RECONSTRUCTION OF THE GEOLOGIC HISTORY OF THE NORTH SIDE OF POAS VOLCANO

The Costa Rica Institute of Electricity (ICE), has drilled about 155 cores on the NE and NW and NE flanks of Poás volcano as part of the site characterization study for the construction of several hydroelectric projects: Hule, Toro 1, 2, 3 and Cariblanco. The cores, up to 275 m in depth, sample the last 400 ka of eruptive and erosional history of Poás. In addition to the cores, there is also a 6 km horizontal tunnel from the Cariblanco project that has been logged and sampled in detail. A detailed stratigraphic section of the NE flank of Poás volcano has been constructed using petrographic descriptions from the cores and tunnel logs. The stratigraphic section is composed of four geological units, which, according to preliminary dating from the oldest to the youngest, are: Rio Sarapiqui (>400 ka) composed principally of breccias and tuffs with subordinate lavas and an ignimbrite deposit; Paleo-Poas (400 ka aprox), consisting primarily of lavas, breccias, and tuffs. One of the cores through Paleo-Poas samples seven different lava flows separated by paleosoils, which together account for 260 m of total thickness. The von Fantzius unit (40-10ka) with a maximum thickness of 70 m is made of lava flows and some pyroclastic deposits in the upper part of the unit. The Congo unit is about 6 ka and is made of pyroclastic and explosive deposits. Petrologically, the lavas of these units range from basalts to andesites. The initial correlations and profiles based of these units will be tested and refined using geochemical data, 40Ar/39Ar, 14C dating and paleomagnetic inclination measurements of the lava flows contained within these four units. We have collected more than 350 oriented samples for paleomagnetic studies and an additional 150 samples for geochemistry and radiometric dating. The samples are from fifteen cores from Cariblanco project, five cores from Hule project and eight cores from the Toro-3 project. In addition, the walls of the Toro-3 horizontal tunnel have been sampled. With the integration of these data it should be possible to constrain the eruption rates, volume fluxes, and erosion events (indicated by soil horizons) and produce a corrected extrusive volcanic flux-rate model of the Poás volcano. The flux-rate model of the Poas volcan volcano will further constrain the element fluxes from the volcanic front of Nicaragua and Costa Rica