THRUSTING AND SEDIMENTATION: A MODEL FROM THE NORTHERN CALCAREOUS ALPS (NCA)

A ramp-flat model can be used to describe thrusting and coeval sedimentation in a deep marine environment: As the frontal part of a nappe climbs up a ramp, water depth above the area of structural thickening decreases. Isolated carbonate platforms or shortlived carbonate buildups develop on the evolving structure, shedding biogenic detritus into the surrounding areas. Further growth will uplift the nappe above the ramp and upper footwall flat above sea level. The nappe covers the upper footwall flat and ends deposition there. The synorogenic sedimentary successions of the western NCA can be compared to specific positions in such a model:

Upper Footwall sedimentation: On the upper footwall flat below the thrusted units, conformable onset of synorogenic sedimentation records distant onset of contraction related to orogeny, and deposition of shallow water biogenic detritus shows the approaching nappe. The youngest sediments below the thrust record the local maximum age of thrusting. This situation is comparable to Aptian-Albian synorogenic sedimentation of the Tannheim and Losenstein Fms. on top of the Allgäu thrust sheet, which are overlain by the Lechtal thrust sheet, and to Albian-Cenomanian synorogenic sedimentation of the Lech Fm. on top of the southern Lechtal thrust sheet, which is overlain by the Inntal thrust sheet. The uppermost Lech Fm. locally contains shallow water detritus (“Urgonian”) transported by gravity flows and thereby records the destruction of carbonate buildups at the flanks of the approaching Inntal thrust sheet.

Thrust-sheet-top sedimentation: On top of the thrust unit, unconformable transgression of terrestric sediments on deeply eroded older rocks records exhumation. Growth geometries in thrust-sheet-top deposits record internal shortening of the thrust sheet after emplacement. The Branderfleck Fm. on top of the northern Lechtal nappe and of the Gosau Group on top of the Inntal nappe are found in this structural position. Undisturbed synorogenic sedimentation will continue beyond the areas of active structural growth.

A problem remains subsidence during shortening of the NCA. In the Cretaceous, the NCA were a foreland fold-and-thrust belt with respect to the closure of the Meliata ocean, but also in the hanging wall of the ?retreating Penninic subduction that caused subsidence.