand dunes are assumed to have covered wide areas of the GDT and large amounts of sand accumulated especially in the wind shadow formed by the mountain ridges west of the GDT1 and the Markgrafneusiedl fault scarp. Furthermore, wide areas of the Obersiebenbrunn and Lassee basins are still covered by sand dunes (Fink, 1955). Although the dunes were mobilized repeatedly during historical times by agriculture, most of them are believed to have their origin in the late Pleistocene (Wiesbauer et al., 1997). However, age control by numerical dating is still missing in this region. In its northern parts, the GDT surface is structured by shallow depressions whereas dry valleys corrugate its margins (Fig. 2a). In recent times, no distinctive drainage pattern could have developed because the sandy and gravelly terrace body conducts the meteoric water quickly into the subsurface. However, during permafrost conditions, the frozen ground could not absorb rain and meltwater. As a consequence, erosional processes led to the development of superficial drainage patterns. Therefore, clearly visible dells in the central part of the terrace and dry valleys corrugating its margins (Fig. 3a) can be interpreted as typical periglacial landforms (French, 2007). Interestingly, all roundish and clam shaped depressions are situated in the northern part of the GDT (T1-3 in Fig. 3a), which is covered by reworked silt and loess, whereas the sand-covered central parts and the southern margins of the GDT do not show comparable morphological features. It seems that those diverse forms of periglacial phenomena in different areas of the terrace surface developed due to specific sediment cover and formation conditions. Obviously, the late Pleistocene morphology is only preserved in its elevated parts and therefore in the footwall of the bounding normal faults. At the GDT2, both the Markgrafneusiedl Fault and the ABF delimit the terrace body from the Quaternary basins in the hanging walls of the faults. These basins are filled up with thick Pleistocene and Holocene growth strata up to 80 m (Decker et al., 2005), composed of Danube river sediments and sandy alluvium, which was transported by creeks from the northern hills.

for a gas pipeline across the northern Marchfeld provided access to sedimentary deposits of the GDT2 and its adjacent basins towards the east and west. The pipeline trench cuts the ABF close to its NE lateral termination (Fig. 3a), but no major offset is visible at the gas pipeline. Nevertheless, some minor layer offsets are observed at the pipeline trench walls (Brauneis and Hudribusch, 2009; Posch€ zmüller and Peresson, 2010). For example, about 1.7 km south Tro of the village of Bockfliess, faults and deformation bands (Fossen, 2010) offset the top of the terrace gravel and the overlying, and partly cryogenic deformed, alluvial sand layers for up to 12 cm (Figs. 4 and 5). One of these faults (F2 in Fig. 3a and Fig. 4) dips towards NNW (340/70), paralleling the strike of the Miocene Matzen Fault System (location see Fig. 3b) spreading some kilometres further north. Accordingly, the offsets may be interpreted as surface expression of a branch fault of this fault system. Concave deflected sand layers can be interpreted as flanking folds with reverse fault drag (Grasemann et al., 2005). Another fault further west has a fault dipping of 286/62 paralleling roughly the strike of the ABF (F1 in Figs. 3a and 5). This fault also offsets alluvial sand layers for about 10 cm. The available data therefore suggest that the ABF and even some of the branch faults of the Matzen Fault System might have been active faults capable offsetting the Late Pleistocene alluvial cover. The NW-dipping morphological scarp of the ABF is very gently dipping and subtle with a maximum height of 5 m. The scarp height systematically decreases from SW to NE towards the tip of the fault. Thus, precise localization of the fault is not easy to determine solely based on topography. Borehole and industrial seismic data (Hinsch

4.2. Trenching at the fault-bounded margins of the GDT2 The GDT2 is bounded by the ABF and the Markgrafneusiedl Fault towards the NW and SE, respectively. A 3 m deep trench excavated

Fig. 5. Distinct fault related offset cut by the gas pipeline. Fault F1 (location see Fig. 3a).

