Plate 75

U-shaped channel form

We are in the same setting as plate 74, with the only variation represented by the shape of the scour. This is a rare occurrence, anyway, because the structure is visible in three dimensions (and has the curious aspect of a big tongue, reminding one of papier-ma;afche;aa decorations on carnival floats).

The causal agent of erosion is again a pyroclastic surge or a "nuée ardente" (burning cloud) flowing down a steep volcano slope (the slope is still the original one: see 75 A). The fill is multilayered, which would suggest a relatively permanent channel; it cannot be excluded, however, that the depositional events succeeded one another in a rapid sequence, possibly during the same volcanic eruption. This seems to be true, at least, for the conformable bedset shown by the longitudinal section (A); below it, there is a thin pavement of ash conformably mantling the scour bottom. Actually, we have here a two-phase filling, separated by a discontinuity (downlap); then, there could be a time gap between phase 1 and phase 2. The second and main phase was probably continuous, and shows an example of retrograde filling, or back-filling,  as indicated by the upslope dip of the beds. Mark these backset beds,  as their occurrence is very rare.

The U-shaped section can give us further suggestions: this profile is typically produced by laminar flow of a viscous fluid, from lava flows to glacier ice. In sedimentary environments, cohesive debris flows are mainly responsible for it. It is thus possible, in our case, that a high-concentration pyroclastic flow or a cold lahar scoured a channel, which was filled later on by more dilute surge flows.

In conclusion, the channel form illustrated here could represent either a scour-and-fill or a channel. There is no overwhelming evidence in favor of one or the other case.