GEOCON 2011: Geology Working For A Resilient Society The geology of Biri Island, Northern Samar, Philippines A.R.C. Fernandez1, A.G.S. Fernando1, A.M. Peleo-Alampay1, M.L.C. Abad1, R.J.M. Antonio1, A.C. Balota2 and D.N. Tangunan1 1National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 2Gesellschaft für Internationale Zusammenarbeit (GIZ) Situated in the San Bernardino Strait, Biri Island is one of the small islands off the municipality of Lavezares in Northern Samar, Philippines. Previous studies in the area (i.e., Schenck [1922], Travaglia et al. [1978] and Garcia and Mercado [1981]) included the rocks of Biri Island under the Late Miocene to Early Pliocene Catbalogan Formation, the most widespread sedimentary unit comprised of sandstones, siltstones, shales and occasional conglomerates in mainland Northern Samar. Based on the mapping done in Biri Island by the Geology 215 (Advanced Field Geology) Class of UP-NIGS in 2010, however, the island is actually composed of two distinct lithologic units: (1) an Early Miocene sedimentary unit unconformably overlain by (2) an undated andesitic complex comprised of a shallow intrusions and agglomerates. The sedimentary unit consists of very thickly- to thinly-bedded, steeply dipping to the northwest, rhythmic sequences of calcareous sandstones, siltstones and shales, with occasional conglomerates and olistoliths of bedded sedimentary rocks. Good exposures are found in the northern part of Biri, especially the islands/rock formations along the northeastern coast of the island. The sedimentary units were dated as Early Miocene based on calcareous nannofossils, corresponding to the NN1 to NN3 nannofossil zones (Fernandez et al., 2010). The andesitic deposits, on the other hand, consist of a agglomerate member and a shallow intrusive member. The agglomerate units are plagioclase-phyric with occasional secondary biotite minerals and occasional glassy rinds and autobrecciation features. They are mapped in the central part of the island at elevations >80m. The shallow intrusive is found locally in the western part of the island, with similar mineralogy as the agglomerate member. The two lithologic units are proposed as new formational units in the present study: “Macadlao Formation” for the sedimentary units and the “Biri Volcanic Complex” for the andesitic agglomerates and intrusions. The 24th Annual Geological Convention of the Geological Society of the Philippines | 8-9 December 2011 | 93 .