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et al., 2005a) was used together with geomorphological analyses of the LIDAR-based DEM in order to select an optimal site for a palaeoseismological trench. The final trench site is located in the highest part of the fault scarp within the city of Deutsch Wagram (Fig. 3a). In 2014, two approximately 2.5 m deep trenches, DW1 and DW2, were excavated with lengths of 78 m and 65 m, respectively (Fig. 6a and b). Both trenches show strong effects of cryoturbation throughout the exposed trench walls, which makes the interpretation challenging (Fig. 7a). However, the trench DW1 clearly shows sedimentary deposits comparable to those typically associated with the Middle and Late Pleistocene terrace body of the GDT (Fink, 1955). In the lower part, grey gravels with sandy channel fills were exposed (Fig. 6c). These terrace gravels are superficially eroded and overlain by light brown sand layers, which can be interpreted as fluvial deposits due to cross bedding and coarse-grained channel fills. Reworked and eroded Miocene mollusc shells (see Section 4.3) were found in these channels, and they indicate a catchment area further north within Miocene deposits. The terrace gravels and the overlying fluvial sand deposits show partly strong deformations by

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cryoturbation. In two large ice wedge casts (running meter 80 and 110 in Fig. 6c) the brown sandy deposits are bulged more than 2 m deep into the terrace gravels. The stratigraphic record in trench DW2 can be divided into two parts: at the eastern end, the deposits resemble those of DW1, whereas further towards the west, approximately around running meter 39, the terrace deposits are covered by brownish alluvium that seemed to intermix with terrace gravels (Fig. 7a). Intense cryoturbation processes seem to have blurred the original depositional structures and contacts. Therefore, even though there might have been an indicator for a tectonic contact, it would have been overprinted by permafrost processes. An electrical resistivity tomography section with a length of 200 m (ERT 1) was measured in the higher part of the terrace (Fig. 6a). A Wenner array with electrode space of 2 m was applied to get sufficient subsurface penetration and resolution. This first section showed clearly the contrast between different electrical conductivity values of Pleistocene gravels or sands and the underlying clayey sediments of the Upper Miocene (Tortonian/Pan€zmüller and Peresson 2010; Grill, 1968). nonian) (Posch-Tro

Fig. 6. Paleoseismological trench on the Aderklaa-Bockfliess Fault (ABF) at Deutsch Wagram. a) Location of the geoelectrical sections (ERT 1e3) and the trench site (DW1&2) (Lidar 1
1 m, courtesy of the province government of Lower Austria). b) Detail showing the situation of trench segments DW1&2 and electrical resistivity tomography ERT 3 (satellite image: Google Earth, date 8/29/2016). c) Schematic and vertically (1.5
) exaggerated north section of the trench. The section shows the terrace gravels (unit 1), alluvial sandy sediments covering the terrace (unit 2), and loamy sediments of the Paleo-Russbach (unit 3). This is covered with an abluation horizon (unit 4). The entire sequence is covered with aeolian sands and chernozem (unit 5). Finding spots of Miocene mollusc shells are marked with spirals.

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Fig. 7. Detail of the north section of trench DW 2 (running meter 20e50) and electrical resistivity tomography crossing the Aderklaa-Bockfliess Fault. a) photo mosaic and schematic section. b) ERT 3 section showing resistivity. The position of the section detail of trench DW2 is marked with a rectangle.

The contact between Miocene and Pleistocene deposits is located at a depth of 10e12 m below the terrain surface. Because this first section did not show any clear lateral cut-off of the horizontal layering, the fault was supposed to be located further west. Therefore, an additional electrical resistivity tomography section (ERT 2) was measured at the assumed hanging wall in about 400 m distance towards the north (Fig. 6a) using the same equipment and

array as for ERT 1. In the hanging wall, the surface of the Miocene sediments was located in a depth of 19 m below the surface. Overlying sediments with lower resistivity could be interpreted as alluvium deposited by the Russbach Creek. Comparing profiles of ERT 1 and ERT 2, a distinct offset of about 10 m could be expected with regard to the different base levels of the Quaternary sediments in the hanging wall and the footwall,

Fig. 8. a) Exaggerated profile along the gas pipeline with details showing the Aderklaa-Bockfliess Fault (ABF) and the Markgrafneusiedl Fault (MNF). OSL/IRSL sample locations are marked with arrows. Sediment cover of the terrace and the adjacent basins show strong relief terrain due to tectonic subsidence and permafrost degradation. b) Detail of the western margin of the GDT2, showing the ABF and OSL ages at AIP 18/19. c) Detail of the eastern margin of the GDT2, showing the MNF and OSL/IRSL ages at various sampling locations.

