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

Extremes in dune preservation: controls on the completeness of fluvial deposits
Extremes in dune preservation: controls on the completeness of fluvial deposits
Understanding sedimentary preservation underpins our ability to interpret the ancient sedimentary record and reconstruct paleoenvironments and paleoclimates. Dune sets are ubiquitous in preserved river deposits and are typically interpreted based on a model that describes the recurrence of erosion in a vertical sequence, but without considering spatial variability. However, spatial variability in flow and sediment transport will change the recurrence of erosion, and therefore dune preservation. In order to better understand the limits of these interpretations and outline the causes of potential variability in preservation potential, this paper reviews existing work and presents new observations of an extreme end-member of dune preservation: ‘form-sets’, formed by dunes in which both stoss- and lee-slopes are preserved intact. These form-sets do not conform to models that are based on the recurrence of erosion, since erosion does not recur in their case, and can therefore be used to evaluate the assumptions that underpin sedimentary preservation.

New Ground Penetrating Radar data from the Río Paraná, Argentina, show dune fields that are buried intact within larger scale barforms. These trains of form-sets are up to 300 m in length, are restricted to unit-bar troughs in the upper 5 m of the channel deposits, occur in > 5% of the mid-channel bar deposits, show reactivation surfaces, occur in multiple levels, and match the size of average-flow dunes. A review of published accounts of form-sets highlights a diversity of processes that can be envisaged for their formation: i) abandonment after extreme floods, ii) slow burial of abandoned dune forms by cohesive clay in sheltered bar troughs and meander-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 burial processes highlights that form-sets need not be indicative of extreme floods. Instead, form-sets are closely associated with surrounding geomorphology such as river banks, meander-neck cut-offs, and bars because this larger-scale context controls the local sediment budget and the nature of recurrence of erosion. Locally enhanced preservation by the ‘extreme’ dominance of deposition is further promoted by finer grain sizes and prolonged changes in flow stage. Such conditions are characteristic, although not exclusive, of large lowland rivers such as the Río Paraná. The spatial control on dune preservation is critical: although at-a-point models adequately describe near-horizontal sets of freely migrating dunes in uniform flows, they are unsuitable for inclined dune co-sets and other cases where multiple scales of bedforms interact. Spatial and temporal variations in flow and sediment transport between the thalweg and different positions on larger bar-forms can change the preservation potential of dunes within river channels. Therefore, dune set thickness distributions are likely grouped in larger-scale units that reflect both formative dune geometries and bar-scale variations in preservation potential. The multi-scale dynamics of preservation highlighted herein also provides a useful comparison for other sedimentary systems.
dunes, unit bars, preservation, river deposits, stratigraphic completeness
0012-8252
652-665
Reesink, A.J.H.
e9a3724d-3430-4fad-b85e-b27ed7343e53
Van den Berg, J.H.
eb44a350-601d-41b9-9961-93630143c7b3
Parsons, D.R.
8bc551f4-ae49-48cd-8533-82a34e749d74
Amsler, M.L.
d0a9a7f6-69a5-4f95-9b48-67c5e8512e94
Best, J.L.
36ba040c-3d2e-4393-963e-f575ba5afad0
Hardy, R.J.
87eecdb0-e674-4507-a6e2-dc3bc8085a5d
Orfeo, O.
06d21dd3-6fb2-4f8f-843a-c12dfe39cbce
Szupiany, R.N.
95215670-d60f-4f1c-8a3a-747b62b3c4a3
Reesink, A.J.H.
e9a3724d-3430-4fad-b85e-b27ed7343e53
Van den Berg, J.H.
eb44a350-601d-41b9-9961-93630143c7b3
Parsons, D.R.
8bc551f4-ae49-48cd-8533-82a34e749d74
Amsler, M.L.
d0a9a7f6-69a5-4f95-9b48-67c5e8512e94
Best, J.L.
36ba040c-3d2e-4393-963e-f575ba5afad0
Hardy, R.J.
87eecdb0-e674-4507-a6e2-dc3bc8085a5d
Orfeo, O.
06d21dd3-6fb2-4f8f-843a-c12dfe39cbce
Szupiany, R.N.
95215670-d60f-4f1c-8a3a-747b62b3c4a3

Reesink, A.J.H., Van den Berg, J.H., Parsons, D.R., Amsler, M.L., Best, J.L., Hardy, R.J., Orfeo, O. and Szupiany, R.N. (2015) Extremes in dune preservation: controls on the completeness of fluvial deposits. Earth-Science Reviews, 150, 652-665. (doi:10.1016/j.earscirev.2015.09.008).

Record type: Article

Abstract

Understanding sedimentary preservation underpins our ability to interpret the ancient sedimentary record and reconstruct paleoenvironments and paleoclimates. Dune sets are ubiquitous in preserved river deposits and are typically interpreted based on a model that describes the recurrence of erosion in a vertical sequence, but without considering spatial variability. However, spatial variability in flow and sediment transport will change the recurrence of erosion, and therefore dune preservation. In order to better understand the limits of these interpretations and outline the causes of potential variability in preservation potential, this paper reviews existing work and presents new observations of an extreme end-member of dune preservation: ‘form-sets’, formed by dunes in which both stoss- and lee-slopes are preserved intact. These form-sets do not conform to models that are based on the recurrence of erosion, since erosion does not recur in their case, and can therefore be used to evaluate the assumptions that underpin sedimentary preservation.

New Ground Penetrating Radar data from the Río Paraná, Argentina, show dune fields that are buried intact within larger scale barforms. These trains of form-sets are up to 300 m in length, are restricted to unit-bar troughs in the upper 5 m of the channel deposits, occur in > 5% of the mid-channel bar deposits, show reactivation surfaces, occur in multiple levels, and match the size of average-flow dunes. A review of published accounts of form-sets highlights a diversity of processes that can be envisaged for their formation: i) abandonment after extreme floods, ii) slow burial of abandoned dune forms by cohesive clay in sheltered bar troughs and meander-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 burial processes highlights that form-sets need not be indicative of extreme floods. Instead, form-sets are closely associated with surrounding geomorphology such as river banks, meander-neck cut-offs, and bars because this larger-scale context controls the local sediment budget and the nature of recurrence of erosion. Locally enhanced preservation by the ‘extreme’ dominance of deposition is further promoted by finer grain sizes and prolonged changes in flow stage. Such conditions are characteristic, although not exclusive, of large lowland rivers such as the Río Paraná. The spatial control on dune preservation is critical: although at-a-point models adequately describe near-horizontal sets of freely migrating dunes in uniform flows, they are unsuitable for inclined dune co-sets and other cases where multiple scales of bedforms interact. Spatial and temporal variations in flow and sediment transport between the thalweg and different positions on larger bar-forms can change the preservation potential of dunes within river channels. Therefore, dune set thickness distributions are likely grouped in larger-scale units that reflect both formative dune geometries and bar-scale variations in preservation potential. The multi-scale dynamics of preservation highlighted herein also provides a useful comparison for other sedimentary systems.

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More information

Accepted/In Press date: 20 September 2015
e-pub ahead of print date: 25 September 2015
Published date: November 2015
Keywords: dunes, unit bars, preservation, river deposits, stratigraphic completeness
Organisations: Geography & Environment

Identifiers

Local EPrints ID: 383194
URI: https://eprints.soton.ac.uk/id/eprint/383194
ISSN: 0012-8252
PURE UUID: ab57d212-6d0b-42d9-a897-25546fe3d028

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Date deposited: 27 Oct 2015 15:34
Last modified: 09 Sep 2019 18:35

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