Morphology and curvature of delta slopes in Swiss lakes: lessons for the interpretation of clinoforms in seismic data

Seismic surveys were conducted and bathymetric data obtained from four alpi ne lakes in Switzerland. The curvature of the delta slopes was analysed wit h mathematical equations. Linear or exponential profiles are observed, repr esenting planar or concave morphologies respectively. Planar profiles are i nterpreted to represent sediment that rests at the angle-of-repose. The slo pe angle of these profiles shows a correlation with sediment calibre. Expon ential profiles do not show a clear correlation between sediment calibre an d slope angle; they do not rest at the angle-of-repose, and different kinds of sediment can rest at the same slope angle. At the transition from lower slope to toe-of-slope, the exponential equation fails to predict the prese nt-day morphology. The toe-of-slope lies above the predicted trend. This is attributed to a drastic increase in turbidite deposition that provides add itional sediment and raises the basin-floor profile above the predicted tre nd. The breaks between delta plain and slope are sharp, reflecting an abrup t change from transport by river now and waves to gravity-driven transport. In these lakes, the base-level fluctuations relative to supply are small a nd insufficient to alter this sharp topographic break. The absence of sigmo idal profiles on the Swiss deltas is attributed to the high rate of prograd ation coupled with small fluctuations in base level.