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respectively. Thus, a third electrical resistivity tomography (ERT 3) was taken, spanning the entire fault scarp, and partly overlapping ERT 1 and ERT 2 in the footwall and hanging wall (Fig. 6a and b). This time, the ERT had the length of 400 m and an electrode spacing of 4 m (Fig. 7b). The results clearly show the expected offset of 10 m (138/148 m a.s.l.) generated by tectonic activity of the ABF. In the western part of ERT 2 the surface of the subsided Miocene sediments can be located in a depth of 19 m below the terrace surface (138 m a.s.l.). Overlying sediments with lower resistivity can be interpreted as alluvium deposited by the Russbach Creek. Using geoelectrical methods, the ABF could be verified 50 m east of the Fabrikstrasse in Deutsch Wagram (48 190 10.3300 N, 16 340 32.0200 E). This coincides with the blurred transition of light grey gravels to the brownish clay-rich sediments in the trench DW2 west of running meter 39 (Fig. 7a). At the eastern margin of GDT2, the fault scarp of the Markgrafneusiedl Fault is more pronounced. The cut-off of the Pleistocene terrace deposits by this Fault is clearly visible in the gas pipeline trench (Fig. 9). Earlier geophysical investigations and paleoseismological trenching along the Markgrafneusiedl Fault not only exposed the SE-dipping normal fault that cuts off the Pleistocene terrace gravels, but also made possible to visualize the stratigraphy of Quaternary sediments in the hanging wall (Hintersberger et al., 2014). 4.3. Stratigraphy In the uppermost 3e4 m, terrace gravels of the GDT2 are affected by cryoturbation, often visible especially in the central terrace areas, whereas specific cryogenic deformations are usually missing on the terrace rims due to erosion. A 0.2 m thick gravel layer consistently covering the zone of cryoturbation (Figs. 10 and 11) shows certain similarities in terms of sedimentary characteristics to abluation horizons described by Liedtke (1990) for moraine landscapes in Germany. Therefore, this gravel layer likely represents the terrace surface during the last glacial period. During tectonic subsidence, the sandy gravels in the basins accumulated to a thickness of about 25e30 m in the Aderklaa Basin and 50e60 m in

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the Obersiebenbrunn Basin (Fig. 2b) (Fink, 1955; Decker et al., 2005). After precise localization of the ABF via geophysical methods, detailed stratigraphic analysis of sedimentary deposits can be used to shed new light on the complex situation exposed in the trench walls. The clayey layers at the western part of DW2 (unit 3 in Fig. 11) seem to be alluvial deposits that also overlie higher parts of the terrace east of the fault. In DW1, east of running meter 85 these clayey layers are replaced gradually by partly cross-bedded fluvial sands (unit 2 in Fig. 10). These clayey deposits and fluvial sands can be interpreted as channel fills that were later deformed by cryoturbation processes. The channel deposits of DW1 and DW2 contain a considerable quantity of reworked marine mollusc shells (e.g. Fig. 6). Most of them are eroded gastropod shells of the genus Potamides (Pirenella) dating to the Sarmatian (Upper Serravallian) stage of the Middle Miocene). In addition, one mollusc shell (Terebralia bidentata) of the older Badenian (Langhian and lower Serravallian) stage was found, as well as some shells of freshwater bivalve (Pisidium amnicum) indicating younger depositions of the Pannonian (Tortonian) stage or later freshwater environments. Today, the Russbach cuts into Sarmatian deposits about 10 km northeast of the trench site. Therefore, Miocene sand and silt together with mollusc shells were transported at least 10 km and then redeposited within the Aderklaa Basin and also on the margins of the GDT2 (Fig. 2a and Fig. 3). In the sections exposed in the trenches DW1 and DW2, the more distinct channels in the upper part of the terrace (unit 2 in Figs. 6 and 11) are exposed on both walls of the trench. Correlations of channels between the two trench walls and cross-bedding orientations suggest a nearly NE-SW flow direction of former streambeds of a Paleo-Russbach, i.e., sub-normal to the current slope of the fault scarp and apparently parallel to the strike of the ABF fault plane. In any case, the evidence of streambeds east of the fault, and therefore in the foot wall, at higher levels than in the hanging wall, shows that the uppermost alluvial sediments (unit 2) were deposited after the formation of the river terrace, still before the terrace body started to be offset due to tectonic subsidence. Even small subsidence of the Aderklaa Basin in relation to the GDT would

