Environmental change and potential climate signal in the Burdigalian Upper Marine Molasse (North Alpine Foreland Basin)

Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics

Observed paleoenvironmental fluctuations in the North Alpine Foreland Basin, as one of the largest sedimentary archives of the Oligocene and Miocene are mainly controlled by regional factors. Global climate signals are usually less prominent than local tectonics and sedimentary input, caused by the enclosed paleogeographic setting of the Paratethys. Moreover are stratigraphic concepts still under debate, disabling a precise correlation of observed regional environmental changes to global climate patterns. In this study, a multi-proxy approach is used to achieve an accurate chronostratigraphy of regional formations and to verify whether global signals can be detected in the North Alpine Foreland Basin. Therefore, a detailed paleoenvironmental and biostratigraphic study of an 18 m-thick section of marine Miocene deposits (Neuhofen Formation) was carried out, using micropaleontology, sedimentology and geochemistry. In total 39 samples, yielding 68 foraminifera species and 47 ostracod species were processed together with 32 nannoplankton samples. Additionally, 34 ostracods and 49 benthic foraminifera were used for the analysis Oxygen and Carbon isotopes. Furthermore, 50 samples of six different sites in the Neuhofen Formation were used for statistical analyses of benthic foraminifera to assess supra-regional environmental correlations. Finally, the chronostratigraphic concept of the Neuhofen Formation was revised using magnetostratigraphic data from four sections, nannoplankton biostratigraphy and Sr-Isotope stratigraphy from previous studies as well as 3D-modelling using previous data additionally to 29 drillings. For the hypothesis that regional environmental patterns are correlating with global climate signals, environmental indices of the Neuhofen Formation (Isotopes, Diversity, Infaunalisation, Abundancy) were compared with global isotope values and Milankovic Cycles. The new stratigraphic concept of the Neuhofen Formation yielded an age of 18.1 – 17.6 Ma with a depositional time of 500,000 years. It was shown by a cluster analysis that strong faunal differences exist between the single localities, indicating separate paleoenvironments. These environmental differences are rather caused by regional factors. Occasionally, e.g. at 17.67 Ma, throughout the deposition of shallow marine sediments in the Neuhofen Formation the influence of global climate change can be inferred.

An integrative palaeoenvironmental and chronostratigraphic study of the Lower Miocene in the North Alpine Foreland Basin – Are global climate signals detectable?

Abstract. Geophysical properties of the subsurface and the vertical stress acting within are key prerequisites to understanding fundamental geological processes and mitigating risks associated with the economic usage of the subsurface. In SE Germany, the North Alpine Foreland Basin (NAFB) is a well-studied sedimentary basin, which was extensively explored for oil and gas in the last century and which is currently explored and exploited for deep geothermal energy. The up to 5 km thick Cenozoic basin fill comprises mostly shales, marls, sandstones, carbonates, and coarse-grained clastics; in particular, Oligocene–Miocene age sediments display significant lateral lithological variability due to two marine transgressions. In addition, Cenozoic marine sediments in the eastern part of the basin are significantly overpressured. The basin sediments overlay Mesozoic passive margin sediments. Here, karstified Upper Jurassic carbonates represent the main target for deep geothermal exploration and production. Even though the North Alpine Foreland Basin has been well studied during its economic development, the relationships between basic geophysical parameters, such as bulk density and seismic velocity, both of which are key for seismic imaging and the prediction of physical rock properties, have not yet been systematically investigated. The same is true for the distribution of vertical stress gradients, a key input parameter for geomechanical modelling and the prediction of natural and induced seismicity. To improve the understanding of density–velocity relationships and the distribution of vertical stress gradients, we systematically analysed 78 deep wells with total depths of 650–4800 m below ground level, which form two overlapping datasets: bulk density and sonic velocity data from 41 deep boreholes were used to establish velocity–density relationships for the main lithological units in the North Alpine Foreland Basin in SE Germany. We applied these newly derived relationships to velocity data of a second set of 55 wells, which at least penetrated the Cenozoic basin fill section in the study area and spliced resulting bulk densities with measured but scarcer measured bulk density data. We integrated these spliced bulk density profiles to vertical stress to investigate the spatial distribution of vertical stress gradients. Thereby, we observed an eastward decrease in vertical stress gradients, which correlates well with the geological configuration of the North Alpine Foreland Basin in SE Germany. In addition, we investigated the distribution of vertical stress gradients at the top of the economically important Upper Jurassic carbonates. As a practical result, we provide lithologically constrained velocity–bulk density relationships and depth-dependent vertical stress gradient models, which can be used as an improved input for future geophysical, geomechanical, geological, and rock physics studies in the North Alpine Foreland Basin, both in fundamental and applied research contexts.

