On the Freezing of Pyroclastic Flows

Processes of sedimentation in pyroclastic flow are still poorly understood, although their massive deposits have been extensively studied. It is unclear whether the flows freeze en masse or gradually sediment particles, aggrading their deposits in a layer-by-layer fashion. Arguments for en masse deposition include the poorly sorted, structureless nature of deposits and the common presence of coarse-tail grading of lithics and/or pumices. Sedimentation by freezing of the flow implies that the deposit facies is directly representative of the dynamical state of the moving flow. Arguments for deposition by aggradation are the existence of compositionally distinct units within some massive deposits and the onlap structure of these units on the preexisting topography. The boundary between compositional units is interpreted as a timeline within the massive deposit. Remarkable exposure of pyroclastic deposits at Toluca Volcano, Mexico, allows 3D reconstruction of the facies spatial distribution. Sections perpendicular to flow axes display a transition of facies from a thin marginal stratified deposit to a thick massive deposit to multiple massive units in the flow interior. While stratified deposits are cross-bedded, massive deposits feature coarse-tail reverse grading of lithics and coarse-tail normal grading of pumices. Multiple units are also coarse-tail graded and are separated by thin stratified veneers. In sections parallel to flow axes, the flow interiors show a gradual transition from massive facies proximally to crudely stratified facies to stratified facies distally, without significant changes in thickness. Since perpendicular sections feature evidences of timelines in the middle of coarse-tail graded deposits, these massive deposits cannot be the result of en masse deposition. Parallel sections show a continuum between massive and stratified facies, implying the existence of multiple depositional units within a structureless deposit. Since these units can be traced into each individual layers of a stratified deposit, we interpret the Toluca pyroclastic deposits being entirely a product of deposition by aggradation. These results suggest that the aggradation process does not only produce massive and/or stratified deposits but is also able to generate coarse-tail grading.