Erosion, transport and segregation of pumice and lithic clasts in pyroclastic flows inferred from ignimbrite at Lascar Volcano, Chile

Abstract

Investigations have been made on the distribution of pumice and lithic clasts in the lithic rich Soncor ignimbrite (26.5ka) and the 1993 pumice flow deposits of Lascar Volcano, Chile. The Soncor ignimbrite shows three main lithofacies which grade into one another. Coarse lithic breccias range from matrix poor stratified varieties, irregular shaped sheets and elongate hummocks in proximal environments, to breccia lenses with pumiceous ignimbrite matrix. Massive, lithic rich facies comprise the bulk of the ignimbrite. Pumice rich facies are bimodal with abundant large pumice clasts (often with reverse grading), rare lithic clasts and occur distally and on high ground adjacent to deep proximal valleys. In the 1993 pyroclastic flow deposits lithic rich facies are deposited on slopes up to 14° whereas pumice rich facies are deposited only on slopes <4°. Lithic rich parts show a thin pumice rich corrugated surface which can be traced into the pumice rich facies. The high lithic content in the Soncor ignimbrite is attributed to the destruction of a pre-existing dome complex, deep explosive cratering into the interior of the volcano and erosion during pyroclastic flow emplacement. Lithic clasts incorporated into the flows during erosion of the basement substrate have been distinguished from those derived from the vent. Categorisation of these lithics and knowledge of the local geology allows these clasts to be used as tracers to interpret former flow dynamics. Lithic populations demonstrate local flow paths and show that lithics are picked up preferentially where flows move around or over obstacles, or through constrictions. Eroded lithics can be anomalously large, particularly close to the location of erosion. Observations of both the Soncor ignimbrite and the 1993 deposits show that lithic rich parts of flows were much more erosive than pumice rich parts. Both the Soncor and 1993 deposits are interpreted as resulting from predominantly high concentration granular suspensions where particle–particle interactions played a major role. The concentrated flows segregated from more expanded and turbulent suspension currents within a few kilometres of the source. During emplacement some degree of internal mixing is inferred to have occurred enabling entrained lithics to migrate into flow interiors. The facies variations and distributions and the strong negative correlation between maximum pumice and lithic clast size are interpreted as the consequence of efficient density segregation within the concentrated flows. The frictional resistance of the lithic rich part is greater so that it deposits on steeper slopes and generally closer to the source. The lower density and more mobile pumice rich upper portions continued to flow and sequentially detached from the lithic rich base of the flow. Pumice rich portions moved to the margins and distal parts of the flow so that distal deposits are lithic poor and non-erosive. The flows are therefore envisaged as going though several important transformations. Proximally, dense, granular flow, undercurrents are formed by rapid sedimentation of suspension currents. Medially to distally the undercurrents evolve to flows with significantly different rheology and mobility characteristics as lithic clasts are sedimented out and distal flows become dominated by pumice.