History of sedimentary infilling and faulting in Subic Bay, Philippines revealed in high-resolution seismic reflection profiles

Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-level curves. The oldest sedimentary packages are marine sediments subaerially exposed and eroded 20 ka. Fluvio-marine to wholly marine sediments were deposited during the ensuing transgression, and prograding units were deposited during stillstands or minor sea-level falls. Faults within the bay have three age ranges. The oldest set cuts through the pre-δ18O Stage 2 rock units, >18 ka; a second disrupts 10.2-11.3 ka sediments; and the youngest, which cut the uppermost sedimentary package, show that movements occurred about every 2 ky, most recently about 3 ka. Northwest-southeast faults that parallel onshore structures associated with Paleogene emplacement of the Zambales Ophiolite Complex to the west and north likely represent rejuvenated tectonism. The northern coastline and north-south-trending axial bay islands appear related to a lineament that dissects Mt Pinatubo farther northeast. A breach in the caldera of Mt Natib is the most likely source of a presumed pyroclastic deposit in the eastern bay that is associated with sediments about 11.3-18 ka, indicating that a Natib eruption occurred much more recently than previously documented for this volcano.