Geomorphic and sedimentary evidence of a gigantic outburst flood from Towada caldera after the 15 ka Towada–Hachinohe ignimbrite eruption, northeast Japan

Abstract

This study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from Towada caldera, Honshu Island, northeast Japan, as observed along the Oirase River downstream from the outlet of the caldera lake. A number of features of the Oirase River are inconsistent with its present discharge, including the occurrence of 1) hanging valleys and a horseshoe-shaped waterfall in the upstream gorge area, 2) boulder bars and scattered flood boulders, 3) a dry valley at a high elevation in the gorge, and 4) a large fan (the Sanbongi fan) in the downstream area of the river. The Sanbongi fan is composed of thick, lithic-rich hyperconcentrated flow deposits that include pumice clasts derived from the 15 ka Towada–Hachinohe ignimbrite and well-rounded meter-sized (and outsized) boulders derived from bedrock of welded ignimbrite. The deposits are entirely aggradational, with no major channels, indicating the absence of a major hiatus during sedimentation. The depositional facies also indicate that a single sheet-like flood event deposited the sediment within the Sanbongi fan area. Based on the age of the Sanbongi fan, the flood occurred between 15 and 12 ka, after eruption of the Towada–Hachinohe ignimbrite. The most probable water source for the flood is Towada caldera lake, as suggested by landforms and sediments along the lake outlet. A paleohydrological analysis indicates that at least 6 km³ of water was released from the caldera at a peak discharge of > 2 × 10⁴ to 3 × 10⁵ m³ s^− 1 during the breakout flood. Although the Sanbongi fan was previously considered to be a “normal” alluvial fan that formed during a stage of low-stand sea level, the present results show that the formation of the fan was closely related to a catastrophic flood from Towada caldera rather than perennial fluvial activity, climate change, or a change in relative sea level.

Introduction

Volcanic eruptions supply a large volume of debris to surrounding basins by various eruptive and fluvial processes, thereby strongly modifying the landscape and river morphology (Smith, 1991, Newhall and Punongbayan, 1996). After such eruptions, the resedimentation of volcaniclastic material by hydrological process (i.e., lahars) may be induced by heavy rain (e.g., Newhall and Punongbayan, 1996, Lavigne and Thouret, 2002); snow and ice melt resulting from heat supplied by pyroclasts, lava, or geothermal activity (Major and Newhall, 1989, Pierson et al., 1990); direct draining of crater- or caldera-lake water during an eruption (Cronin et al., 1997, Thouret et al., 1998); dam failure at a crater- or caldera-lake (Waythomas et al., 1996, Manville et al., 1999); the release of volcanically dammed impoundments (Park and Schmincke, 1997, Macías et al., 2004, Kataoka et al., 2008); or the direct influx of debris avalanche deposits that evolve into debris and/or flood flows (Vallance and Scott, 1997).

Among these processes, catastrophic outburst floods associated with the breaching of a volcanic dam or caldera rim tend to be large-scale events because of the significant volume of impounded water and loose pyroclastic sediments. Extreme floods of this type can travel long distances from the source volcano, modifying landforms and the hydrology of downstream areas, and threatening human life and infrastructure. Many countries with active volcanoes are at risk of such catastrophic floods arising from dam failure. Compared with the large numbers of studies on primary eruption processes, igneous petrology, magma geochemistry, and “primary” volcanic hazards, few researchers have focused on volcanogenic debris flows and floods as natural hazards, although the number of reports has increased in recent decades (see Manville et al., 2009 and references therein).

The Japanese Islands are located in an active arc setting; consequently, volcanism has strongly affected the sedimentary environment and landforms in the region. However, few studies have examined the sedimentological, geomorphological, and hydrological implications of a sudden, voluminous input of volcanic debris to sedimentary basins and associated terrace- and fan-forming processes, except for a small number of recent sedimentological studies (Nakayama and Yoshikawa, 1997, Kataoka and Nakajo, 2002, Kataoka, 2005, Kataoka et al., 2009) and geomorphological studies (Naito, 1966, Yokoyama, 1999, Yoshida et al., 2005, Kataoka et al., 2008). In other words, most of the studies in this field have focused on the relationship between local tectonics, global eustatic sea-level change, and climate change during the Quaternary.

Towada volcano, in northeast Japan, contains a large caldera that formed mainly during the 15 ka Towada–Hachinohe eruption, which emplaced ~ 40 km³ of non-welded ignimbrite (Hayakawa, 1985), filling nearby rivers and having a strong influence on the surrounding sedimentary environment. The present paper reports on unusual geomorphological and sedimentological features in the Oirase River catchment, which resulted from a catastrophic volcanogenic flood from Towada caldera lake after the 15 ka ignimbrite eruption. The landforms and deposits produced by the flood provide clues to the magnitude of paleohydraulic discharge. This paper also aims at understanding the formation processes of alluvial fans and fluvial terraces linked to volcanism. Finally, the potential consequences of future volcanic hazards related to extremely large volcanogenic floods are discussed.