Arbeitsbericht NAB 21-18

Morphology of the Overdeepened Gebenstorf-Stilli Trough (Lower Aare Valley) from Seismic Surface Wave Investigations

The confluence area of the rivers Aare, Reuss and Limmat ("Wasserschloss") and the northwards adjacent Lower Aare Valley in central Northern Switzerland are a key region of Alpine Quaternary geology. Below the area lies the Gebenstorf-Stilli Trough, a ~ 10 km-long, N-S trending overdeepening with a depth ≥ 112 m below surface. In the light of the unique underlying bedrock geology, including Mesozoic strata, this trough is of paramount interest for investigating subglacial erosion processes and relative rock erodibilities. To reveal its morphology, we targeted it with a set of seismic cross-sections featuring analyses of seismic surface waves. 

Our application of seismic surface wave methods included measurements of the ambient noise wavefield, computation of the horizontal-to-vertical spectral ratio (HVSR) and analyses of dominant frequencies. We focused our investigations on imaging the Base Quaternary-bedrock interface on eight 2D sections that cross the Gebenstorf-Stilli Trough. Using independent active and passive measurements, we found shear wave velocity contrasts to the underlying bedrock (consisting of Tertiary or Late Jurassic sediments) that validated our findings and supported depth conversion of the dominant frequencies recorded. The procedure resulted in 50 m spaced depth measurements that were connected to build continuous Base Quaternary horizon models along 2D cross-sections. The results were validated against available borehole logs.

A first geological interpretation integrated these 2D cross-sections with surface geology and borehole data. The combined profiles greatly improved our knowledge of the dimensions and morphology as well as the infill of the Gebenstorf-Stilli Trough. The dataset allowed distinction of two separate sub-basins separated by a ridge located at Lauffohr. We distinguished between the southern, ≥ 112 m deep Gebenstorf Basin and the northern, ≥ 76 m deep Stilli Basin, with marked differences in underlying geology and infill sedimentology. This report provides the basis for detailed studies interpreting the shape and sedimentary infill of the Gebenstorf-Stilli Trough with respect to the bedrock geological and paleoglaciological conditions.