Abstract: Development of Inverse Grading in Grain Flows
A. H. Sallenger, Jr.
Inverse grading has been used as an indicator of grain-flow deposits in various sedimentary environments (i.e., beach foreshore, basal parts of some turbidites, slipface deposits of eolian dunes). There is disagreement, however, on the process responsible for the development of inverse grading. Bagnold suggested that, because grains are supported in a grain flow by dispersive pressures, the largest grains (those with the greatest dispersive pressure) should migrate to the top of the flow (a position of low shear stress and, therefore, low pressure), whereas the smaller grains should go to the bottom of the flow (high shear stress and pressure). Middleton hypothesized a kinetic-sieve mechanism whereby the finer grains fall downward between the larger during grain flow. However, the two hypotheses lead to different consequences when applied to a mixture of heavy and light grains. Under Bagnold's hypothesis, a heavy grain of density (^rgrH) and diameter (DH) would be associated at any level in an inversely graded bed with a lighter grain of density (^rgrL) and diameter (DL) as (^rgrL/^rgrH)½ = DH/DL. A heavy grain, therefore, should be smaller than as associated light grain. However, Middleton's hypothesis suggests that the heavy grain should be equal to or larger than an associated light grain. Laboratory experiments employing a grain flow of particles of different densities and diameters show ot only that heavy grains are smaller than associated lights at any level in the bed, but also that agreement with Bagnold's theory is good quantitatively.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC