Indications of Deep Marine Fans in the Early Miocene Foredeep of Lower Austria: A Potential New Play

The petroleum province in Lower Austria resulted from the Alpine collision and the subsequent formation of the Vienna Basin. OMV is active in this area since its foundation in 1956. Several plays have been successfully tested and produced in this complex geological region. The main exploration focus is currently on the deep plays. However, this paper proposes a so far unrecognized and therefore undrilled play in a shallower level to broaden OMV's portfolio in Austria. Seismic re-interpretations of reprocessed 3D seismic data and structural reconstructions were used to review some of the existing plays and get novel ideas from improved understanding of processes. In the frontal accretion zone of the Alpine wedge, the Waschberg-Ždánice zone discoveries are limited to the frontal thrust unit and associated structures. The more internal parts of the thrust belt have only sparsely been drilled and are perceived not to have high-quality reservoir rocks. The detailed structural interpretations indicated that the foredeep axis during the Early Miocene was positioned in the thrust sheet located directly in front of the advancing Alpine wedge (comprising the eroding Rhenodanubian Flysch in its frontal part). Seismic amplitude anomalies can be interpreted to represent Lower Miocene basin floor and slope fans. Nearby wells did not penetrate these fans but drilled instead shale-dominated lithologies. Thus, the presence of potential sand-rich fans in front of the advancing alpine wedge is considered a potential new play in Lower Austria. Analogues are found in Upper Austria some 250 km to the West, where several large gas fields in Lower Miocene deposits located in front of the advancing Alpine wedge have been discovered by another operator. In that area the fans are only partly involved in the fold-thrust belt. In Lower Austria, these fans are located within the rear thrust sheet(s), providing a structural component to a mixed structural-stratigraphic trap. Two potential charge mechanism can be considered: a) biogenic gas charge from the organic matter of surrounding shales (like the Upper Austria analogues) or b) oil charge via the thrust fault planes from the Jurassic Mikulov Formation (the proven main source rock in the broader area). Our results add to the understanding of the Miocene structural-stratigraphic evolution of the Alpine collision zone. The definition of a potential new play may add significant value to OMV's upstream efforts in a very mature hydrocarbon province.

Sarmatian and Pannonian mollusks from Pécs-Danitzpuszta, southern Hungary: a unique local faunal succession

As the almost 200-year palaeontological research revealed, the geographical distribution of various fossil mollusk faunas in deposits of the late Neogene Lake Pannon displays a regular pattern. The lake basin was filled by lateral accretion of sediments, resulting in condensed sedimentary successions in the distal parts of the basin and successively younger shallow-water deposits from the margins towards the basin center. Exposed intra-basin basement highs, however, broke this strict pattern when they acted as sediment sources during the lake’s lifetime. The Mecsek Mts in southern Hungary was such an island in Lake Pannon during the early late Miocene. Deposition of the 200 m thick Sarmatian–Pannonian sedimentary succession in Pécs-Danitzpuszta at the foot of the Mecsek Mts was thus controlled by local tectonic and sedimentary processes, resulting in a unique succession of facies and mollusk faunas. A typical, restricted marine Sarmatian fauna is followed by a distinct freshwater or oligohaline interval, which, according to micropalaeontological evidence, still belongs to the Sarmatian. Although poor preservation of fossils does not allow firm conclusions, it seems that freshwater Sarmatian snails were the ancestors of the brackish-water-adapted early Pannonian pulmonate snail taxa. The successive “Sarmatian-type” dwarfed cockle fauna is similar to those widely reported from the Sarmatian–Pannonian boundary in various parts of the Pannonian Basin; however, a thorough taxonomic study of its species is still lacking. The bulk of the sedimentary succession corresponds to the sublittoral to profundal “white marls,” which are widespread in the southern Pannonian Basin. In Croatia and Serbia, they are divided into the Lymnocardium praeponticum or Radix croatica Zone (11.6–11.4 Ma) below, and the Congeria banatica Zone (11.4–9.7 Ma) above; this division can be applied to the Pécs-Danitzpuszta succession as well. Sedimentation of the calcareous marl, however, ceased at Pécs-Danitzpuszta at about 10.5–10.2 Ma ago (during the younger part of the Lymnocardium schedelianum Chron), when silt was deposited with a diverse sublittoral mollusk fauna. Similar faunas are known from the Vienna Basin, southern Banat, and other marginal parts of the Pannonian Basin System, but not from Croatia and Serbia, where deposition of the deep-water white marls continued during this time. Finally, the Pécs-Danitzpuszta succession was capped with a thick, coarse-grained sand series that contains mollusk molds and casts representing a typical littoral assemblage. This littoral fauna is well-known from easternmost Austria, northern Serbia, and northwestern Romania, but never directly from above the sublittoral L. schedelianum Zone. The fauna is characteristic for the upper part of the Lymnocardium conjungens Zone and has an inferred age of ca. 10.2–10.0 Ma. The Pécs-Danitzpuszta succession thus allows to establish the chronostratigraphic relationship between mollusk faunas that have not been observed in one succession nor in close proximity to each other in other parts of the Pannonian Basin.

