Bypass thresholds for gravity flows: implications for sand distribution in turbidite systems

Abstract:

The stratigraphic architecture of turbidite and other systems dominated by gravity flows is underpinned by the ability to transport sediment as suspended load. The ability to transport sediment is crucial to concepts of flow efficiency and equilibrium profile. Predominately bypass-erosional flows transport their sediment load downslope to the basin; forming sand-starved slope channels and canyons. In contrast, flows that cannot suspend their sediment are depositional on inbound slopes leading to constructional slope systems with sand-filled slope conduits and potentially sand starved basins.

An experimentally validated mathematical model for polydisperse turbulent suspension is used to estimate the critical shear velocity (u_*c) for the threshold between depositional and erosional regimes in gravity driven flows. The model shows that the criterion is a function of flow concentration and grain-size distribution increasing as stratification increases within flows: arising from either higher particle concentrations or maximum sizes (increased mean, standard deviation and decreased skew). Predicted values of u_*c are as much as two orders of magnitude greater than those derived using standard monodisperse sediment transport models (e.g., Rouse criterion). The results presented better constrain the potential for total sediment bypass on slopes in turbidite systems. Representative values of velocity and density from monitored natural sandy turbidity currents show total sediment bypass or erosion may be typical in the head and body of supercritical flows on average canyon slopes.