TY - GEN

T1 - New perspectives on volume and emplacement dynamics of the Köfels rockslide deposits by combined geophysical–geological studies.

AU - Oswald, Patrick

AU - Sanders, Diethard

AU - Mair, Gregor

AU - Wacker, Patricia

AU - Moernaut, Jasper

AU - Strasser, Michael

AU - Geitner, Clemens

AU - Scholger, Robert

AU - Poscher, Gerhard

PY - 2024

Y1 - 2024

N2 - The Köfels rockslide (Eastern Alps, Austria) represents one of the largest prehistoric rockslides in the Alps andhas been extensively investigated for its surface geology, geomorphology, event age, and detachment- anddeposit volumes. However, the interaction of the rockslide mass with the pre-existing topography and valley-fillsubstrate, as well as the emplacement mechanisms, remain unclear.This study presents new geophysical data and documentation of temporary outcrops in the upstream valleys ofthe rockslide to elucidate the dynamics of the rockslide emplacement. New reflection seismic data image thesubsurface of the Längenfeld basin, revealing rockslide deposits several kilometers south (upstream) of the mainrockslide area. This indicates that after halting at the opposite valley side near Niederthai, the rockslide masscollapsed and divided into two branches. The downstream branch traveled at least 1.9 km, as evidenced by asurface outcrop, while the upstream branch extended 2.3 km upstream. The rockslide dammed the Ötz river,causing subsequent burial of the rockslide deposits under an up to 238 m thick, generally coarsening-upwarddeltaic backwater sedimentary sequence. Seismic data reveal no postglacial sediments between the rockslideand bedrock beneath the central rockslide, indicating extensive scraping of the pre-rockslide valley infill. Theupstream branch's basal shear surface displays a south-verging ramp-flat geometry with at least three rampsaccompanied by low-angle thrusts. Thrusting and buckling of the rockslide topography resulted in the formationof toma-hills, with normal fault sets forming depressions between the hills. These observations indicate that theinternal collapse dynamics of the rockslide mass are governed by a combination of thrusting (buckling) andnormal faulting, with the rockslide mass thinning progressively with distance.Electric resistivity tomography and ground-penetration radar investigations of the Niederthai plain (~450 mabove the present-day valley floor) reveal no sign of lake sedimentation typically expected from a rockslidedammedbackwater lake. Instead, observations in a temporary construction pit show that the uppermostsuccession of the plain is composed of an overall graded but largely homogeneous silt-sized sedimentary depositwith abundant dykes and large flame structures filled with coarser-grained sands and rounded pebbles of upperÖtz valley provenance. Geophysical data reveal superposition and lateral thrusting geometries of subsurfaceintervals with high resistivity overlying/overthrusting moderate resistivity packages in the deeper subsurface.These findings indicate significant mobilization, bulldozing, and fluidization of the pre-event fluvial sediment inthe Ötz valley floor.Three-dimensional mapping of all available data estimates the minimum rockslide depositional volume at 5.8km³, at least 1.5 times larger than the estimated detachment volume (3.1-4.0 km³). This discrepancy cannotsolely be explained by the tendency of the rockslide mass to gain volume during progressive destabilization buthighlights the significant erosion and incorporation of pre-rockslide water-saturated sediments into the movingmass, which in turn likely further influenced the rockslide’s mass propagation and depositional behavior. Thus,our findings enhance the understanding of the Köfels rockslide's emplacement mechanisms and highlight theinteraction between the rockslide and the pre-existing valley infill.

AB - The Köfels rockslide (Eastern Alps, Austria) represents one of the largest prehistoric rockslides in the Alps andhas been extensively investigated for its surface geology, geomorphology, event age, and detachment- anddeposit volumes. However, the interaction of the rockslide mass with the pre-existing topography and valley-fillsubstrate, as well as the emplacement mechanisms, remain unclear.This study presents new geophysical data and documentation of temporary outcrops in the upstream valleys ofthe rockslide to elucidate the dynamics of the rockslide emplacement. New reflection seismic data image thesubsurface of the Längenfeld basin, revealing rockslide deposits several kilometers south (upstream) of the mainrockslide area. This indicates that after halting at the opposite valley side near Niederthai, the rockslide masscollapsed and divided into two branches. The downstream branch traveled at least 1.9 km, as evidenced by asurface outcrop, while the upstream branch extended 2.3 km upstream. The rockslide dammed the Ötz river,causing subsequent burial of the rockslide deposits under an up to 238 m thick, generally coarsening-upwarddeltaic backwater sedimentary sequence. Seismic data reveal no postglacial sediments between the rockslideand bedrock beneath the central rockslide, indicating extensive scraping of the pre-rockslide valley infill. Theupstream branch's basal shear surface displays a south-verging ramp-flat geometry with at least three rampsaccompanied by low-angle thrusts. Thrusting and buckling of the rockslide topography resulted in the formationof toma-hills, with normal fault sets forming depressions between the hills. These observations indicate that theinternal collapse dynamics of the rockslide mass are governed by a combination of thrusting (buckling) andnormal faulting, with the rockslide mass thinning progressively with distance.Electric resistivity tomography and ground-penetration radar investigations of the Niederthai plain (~450 mabove the present-day valley floor) reveal no sign of lake sedimentation typically expected from a rockslidedammedbackwater lake. Instead, observations in a temporary construction pit show that the uppermostsuccession of the plain is composed of an overall graded but largely homogeneous silt-sized sedimentary depositwith abundant dykes and large flame structures filled with coarser-grained sands and rounded pebbles of upperÖtz valley provenance. Geophysical data reveal superposition and lateral thrusting geometries of subsurfaceintervals with high resistivity overlying/overthrusting moderate resistivity packages in the deeper subsurface.These findings indicate significant mobilization, bulldozing, and fluidization of the pre-event fluvial sediment inthe Ötz valley floor.Three-dimensional mapping of all available data estimates the minimum rockslide depositional volume at 5.8km³, at least 1.5 times larger than the estimated detachment volume (3.1-4.0 km³). This discrepancy cannotsolely be explained by the tendency of the rockslide mass to gain volume during progressive destabilization buthighlights the significant erosion and incorporation of pre-rockslide water-saturated sediments into the movingmass, which in turn likely further influenced the rockslide’s mass propagation and depositional behavior. Thus,our findings enhance the understanding of the Köfels rockslide's emplacement mechanisms and highlight theinteraction between the rockslide and the pre-existing valley infill.

KW - Köfels

KW - rockslide

KW - sediment dynamics

KW - Geophysics

KW - volume estimation

M3 - Conference contribution

SP - 133

BT - Pangeo / DEUQUA 2024 Abstracts

T2 - Pangeo /DEUQUA 2024

Y2 - 23 September 2024 through 27 September 2024

ER -