Fig. 9. OSL and IRSL samples AIP 25 and 26 within the gas pipeline trench, from the loess cover in the Markgrafneusiedl Fault (MNF) foot wall.

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Fig. 10. Photomosaic of the north section of trench DW 2 (hanging wall), running meter 20e23. Grid meshes are 1
1 m. Unit 3: loamy and sandy fluvial deposits of the PaleoRussbach, and reworked terrace gravels. Unit 4: gravel layer (abluation horizon). Unit 5: dark-brown chernozem evolved from aeolian sand. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Photomosaic of the north section of trench DW 1 (foot wall), running meter 100e104. Grid meshes are 1
1 m. Unit 1: grey fluvial gravel and sand of the Gaenserndorf Terrace. Unit 2: cross bedded brown sand (fluvial deposits of the Paleo-Russbach). Unit 4: gravel layer (abluation horizon). Unit 5: dark-brown chernozem evolved from aeolian sand. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

deflect the originally south- and eastward direction of the drainage towards the developing basin in the West. Therefore, we assume that distributaries of the Paleo-Russbach had been formed at higher levels on the terrace surface. Later, the branches of this creek were dislocated westwards to deeper levels during the progressive subsidence of the Aderklaa Basin. On the terrace and in the basin, the sediments of the PaleoRussbach are intensely overprinted by cryoturbation features (unit 3 in Fig. 10). East of running meter 39, sandy and clayey layers retrace former incisions into the terrace gravels. West thereof, eroded and reworked coarse grained material transported down from the terrace was redeposited in gravel-dominated channel fills. Just south of the trench site, via a deep dell large amounts of gravelrich material were transported from the terrace to the basin. In the electrical resistivity tomography sections those reworked gravels are distinguishable as near-surface areas of higher resistivity (Fig. 7b). 4.4. Dating Twenty samples were analyzed by luminescence dating techniques within this study. The results are provided in Table 1. For a number of samples, ages ranging around 15e20 ka could be determined using both quartz and feldspar based dating techniques. These ages are in good agreement and point towards a high degree of reliance for this dating. However, it needs to be stressed

that all feldspar based ages were not corrected for fading. Fading describes an anomalous signal loss very commonly observed for potassium-rich feldspar (Wintle, 1973). If not corrected for, fading leads to the underestimation of the burial age. Although several methods have been proposed to correct for fading (e.g. Huntley and Lamothe, 2001), the application of fading correction for samples in a high dose range remains debatable. Therefore no fading correction was applied in this study. Ages of the higher age range, where no age control from quartz measurements is available (especially around and above 200 ka, see Table 1), should be interpreted as minimum ages. The exact ages of the Quaternary river terraces in the central Vienna Basin were unknown due to the lack of numerical ages dating until recent times. With regard to a better understanding of the younger geomorphological evolution during the Pleistocene and the overall climatic conditions in the Alpine-Carpathian transition, several sites along the GDT have been investigated during the last years and dated by means of optically stimulated luminescence techniques. The resulting chronological sequences comprise ages from Danube gravel bodies, as well as from silty, and sandy overbank sediments. Such ages should be interpreted with regard to tectonic subsidence and erosional/sedimentary processes. North of Vienna, the sequence of the Gaenserndorf and Schlosshof gravel terraces have been dissected by normal faults (Decker et al., 2005) (Fig. 12). Similar sedimentological characteristics observed within several gravel pits on those terraces suggest

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Table 1 List of OSL and IRSL samples from gravel pits. (n) is the number of accepted aliquots. Sample AIP

Protocol (mineral)

Location

Depth (m)

n

Moisture (%)

