High-resolution seismic imaging of outcrop-scale channels and an incised-valley system within the fluvial-dominated Lower Freshwater Molasse (Aquitanian, western Swiss Molasse Basin)

Abstract

High-resolution seismic reflection surveys on Lake Geneva near Lausanne, Switzerland, allow the interpretation of outcrop-scale sedimentary features within the fluvial-dominated Lower Miocene, Lower Freshwater Molasse bedrock. Seismic modelling of an outcrop is used to calibrate the seismic data and assist with interpretation. Seismic resolution is sufficient to image sandstone channel-fill complexes 4–5 m thick. Five seismic facies are recognized and interpreted as fluvial architectural elements of the Lower Freshwater Molasse. Two of these facies are characterized by fairly continuous reflections of variable frequency and low-to-moderate amplitude, interpreted as metre-thick channel-fill complexes. The lateral extent of such channels ranges from 100 to 170 m. The other three seismic facies are characterized by moderate-to-high amplitude, sub-parallel continuous reflections. The three facies are mainly distinguished using reflection frequency and continuity. They correspond to distinct floodplain settings, i.e. proximal, intermediate and distal, with respect to the active channel belt. On a larger scale, a portion of a ca. 50-m deep incised-valley system, probably caused by a relative base-level fall, was identified. Two distinct valley-fill units are recognized. The lower unit was interpreted as comprising valley-edge slump units by comparison with outcrop data. The upper unit is interpreted as the fluvial fill deposited in the most landward portion of the valley.