Emerged Coral Terraces in West Luzon, Philippines and Their Implications for Sea Level Changes and Coastal Uplift

Abstract:

We examined emergent coastal geomorphic features in Ilocos Norte, Ilocos Sur, and La Union to understand their implications for sea level changes and seismotectonics of the Manila trench subduction zone and associated upper plate structures. Raised coral terraces with recognizable notches and meter-scale terrace risers are often exposed and well preserved along promontories and nearby smaller islands. Varying elevations of the lower terrace were measured in Burgos and Badoc, Ilocos Norte (2 to 3 m amsl), San Esteban, Ilocos Sur (1.4 m amsl), and Bacnotan, La Union (1.2 m amsl). Initial ¹⁴C- and ²³⁰Th-dating of fossil corals from the lower terraces in Ilocos Norte, Ilocos Sur, and La Union reveals ages that range from ~7.5 ky to ~3.5 ky. Young coral ages (~1 ky) were meanwhile obtained from the upper terraces of Badoc Island, Ilocos Norte at 4 to 5 m amsl. Based on preliminary coral ages, we infer that the older (~7.5 ky to ~6 ky) fossil corals are related to the mid-Holocene marine transgression while younger attached corals (~1 kyr) could represent the age of the terrace surface prior to emergence. Alternatively, a large wave event may have also transported the young fossil corals on the mid-Holocene transgressive reef platform. Emergence of the coral terraces may be attributed to regional sea level changes and possibly accumulated uplift from aseismic and seismic processes along the Manila Trench and nearby faults. Varied deformation patterns of the raised coral terraces further provide constraints on local tectonic controls and upper plate structures of the subduction zone.