Styles of soft-sediment deformation on top of a growing fold system in the Gosau Group at Muttekopf, Northern Calcareous Alps, Austria: Slumping versus tectonic deformation

Abstract

Styles of soft-sediment deformation in a syngrowth coarse-grained sediment gravity flow system are described from outcrops of the Upper Gosau Subgroup of the Northern Calcareous Alps. The deepwater sediments were deposited along the flanks and top of growing folds. Beside load and fluid escape structures, which are commonly found in sediment gravity flow systems, soft-sediment deformation is mainly related to fluidization of coarse-grained beds and downslope gliding of meter-thick sediment packages. Common structures affecting single or several beds are: hydroplastic folding of fine-grained beds in a coarse-grained fluidized matrix, symmetric or asymmetric mullions at the base of coarse-grained beds, and folding of heterolithic units into coarse-grained beds. Folds have varying geometries and styles, and fold axes are scattered. Soft-sediment folds also affect thick sediment packages. These folds are uniformly verging asymmetric metric to dekametric folds, formed at different degrees of lithification. Rheology of sandstones in such folds ranges from ductile to semi-brittle and brittle. Fold axes tightly cluster about one direction. The first group of deformational structures is attributed to slump-related deformation, whereas the second group represents folds related to tectonic deformation. A dislocation model and a shear zone model are used to interpret orientation of folds related to slump deformation. This considers processes taking place on the scale of the whole slump sheet and within the shear zone at the base of gliding units. Both models predict rotation of fold axes toward the transport direction of the slump. Field data support that fold axes scatter either about the downslope transport direction or about the strike of the slope.