Petrographic Character of the Upper Holocene Tephra Beds from Paitan Lake, Central Plain of Luzon, the Philippines

Wednesday, 17 December 2014
Takashi Tashiro and Kyota Uemura, Nihon University, Tokyo, Japan
This presentation will provide a report on the petrographical features of the tephras NLPa-1 (depth 3.14 to 3.16 meters, approximately 2,490 BP by sedimental ratio) and NLPa-2 (depth 2.88 to 3.1 meters, 2,385 14CcalBP), which were discovered in the boring core (excavation depth of 10 meters) as reported by Tashiro et al. (2014). Both tephras have fine grained, white glassy lithofacies with no stratification, and they have been identified as ash fall deposits because there is no sedimentary structure and no internally encapsulated units. Mineralogical analyses, particle size composition analyses, chemical composition analyses of the main components, and gradient index analyses of the volcanic glass and minerals were performed. The results described above led us to believe that both tephras are potentially composed of ash fall from a caldera eruption. Furthermore, both tephras can be thought of as sedimentations from multiple eruptions whereby the same volcano was the supply source of the sediments since the time intervals are confirmed for both tephras and there is a high degree of similarity in their petrographical features. A comparison of the main component composition of volcanic glass from tephras of NLPa-1 and NLPa-2, as well as those that have been described both within and outside the Philippines, was conducted next to investigate the supply source of the sediments. The findings point to a high degree of similarity of these tephras with the Iro and Ins series originating in the Irosin Caldera (Mirabueno et al., 2011, etc.). A direct comparison, however, was considered impossible since the sedimentation era of the Iro and Ins series is about 41ka and is therefore significantly different from that of these tephras. While the results of this research did not allow us to compare NLPa-1 and NLPa-2 because of the reasons described above, it is hoped that what is described here will contribute toward the building of the tephrochronology in the Philippines.