CECOAL   02625
CENTRO DE ECOLOGIA APLICADA DEL LITORAL
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Does large scale alluvial morphology control dune-set preservation?
Autor/es:
REESINK, A.; ASHWORTH, P.; AMSLER, M.; BEST, J.; HARDY, R,; LANE, S.; NICHOLAS, A.; ORFEO, O.; PARSONS, D.; SMITH G, SAMBROOK; SANDBACH, S.; SZUPIANY, R,
Lugar:
S. M. de Tucumán, Argentina
Reunión:
Congreso; 9th International Conference on Fluvial Sedimentology; 2009
Institución organizadora:
Univ. Nac. de Tucumán - Fundación Miguel Lillo
Resumen:
The thickness distributions of cross-strata sets underlie paleo-hydraulic interpretations and predictions of permeability heterogeneity. Previous work on the proportion of a bedform that is preserved as a set stresses the importance of the variation in bedform height and the deposition rate relative to the migration rate. Although no real consensus exists on the relative importance of the factors controlling set preservation, it is generally accepted that sets scale to their formative bedforms, and that both follow log-normal distributions. This paper compares the thicknesses distributions of cross-strata sets with ground-penetrating radar (GPR) surveys. Cross stratatified sets are classified into three groups based on radar facies, and these three groups are interpreted as: nearhorizontal dune sets, inclined dune sets, and angle-of-repose unit-bar sets. The distributions of medium- and largescale set thicknesses were measured from 28, ~4 m long suction-cores taken in mid-channel bars in the Paraná River, Argentina, and 36 ~ 3m long cores taken in the South Saskatchewan River, Canada. Results show that the total distribution of thicknesses of sets formed by dunes and bars does not follow a log-normal distribution. Instead, the total distribution is better described by three separate log-normal distributions: [1] dune sets associated with near-horizontal (<4 º ) GPR reflections, formed by dunes on nearhorizontal surfaces, and; [2] dune sets associated with inclined (>4º) GPR reflections, formed by dunes on slopes; [3] unit-bar sets, identified by their large lateral extent, large thickness compared to bankfull levels, and association with fine-grained bottomsets. No clear relationship was found between set thickness and vertical position in the deposit. Dune sets associated with inclined GPR reflections [2] are 2-2.5 times thicker on average than dune sets associated with near-horizontal surfaces [1]. This increase in set thickness can be explained by increased preservation of sets on slopes, which is attributed to the decrease in migration rate, height and length of dunes migrating down large-scale lee slopes, and the locally increased deposition rates caused by flow expansion over the lee slope. Inclined dune sets made up 10- 20% of the investigated river channel deposits which equals roughly 40% of all dune sets. It is therefore suggested here that largescale alluvial morphology forms a dominant control on the preservation of dune sets.