The North Alpine Foreland Basin in SE Germany is Germany’s most active deep geothermal province. However, in its southern and eastern part the basin is considerably overpressured, which is a significant challenge for drilling deep geothermal wells. In this study, we combine drilling data and velocity-based pore pressure analyses with 3D basin modeling to assess the predictability and controlling factors of overpressure in the sub-regional context (area of 80 km × 50 km) around the Geretsried GEN-1 well, a deep geothermal exploration well in the southern part of the North Alpine Foreland Basin in SE Germany. Drilling data and velocity-based pore pressure analyses indicate overpressure maxima in the Lower Oligocene (Rupelian and Schoeneck Formation) and up to mild overpressure in the Upper Oligocene (Chattian) and Upper Cretaceous, except for the hydrostatically pressured northwestern part of the study area. 3D basin modeling calibrated to four hydrocarbon wells surrounding the Geretsried GEN-1 well demonstrates the dominating role of disequilibrium compaction and low permeability units related to overpressure generation in the North Alpine Foreland Basin. However, secondary overpressure generation mechanisms are likely contributing. Also, the impact of Upper Cretaceous shales, which are eroded in the northwestern part of the study area, on overpressure maintenance is investigated. The calibrated basin model is tested against the drilling history and velocity (VSP) data-based pore pressure estimate of the Geretsried GEN-1 well and reveals that pore pressure prediction is generally possible using 3D basin modeling in the North Alpine Foreland Basin, but should be improved with more detailed analysis of lateral drainage systems and facies variations in the future. The results of the study are of relevance to future well planning and drilling as well as to geomechanical modeling of subsurface stresses and deep geothermal production in the North Alpine Foreland Basin.

A new well-preserved ostracod fauna from the middle Burdigalian (lower Miocene) of the North Alpine Foreland Basin

The North Alpine Foreland Basin is the peripheral foredeep of the Northern Alps, extending from Lake Geneva in the West to Upper Austria in the East. The largest portion of the foredeep consists of an undeformed part, called Foreland Molasse, and a small, deformed belt along the North Alpine Thrust Front, called Subalpine Molasse. Spanning up to 150 km in N-S direction, the North Alpine Foreland Basin has its widest extent in SE Germany (Bavaria). Here, the physical properties of the Cenozoic basin fill and its underlying Mesozoic passive margin sediments display a high degree of heterogeneity in both the Foreland Molasse and Subalpine Molasse parts. Since 2016, we systematically analysed data from more than 300 deep wellbores, with vertical depths up to 5 km below ground level, to understand the distribution and interplay of these heterogeneities: We used minimum stress magnitude measurements such as formation integrity and leak-off tests in combination with geophysical borehole measurements such as density and velocity to infer the distribution of lateral and vertical stresses in the SE German part of the North Alpine Foreland Basin. Collection of pore pressure indicators and measurements such as drilling mud weights, drilling problems, well tests and wireline formation tests and their correlation with vertical stress and sediment compaction allowed us to also infer the regional distribution of pore pressure and to model the variable styles of deformation of the Subalpine Molasse along the North Alpine Thrust Front. In this contribution, we give a graphical overview of how stress, pore pressure and deformation are linked and driven by sediment composition and compaction. We also set our findings into context with high frequency, large amplitude variations of temperature and fluid flow patterns, proposing an updated model for the distribution and interference of physical properties and processes in the North Alpine Foreland Basin in SE Germany.

