BASIN-WIDE STUDY OF HIGH-FREQUENCY DELTA SWITCHING AND FLUVIAL AVULSION IN LINGAYEN GULF, NORTHWESTERN PHILIPPINES: POTENTIAL FOR IDENTIFYING TECTONIC, CLIMATIC, AND SEA LEVEL CONTROLS

The onshore and offshore evolution of the Lingayen Gulf bayhead plain in northwestern Philippines is established using remotely sensed images, maps, bathymetric charts, written and anecdotal accounts, surface sediment samples, high-resolution seismic profiles and core data.

Delta progradation and switching accompanied by meander belt migration formed the present bayhead plain. Historical cases of natural and man-induced delta switching occurred between the mid-19th to early 20th century and around 1935. Delta retreat and progradation at rates of 19 to 25 m/yr are documented by historical changes in shoreline position and marked by transgressive and regressive types of beach ridge sets. Similar and older ridges, which occur as far as 16 km inland, are used to reconstruct former delta lobes. Five kilometers from the present shoreline, at least 17 delta lobes were built successively or simultaneously by the major bayhead rivers: Agno to the west, Dagupan at the center and Bued-Patalan to the east.

Offshore, very high-resolution (Parasound) profiles reveal a former delta-plain formed during a lower sea stand. Rapid sea-level rise prior to 7,575±185 cal yBP transgressed this surface and deposited a transparent package with basal lensoidal fine sands overlain by offshore muds. This also translated the shoreline approximately 16 km inland, from a previous position approximately 17 km seaward and 60 m lower than the present. Subsequent deceleration and fall to the present sea level induced progradation, partly represented by four vertically stacked muddy prodelta lobes defined by truncated bi-directional reflections and correlated with the four youngest Dagupan deltas.

Associated with these deltaic units are seven meander belts that display a general migration to the southwest but all emanate from the Agno Alluvial Fan. The switching of deltas and migration of river systems are attributed to the alternate switching of the active distributary channel of the alluvial fan, which can be controlled by tectonism, sea level and climate change. The nature and interaction of these controls may potentially be revealed by such high-frequency events, pending better age controls using radiocarbon or luminescence dating methods.