The effect of paleotopography on lithic distribution and facies associations of small volume ignimbrites: the WTT Cupa (Roccamonfina volcano, Italy)

Abstract

The distribution of lithic clasts within two trachytic, small volume, pumiceous ignimbrites are described from the Quaternary `White Trachytic Tuff Cupa' formation of Roccamonfina volcano, Italy. The ignimbrites show a downslope grading of lithics, with a maximum size where there is a major break in the volcano's slope, rather than at proximal locations. This is also the location where ignimbrites are thickest and most massive. The break in slope is interpreted to have reduced flow capacity and velocity, increasing the sedimentation rate, so that massive ignimbrite formed by hindered settling sedimentation. Ignimbrite Cc, exhibits no vertical grading of lithics, though it does show downslope grading with maximum size at the major break in slope and a rapid decrease further downslope. Ignimbrite Cc thins away from the break in slope, and shows an upward fining of the grain size within the topmost few decimeters of the unit. The ignimbrite is stratified proximally, and grades to massive facies at the break in slope, and distally to stratified facies with numerous inverse-graded beds. The simplest mechanism accounting for these downslope variations is progressive aggradation from a quasi-steady, nonuniform pyroclastic density current. The changes in deposit thickness and facies are interpreted to record downcurrent changes in sedimentation rate. The upward fining reflects waning flow. Inversely graded, bedded depositional facies in distal areas is interpreted to reflect flow unsteadiness and a decrease in suspended sediment load. Ignimbrite Cd shows vertical, as well as downslope grading of lithics. This characteristic, coupled with the widespread massive facies of the deposit and the tabular unit geometry are features that can be reconciled with both the debris flow/plug analogy for pyroclastic flows ( Sparks, 1976) and the progressive aggradation model ( Branney and Kokelaar, 1992). However, none of them appears to satisfy completely the field evidences, implying that when dealing with massive ignimbrites, other evidence than lithic grading needs to be presented to better understand the related transport and depositional processes.