Palynological and calcareous nannofossil investigation of the Gosau Group deposits from three boreholes in the Vienna Basin, Austria

Palynological and calcareous nannofossil investigations on samples from the basal part of the Gosau Group succession in three boreholes (Glinzendorf T1, Gänserndorf T3 and Markgraf Neusiedl T1) in the Vienna Basin provide calibrated age assessment of late Coniacian to early Campanian age for this thick non-marine to marginal-marine siliciclastic interval.

Applicability and challenges for the authigenic 10Be/9Be dating as revealed by studies from the Pannonian Basin realm

<p>Bourlès et al. (1989:<em> Geochim. Cosmochim. Acta</em>) suggested that authigenic <sup>10</sup>Be/<sup>9</sup>Be ratio could provide a geochronological tool to date deposition of clay-bearing sediment settled in a water column up to 14 Ma old. It is based on ratio of atmospheric cosmogenic radionuclide <sup>10</sup>Be delivered to depositional environments by precipitation and stable <sup>9</sup>Be extracted from rock massifs by chemical weathering. Determination of the initial <sup>10</sup>Be/<sup>9</sup>Be ratio is essential for efficient application of the dating and may vary spatially as well as in time due to changes in drainage basins, depositional environments, climate, and other factors. The potential of the authigenic <sup>10</sup>Be/<sup>9</sup>Be dating was evaluated during last years in the Pannonian Basin realm, located in Central Europe. This contribution summarizes successful applications as well as discovered problems and challenges, which motivate the ongoing research.</p><p>Two initial <sup>10</sup>Be/<sup>9</sup>Be ratios were established from Holocene alluvial and lacustrine clays in the Danube Basin (Šujan et al., 2016: <em>Glob. Planet. Change</em>). The dating was applied to shallow to deep-water sediments deposited in Lake Pannon within the Danube Basin, and helped to constrain paleogeographic changes in the age range of 11.6–3 Ma. Application of the method to the post-rift alluvial succession with high subsidence rates of 50–400 m/Ma in the range of ~9.5–6.0 Ma yielded data consistent with other geochronological proxies (Šujan et al., 2020: <em>Sed. Geol.</em>; Joniak et al., 2020: <em>Palaeo<sup>3</sup></em>). The fast accumulation and tectonic quiescence likely provided stable environmental conditions favorable for the dating method applicability.</p><p>Lacustrine and deltaic deposits of Lake Pannon were analyzed from cores of Paks boreholes in the central part of the Pannonian Basin. The resulting authigenic <sup>10</sup>Be/<sup>9</sup>Be ages are generally in agreement with magnetostratigraphic age constraints correlated using seismic stratigraphy (Magyar et al., 2019: <em>Földt. Közl.</em>). Outliers with relative enrichment of <sup>10</sup>Be appear in most distal facies, where low terrestrial <sup>9</sup>Be input is expected.</p><p>A study of turbidite deposits from the Transylvanian Basin allowed to compare the established lacustrine initial <sup>10</sup>Be/<sup>9</sup>Be with a ratio independently calculated from Ar/Ar dated horizon (Botka et al., 2019: <em>Austrian J. Earth. Sci.</em>). Majority of samples provided a good fit with other age proxies, while one sedimentary interval exhibits twofold increase of <sup>10</sup>Be/<sup>9</sup>Be probably indicating variability in the environmental conditions (Baranyi et al., 2021: <em>Rev. Palaeobot. Palyn.</em>).</p><p>An order of magnitude higher authigenic <sup>10</sup>Be/<sup>9</sup>Be comparing to the established initial ratios were obtained from supposed early Pleistocene sediments from the locality Sollenau in the Vienna Basin. The visual appearance implies, that secondary pedogenic processes might be responsible for a post-depositional input of <sup>10</sup>Be (Willenbring, von Blanckenburg, 2010: <em>Earth. Sci. Rev.</em>). Another case of high <sup>10</sup>Be/<sup>9</sup>Be preventing age calculation was observed in a Pleistocene alluvial environment with intense loess input.</p><p>An ongoing research aims to determine the effects of changes in depositional process, sediment source proximity and provenance on the applicability of the dating method. This research was financially supported by the Slovak Research and Development Agency under contract APVV-16-0121 and by the Hungarian National Research, Development and Innovation Office under contract NKFIH-116618.</p>

