Sedimentary structures and facies interpretation of some molasse deposits: Sense- Schwarzwasser area, Canton Bern, Switzerland

Abstract

Burdigalian sand- and siltstones, belonging to the Upper Marine Molasse, have been studied in the Swiss "Mittelland", to get an insight into the relationships between primary sedimentary structures, their genesis and facies. To obtain information on the conditions, prevailing during the deposition of sand and silt in the investigated area, a synopsis is given from the paleogeography and paleoclimate. This synopsis together with the estimation of the paleocurrent pattern and the petrographic characteristics is given in chapter I. In chapters II, III and IV, the observed structures have been analysed. In these chapters the textural properties of the sediments have also been given and their mode of formation is discussed. Four main structural types have been distinguished: Giant ripples of fluviatile origin (chapter II). Mega flaser structures, belonging to deposits formed in the deeper part of the shallow marine zone (chapter III, § 1). Giant ripples, belonging to offshore bars (chapter III, § 2). Parallel bedded structures, belonging to lagoons and/or bays, cut off from the main basin by formerly built offshore bars (chapter IV, § 1). The gullies described in chapter IV, § 2 are the channels in the coastal area, through which the feeding rivers discharged their material into the basin. In relation with the tectonic movement of the basin floor, the sequence of the formation of the different structural types has been discussed (chapter V). In this chapter we also come to the conclusion that the Sense-Schwarzwasser area during the Burdigalian (Miocene) can be characterized as the coastal zone of a shallow marine inland sea (the peri-Alpine depression). Rivers supplied the material, from which part accumulated as talus fans and giant ripples, whereas the finer material was dumped in the sea and reworked towards the coast forming a wave built terrace. The cyclic sedimentation pattern is more or less similar to that of the cyclothems of the Carboniferons.