Tectonic vs gravity - driven deformation of Upper Jurassic Oberalm Fm limestones, Northern Calcareous Alps, Austria

In this work we describe the deformation observed within the Upper Jurassic limestones of the Oberalm Fm located SE of Bad Ischl (central Northern Calcareous Alps, Eastern Alps, Austria). The Oberalm Fm have been documented to record incipient contractional deformation in the Northern Calcareous Alps (NCA), and provide the evidence for the earliest Alpine deformation in the Upper Austroalpine unit (part of the western ALCAPA domain).
Calcarenites and marls of the Oberalm Fm crop out along the Grabenbach valley. They unconformably onlap onto the Triassic Dachstein Limestones, and infill a syn-tectonic basin in the footwall of the Grabenbach thrust. Syndepositional deformation of the Oberalm Fm along the unconformity is observed with small scale folds of varying orientation and olistoliths of Dachstein limestones. Above, the Oberalm Fm is regularly bedded, with beds few centimeters thick and almost perfectly parallel to each other.
Within the parallel-bedded interval of Oberalm Fm, intense folding is observed along a 70 m long outcrop. Along this outcrop, other than for folds, bedding is roughly vertical to sub-vertical. Two sets of folds can be distinguished based on the axial planes orientations: first have axial planes dipping at a low angle towards north and south whereas the second have them nearly vertical. Folds of first set are typically larger, with amplitudes up to 2 m, can be traced along the outcrop wall for up to 10 m and have a chevron geometry with well visible hinge collapse structures. Normal faulting acting as a compensation is often associated with hinge collapse. No pressure solution, outer arc extension nor slickensides on layer surfaces was observed at a macroscopic scale. The second set of folds is represented by folds of amplitudes typically up to 1 m, usually with gentler hinges, without collapse-related structures. All folds have a consistent trend of the fold axes towards the ESE.
Calcite veins usually cross cut layering at high angle, without any relationship to folding. This suggest that they probably predate folding. Within unfolded parts of the outcrop fish tail structures can be observed. The whole folded section is juxtaposed in the northern part along a S-dipping thrust with rocks of the same formation without any major signs of folding. To the top, the whole succession is erosively truncated and unconformably overlain by a flat-lying, undeformed, coarse carbonate breccia bank of the Tressenstein unit (of similar age to that of the Oberalm Fm).
Deformation observed in the Oberalm Fm can be consistent with both gravity driven sliding and tectonic deformation. The deformation style and the erosive truncation of the structures indicates that deformation occurred in very shallow burial conditions (less than 500m, and atmospheric temperatures) and soon after deposition. However, the origin of the folds remains elusive, with the deformation being reminiscent of gravity-driven deformation, but the coherent fold axis orientation being consistent with a tectonic origin of deformation.
Irrespective of the origin, this example presents an excellent illustration of Late Jurassic syn-depositional to early-burial contractional deformation in the Alpine domain.