P-PINI: a new inversion method for sediment-burial dating

<p>For sediment-burial dating with a cosmogenic nuclide pair, the isochron burial method performs well provided that the sediment source has undergone (1) steady erosion and (2) continuous exposure to cosmic rays. These conditions exert important limitations on applications of the method. And yet, in mountainous fluvial and glacial landscapes, it is commonly found that the source area has experienced landsliding or glacial quarrying (i.e., non-steady erosion), and/or intermittent sediment storage or burial beneath glaciers (i.e., discontinuous exposure). As well as breaching the assumptions of the isochron method, such processes tend to yield low nuclide concentrations in the sample, which further limits its workability.</p><p>Here we present a more flexible method that accommodates complex, non-steady pre-burial erosion and exposure histories: conditions that exclude the isochron burial method. P-PINI (Particle Pathway Inversion of Nuclide Inventories) is a Monte Carlo-based inversion model that employs a source-to-sink approach for estimating the depositional age of fluvial and glaciogenic sediments. This method has been successfully applied to the Deckenschotter in the northern Alpine foreland (see Knudsen et al. 2020, Earth & Planetary Science Letters 549, 116491). As with the isochron burial method, P-PINI exploits an ensemble of paired nuclide (e.g., <sup>10</sup>Be-<sup>26</sup>Al) concentrations measured in different samples from the same depth in a sedimentary sequence. But unlike the isochron method, P-PINI applies a stochastic approach to simulate a wide range of possible pre-depositional exposure and erosion histories for each individual sample. These different pre-burial histories (unique to each sample) are then integrated with the constraint that all samples share a common burial history at the sink. Where cosmogenic nuclide data (or other chronometric data, e.g., OSL) are available for multiple sites, Bayesian inference modelling can impose a priori relative age constraints, or estimates on the maximum duration of sediment storage.</p><p>In this presentation, we extend P-PINI to explore how sediment storage and reworking (i.e., a range of burial depths and durations) between source and sink affects burial age estimates. Significant intermediate storage is characteristic of large river systems, such as the Danube River. Using cosmogenic <sup>10</sup>Be-<sup>26</sup>Al concentrations measured in fluvial gravels at Gänserndorf and Schlosshof, two terraces along the Danube River in the Vienna Basin (Braumann et al., 2019. Quat. Int. 509. 87-102), we examine how the burial ages at these two sites are a function of the pre-burial history experienced by the samples.  </p>

The Evolution of the Paleo-Danube Deltas of the Lower Pannonian in the Vienna Basin

<p><strong>ABSTRACT</strong></p><p>The Vienna Basin is a rhombohedral SSW-NNE oriented Neogene extensional basin that formed along sinistral fault systems during Miocene lateral extrusion of the Eastern Alps. The basin fill consists of shallow marine and terrestrial sediments of early to late Miocene age reaching a thickness of 5500 m in the central part of the basin. The early Pannonian was a crucial time in the development of the Vienna Basin, as It coincided with the formation of Lake Pannon. The lake formed at 11.6 Ma when a significant regressive event isolated Lake Pannon from the Paratethys Sea, creating lacustrine depositional environments. At that time the delta of the Paleo-Danube started shedding its sediments into the central Vienna Basin. Based on an existing age model delta deposition commenced around 11.5 Ma and continued until 11.1 Ma. These subsurface deltaic deposits of the Hollabrunn-Mistelbach Formation represent the coeval fluvial deposits of the Paleo-Danube in the eastern plains of the North Alpine Foreland Basin. Therefore, the Palaeo-Danube represents an extraordinary case in where coeval fluvial and deltaic deposits of a Miocene river are continuously captured.</p><p>This study provides an interpretation of depositional architecture and depositional environments of this delta in the Austrian part of the central Vienna Basin based on the integration of 3D seismic surveys and well data. The mapped delta has an area of about 580 km<sup>2</sup>, and solely based on the geometry we classify the delta as a mostly river – dominated delta with significant influence of wave – reworking processes. For seven time slices paleogeographic maps are created, showing the interplay between the lacustrine environments of Lake Pannon, delta evolution and fluvial systems incising in the abandoned deltaplain. Onlaps between single deltalobes indicate a northward-movement of the main distributary channel. Approximate water-depth estimates are carried out with in-seismic measurements of the true vertical depth between the topset deposits of the delta and the base of the bottomset deposits. These data suggest a decrease of lake water depth from about 170 m during the initial phase of delta formation at 11.5 Ma to about 100 m during its terminal phase at 11.1 Ma. A major lake level rise of Lake Pannon around 11.1 Ma caused a flooding of the margins of the Vienna Basin, resulting in a back stepping of riverine deposits and termination of delta deposition in the study area.</p><p> </p>