Tectonic Control of High-Frequency Holocene Delta Switching and Fluvial Migration in Lingayen Gulf Bayhead, Northwestern Philippines

Abstract

Remotely sensed images, maps, charts, and historical accounts document the evolution of the Lingayen Gulf bayhead plain in the northwestern Philippines. Beach ridges and relict channel patterns record delta progradation and switching that, together with meander belt migration, constructed the bayhead plain. The latest delta switching occurred after 1935, when the downstream portion of the Agno River, the largest river discharging into the bayhead, was artificially diverted to a more direct route. The shoreline retreated in the abandoned delta; in contrast, paired sets of beach ridges that diverge toward the new river mouth formed a cuspate delta. In the more landward portions of the coastal plain, similar but older pairs of wedge-shaped beach ridge sets occur between more continuous to parallel sets that also truncate their apical ends. The pair of wedge-shaped sets was used to map paleodelta lobes; the continuous-parallel sets record transgressive events. Within 7 kilometers of the bayhead plain, at least 15 paleodelta lobes were alternately, successively, or simultaneously built by three river systems. These paleodeltas formed during the sea level fall from 2.4 ka to the present. Before this, westward tilting because of movements along the two faults that bound the alluvial plain and switching of the main distributary channel of the Agno alluvial fan caused the lower Agno River to migrate episodically to the southwest. Continued channel avulsions in the alluvial fan changed the sediment loads of the multiple river pathways created by meander belt migration, leading to delta switching downstream. Contemporaneous 220-year recurrences of delta switching and high-magnitude Philippine Fault earthquakes indicate tectonic control of the bayhead plain evolution.