Detecting fold structures at the southern flank of the Neogene Vienna Basin in eastern Austria using near-subsurface geophysical methods

Abstract

At the southern flank of the Vienna Basin, spectacular fold structures were exposed in the former Steinbrunn sand pit. The succession of Upper Pannonian age consists of decimeter- to meter-thick sandy, silty, and clayey beds, which are overlain by sandstone beds. Previously, these fold structures have been interpreted differently as of tectonic but also gravitational origin. To gain a more detailed insight into the 3D distribution and orientation of the folds, high-resolution geophysics such as electrical resistivity tomography (ERT), ground-penetrating radar (GPR), and electromagnetics (EM) were applied for mapping the subsurface in the surroundings of the sand pit. The EM results found that the uppermost layer was more clayey northwest and sandier southeast of the sand pit. To directly compare ERT and GPR results with the lithology of the fold structures observed in the sand pit, reference profiles behind the wall of the sand pit were performed. Both methods clearly revealed fold structures parallel to the folded Pannonian strata of the outcrop. Although the GPR data displayed boundaries between sandy and clayey sediments, and thus clearly imaged the fold geometry of the succession, the resistivities in the ERT profiles portrayed a smoother image. Almost all GPR profiles revealed wavelike structures with axes in the southeastern direction and dome-shaped structures with axes in the southwestern direction, deepening toward the west. This pattern was comparable with the sections of rounded buckle folds, which underlie almost the entire investigation area of approximately [Formula: see text] in size. Compared with fold structures as documented from a neighboring abandoned coal mine, the geophysical investigations around the Steinbrunn sand pit supported the hypothesis of a tectonic origin, formed by simple buckling of a planar surface and its confining volume along the southeastern flank of the Vienna Basin in Plio/Pleistocene times.