Morphology, sedimentology and palaeohydraulic significance of large gravel dunes, Altai Mountains, Siberia

Abstract

Coarse‐gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two‐dimensional (2‐D) transverse dunes and 3‐D cuspate and lunate dunes developed in coarse gravels is comparable with sand‐dune morphology within lesser‐scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2‐D dunes was relatively uniform but over 3‐D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3‐D dunes. Cross‐beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss‐entrained gravel transported over the crests rolling‐down and depositing on the lee slopes. Lee‐side sediments are often finer than the stoss‐slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly planed‐down during falling stage because lee‐side cross‐beds tend to be steeper than the angle of the preserved lee slope. However, silt‐rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post‐flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs.

Sedimentología y estratigrafía de alta resolución del subgrupo río Neuquén (Cretácico Superior) departamento confluencia, provincia de Neuquén, Argentina

“…Interpretación: Este elemento comúnmente representa la migración de formas de lecho transversales de cresta sinuosa (3D) (Harms et al, 1975;Carling, 1996;Todd, 1996) o barras linguoides formadas durante la disminución de la velocidad del flujo y de carga de sedimentos, como es común en las etapas posteriores a un evento de inundación (Manier et al, 1993).…”

“…Estos procesos están ampliamente determinados por la descarga, porque existe una relación inversa entre la descarga y la población de dunas (Gabel, 1993). La preservación de este tipo de dunas gravosas en el registro estratigráfico ha sido interpretada como el resultado de condiciones de rápido abatimiento en el flujo posterior a un pico de descarga (Carling, 1996). En ríos modernos la ocurrencia de campos de dunas '3D' requiere aguas profundas (Kraus, 1984;Middleton y Trujillo, 1984) y descarga estacional; en la cual el repentino ascenso de la descarga es seguido por una caída en el nivel de la misma con pocos días de diferencia (Kadkikar et al, 2000).…”

Coupled flood and sediment transport modelling with adaptive mesh refinement

“…This megaflood was modelled by Carling et al [36] using integrated 1D and 2D hydraulic models over a presumed fixed bed. Yet, a megagflood is certainly highly energetic, capable of triggering very active sediment transport and intense geomorphic change [35], which in turn modifies the flood flow significantly. Even a single flood event can result in rapid formation of a modern bedrock canyon [37].…”

“…The background of this megaflood was presented before [34][35][36]. This megaflood was modelled by Carling et al [36] using integrated 1D and 2D hydraulic models over a presumed fixed bed.…”

Structural and textural response to dynamics of fluvioglacial processes of the Rzymsko esker sediments, Central Poland

“…Sand was carried away in suspension, but energy of flow was too low to move bigger clasts (Wyżga 1993). Sediments of the Gt lithofacies originated as a result of migrating three-dimensional gravelly ripplemarks in channels of considerable depth and high flow competence (Brennand & Shaw 1996, Carling 1996, Rudoy 2002, Fard & Gruszka 2007. Directions of long axes of clasts are parallel to the general orientation of the form.…”