Abstract

Inverse grading involves an upward increase in either or both of clast size and percentage. Debris flows possess high viscosity and yield strength; hence dispersive pressure, kinetic sieving, and fluid dynamic boundary effects are regarded as unsatisfactory mechanisms for inverse grading. The strength loss (sensitivity) that clay suffers on deformation can explain inverse grading; the lowest, most strongly sheared layers of debris are weakest and support relatively small clasts. Other processes which may contribute are reduced buoyancy due to dilation, strength/fabric anisotropy, variable clast fall-out rate, inherited stratification (e.g., mud decollement in slumps) and sediment assimilation during flow. The sensitivity mechanism may also apply to density-modified muddy grain-flows. It implies that coarse lag deposits should occur upslope of inversely graded deposits.

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