Cuspate-lobate folding in glacial sediments revealed by a small-scale 3-D seismic survey

Abstract

We carried out a small-scale 3-D seismic survey (120 × 120 m2, bin size 1.5 m) in advance of a research borehole. The target consists of 150 m of Quaternary sediments in a glacially overdeepened valley. We used a wheelbarrow-mounted electrodynamic vibrator as seismic source and chose a simple orthogonal layout. During one week of acquisition, we fired 1024 shots into 384 vertical geophones. The key processing step was the interpolation and regularization of traces, realized by common reflection surface (CRS) processing. This enhances data quality in low fold regions at small offsets. Despite the small source, the entire Quaternary fill and the base of the valley is imaged well. At a depth of 20–50 m, glaciotectonic deformation, in the form of cuspate-lobate folds, is visible, which was not recognized previously in a 2-D seismic profile that runs along the edge of the 3-D area. The folding indicates that compressional glacial stresses acted on layers of stiff till and less competent clastics. We interpret that the varying fold axes' directions indicate varying stress fields during the Last Glacial Maximum. Cuspate-lobate folding has hitherto not been used to describe the deformation of glacial sediments.