Search

Abstract: We conducted systematic mapping of Holocene marine terraces in eastern Mindanao Island, Philippines for the first time. Raised marine platforms along the 80-km-long coastline of eastern Davao Oriental Province are geomorphic evidence of tectonic deformation resulting from the westward subduction of the Philippine Sea plate along the Philippine trench. Holocene coral platforms consist of up to four terrace steps: T1: 1–5m, T2: 3–6m, T3: 6–10m, and T4: 8–12mamsl, from the lowest to highest, respectively. Terraces are subhorizontal, exposing cemented coral shingle and eroded coral heads, while terrace risers are 1–3m high. Radiocarbon ages, 8080–4140calyr BP, reveal that erosional surfaces were carved onto the Holocene transgressive reef complex which grew upward until ∼8000years ago. The maximum uplift rate is ∼1.5mm/yr based on the highest Holocene terrace at <11.4mamsl. The staircase topography and meter-scale terrace risers infer that at least four large earthquakes have uplifted the coast in the past ∼8000years. The deformation pattern of the terraces further suggests that seismic sources are probably located offshore. However, historical earthquakes as large as M W 7.5 along the Philippine trench were not large enough to produce meter-scale coastal uplift, suggesting that much larger earthquakes occurred in the past. A long-term tectonic uplift rate of ∼1.3mm/yr was also estimated based on Late Pleistocene terraces. [Copyright &y& Elsevier]

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 cm. 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. • Potential tsunami deposits in SW Mindanao Island are documented for the first time. • Inferred 1976 tsunami deposits were observed in a mangrove swamp and a coastal plain. • Washover deposits are sand layers that contrast with background mangrove sediments. • A deeper sand unit is potentially associated with a prehistoric extreme wave event. • Mangrove environments pose challenges in the interpretation of washover deposits. [ABSTRACT FROM AUTHOR]