Large-scale internal structure in volcanogenic breakout flood deposits: Extensive GPR survey on volcaniclastic deposits

Large-scale outburst floods from volcanic lakes such as caldera lakes or volcanically dammed river-valleys tend to be voluminous with total discharge of > 1-10s km3 and peak discharge of >10000s to 100000s m3 s-1. Such a large flood can travel long distance and leave sediments and bedforms/landforms extensively with large-scale internal structures, which are difficult to assess from single local sites. Moreover, the sediments and bedforms/landforms are sometimes untraceable, and outcrop information obtained by classical geological and geomorphological field surveys is limited to the dissected/terraced parts of fan body, road cuts and/or large quarries. Therefore, GPR (Ground Penetrating Radar), using the properties of electromagnetic waves' propagation through media, seems best adapted for the appraisal of large-scale subsurface structures. Recently, studies on GPR applications to volcanic deposits have successfully captured images of lava flows and volcaniclastic deposits and proved the usefulness of this method even onto the volcanic areas which often encompass complicated stratigraphy and structures with variable material, grainsize, and ferromagnetic content. Using GPR, the present study aims to understand the large-scale internal structures of volcanogenic flood deposits. The survey was carried out over two volcanogenic flood fan (or apron) sediments in northeast Japan, at Numazawa and Towada volcanoes. The 5 ka Numazawa flood deposits in the Tadami river catchment that has been emplaced by a breakout flood from ignimbrite-dammed valley leaving pumiceous gravelly sediments with meter-sized boulders in the flow path. At Towada volcano, a comparable flood event originating from a breach in the caldera rim emplaced the 13-15 ka Sanbongi fan deposits in the Oirase river valley, which is characterized by a bouldery fan deposits. The GPR data was collected following 200 to 500 m long lateral and longitudinal transects, which were captured using a GPR Pulse-Ekko-Pro mounted with 50 MHz and 100 MHz antennas. We obtained radargrams up to 15 m depth, where the dielectric properties of the material were allowing a good penetration of electromagnetic waves. GPR profiles show large internal cross structures with 2-5 m amplitude and 10s m wavelength indicating lateral/downstream accretion in flood fan deposits. Some of them are slightly inclined towards upstream (backsets) which may suggest upstream migration of bedforms. These cross stratifications revealed by the GPR profiles correspond with very low-angle cross-stratifications or parallel stratifications in outcrops. The GPR images also containing hyperbolic reflectors suggest 1-3 m diameter boulders scattered into the fan deposits. Both the 5 ka Numazawa flood deposits and 13-15 ka Sanbongi fan deposits have underlying ignimbrites emplaced by the eruptions. Boundaries between fan deposits and ignimbrites can be detected in reflectors, as the latter blocks the electromagnetic waves. The fact indicates that the GPR survey on volcaniclastic material is useful to understand paleotopographies, volumes of buried ignimbrite and overlying breakout flood or lahar sediments.