GEOLOGICAL SOCIETY OF THE PHILIPPINES
Deciphering the Origin of Coal and Manganese Deposits in Nagtipunan, Quirino Province, Luzon:
Clues from Stratigraphic and Tectonic Studies
Field investigation of Nagtipunan area in the southern tip of the northern Sierra Madre (NSM) reveals the presence of coal (lignite) and manganese within the Eocene Caraballo Formation. The existence of coal within the Caraballo Formation follows the discovery of coal deposits in the Upper Oligocene to Lower Miocene Lubuagan Formation. Other formations recognized in the region include the Oligocene Ibulao Limestone, the Middle Miocene Aglipay Limestone and the Upper Miocene to Lower Pliocene Cabagan Formation.
The coal occurs as seams within the sandstone-shale interbeds of the Caraballo Formation. Its occurrence clearly reflects the varying facies (from shallow to deep marine) within the rock unit. The manganese mainly occurs as a thin layer sandwiched between the turbiditic beds of the Caraballo Formation. In some sections of the formation, however, nodular or disseminated types exist. Geochemical analysis of the manganese samples indicate a relatively high Mn content (69.9%). The manganese deposits are presumed to be products of submarine volcanogenic processes that accompanied intra-arc rifting in northern Luzon and the subsequent formation of the Cagayan Valley Basin.
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