PYROCLASTIC FLOW ANALOGY FOR DEPOSITION IN THE 1992 NICARAGUA TSUNAMI

Event deposits provide data for understanding the exceptional and often disastrous geological processes that create them. Deposits from tsunamis and pyroclastic flows have been studied extensively because these events cause much damage and loss of life. The generalized internal structure of the 1992 Nicaragua tsunami deposit resembles that of some pyroclastic flows. This similarity suggests the tsunami deposit formed from a slurry or grains in traction, as opposed to a diffuse suspension.

The 1992 Nicaragua tsunami deposit represents a single wave. Because a beach ridge prevented erosive withdrawal of water, the deposit of this wave is well preserved. Our data for describing structure includes direct field observation as well as detailed sampling and settling velocity analysis.

The tsunami has two primary layers: a lower coarser layer with complex internal structure, and an upper layer composed of finer material. Patches of very coarse material are emplaced discontinuously at the base of the deposit, usually in depressions or crab burrows. Sand is usually normally graded but sometimes ungraded, and there is a sharp grain size break at the separation between layers. Where dense minerals are common, dark laminae often occur at the base of the deposit, and sometimes at the separation between the upper and lower layers, as well as discontinuously in between. Rip-up clasts are sometimes concentrated in a layer above or below the separation. If present through the lower layer they are inversely graded or ungraded.

Pyroclastic flow structures have been described and explained in greater detail than tsunamis. One way of categorizing pyroclastic flow deposits would associate structures like those in the lower section of the Nicaragua tsunami deposit with traction processes. The inversely graded rip-ups are sometimes interpreted as evidence for a slurry dense enough to float low-density clasts, but may instead represent density and size biased overpassing processes during aggradation. Regardless, the structure in the lower portion of the deposit is inconsistent with sediment raining out of suspension. However, models that assume this might apply well to the upper, finer layer.