Wave-, Tide-, and Current-Controlled Sandbodies of Miocene Molasse, Western Switzerland

Abstract

The Miocene (Burdigalian) clastic sediments of western Switzerland were deposited in a wide variety of environments ranging from fan deltas to offshore coquina banks. Conglomerates and sandstones of the proximal fan-delta facies were deposited in distributary channels that were influenced by tides to varying extents. Tidal sand waves exhibiting bipolar paleocurrent patterns and intertidal sand flats are the most conspicuous features of the coastal facies belt. The nearshore facies is composed of thick, sandy subtidal shoals built of current-generated megaripples, intershoal swales containing horizontally stratified fine sandstones and mudstones, and swash bars or flood ramps with low angle accretionary bedding. The offshore facies belt contains very large coquina banks ashioned by vigorous (? tidal) currents and reworked by fair-weather waves. Wave power, calculated from preserved ripple marks, indicates a moderate-energy sea of limited fetch, perhaps 100 km. The ratio of wave power to reasonable estimates of discharge effectiveness index suggests wave-dominated rather than fluvial-dominated deltaic systems. Additionally, comparison of wave power with interpreted tidal range (microtidal to mesotidal) indicates strong influence from both tides and waves but little influence from river outflow. The predicted association of sand-filled distributaries and laterally persistent nearshore features is in agreement with the facies described from the Burdigalian Molasse of the Fribourg area, western Switzerland.