Integrated high-resolution stratigraphy of a Middle to Late Miocene sedimentary sequence in the central part of the Vienna Basin
In order to determine the relative contributions of tectonics and eustasy to the sedimentary infill of the Vienna Basin a high-resolution stratigraphic record of a Middle to Late Miocene sedimentary sequence was established for a well (Spannberg-21) in the central part of the Vienna Basin. The well is located on an intrabasinal high, the Spannberg Ridge, a location that is relatively protected from local depocentre shifts. Downhole magnetostratigraphic measurements and biostratigraphical analysis form the basis for the chronostratigraphic framework. Temporal gaps in the sedimentary sequence were quantified from seismic data, well correlations and high-resolution electrical borehole images. Stratigraphic control with this integrated approach was good in the Sarmatian and Pannonian, but difficult in the Badenian. The resulting sedimentation rates show an increase towards the Upper Sarmatian from 0.43 m/kyr to > 1.2 m/kyr, followed by a decrease to relatively constant values around 0.3 m/kyr in the Pannonian. The sequence reflects the creation of accommodation space during the pull-apart phase of the basin and the subsequent slowing of the tectonic activity. The retreat of the Paratethys from the North Alpine Foreland Basin during the Early Sarmatian temporarily increased the influx of coarsergrained sediment, but eventually the basin acted mostly as a by-pass zone of sediment towards the Pannonian Basin. At a finer scale, the sequence exhibits correlations with global eustasy indicators, notably during the Sarmatian, the time of greatest basin subsidence and full connectivity with the Paratethyan system. In the Pannonian the eustatic signals become weaker due to an increased isolation of the Vienna Basin from Lake Pannon.
A new and precisely defined chronometric subdivision of the Badenian (Middle Miocene, regional stage of Central Paratethys) is proposed. This uses global events, mainly geomagnetic polarity reversals as correlated chronometric boundaries, supported by climatic and sea-level changes in addition to isotope events and biostratigraphic data. The Karpatian/ Badenian boundary lies at 16.303 Ma, at the top of Chron C5Cn.2n, which is near the base of the Praeorbulina sicana Lowest-occurrence Zone (LOZ). The Badenian/Sarmatian boundary is placed at the top of polarity Chron C5Ar.2n, thus at 12.829 Ma. In relation to three sea level cycles TB 2.3, TB 2.4 and TB 2.5 and astronomically confirmed data, the Badenian can be divided into three parts of nearly equivalent duration. The Early Badenian as newly defined here ranges from 16.303 to 15.032 Ma (top of polarity Chron C5Bn.2n). The younger boundary correlates roughly to the base of the planktonic foraminifera Orbulina suturalis LOZ at 15.10 Ma, the HO (Highest Occurrence) of the nannofossil Helicosphaera ampliaperta at 14.91 Ma (NN4/NN5 boundary) and the Lan2/Ser1 sequence boundary at 14.80 Ma. The subsequent Mid Badenian ranges from 15.032 Ma to 13.82 Ma; the latter datum correlates with the base of the Serravallian, characterized by a strong global cooling event reflected in the oxygen isotope event Mi3b. The main part of cycle TB 2.4 falls into the Mid Badenian, which can be subdivided by a short cooling event at 14.24 Ma during the Middle Miocene Climate Transition (14.70 to 13.82 Ma). The HCO (Highest common occurrence) of the nannofossil Helicosphaera waltrans at 14.357 Ma supports this division, also seen in the tropical plankton Zones M6 Orbulina suturalis LOZ and M7 Fohsella peripheroacuta LOZ that correspond roughly to the lower and upper Lagenidae zones in the Vienna Basin, respectively. The Late Badenian is delimited in time at the base to 13.82 Ma by the Langhian/Serravallian boundary and at the top by the top of polarity Chron C5Ar.2n at 12.829 Ma. The Mediterranean Langhian/Serravallian boundary can be equated with the Mid/Late Badenian boundary at 13.82 Ma. However, the Karpatian/Badenian boundary at 16.303 Ma, a significant event easily recognizable in biostratigraphy, paleoclimate evolution and sequence stratigraphy, cannot be equated with the proposed global Burdigalian/Langhian, and thus Early/Middle Miocene boundary, at 15.974 Ma
