On traction-carpet sedimentation

Abstract

Traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows. They have a convex-up velocity profile and comprise a lower frictional and an upper collisional region. The frictional region, having a particle concentration more than 80% of the packed bed, is characterized by nearly continuous grain contacts, low strain rate, and hampered grain segregation. On the other hand, the collisional region has a particle concentration between 15% and 80% of the packed bed and is characterized by active grain collisions and higher strain rate. Deposition from the traction carpets occurs via progressive aggradation of the bed, rather than via en masse freezing, while the downward grain flux from the overlying flow maintains them. The thickness of a traction-carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet. Inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure, which is related to nonuniform distribution of particles. When a thick frictional region develops, however, the inverse size distribution in the collisional region is poorly recorded in the deposits. Depositional features of traction-carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions. The thickness ratio is further controlled by the applied shear stress, sediment fallout rate, and grain size. Generally, a collisional region is better developed beneath a highly competent and coarse-grained (gravelly) flow, whereas a thick frictional region is developed beneath a heavily sediment-laden fine-grained (sandy) flow. This explains the more common occurrence of inverse grading in coarse-grained deposits.