Tectonic processes, variations in sediment flux, and eustatic sea level recorded by the 20 Myr old Burdigalian transgression in the Swiss Molasse basin
Authors: Garefalakis Philippos , Schlunegger Fritz 
Published: November 2019

Abstract. The stratigraphic architecture of the Swiss Molasse basin, situated on the
northern side of the evolving Alps, reveals crucial information about the
basin's geometry, its evolution, and the processes leading to the deposition
of the siliciclastic sediments. Nevertheless, the formation of the Upper
Marine Molasse (OMM) and the controls on the related Burdigalian
transgression have still been a matter of scientific debate. During the time
period from ca. 20 to 17 Ma, the Swiss Molasse basin was partly flooded by a
shallow marine sea striking SW–NE. Previous studies have proposed that
the transgression occurred in response to a rise in global sea level,
a reduction of sediment flux, or an increase in tectonically controlled
accommodation space. Here, we readdress this problem and extract
stratigraphic signals from the Burdigalian molasse deposits that can be
related to changes in sediment supply rate, variations in the eustatic sea
level, and subduction tectonics. To achieve this goal, we conducted
sedimentological and stratigraphic analyses of several sites across the
entire Swiss Molasse basin. Field investigations show that the transgression and the subsequent
evolution of the Burdigalian seaway was characterized by (i) a deepening and
widening of the basin, (ii) phases of erosion and non-deposition during Lower Freshwater Molasse (USM),
OMM, and Upper Freshwater Molasse (OSM) times, and (iii) changes in along-strike drainage reversals. We
use these changes in the stratigraphic record to disentangle
tectonic and surface controls on the facies evolution at various scales. As
the most important mechanism, rollback subduction of the European mantle
lithosphere most likely caused a further downwarping of the foreland plate,
which we use to explain the deepening and widening of the Molasse basin,
particularly at distal sites. In addition, subduction tectonics also caused
the uplift of the Aar massif. This process was likely to have shifted the
patterns of surface loads, thereby resulting in a buckling of the foreland
plate and influencing the water depths in the basin. We use this mechanism
to explain the establishment of distinct depositional settings,
particularly the formation of subtidal shoals wherein a bulge in relation to
this buckling is expected. The rise of the Aar massif also resulted in a
reorganization of the drainage network in the Alpine hinterland, with the
consequence that the sediment flux to the basin decreased. We consider
this reduction in sediment supply to have amplified the tectonically controlled deepening of the Molasse basin. Because the marine conditions
were generally very shallow, subtle changes in eustatic sea level
contributed to the formation of several hiatuses that chronicle periods of
erosion and non-sedimentation. These processes also amplified the
tectonically induced increase in accommodation space during times of global
sea level highstands. Whereas these mechanisms are capable of explaining the
establishment of the Burdigalian seaway and the formation of distinct
sedimentological niches in the Swiss Molasse basin, the drainage reversal
during OMM times possibly requires a change in tectonic processes at the
slab scale, most likely including the entire Alpine range between the Eastern
and Central Alps. In conclusion, we consider rollback tectonics to be the main driving force
controlling the transgression of the OMM in Switzerland, with contributions
by the uplift of individual crustal blocks (here the Aar massif) and by a reduction of sediment supply. This reduction of sediment flux was likely to have been controlled by
tectonic processes as well when basement blocks became uplifted, thereby
modifying the catchment geometries. Eustatic changes in sea level explain
the various hiatuses and amplified the deepening of the basin during
eustatic highstand conditions.