Upper‐regime parallel lamination as the result of turbulent sediment transport and low‐amplitude bed forms

Abstract

A series of laboratory experiments has been carried out in which parallel‐laminated deposits were produced from an upper‐regime plane bed. The laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area (0–3 m by 1–5m). Fluctuations in bed elevation were measured both during deposition and at equilibrium; much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence, as most theories for lamina formation would require. We suggest instead that extremely low‐amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination. All the lamination produced in the laboratory experiments was normally graded. Using high‐speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain‐diameters thick followed by the slow sieving‐out of a well packed surface layer of finer sand. The initial deposition is the result of small‐scale turbulent fluctuations in boundary shear stress. The sieving‐out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition; we term this process ‘glazing’. The alignment of small‐scale turbulent scour‐and‐fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination.

Fluid and sediment dynamics of upper stage plane beds

“…Such interaction between turbulence and sediment transport has a bearing on the prediction of sediment transport rate and flow resistance and may well be related to the very existence of upper stage plane beds rather than beds covered by tipples or dunes [Bagnold, 1966;Engelund andFredsoe, 1974, 1982;Allen and Leeder, 1980;Bridge and Best, 1988]. It has also been recognized recently that low-relief (millimeter high) bed waves migrate over upper stage plane beds and are responsible for the formation of planar lamination under depositional conditions Best, 1988, 1997;Paola et al, 1989;Best and Bridge, 1992].…”

Fluvial responses to the Weichselian ice sheet advances and retreats: implications for understanding river paleohydrology and pattern changes in Central Poland

“…Thus, the stable upper-plane bed and inclined parallel stratification of point-bar sequences (lithofacies Si) were formed due to the migration of low-relief bed waves of up to 20-25 mm high and 1.3 m long (see Bridge and Best 1988;Paola et al 1989;Best and Bridge 1992). Similar supercritical water flow conditions were responsible for the deposition of massive sands (lithofacies Sm; Chobielin site; Figs.…”

Analisis estratigrafico secuencial de las formaciones Anacleto y Allen (Cretacico Tardio) en el borde nororiental de Cuenca Neuquina, Argentina

“…Interpretación: la estratificación paralela o entrecruzada de bajo ángulo resulta de la depositación a partir de flujos de alto régimen, de lecho plano o en la transición antiduna a lecho plano durante episodios de alta descarga (Paola et al, 1989;Bridge y Best, 1997;Marshall, 2000); también ha sido asociada con la transición del flujo subcrítico al supercrítico, presentando estabilidad bajo condiciones de velocidad de alrededor de 1 m/seg y profundidades de entre 0,25 a 0,5 m cuando las areniscas son de grano medio (Miall, 1996). Los intraclastos pelíticos son el producto de la remoción por flujos turbulentos de los depósitos de la llanura de inundación (Alexander et al, 2001;Fielding, 2006).…”