The depositional environments and hydrocarbon potential of the siliciclastic, clayey and carbonate sediments from the Middle Miocene succession in the Varna-Balchik Depression, located in the south-eastern parts of the Moesian Platform, were studied using core and outcrop samples. Based on the lithology and resistivity log the succession is subdivided from base to top into five units. Siliciclastic sedimentation prevailed in the lower parts of units I and II, whereas their upper parts are dominated by carbonate rocks. Unit III is represented by laminated clays and biodetritic limestone. Units IV and V are represented by aragonitic sediments and biomicritic limestones, correlated with the Upper Miocene Topola and Karvuna Formations, respectively. Biogenic silica in the form of diatom frustules and sponge spicules correlates subunit IIa and unit III to the lower and upper parts of the Middle Miocene Euxinograd Formation. Both (sub)units contain organic carbon contents in the order of 1 to 2 wt. % (median: 0.8 for subunit IIa; 1.3 for unit III), locally up to 4 wt. %. Based on Hydrogen Index values (HI) and alkane distribution pattern, the kerogen is mainly type II in subunit IIa (average HI= 324 mg HC/g TOC) and type III in unit III (average HI ~200 mg HC/g TOC). TOC and Rock Eval data show that subunit IIa holds a fair (to good) hydrocarbon generative potential for oil, whereas the upper 5 m of unit III holds a good (to fair) potential with the possibility to generate gas and minor oil. The rocks of both units are immature in the study area. Generally low sulphur contents are probably due to deposition in environments with reduced salinity. Normal marine conditions are suggested for unit III. Biomarker composition is typical for mixed marine and terrestrial organic matter and suggests deposition in dysoxic to anoxic environments.
The Miocene evolution of the area transitional from the Eastern Alps to the Pannonian Basin System was studied through the paleogeographic evolution of the Slovenj Gradec Basin in northern Slovenia. It is based on mapping, section logging, nannoplankton biostratigraphy, and petrography. The results are correlated with the lithological column of the borehole MD-1/05. The evolution of the basin is connected with the development of the Pannonian Basin System, and the global 3rd order cycles, which influenced the connection with the Mediterranean Sea. Sedimentation started in the Karpatian in a fluvial to lacustrine environment and terminated at the end of the Early Badenian. During this period, three transgression–regression cycles were recorded. The first transgression occurred in the Karpatian and corresponds to the TB 2.2. cycle. The sediments reflect proximity of the hinterland. After a short break in sedimentation, the Early Badenian deposition followed. It marks the second transgression into the SGB, the first Badenian, correlated with the TB 2.3 cycle. There are signs of a transitional environment, which evolved to marine in advanced stages. At the high-stand system tract, the sea flooded the entire Slovenj Gradec Basin. Subsequent reduced quantity and diversity of the microfossils marks the onset of the second regression stage. It is followed by the third transgression, the second in the Badenian, correlated with the TB 2.4 cycle. The late Early Badenian deposition continued in the lower-energy, though occasionally still turbulent environment. Silty sediments with upward increasing content of organic matter indicate shallowing of the basin, until its final diminishing. Layers of fresh-water coal already bear witness to the existence of restricted swamps. After the Early Badenian, the area of the Slovenj Gradec Basin became dry land, exposed to erosion.
Massive evaporites were discovered in the Soltvadkert Trough (Great Plain, Hungary) correlating to the Badenian Salinity Crisis (13.8 Ma, Middle Miocene) on the basis of nannoplankton and foraminifera biostratigraphy. This new occurrence from Hungary previously thought to be devoid of evaporites is part of a growing body of evidence of evaporitic basins inside the Carpathian Arc. We suggest the presence of evaporites perhaps in the entire Central Paratethys during the salinity crisis. Different scenarios are suggested for what subsequently happened to these evaporites to explain their presence or absence in the geological record. Where they are present, scenario A suggests that they were preserved in subsiding, deep basins overlain by younger sediments that protected the evaporites from reworking, like in the studied area. Where they are absent, scenario B suggests recycling. Scenario B explains how the supposedly brackish Sarmatian could have been hyper/normal saline locally by providing a source of the excess salt from the reworking and dissolving of BSC halite into seawater. These scenarios suggest a much larger amount of evaporites locked up in the Central Paratethys during the salinity crisis then previously thought, probably contributing to the step-like nature of cooling of the Mid Miocene Climate Transition, the coeval Mi3b.
