HAND, Bryce M., Dept. Earth Sciences, Syracuse Univ., Syracuse, NY 13244-1070.

Inverse grading in turbidites can be explained in terms of the differing transport velocities of different sediment grainsize fractions within any particular flow. The mechanism depends on the fact that suspended sediment (other than continuously suspended fines) is preferentially transported by the lower, low-velocity portions of the flow. The effect of fall velocity on the vertical distribution of sediment is to cause coarse fractions to travel more slowly than fine. Size fractions traveling slower than the head of a turbidity current will not be present at the front of the flow, and sediment deposited before such fractions arrive should be inversely graded.

This transport-lag model permits one to derive, from coarsest grains at the bottom of an inversely graded interval, an upper limit for strength of flow. Combined with a minimum estimate of flow strength based on the suspension threshold (from the coarsest grains in the bed as a whole), this allows fairly tight bracketing of shear velocity.

In turbidites whose texture was limited by flow strength (rather than by availability), and whose accumulation began immediately upon passage of the head, fall velocities of the coarsest grains at bottom and top of an inversely graded interval should differ by a factor of 4. A ratio less than 4 may mean that deposition did not begin until after arrival of material unable to keep up with the head. Alternatively, it may mean that availability (source area) determined the coarsest material in the bed. Source area restriction should be suspected whenever an ungraded interval is present.

Transport lag may also produce inverse grading in ashflow tuffs.