Salt welds from up close: Examples from the Eastern Alps

Salt welds are frequent features in basins with halokinesis. They are profusely observed on reflection seismic and are common features on maps of salt-rich geological provinces (even though they may not always have been identified as such). Despite this abundance, detailed studies of salt weld outcrops are remarkably scarce, in part due to outcrop conditions being hampered by vegetation and weathering. This results in a significant paucity in the description of the structure of salt welds at the meter- to centimeter-scale.

In this contribution we present our observations on three non-primary salt welds from the Northern Calcareous Alps (NCA) in the Eastern Alps, an area that evolved from a Permian-Jurassic passive margin setting to the Cretaceous-Recent Alpine orogenesis. The NCA are dominated by Triassic to Jurassic shallow to deep water carbonates, underlain by an Upper Permian evaporitic unit (mainly salt, anhydrite and shales). The evaporite unit is preserved at present mostly in broad, poorly outcropping salt bodies (salt walls?) located between blocks of Triassic platforms, and in diapirs and allochthonous bodies that are mined or quarried for halite and anhydrite. These bodies have been affected by Alpine orogenesis to different degrees, but in general present a strong contractional and/or strike-slip overprint.

In this presentation we discuss welds that are associated to two diapirs and potentially to a strongly overprinted salt wall. Outcrop conditions for the welds are outstanding, with two of these welds being observed in the galleries of two salt mines. All welds occur in Middle to Upper Triassic platform carbonates and contain no major traces of evaporites but do contain either highly sheared syn-evaporite shales or fragments of Lower to Middle Triassic post-salt sediments. Hand samples and outcrop observations have been used to describe the millimiter- to hectometer-scale structure of the welds and provide a unique insight into the detailed architecture of salt welds. Furthermore, it reveals a different tectonic evolution for each weld, with different relative contributions of halokinesis and faulting during the passive margin stage, and of re-activation during Alpine orogenesis.