EP004-0014
Discriminating mid-channel and bank-attached fluvial bars by flow divergence: Implications for the interpretation of stratigraphic records
Monday, 7 December 2020
Poster
Massimiliano Ghinassi1, Alessandro Ielpi2,3, Dario Ventra4,5, Ivan Martini6, Alvise Finotello1,7 and Elena Bellizia1, (1)University of Padova, Department of Geosciences, Padova, Italy, (2)Laurentian University, Harquail School of Earth Sciences, Sudbury, ON, Canada, (3)Laurentian University, School of the Environment, Sudbury, ON, Canada, (4)Utrecht University, Faculty of Geosciences, Utrecht, Netherlands, (5)University of Geneva, Department of Earth Sciences, Geneva, Switzerland, (6)University of Siena, Department of Physical Sciences, Earth and Environment, Siena, Italy, (7)University of Venice, Department of Environmental Sciences, Informatics, and Statistics, Venice, Italy
Abstract:
The characterization of ancient fluvial-channel bars is a fundamental tool of traditional facies models for the interpretation of paleochannel patterns. Investigation of ancient fluvial-channel bars informs predictions of sedimentary facies distribution, and therefore of river morphodynamics through time. Morphodynamic evolution of rivers, shows that sediment storage within active fluvial channels can basically occur either along the banks or along their axial portion, generating bank-attached and mid-channel bars, respectively. In this study, we introduce a metric of flow divergence (Δf), measured as the angle between the azimuths of bar accretionary slopes and local dune migration, and explore the distribution of such metric in clear-water active sand-bedded channel reaches. The dataset is from 103 channel reaches and includes both mid-channel and bank-attached bars, which developed in low-variable peak discharge rivers of Russia and Brazil.
Measurements shows that dune migration transverse to the dip azimuth of bar slopes (Δf≅ 90-110°) occurs both in mid-channel and bank-attached bars, contradicting the tenet that paleoflows nearly orthogonal to the dip azimuth of large-scale inclined strata are indicative of sinuous, single-thread paleochannels. Accordingly, only Δf values significantly higher (> 140°) or lower (< 60°) than 90° discriminate between bank-attached and mid-channel bars, since these values typifies, respectively, in mid-channel bar heads and tails. Comparisons between Δf values and bar-accretionary styles points out that mid-channel bars formed by high- and low-variable discharge rivers tend to preserve axially and laterally accreting deposits, respectively.
We, therefore, propose a new, quantitative approach to better discriminate fluvial bars from sedimentary successions, but we stress that similar approaches needs strongly to be complemented with additional sedimentological information from high-quality outcrops.
