Sedimentological evidence of washover deposits from extreme wave events in Zamboanga del Sur, Mindanao, southern Philippines

Abstract

Abstract The Philippines active tectonic setting and vulnerability to tsunami hazards underscore the necessity to understand tsunami sources and their impacts. Several tsunamigenic earthquakes have devastated coastal communities in the past but geological evidence of these infrequent extreme wave events (EWEs) have yet to be described and analyzed in detail. This study documents and establishes evidence of potential tsunami deposits preserved in a mangrove environment in western Mindanao Island, which was inundated by the 1976 Moro Gulf tsunami. The 1976 Moro Gulf tsunami, by far the worst tsunami disaster in the Philippines, is associated with a Mw 8.1 earthquake along the Cotabato subduction zone. The sedimentological characteristics of potential tsunami deposits in three coastal sites bordering Pagadian and Illana Bays in Zamboanga del Sur, Mindanao, southern Philippines were analyzed and compared to describe the preservation of washover deposits in different coastal systems. The potential washover deposits of the 1976 tsunami were identified based on their sedimentary characteristics and features that contrast with the background (pre-tsunami event) sediments. The potential 1976 tsunami deposits are predominantly sand-sized and coarser than the background sediments. They also contain mud rip-up clasts, magnetite lamina, and an erosive base, similar to reported washover deposits elsewhere. The thickness of the washover deposits ranges from 7 to 12\xa0cm. The thickest sand layer was observed in a mangrove swamp, implying that local topography influenced the distribution and preservation of the deposit. A potentially older EWE deposit was also observed in Pagadian City. The background sediments in both the mangrove swamps and coastal plains are mud to fine sand. While scarcity in historical data and rapidly changing environmental conditions pose challenges in studying washover deposits in a tropical setting, this study highlights essential information and lessons for future researchers of washover deposits in the Philippines. Sedimentological data on the potential EWE washover deposits in western Mindanao Island serve as fundamental information in understanding the processes and mechanisms of extreme wave events in this region. This study further establishes geological evidence of the country s worst historical tsunami, the 1976 Moro Gulf tsunami, which contributes to our knowledge of tsunami deposits in tropical settings.