A Tethys-wide mid-Carnian (Upper Triassic) carbonate productivity crisis: Evidence for the Alpine Reingraben Event from Spiti (Indian Himalaya)?

A significant mid-Carnian lithological change from carbonatic to siliciclastic sedimentation is described from the Spiti Himalaya, N India. The recorded sudden increase of terrigenous influx occurs at the boundary from the Chomule to the Rama Formation; it is dated within the late Lower Carnian (uppermost aonoides Zone) and isochronous all over the Tethys Himalaya. Bio- and chemostratigraphic data point to a synchrony with changes in carbonate productivity known from on area of wide extent along the NW Tethys: The ‘Reingraben Event’ of the Northern Calcareous Alps (Austria) defines a regional decline in carbonate productivity related to both a platform demise and enhanced siliciclastic input. Laminated shales found in a distal part of the Spiti basin document the onset of anoxic conditions within the deeper shelf synchronous to the ‘Reingraben Event’ in Austria. High-resolution biostratigraphy and identical δ13Ccarb-curves are therefore in support of a direct link between the change in Spiti and the Reingraben Event.Our data, combined with a comprehensive review of previously postulated scenarios regarding the ‘Reingraben Event’, suggest linking of climatic change with siliciclastic input and a carbonate productivity crisis, initiated by onset of a relatively humid, megamonsoonal late Lower Carnian “atmospheric system” that affected both the southern and northwestern Tethys and point to a supra-regional trigger of the Alpine ‘Reingraben Event’.