NORMAL GRADING PATTERNS IN DEPOSITS OF THE 1992 NICARAGUA TSUNAMI

The 1992 Nicaragua tsunami left behind normally graded sand sheets which were sampled at several locations near Playa de Popoyo (11° 27’ N, 86° 6’ W). Vertically contiguous samples spaced 5-10 mm apart were analyzed for their settling velocity distributions. These show that the sheet is composed of two layers, with stronger grading in the lower layer than in the upper. This pattern is inconsistent with two common explanations for normal grading, and must be the result of some other process.

Normal grading has been explained by two models: 1) In an environment of rapid sedimentation, faster settling grains settle out of the water column before slower grains, leading to depletion of faster grains in the upper part of the deposit. 2) In an environment where transport energy is decreasing, slow settling grains are more easily maintained in the flow, so deposition is biased to coarser grains early in the depositional process. Taking simple assumptions, these two explanations would produce distinctly different grading patterns and can be viewed as extremes on a spectrum from 1) Complete disequilibrium between sediment carried and transport capacity to 2) Continuous equilibrium between sediment carried and transport capacity. However, the pattern of grading observed near Playa de Popoyo is inconsistent with any combination of these end members. This suggests that a more specific explanation for normal grading in this tsunami deposit is needed. This pattern of grading may be caused by kinetic sieving or dispersive pressure in a rapidly deposited shearing sediment slurry.