, U. (2009): The North Alpine Foreland Basin: SpecialVolume of the 2008 Molasse Meeting. – N. Jb. Geol. Palaont. Abh., 254 : 1–4; Stuttgart.Abstract: The North Alpine Foreland Basin is a classic area for integrative sedimentological andpalaeontological studies of a dynamic Cenozoic basinal setting characterized by rapidly changingmarine and terrestrial conditions. This special publication includes ten contributions presented atthe 2008 Molasse Group meeting at the State Museum of Natural History in Stuttgart, Germany. Thecontributions include various aspects of sedimentology, vertebrate and invertebrate palaeontology,micropalaeontology, stratigraphy, biogeography and palaeoclimatology of the Lower FreshwaterMolasse (USM), Upper Marine Molasse (OMM) and Upper Freshwater Molasse (OSM). In thisintroduction: 1) a short resume of the activities of the Molasse Group is followed by 2) an intro -duction to the scientific contributions, and by 3) an outlook for future scientific research.Key words: Molasse Sea, North Alpine Foreland Basin, Oligocene, Miocene, Paratethys, palaeonto-logy, sedimentology.

The Miocene marine basins of Central and Southeast Europe, once comprising the Paratethys Sea, were gradually filled with sediments during the Neogene and turned to be the catchment area of the proto‐Danube and finally that of the modern Danube. Seismic data from various parts of the large Danube catchment area show that these several hundred meter deep basins were filled by lateral accretion of river‐transported sediments, appearing as shelf edge scale clinoform sets in seismic profiles. The direction of shelf edge progradation is NW to SE (N to S, W to E) in each basin, except for the Dacian basin where NE to SW direction prevails. The age of the clinoform sets is generally younging downstream: 19–18 Ma in the North Alpine Foreland basin, 14–13 Ma in the Vienna basin, 10–9 Ma in the Danube (Kisalföld) basin, 8.6–4 Ma in the Central Pannonian basin (Alföld), ?9–5 Ma in the Dacian basin, and 6–0 Ma in the Euxinian (Black Sea) basin. In spite of this geographical and temporal pattern, only the Danube (Kisalföld) and the western and central part of the Central Pannonian basin were filled by the proto‐Danube shelf accretion. Formation of the Danube, as a longitudinal river of the Alpine foreland that gradually elongated to the east and followed the retreating shoreline of the Paratethys, most probably took place at the beginning of the Late Miocene, ca. 11 Ma ago, thus the Early and Middle Miocene shelf advance in the North Alpine Foreland and Vienna basins, respectively, cannot be attributed to a „paleo‐Danube”. The clinoform systems of the Dacian basin are coeval with those of the upstream Central Pannonian basin, indicating that by the time the Danube sedimentary system reached the Dacian basin, it was already a shallow basin. The vast clinoforms of the northwestern Euxinian shelf also significantly overlap in age with the Pannonian basin ones; only the <4 Ma part of the shelf accretion can be attributed to the Danube sensu stricto.

Late Burdigalian sea retreat from the North Alpine Foreland Basin: new magnetostratigraphic age constraints

For the first time, drilling- and velocity-based well analysis and 3D basin modeling were combined to test the predictability and controlling factors of overpressure in the North Alpine Foreland Basin in SE Germany. More specifically, the techniques were tested in the sub-regional context of the deep geothermal well Geretsried GEN-1 (TVD = 4852 m), located in the south of Munich. A 3D basin model based on a total of 20 wells was calibrated to the pressure distribution of four petroleum wells and tested against the Geretsried GEN-1 well. The results demonstrate that overpressure in the North Alpine Foreland Basin SE Germany can be predicted from a simple 3D basin model calibrated to a minimum number of wells. Thereby, disequilibrium compaction likely acts as the main overpressure mechanism in the study area, underpinned by significantly higher sedimentation rates at overpressured locations. 3D basin modeling also confirms the role of Upper Cretaceous shales, which, if present, serve as an important pressure barrier between the under- to normally pressured Jurassic and overpressured Cenozoic basin fill. In addition, overpressure magnitudes of the Chattian might be higher than previously expected. The results of this study have great impact on future drilling campaigns in the North Alpine Foreland Basin in SE Germany. Minimized non-productive time and drilling cost, improved well planning and increased safety are amongst the most important benefits of accurate pore pressure and overpressure prediction. The newly derived insights on the mechanisms of overpressure will greatly influence future geomechanical and tectonic studies, since pore pressure drives rock strength and principle stress magnitudes. Finally, the study is a great example for the importance of an interdisciplinary approach and the incorporation of geological conditions, when investigating drilling-related problems.

