Initiation and evolution of turbidity current sediment waves in the Magdalena turbidite system

Abstract

This study describes an extensive sediment–wave field in the Magdalena Turbidite System, Caribbean Sea, Colombia, which occupies an area of at least 15 000 km2 on the continental slope (3330–>3800 m). The waves display wavelengths up to 1.9 km, wave heights up to 18 m, and crestlines that are aligned roughly parallel to the regional bathymetric contours. Preferential deposition on the upslope wave flank has led to upslope migration, in the manner of antidunes. The Magdalena sediment waves are interpreted as forming beneath unconfined turbidity currents, which may result from the downslope evolution of slumps and mass flows. The unconfined turbidity currents are derived from several point sources along the continental slope and spread laterally as they flow downslope. This has led to the formation of a laterally extensive sediment–wave field. Simple numerical modelling estimates that the turbidity currents responsible for wave generation were near- or super-critical, with flow thickness and velocity estimated at 40–160 m, and 36–82 cm s−1 respectively. However, later phases of wave growth were not dependent on specific flow conditions. The most important aspect of this study is that the entire sediment–wave unit, from the basal boundary to the present-day seafloor, has been investigated using ultra high-resolution seismic profiles. The sediment–wave unit rests upon an irregular discontinuity that marks a recent change in the sedimentary regime of the Magdalena Turbidite System, from channelised to unchannelised gravity flows. Above this boundary, the sediment waves display a growth pattern characterised by an increase in wave dimensions. In addition, the wave dimensions appear to become more regular through time. However, breaks of slope in the lower bounding surface of the wave field have produced variations in wave morphology that are still visible at the present-day seafloor. This indicates that there is a close relationship between variations in slope angle and turbidity current flow characteristics, which in turn leads to variations in wave morphology