A CASE OF NO-WIND PLINIAN FALLOUT AT PULULAGUA CALDERA (ECUADOR) - IMPLICATIONS FOR MODELS OF CLAST DISPERSAL

The caldera of Pululagua is an eruptive centre of the Northern Volcani c Zone of the South American volcanic arc, located about 15 km north o f Quito, Ecuador. Activity leading to formation of the caldera occurre d about 2450 b.p. as a series of volcanic episodes during which an est imated 5-6 km3 (DRE) of hornblende-bearing dacitic magma was erupted. A basal pumice-fall deposit covers more than 2.2 x 10(4) km2 with a vo lume of about 1.1 km3 and represents the principal and best-preserved plinian layer. Circular patterns of isopachs and pumice, lithic and Md isopleths of the Basal Fallout (BF) around the caldera indicate empla cement in wind-free conditions. Absence of wind is confirmed by an ubi quitous, normally graded, thin ash bed at the top of the lapilli layer which originated from slow settling of fines after cessation of the p linian column (co-plinian ash). The unusual atmospheric conditions dur ing deposition make the BF deposit particularly suitable for the appli cation and evaluation of pyroclast dispersal models. Application of th e Carey and Sparks' (1986) model shows that whereas the 3.2-, 1.6-, an d 0.8-cm lithic isopleths predict a model column height of about 36 km , the 6.4-cm isopleth yields an estimate of only 21 km. The 4.9- and 6 .4-cm isopleths yield a column height of 28 km using the model of Wils on and Walker (1987). The two models give the same mass discharge rate of 2 X 10(8) kg s-1. A simple exponential decrease of thickness with distance, as proposed by Pyle (1989) for plinian falls, fits well with the BF. Exponential decrease of size with distance is followed by cla sts less than about 3 cm, suggesting, in agreement with Wilson and Wal ker (1987), that only a small proportion of large clasts reach the top of the column. Variations with distance in clast distribution pattern s imply that, in order to obtain column heights by clast dispersal mod els, the distribution should be known from both proximal and distal zo nes. Knowledge of only a few isopleths, irrespective of their distance from the vent, is not sufficient as seemed justified by the method of Pyle (1989).