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

Investigations have been made on the distribution of pumice and lithic clas ts in the lithic rich Soncor ignimbrite (26.5 ka) and the 1993 pumice flow deposits of Lascar Volcano, Chile. The Soncor ignimbrite shows three main l ithofacies which grade into one another. Coarse lithic breccias range from matrix poor stratified varieties, irregular shaped sheets and elongate humm ocks in proximal environments, to breccia lenses with pumiceous ignimbrite matrix. Massive, lithic rich facies comprise the bulk of the ignimbrite. Pu mice 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 li thic rich facies are deposited on slopes up to 14 degrees whereas pumice ri ch facies are deposited only on slopes <4<degrees>. Lithic rich parts show a thin pumice rich corrugated surface which can be traced into the pumice r ich 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 how emplac ement. Lithic clasts incorporated into the flows during erosion of the base ment substrate have been distinguished from those derived from the vent. Ca tegorisation of these lithics and knowledge of the local geology allows the se clasts to be used as tracers to interpret former flow dynamics. Lithic p opulations demonstrate local flow paths and show that lithics are picked up preferentially where flows move around or over obstacles, or through const rictions. Eroded lithics can be anomalously large, particularly close to th e location of erosion. Observations of both the Soncor ignimbrite and the 1 993 deposits show that lithic rich parts of flows were much more erosive th an 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 fe w kilometres of the source. During emplacement some degree of internal mixi ng is inferred to have occurred enabling entrained lithics to migrate into flow interiors. The facies variations and distributions and the strong nega tive correlation between maximum pumice and lithic clast size are interpret ed as the consequence of efficient density segregation within the concentra ted 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 ric h 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 envisage d as going though several important transformations. Proximally, dense, gra nular flow, undercurrents are formed by rapid sedimentation of suspension c urrents. Medially to distally the undercurrents evolve to flows with signif icantly different rheology and mobility characteristics as lithic clasts ar e sedimented out and distal flows become dominated by pumice. (C) 2000 Else vier Science B.V. All rights reserved.