238

5 6 7 8 9 9 10 10 11 11 18 19 25 25 26 35 37 38 39 40 41 44 46

IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) OSL (q) IRSL(fs) OSL (q) IRSL(fs) OSL (q) IRSL(fs) IRSL(fs) IRSL(fs) OSL (q) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs) IRSL(fs)

GDT3 GDT3 GDT3 OB OB OB OB OB OB OB ABF ABF MNF MNF MNF SDF1 SDF1 SDF1 SDF1 SDF1 SDF1 SDF1 SDF1

7.0 7.0 5.0 1.7 0.8 0.8 1.7 1.7 1.8 1.8 3.1 0.3 1.0 1.0 0.5 1.5 1.2 3.7 3.6 3.3 2.6 1.9 1.2

6 12 11 6 7 23 7 27 9 24 9 10 8 9 8 5 5 5 10 12 11 8 8

9±6 9±6 9±6 12 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7 12 ± 7 12 ± 7 12 ± 7 12 ± 7 12 ± 7 12 ± 7 12 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7 10 ± 7

0.92 1.13 0.66 1.65 1.74 1.74 1.66 1.66 2.21 2.21 2.08 1.82 2.58 2.58 2.35 1.28 1.07 0.82 1.78 1.84 2.69 1.49 2.39

U (ppm) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.04 0.04 0.04 0.03 0.04 0.04 0.04 0.03 0.02 0.02 0.03 0.03 0.04 0.03 0.04

232

K (%)

2.72 ± 0.08 3.57 ± 0.10 2.08 ± 0.09 8.36 ± 0.23 5.65 ± 0.16 5.65 ± 0.16 5.48 ± 0.16 5.48 ± 0.16 7.35 ± 0.21 7.35 ± 0.21 7.50 ± 0.21 6.60 ± 0.19 8.42 ± 0.23 8.42 ± 0.23 7.62 ± 0.21 6.26 ± 0.18 4.86 ± 0.14 3.54 ± 0.11 7.56 ± 0.21 7.64 ± 0.21 10.46 ± 0.28 5.92 ± 0.17 9.12 ± 0.25

0.94 1.02 0.98 1.45 1.02 1.02 0.94 0.94 1.20 1.20 0.99 1.01 1.12 1.12 1.01 1.65 1.22 1.17 1.18 1.24 1.61 0.91 1.44

Th (ppm)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02

De (Gy)

Dose rate (Gy/ka)

550 ± 35 535 ± 47 583 ± 53 600 ± 36 30.7 ± 2.5 42.0 ± 2.8 31.9 ± 2.5 37.4 ± 2.1 34.2 ± 2.7 41.5 ± 1.5 36.7 ± 1.3 35.2 ± 1.4 38.7 ± 3.7 44.7 ± 2.3 39.0 ± 1.4 554 ± 35 531 ± 45 543 ± 34 274 ± 20 154 ± 5.5 169 ± 5.9 36.7 ± 2.1 43.6 ± 1.6

1.86 2.04 1.81 2.89 1.80 2.42 1.68 2.30 2.08 2.74 2.51 2.47 2.19 2.78 2.59 2.88 2.51 2.13 2.64 2.72 3.45 2.26 3.17

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.16 0.18 0.16 0.27 0.14 0.21 0.13 0.22 0.17 0.25 0.24 0.23 0.18 0.26 0.24 0.26 0.24 0.20 0.25 0.25 0.32 0.22 0.30

AGE (ka) 296 ± 32 262 ± 32 322 ± 41 207 ± 23 17.1 ± 2.0 17.3 ± 1.9 19.0 ± 2.1 16.3 ± 1.8 16.4 ± 1.9 15.2 ± 1.5 14.6 ± 1.5 14.2 ± 1.4 17.7 ± 2.2 16.1 ± 1.7 15.1 ± 1.5 192 ± 21 211 ± 27 255 ± 29 104 ± 12 56.6 ± 5.7 48.9 ± 4.8 16.3 ± 1.8 13.8 ± 1.4

ABF: Aderklaa Basin Fault. GDT3: Gaenserndorf Terrace 3. MNF: Markgrafneusiedl Fault. OB: Obersiebenbrunn Basin. SDF1: trench Siehdichfür 1.

