LA FORMACIÓN ALTO PALOMO: FLUJOS PUMÍTICOS DE LA CORDILLERA VOLCÁNICA CENTRAL, COSTA RICA

Abstract

The Alto Palomo Formation is divided in two members: Palomo (lower) and Palmitos (upper).Both are composed by pumice lapilli tuffs (pumice pyroclastic flows). There are also lithic or pumice basal breccias (co-ignimbritic lag breccias and air fall breccias). At the base of this sequence, there are in some localitiesthin layers of fine to medium-grained ashes (pyroclastic surges). The two members can be recognized accordingto their compositional differences: biotite and accretional lapilli in Palmitos Member and absence of them inPalomo Member. The distribution of the latter suggests its origin in La Picada Caldera. The distribution of components in Palmitos Member shows a strong control by paleo-valleys (observed thickness ≤ 100 m) around La Vieja river. The occurrence of basal lithic and pumice breccias around this locality, suggests that this memberoriginated from Chocosuela Caldera. Platanar volcano built its edifice on its southern rim.