The subsurface geometry of the overdeepened Lower Aare Valley revealed by seismic surface waves
Lukas Gegg1, Lorenz Keller2, Marius W. Buechi1, Thomas Spillmann3, Gaudenz Deplazes3, Herfried Madritsch3, and Flavio S. Anselmetti1 1 Quaternary Geology and Paleoclimatology, Institute of Geological Sciences, University of Bern, Switzerland ([email protected]) 2 roXplore gmbh, Amlikon-Bissegg, Switzerland 3 National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettingen, Switzerland Gebenstorf-Stilli Trough
- S-N trending glacial overdeepening - confluence of Aare, Reuss, Limmat drainages - presumably Mid-Pleistocene in age
Limmat
Gebenstorf- Aare Stilli Trough
Reuss
0 0.5 1 km Birrfeld basin
Base of Quaternary (Pietsch & Jordan, 2014) Background: SwissALTI3D 2 m DEM below 230 m a.s.l. 290 – 310 230 – 250 310 – 330 250 – 270 330 – 350 270 – 290 above 350 Base Quat.10 subcrop
5
5 T a b u l a r 25 J u r a 10
Limmat
Gebenstorf- Aare Stilli Trough 20 60 48 30 60
Reuss 20 M o l a s s e
0 0.5 1 km B a s i n Birrfeld basin
Base of Quaternary (Pietsch & Jordan, 2014) Background: SwissALTI3D 2 m DEM Base Quaternary subcrop: Stratigraphy Main Lithofacies Fault Fault, inferred below 230 m a.s.l. 290 – 310 Molasse Opalinus Clay Limestone Base of Quaternary 230 – 250 310 – 330 Upper Malm Staffelegg Fm. Marl (Pietsch & Jordan, 2014) 250 – 270 330 – 350 Wildegg Fm. Bänkerjoch Fm. Sandstone 330 m 270 m 180 m 270 – 290 above 350 Upper Dogger Schinznach Fm. Shale a. s. l. 240 m 150 m Background: Base of Quaternary 25 m DEM (Pietsch & Jordan, 2014) 300 m 210 m Scientific boreholes S N
QUST
QGVO
QGBR
~ 265 m a.s.l.
~ 255 m a.s.l.
Trough base ~ 225 m a.s.l. Acquisition of seismic cross-sections with surface waves - 3 scientific boreholes reaching the base of the trough - 9 seismic lines planned where boreholes allowed for quality control and calibration QUST - acquisition in 2019 and early 2020 with a combination of three surface wave approaches:
Method HVSR (horizontal- MFA (multiple ESAC (extended to-vertical spectral frequency analysis) spatial auto ratios) correlation)
QGVO Type Passive point Active point Passive L-shaped measurement measurement array (hammer source) measurement Point # ~ 230 ~ 50 10 Target Fundamental Shallow shear Deeper shear QGBR frequency wave velocity wave velocity Reference Nakamura 1989 Dziewonski et al. Ohori et al. 2002 1969
knowledge of fundamental frequency and shear wave velocity 0 0.5 1 km seismic line borehole function allows calculation of depth of impedance contrast Interpretation of seismic data (considering surface and borehole information)
→ differences in valley morphology within rocks of varying lithology and erosional resistance → lithological or structural controls on subglacial overdeepening erosion? → evidence for the processes of subglacial overdeepening erosion?