Colombera, L, Shiers, MN and Mountney, NP (2016) Assessment of backwater controls on the architecture of distributary channel fills in a tide-influenced coastal-plain succession: Campanian Neslen Formation, USA. Journal of Sedimentary Research, 86 (5). pp. 476-497. ISSN 1527-1404
Abstract
The backwater zone of a river is its distal reach downstream of the point at which the streambed elevation reaches sea level. Backwater hydraulics is believed to exert an important control on fluvio-deltaic morphodynamics, but the expressions with which this may be recorded in the preserved stratigraphic record are not well understood. The seaward reaches of modern rivers can undergo flow acceleration and become erosional at high discharges due to drawdown of the in-channel water surface near the river mouth, in relation to the fixed water surface at the shoreline. As coastal-plain distributary channels approach the shoreline they tend to be subject to a reduction in lateral mobility, which could be related to diminished sediment flux at low flow. Current understanding of channel morphodynamics associated with backwater effects, as based on observations from numerical models and modern sedimentary systems, is here used to make predictions concerning the architecture of coastal distributary channel fills in the rock record. On the basis of existing knowledge, distributary channel fills are predicted to be typically characterized by low width-to-thickness aspect ratios, by a clustering of scour surfaces toward their base, by an aggradational infill style, by a facies organization that bears evidence of drawdown-influenced scour filling, possibly resulting in the overprint of tidal signals toward their base, and by co-genetic sand-prone overbank units of limited occurrence, thickness and sand content. To test these predictions, fieldwork was carried out to examine sedimentological characters of channel bodies from an interval of the Campanian Neslen Formation (eastern Utah, USA), which comprises a succession of sandstone, carbonaceous mudstone, and coal, deposited in a coastal-plain setting, and in which significant evidence of tidal influence is preserved. Three types of channel bodies are recognized in the studied interval, in terms of lithology and formative-channel morphodynamics: sand-prone laterally accreting channel elements, heterolithic laterally accreting channel elements and sand-prone aggradational ribbon channel elements. This study concentrates on the ribbon channel bodies since they possess a geometry compatible with laterally stable distributaries developed in the zone of drawdown. Sedimentological and architectural characteristics of these bodies are analyzed and compared with the proposed model of distributary channel-fill architecture. Although conclusive evidence of the influence of backwater processes in controlling the facies architecture of distributary channel fills is not reached, the studied bodies display an ensemble of internal architecture, lithological organization, nature of bounding surfaces and relationships with other units that conforms to the proposed model to a certain extent. The analyzed ribbon sandbodies are all characterized by erosional cut-banks, very limited proportions of mudstone deposits, a lack of genetically related barform units, clustering of scour fills at their base, and a lack of relationships with co-genetic river-fed overbank sandstones. This work provides a guide to future research, which is required to better understand the role of backwater processes in controlling the architecture of distributary channel bodies, their down-dip variations, and how these are expressed in the stratigraphic evolution of prograding coastal plains.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2016, Society for Sedimentary Geology. This is an author produced version of a paper published in Journal of Sedimentary Research. Uploaded in accordance with the publisher's self-archiving policy. |
Keywords: | distributary; channel; delta; backwater; morphodynamics |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute for Applied Geosciences (IAG) (Leeds) |
Funding Information: | Funder Grant number Operating Account N/A Shell International No External Ref |
Depositing User: | Symplectic Publications |
Date Deposited: | 03 Jun 2016 12:16 |
Last Modified: | 21 Jul 2017 11:22 |
Published Version: | http://dx.doi.org/10.2110/jsr.2016.33 |
Status: | Published |
Publisher: | Society for Sedimentary Geology (SEPM) |
Identification Number: | 10.2110/jsr.2016.33 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:99404 |