similar deposition ages for GDT and Schlosshof Terrace. Both terrace complexes show well rounded imbricated gravels and boulders, typical braided-river layering, and cryoturbation within the uppermost 3 m. Therefore similar ages for both terraces could be supposed from stratigraphical reasons although the terraces surface differs in altitude up to 20 m. Generally in Penck and Brückner's system (Penck and Brückner, 1909) the GDT was correlated

with the middle Pleistocene “Hochterrasse”. Consequently, the age of the GDT was assumed to be formed during the Riss glacation (MIS 6) (van Husen and Reitner, 2011) unlike to the higher SHT which was linked to the older Mindel glacation (Wessely and Draxler, 2006). Recent infrared stimulated luminescence (IRSL) data reveals minimum ages between about 200 and 320 ka (MIS 7e9) for the terrace gravels of the GDT (Fig. 12).

Fig. 12. IRSL-(fs) ages [ka] in the Marchfeld area. Circles indicate the Pleistocene river terrace sediments, pentagons colluvial/alluvial sediments in the basins, diamonds aeolian/ alluvial cover from the last glacial period. Bars show trench site locations. Dashed line: trace of the gas pipeline; terraces: G€anserndorf terrace (GDT); Schlosshof terrace (SHT); faults: Aderklaa-Bockfliess fault (ABF); Markgrafneusiedl fault (MNF); Vienna Basin Transfer Fault (VBTF). Basins: Aderklaa Basin (AB); Obersiebenbrunn Basin (OB); Lassee Basin (LB); trench sites: Deutsch Wagram (DW); Siedichfuer 1 (SDF1); small numerals are AIP numbers.

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Samples from a gravel pit at GDT3 provided IRSL ages for the GDT ranging from 262 ± 32 ka to 322 ± 41 ka (Fig. 12 & Table 1). For methological reasons, these ages must be interpreted as minimum ages. However, silty colluvial sediments from the Obersiebenbrunn Basin in the hanging wall of the Markgrafneusiedl Fault provided an IRSL minimum age of 207 ± 23 ka (AIP 8 in Fig. 8). This date can be interpreted as a terminus ante quem for the terrace sedimentation because these silt layers cover the subsided terrace gravels indicating the end of gravel accumulation. It can be assumed that at this time the Danube River had located its streambed further to the south. In the trench SDF1 further south (Fig. 12) a sandy channel fill in the uppermost part of the terrace has a minimum IRSL age of 192 ± 21 ka (AIP 35 in Fig. 13). This age can be interpreted as a minimum age of the terrace because the ages are not corrected for fading, and therefore the underestimating of the age cannot be excluded. Two other samples from this trench provided ages of 255 ± 29 ka and 211 ± 27 ka (AIP 38 and 37, cf. Figs. 12 and 14) for the terrace deposition whereas the lowermost sample from the colluvium in the hanging wall was dated with 104 ± 12 ka (AIP 39 in Fig. 14). Based on available IRSL ages the end of gravel sedimentation on the GDT can be supposed latest at about 200 ka BP. After this age, mainly reworked sand and silt were deposited on the terrace surface, whereas in the tectonically subsided areas, additional sand and silt layers from flooding events from the Danube were still deposited. Trench SDF1 on the Markgrafneusiedl Fault provided in the footwall of the fault terrace-ages between MIS 6 and MIS 10. In contrast, in the subsided part, which is formed by colluvial layers, the lowermost samples provided ages ranging from MIS 2 to MIS 5 ages (Fig. 15). Sediments of the pronounced Riss (MIS 6) can be expected in the subsided parts of the GDT where they are covered now by later deposits of the late Pleistocene and Holocene. The aeolian sediments and alluvial sand layers which cover the terrace gravels were dated using OSL and IRSL techniques. Samples from these cover deposits reveal ages of 14.2 ± 1.4 ka to 17.3 ± 1.9 ka (Fig. 8 & Table 1) indicating a sedimentation during the termination of the last glacial period.