During the Burdigalian the North Alpine Foreland Basin, as part of the central and western Paratethys, underwent various paleogeographic and paleoenvironmental changes which led to the deposition of different sedimentary facies. For instance, the Ottnangian regional stratigraphic stage (18.2 – 17.3 Ma) was characterized by a major transgression in the beginning, with the deposition of several littoral facies along the northern coastline of the North Alpine Foreland Basin. Successively, the late Ottnangian documents the final retreat of the Paratethys Sea from the North Alpine Foreland Basin and the deposition of widely distributed fluviatile units. The outcrop Neustift in southeast Germany bears many different siliciclastic facies and shows the transition of the Ottnangian marine realm towards a riverine-deltaic environment. Despite the exceptional size and complexity of the succession, this outcrop is only poorly studied. Anyhow, this section gives unique potential for the understanding of the facies evolution during the terminal Ottnangian. In total a 70 m long and 15 m high sedimentary log was recorded together with several micropaleontological samples (1 kg each) for the reconstruction of the depositional environment and stratigraphic positioning. The micropaleontological samples yielded 164 genera of benthic foraminifera and 144 species of ostracods. Moreover, these deposits are extremely rich in macrofossils (elasmobranchia, mollusca, brachiopoda, echinoidea) which are also poorly studied. We found out, that the lowermost segment of the section belongs to the marine “Littoral Facies of Holzbach-Höch”, which deposited directly on top of a transgressive layer on the crystalline basement. Several of the observed ostracod species are new to these deposits. Large-scale cross-bedding structures show that this shallow marine environment was affected by strong tidal currents along the rocky shoreline. The fine-grained sediments with wavy and lenticular bedding on top of the littoral deposits show an ongoing transgression, with neritic foraminiferal assemblages and bioturbation. They were assigned by biostratigraphy to the uppermost Neuhofen Formation. Finally, the marine deposition is replaced by a large-scale deltaic system, resulting into the deposition of the fluviatile Ortenburg gravel. This preliminary study should draw some more attention to this unique outcrop in the North Alpine Foreland Basin and the potential for further studies in the realm of sedimentology and paleontology. 

Shallow oil and gas shows are common in the Alpine thrust front (including the Flysch Zone) and the North Alpine Foreland Basin in Switzerland, southern Germany and Austria, but have not hitherto been evaluated systematically. In the vertically‐drained Vienna Basin and the easternmost part of the Flysch Zone, shallow oil and gas shows and seeps often coincide with deeper‐lying hydrocarbon accumulations, and gas shows occur along major faults – for example within the urbanised area of the city of Vienna. The number of gas shows decreases in the Vienna Basin away from (to the south of) the subcrop of the main thermogenic source rock (the Upper Jurassic Mikulov Formation); however shallow accumulations of microbial gas occur in that area. To the west, along the northern margin of the laterally‐drained North Alpine Foreland Basin, oil shows have been recorded in both Austria and Switzerland; microbial gas shows are common in addition to thermogenic hydrocarbons. Typically the shows form regional clusters along river valleys and occur above shallow gas accumulations.A Lower Oligocene organic‐rich interval represents the main source of oil / condensate and thermogenic gas in the Upper Austrian part of the North Alpine Foreland Basin, whereas the composition of oil shows within the Calcareous Alps to the south indicates the presence of mature Mesozoic source rocks within the Alpine nappes. This implies the presence of an additional, as‐yet untested petroleum system. Thermogenic gas, occurring in Permo‐Triassic evaporitic rocks in the Calcareous Alps, as well as microbial gas in younger sediments, has frequently been encountered during salt mining and tunnelling activities.A surprising discrepancy has been found in different parts of the study area between the number of hydrocarbon shows and the number of economic fields. Whereas the number of fields and shows are approximately in proportion in the Vienna Basin and the Austrian sector of the North Alpine Foreland Basin, shows appear to be “under‐represented” in Germany. By contrast in Switzerland, despite a high number of shows especially in the North Alpine Foreland Basin and the Jura fold‐and‐thrust belt, no economic production has been established to date. Future exploration will show whether this is due to poor reservoir/trap quality, or if undiscovered resources are in fact present. The presence of oil shows generated from Mesozoic and Oligocene source rocks in the SW German and Swiss parts of the North Alpine Foreland Basin suggests the occurrence of multiple petroleum systems; these systems should be delineated in future studies.Few surface seeps have been recorded in less populated parts of the study area such as the high Alps, possibly due to sampling bias. However, this bias does not explain the low frequency of recorded hydrocarbon shows in the German part of the North Alpine Foreland Basin. This may be because the geological setting there is in general less favourable for the migration of thermogenic gas into shallow reservoirs and its preservation in shallow traps.

