Extremes in dune preservation: Controls on the completeness of fluvial deposits

REESINK, ARNOLD J.H.; VAN DEN BERG J.M.; PARSONS, DANIELS; AMSLER, MARIO L.; BEST, JAMES; HARDY, RICHARD J.; ORFEO, OSCAR; SZUPIANY, RICARDO N.

EARTH-SCIENCE REVIEWS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015 vol. 150 p. 652 - 665

0012-8252

Understanding sedimentarypreservation underpins our ability to interpret the ancient sedimentary recordand reconstruct paleoenvironments and paleoclimates. Dune sets are ubiquitousin preserved river deposits and are typically interpreted based on a model thatdescribes the recurrence of erosion in a vertical sequence, but without consideringspatial variability. However, spatial variability in flow and sedimenttransport will change the recurrence of erosion, and therefore dunepreservation. In order to better understand the limits of these interpretationsand outline the causes of potential variability in preservation potential, thispaper reviews existing work and presents new observations of an extremeend-member of dune preservation: ?form-sets?, formed by dunes in which bothstoss- and lee-slopes are preserved intact. These form-sets do not conform tomodels that are based on the recurrence of erosion, since erosion does notrecur in their case, and can therefore be used to evaluate the assumptions thatunderpin sedimentary preservation. New Ground Penetrating Radar data from theRío Paraná, Argentina, show dune fields that are buried intact within largerscale barforms. These trains of form-sets are up to 300 m in length, arerestricted to unit-bar troughs in the upper 5mof the channel deposits, occur inN5% of the mid-channel bar deposits, show reactivation surfaces, occur inmultiple levels, and match the size of average-flow dunes. A review ofpublished accounts of form-sets highlights a diversity of processes that can beenvisaged for their formation: i) abandonment after extreme floods, ii) slowburial of abandoned dune forms by cohesive clay in sheltered bar troughs andmeander-neck cut-offs, iii) fast burial by mass-movement processes, and iv)climbing of dune sets due to local dominance of deposition over dune migration.Analysis of these new and published accounts of form-sets and their burialprocesses highlights that form-sets need not be indicative of extreme floods.Instead, form-sets are closely associated with surrounding geomorphology suchas river banks, meander-neck cut-offs, and bars because this larger-scalecontext controls the local sediment budget and the nature of recurrence oferosion. Locally enhanced preservation by the ?extreme? dominance of depositionis further promoted by finer grain sizes and prolonged changes in flow stage.Such conditions are characteristic, although not exclusive, of large lowlandrivers such as the Río Paraná. The spatial control on dune preservation iscritical: although at-a-point models adequately describe near-horizontal setsof freely migrating dunes in uniform flows, they are unsuitable for inclineddune co-sets and other cases where multiple scales of bedforms interact.Spatial and temporal variations in flow and sediment transport between thethalweg and different positions on larger bar-forms can change the preservationpotential of dunes within river channels. Therefore, dune set thicknessdistributions are likely grouped in larger-scale units that reflect bothformative dune geometries and bar-scale variations in preservation potential.The multi-scale dynamics of preservation highlighted herein also provides auseful comparison for other sedimentary systems.