Evolution and mechanics of a Miocene tidal sandwave

Abstract

A remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi‐diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near‐symmetrical combined‐flow ripple marks in the flood apron. Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple‐dune transition and the dune‐plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec−1 for ebbs and up to 0·5 m sec−1 for floods. The enhancement by waves of bed shear stress (τwc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec−1. Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m−1 sec−1) is over 10 times greater than at neaps (0·002 kg m−1 sec−1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to those of the present Dutch tidal estuaries.

Simulation of Tidal Flow and Circulation Patterns in the Early Miocene (Upper Marine Molasse) of the Alpine Foreland Basin

“…The palinspastic reconstruction of the Burdigalian Seaway hence results in a width of approximately 70 km. This width fits well with results from Allen & Homewood (1984), who reconstructed marine conditions near Fribourg. In western Switzerland the Alpine thrust front was in a fixed position southeast of Lausanne and the 'pinch-out' of Molasse strata to the northwest markedly slowed down between 22 and 12 Ma (Burkhard & Sommaruga, 1998).…”

2004 Indian Ocean tsunami inflow and outflow at Phuket, Thailand

“…Through, threshold of sediment motion under unidirectional currents has been suggested (e.g. Miller et al, 1977;Allen and Homewood, 1984), equations suggested by Komar and Miller (1973) were applied to this work because they considered wavelength and mean diameter of sediment as factors for controlling near bottom threshold flow velocity:…”

Early and Middle Miocene dinoflagellate cyst stratigraphy of the Central Paratethys, Central Europe

“…These meso-and macrotidally controlled conditions extended from the Austrian part of the Alpine foreland basin westward into the Swiss and French Molasse during the late Early Miocene (e.g. Allen & Homewood, 1984;Lesueur et al, 1990). This hydrodynamic regime resulted from an effective but shortlived connection of the Paratethys with the western Mediterranean Sea via the Rhône valley (Rögl, 1998) and also from a connection with the Eastern Mediterranean (Martel et al, 1994).…”