Abstract

Professor R. M. Shackleton commented on the interest of the density relations of the ignimbrites and the sediments into which they sank. The authors showed that the ignimbrites were welded and flattened after they sank into the sediments, so their density, before the loss of volatiles associated with this welding and flattening, must have been greater than that of the sediments into which they sank. Allowing for the effects of tectonic deformation, which perhaps doubled vertical dimensions, the ignimbrites sank about 50 m into the sediments. A minimum average density for this thickness of muds would be about 1.8 so the density of the ignimbrite when it sank into the sediments must have been at least 2.00. The geometry of the structures suggests that the ignimbrite was still mobile when it sank. If a flow with such a density and mobility reached a body of water it could not flow over the surface but would presumably flow along the bottom like a turbidity current. The flow might originally have contained far more volatiles, only sinking into the sediments when loss of volatiles raised the density to about 2.0, but the occurrence described by the authors does suggest the likelihood of subaqueous ignimbrite flow. Would the authors comment?

The Authors agreed with Professor Shackleton that the density of the flow before the loss of volatiles must have been greater than that of the underlying sediments: the downwarp structures make this self-evident. It is commonly believed that pyroclastic flows contain low ration

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