5. Conclusions In the central Vienna Basin, normal faults define the eastern and western margin of a Pleistocene Danube terrace, the central segment of the so-called Gaenserndorf Terrace (GDT). At its eastern margin this terrace segment is offset by the SE-dipping Markgrafneusiedl normal fault which forms an up to 17 m high fault scarp. The western boundary is formed by the NW-dipping Aderklaa-Bockfliess normal fault (ABF). The associated morphological fault scarp has a height of up to 5 m. Fault-bounded Quaternary basins formed at the surface of the hanging walls of these faults. These faults were investigated by trenching and geophysical methods. The ABF could be precisely localized by electrical resistivity tomography (ERT), while, due to cryoturbation, the fault plane could not unambiguously constrained in the sections exposed in the trenches. The ERT indicates an offset in the Quaternary river gravels of about 10 m (Fig. 7a and b). Assuming that this offset resulted from tectonic subsidence after gravel accumulation on the GDT, the maximum slip rate for the ABF can be estimated. Using the youngest minimum IRSL ages obtained for the GDT of approximately 200 ka for the end of gravel accumulation on GDT, the maximum slip rate for the ABF would be about 0.05 mm/a for the last 200 ka. The sections of the paleoseismological trench in Deutsch Wagram show Pleistocene depositions of the Russbach Creek at high levels above the recent streambed. From this evidence and from sections along the gas pipeline, it became clear that the creeks coming from the Tertiary hillsides in the north have deposited layers of sand and silt on the terrace surface after the shift of the Danube to the south and therefore after the end of river gravel sedimentation. Due to progressive subsidence in the hanging walls of the dissecting fault system, the creeks dislocated their streambeds to lower levels within the basins. Periglacial morphology is preserved in elevated parts of the terrace, in the footwall of the bounding normal faults. Draining valleys corrugating the fault scarps are an indication of the advanced subsidence process of the Aderklaa and Obersiebenbrunn Quaternary basins before the last glacial period. During the Pleistocene, before subsidence and erosion formed the recent streambeds, the Russbach and Weidenbach Creeks also flowed above the

Fig. 13. Highest part of trench SDF1 at the eastern margin of GDT2 (location see Fig. 12) with western limit of the S-section. In a sandy channel fill IRSL sample AIP 35 (192 ± 21 ka) is marked with an ellipse. Grid meshes are 0.5
0.5 m.

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Fig. 14. Schematic S-section of trench SDF1 on the MGF showing ISRL sample locations. The terrace age is represented by sample AIP 38 from a sand layer in the foot wall. The IRSL samples from sandy colluvial layers in the hanging wall give a chronological sequence from the deepest to the highest layer.

Fig. 15. Diagram showing OSL and IRSL ages from the foot wall (GDT2 and GDT3), from the hanging wall (marked with an ellipse), and the alluvial/aeolian sediment cover.

recent terrace surface. The Pleistocene river terrace is covered in parts with reddish and often loamy sediments containing marine mollusc shells which have their origin in the Miocene sediments further north. Cryogenic landforms on the terrace are only preserved in elevated parts located in the footwall of the bounding normal faults. IRSL age data reveal minimum ages between about 200 and 320 ka (Table 1) for the GDT. In the subsided parts of the GDT, colluvial sediments show significantly younger ages. The terrace surface and the basin grounds are locally covered with alluvium and loess of the last glacial period revealing OSL and IRSL ages from 14.2 ± 1.4 ka to 17.3 ± 1.9 ka (e.g. Fig. 8). Therefore, numerical dating indicates that the apparently contemporaneous sedimentation of all terrace segments from the terrace west of Seyring to the Morava

River in the east, which was formerly correlated with MIS 6, formed mainly during MIS 8 or even earlier.

Acknowledgements The research projects on active faults in the Vienna Basin and the city of Vienna was funded by the OMV (4AH/1637495-2000) and the County of Lower Austria (NC 81-2012). In particular we would like to thank Bernhard Grasemann and Erich Draganits for fruitful discussions and help. In addition, we would like to thank the Department for Geography and Regional Research at the University of Vienna for the supply of the geophysical equipment.

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Please cite this article in press as: Weissl, M., et al., Active tectonics and geomorphology of the Gaenserndorf Terrace in the Central Vienna Basin (Austria), Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2016.11.022