Sediments belonging to the Oligocene Vima Formation (located in the north-western part of the Transylvanian Basin, Romania) have been investigated for calcareous nannofossils content. Biostratigraphically, the sedimentary succession is late Rupelian–Chattian in age, belonging to the NP24 — Sphenolithus distentus and NP25 — Sphenolithus ciperoensis biozones, to CP19a — Cyclicargolithus floridanus and CP19b — Reticulofenestra bisecta Subzones and to the interval from CNO4 — Sphenolithus distentus/Sphenolithus predistentus CRZ to CNO5 — Sphenolithus ciperoensis TZ. The palaeoenvironment of the Fântânele section was reconstructed by means of calcareous nannofossils and statistics. Multivariate statistics were applied to the composition of autochthonous assemblages and the obtained clusters were used to assess the palaeoecological preferences of the nannofossils. We document changes from more stable open-marine regime, with temperate sea-surface temperatures interfering locally with influx of cooler water and enriched cool-nutrient supply for the late Rupelian–earliest Chattian (NP24), to shallower and possibly warmer near-shore marine eutrophic environment, with salinity fluctuations, increased terrigenous material run-off and freshwater influx for the remaining early Chattian (NP25).
The formations underlying the Neogene infill of the Vienna Basin are still poorly documented. Until now correlation of subsurface lithostratigraphic units with those of the Rhenodanubian nappe system and the Magura nappe system, outcropping at the basin margins, has been based on extrapolations. A recent drilling campaign in the Bernhardsthal oil field of the northern Vienna Basin in Austria reached the pre-Neogene basement and provided cuttings for biostratigraphic and paleoecological analyses. Based on these data, acquired by using detailed micro- and nanno-paleontological analyses, a Lutetian age (middle Eocene) and a bathyal depositional environment for the Flysch of the Harrersdorf Unit was documented. The lithological similarity of the drilling with the Steinberg Flysch Formation of the Greifenstein Nappe and its Lutetian age suggests, that the middle Eocene part of the Harrersdorf Unit represents a continuation of the Greifenstein Nappe of the Rhenodanubian Flysch, rather than a frontal part of the Rača Nappe of the Magura Flysch as previously thought.
The Neogene Transylvanian Basin (TB), enclosed between the eastern and southern Carpathians and the Apuseni Mountains in Romania, is a significant natural gas province with a long production history. In order to improve the (bio) stratigraphic resolution, correlations and dating in the several 100-m-thick upper Miocene (Pannonian) succession of the basin, the largest and most fossiliferous outcrop at Guşteriţa (northeastern part of Sibiu) was investigated and set as a reference section for the Congeria banatica zone in the entire TB. Grey, laminated and massive silty marl, deposited in the deep-water environment of Lake Pannon, was exposed in the ~55-m-high outcrop. The uppermost 25 m of the section was sampled in high resolution (sampling per metres) for macro- and microfossils, including palynology; for authigenic 10Be/9Be dating and for magnetostratigraphy; in addition, macrofossils and samples for authigenic 10Be/9Be isotopic measurements were collected from the lower part of the section as well. The studied sedimentary record belongs to the profundal C. banatica mollusc assemblage zone. The upper 25 m can be correlated to the Hemicytheria tenuistriata and Propontoniella candeo ostracod biozones, the uppermost part of the Spiniferites oblongus, the entire Pontiadinium pecsvaradense and the lowermost part of the Spiniferites hennersdorfensis organic-walled microplankton zones. All samples contained endemic Pannonian calcareous nannofossils, representing the Noelaerhabdus bozinovicae zone. Nine samples were analysed for authigenic 10Be/9Be isotopic measurements. The calculated age data of six samples provided a weighted mean value of 10.42 ± 0.39 Ma. However, three samples within the section exhibited higher isotopic ratios and yielded younger apparent ages. A nearly twofold change in the initial 10Be/9Be ratio is a possible reason for the higher measured isotopic ratios of these samples. Magnetostratigraphic samples showed normal polarity for the entire upper part of the outcrop and can be correlated with the C5n.2n polarity chron (11.056–9.984 Ma, ATNTS2012), which is in agreement with the biostratigraphic data. Based on these newly obtained data and correlation of the biozones with other parts of the Pannonian Basin System, the Guşteriţa section represents the ~ 11.0–10.5 Ma interval, and it is a key section for correlation of mollusc, ostracod, dinoflagellate and calcareous nannoplankton biostratigraphic records within this time interval.