Formation of inversely graded basal layers in ignimbrites by progressive aggradation

A model for the formation of coarse-depleted, inversely graded basal layers in ignimbrites is presented. It is proposed that the deposit forms by progressive aggradation of sediment below the flow, and that the basal layer at any place is deposited from the leading part of the flow. Sinking of large lithics within the flow produces a normal grading, which is reversed in the basal deposit by the caterpillar-like motion of the flow. Large pumices rise to the top of the flow and migrate to the head, where many are pushed aside and accumulate in levées. Pumices, which deposit from the head are mostly re-entrained into the flow due to their buoyancy but, exceptionally, some may be trapped at the base of the aggrading deposit. We describe concentrations of large pumices in the basal layer of small-volume ignimbrites, which give support to this model. The model is an alternative to the traditional hypothesis which assumes that coarse-depleted, inversely graded basal layers in ignimbrites are the result of the upward migration of large particles out of the basal layer, in a flow which later freezes en masse.