<p>As one of the most promising settings for deep geothermal energy, the North Alpine Foreland Basin in Bavaria (SE Germany) is currently a location of ongoing planning, development, and operation of hydrothermal energy projects. The high risks associated with the more than 3000 m deep wells, drilled into the reservoir still limits an accelerated exploration and usage of the reservoir’s potential. The heterogeneities of rock mass and thus uncertain subsurface conditions threaten the wellbore integrity and lower the predictability of hydraulic reservoir stimulation and its success. Characterizing the fracturing properties of in situ samples and analogue rocks, extracted from nearby quarries, aims to increase certainty in expected fracturing processes.  </p><p>To determine the fracture toughness and fracture energy in tensile (mode I) and shear mode (mode II) two experimental setups are used. With the double edge-notched Brazilian disk (DNBD) test the mode II fracture toughness is measured by inducing shear failure through uniaxial compression of a bi-notched Brazilian disk. The semi-circular bend (SCB) test applies tensile forces on a notched semi-cylindrical disk by bending the sample around three roller supports and produces mode II fracture. A high-speed camera records the test procedure for further characterizing the fracture propagation and the influence of inhomogeneities.</p><p>Together with elastic and strength properties of the analog rocks, extracted from further rock mechanical tests, the fracture energy values serve as input for a numerical study of borehole stability. By using the hybrid FEM-DEM method, complex 2D and 3D borehole models are computed without losing the relation to realistic processes. In multiple scenarios, that are representative of the geothermal projects in the North Alpine Foreland Basin, the rock mass behavior during borehole excavation is simulated with the Irazu software. Additionally, pre-existing fracture networks, in situ stresses and pore-fluid pressure are integrated, which influence the resulting fracture pattern and fracturing degree. The coupled mechanical-hydraulical model will later be extended to additionally incorporate thermal effects.</p><p>When considered during future drill operations these results can help to lower the economic risk that is still associated with deep geothermal operation and consequently increase the development speed to a sustainable heat energy provision in SE Germany.</p><p>This work is part of the Geothermal-Alliance Bavaria and funded by the Bavarian State Ministry of Science and Arts (StMWK).</p><p> </p><p>Keywords: numerical FDEM simulation; laboratory testing; fracture energy; geothermal wellbores; North Alpine Foreland Basin, SE Germany</p>

Within the Lower Austrian part of the North Alpine Foreland Basin (NAFB), up to 1000 m of sediments were deposited throughout the Ottnangian (Early Miocene, Burdigalian). According to homogeneous compositions and sparse biostratigraphic resolution, a consistent stratigraphic concept from the basin margins into the foreland depocenter was still lacking. New investigations on several deep drill cores throughout the basin provide comprehensive sedimentological, mineralogical, chemical and micropaleontological data. A calcite poor, fossil- and pyrite-free, smectite-rich, up to 800 m thick interval was identified and correlated to the time interval of the late Ottnangian brackish Rzehakia Lake System. For this section, we introduce the term Calcite Minimum Interval (CMI). We define the onset of the CMI by a sharp decrease of calcite contents and the disappearance of autochthonous (and reworked) calcareous nannofossils. We define the termination of the CMI by the permanent increase of pyrite contents and the reappearance of calcareous nannofossils. The CMI as a litho- and chemostratigraphical marker for the Rzehakia Lake System constitutes a stratigraphic key horizon. Within the NAFB in Lower Austria, its onset corresponds to the middle/upper Ottnangian transition while its termination correlates roughly to the Ottnangian / Karpatian boundary. This allows a precise definition, identification and correlation of (upper) Ottnangian stratigraphic units of the NAFB. For the central basinal parts of the Rzehakia Lake System, we introduce the new lithostratigraphic term Wildendürnbach Formation which correlates to the marginal Traisen Formation.