The central Styrian Basin (Gnas Subbasin): Structure and stratigraphy revealed by seismic and borehole data

Florian Dax

Research output: ThesisMaster's Thesis

Abstract

The Gnas Subbasin represents the central part of the Neogene Styrian Basin in the southeast of Austria. Despite decades of exploration activity for hydrocarbons, the formation mechanisms and the basin architecture are still poorly understood. Aim of this thesis, therefore, is to determine depth and geometry of the pre-Neogene basement, tectonic patterns, as well as stratigraphic characterizations of the basin fill including a sequence stratigraphic interpretation of the Sarmatian strata. For this purpose, an extensive, largely unpublished data set of seismic lines and well information, covering major parts of the Gnas Subbasin, was provided generously by RAG Austria AG and OMV. The subsurface data reveals that the basin depth exceeds 4 km below ground level. Moreover, it shows the presence of an additional high in the northern Gnas Subbasin, termed Wollsdorf High. Seismic interpretation further allowed to reconstruct the tectonic evolution of the basin. Extreme crustal extension in the lower Miocene initiated basin evolution by activating a detachment horizon within the pre-Neogene basement of the Gnas Subbasin. The detachment horizon dips towards southeast, indicating extension in NW-SE direction. The upper plate (hanging wall) of the detachment horizon experienced significant offset and brittle deformation. Low angle normal faults cut through the upper plate and disintegrated it into fault blocks. Ongoing extension caused rotation of fault blocks along the low angle normal faults, which resulted in a pronounced relief of the pre-Neogene basement. Consequently, coarse-grained sediments were deposited in alluvial fans and fan delta complexes in early Miocene time. Rapid subsidence resulted subsequently in a deeper marine environment where the pelitic Styrian Schlier was deposited. Sedimentation in fan delta complexes continued simultaneously in marginal areas. Seismic data show that this synrift strata covers huge parts of the subsurface of the Gnas Subbasin and reaches a thickness of at least 2 km in the depocenter of the basin. Low angle normal faults, which also cut through the synrift strata, appear to be arranged in a radial, most likely en-echelon pattern around the basin centre. Major fault activity ceased around the Karpatian/Badenian boundary (i.e., Styrian Tectonic Phase). Minor fault activity is portrayed by the seismic data from the Badenian onwards except for a (mainly?) Sarmatian, large offset fault in the northern Gnas Subbasin, which represents a second - minor - phase of extension. The Badenian is characterized by the deposition of turbidites in central parts of the basin. A lower Badenian basin floor fan facies of likely sand-rich turbidites grades into a middle and upper Badenian mud-rich slope fan facies. Lower Badenian lava flows were shed simultaneously with turbidite deposits into the basin centre from the Gleichenberg volcano in the east. Different facies of Leitha Limestone formed on isolated highs during the lower and middle Badenian. The Badenian strata is usually around 1100 to 1200 m thick, but its thickness decreases significantly above highs. The Sarmatian is characterized by shallow marine sediments, up to 800 m thick in the basin centre, and a merging of the various depositional environment into a unified one. The Sarmatian can be described by a 3rd order sequence which further subdivides into five 4th order sequences. The Pannonian is characterized by a uniform, fluvial-limnic-deltaic depositional environment with thicknesses of up to 360 m below seismic reference datum (300 m above sea level). Even higher values are found where incised valleys cut into the underlying Sarmatian strata and were later filled with Pannonian sediments. Post-Pannonian compression resulted in crustal-scale folding and significant uplift along the basin margins. In contrast, inversion of faults cannot be observed in s
Translated title of the contributionDas zentrale Steirische Becken (Gnaser Teilbecken): Strukturelle und stratigraphische Charakterisierung basierend auf Seismik und Bohrlochdaten
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Sachsenhofer, Reinhard, Supervisor (internal)
Award date17 Dec 2021
Publication statusPublished - 2021

Bibliographical note

no embargo

Keywords

  • Styrian Basin
  • Gnas Subbasin
  • Seismic interpretation
  • Tectonics
  • Stratigraphy

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