Origin and significance of fully-preserved dunes in large-river deposits
A.J.H. Reesink, SET - University of Brighton & GEES - University of Birmingham,
UK, email: a.j.h.reesink@brighton.ac.uk
Fully-preserved dunes are single cross sets that have maintained the shape of their
formative dunes and their existence can potentially provide important information
about the controls on bedform preservation. Unfortunately, few descriptions of fullypreserved dunes in sandy alluvial deposits exist. Ground-penetrating radar (GPR) data
from mid-channel bars in the Paraná River near Corrientes, Argentina, are used to
show the sedimentological context in which fully-preserved dunes are found and
some explanations of their formation are presented.
The fully-preserved dunes in the sandy Paraná River are characterised by a strong
reflection from the top surface, lower-amplitude basal reflections and loweramplitude, high-angle internal reflections. These fully-preserved dunes have heights
of 0.6 ± 0.2 m with a maximum of 1.2 m, lengths of 24 ± 11m with a maximum of 54
m, and widths in the same order of magnitude as the lengths. Approximately 90% of
the fully-preserved dunes are overlain by large-scale unit-bar foresets with heights
exceeding 0.5 m. Some of these overlying unit-bar foresets comprise several
reactivation surfaces that are interpreted as low-flow structures, and dune heights vary
systematically as much as 0.8 m along trains of dunes, which suggests that the dune
forms are buried over multiple high- and low-flow events.
Three potential mechanisms for full preservation are: [1] abandonment of the dunes
during low flow and burial by fine-grained cohesive sediment that prevents further
erosion; [2] rapid burial of the dunes; and, [3] stagnation of the dunes in the wake
zone in the lee of (unit) bars and subsequent burial by the unit-bar bottomsets and
foresets. Differences in the relative importance of these mechanisms point at
respectively [a] stage variability [b] aggradation rate, and [c] migration rate, as the
key factors controlling bedform preservation. Based on the GPR data, it is suggested
here that the effect of bar-scale morphology on dune migration, and therefore
preservation, is currently underestimated in fluvial sedimentology.