Most of the major valleys in Switzerland contain a complex infill of glacial and fluvial Quaternary sediments. Information from borcholes provide some insights into the internal structure of those valleys, but reflection seismic profiling often yields a more complete and detailed image of the complex internal sediment architecture. To obtain an overview of the internal structure of the Birrfeld valley, we shot two perpendicular high-resolution reflection seismic profiles in the central part of the valley with a total length of 5.1 km. Based on reflection characteristics, we distinguish five different seismic units on the seismic sections interpreted together with information from borcholes and outcrops located near the profiles. An uppermost unit S1, 30 to 40 m thick, consists of fluvial and glaclofluvial gravel. Separated from this unit by a strong reflection, unit S2 consists mainly of (glacio-) lacustrine sediments, which reach a thickness of more than 100 m. In places, a third unit (S3) of subglacial deposits, is identified below S2. It is not continuous across the profiles and generally less than 100 in thick. It is not clear whether all parts of S3 correspond to the same deposits. Unit S4, characterized by a band of strong reflections, is interpreted as a Molasse (Tertiary) layer. Underneath the Molasse Jurassic sediments (Malm) are expected. However, because of the limited strength of our seismic source we observe these sediments (S5) only in the northern part of Profile 1. The top of Maim, which outcrops further north, coincides with a strong seismic reflection. These high-resolution reflection data provide insight into the complex three-dimensional structure of the Quaternary package of sediments variation and help to elucidate the complex palaeoclimatic history.
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