Preliminary report on volcanic ash layers in the northeastern part of the Higashi-Ibaraki Terrace, Mito District, northeastern Kanto Plain, eastern Japan

Loam (i.e., tephric loess deposits), terrace deposits, and three volcanic ash layers occur in three outcrops in the northeastern part of the Higashi-Ibaraki Terrace in Mito district, northeastern Kanto Plain.Analysis of the sedimentary facies, mineral compositions, and refractive index values of volcanic glass shards and heavy minerals suggest that the Akagi-Kanuma Pumice (Ag-KP) occurs at all three sites, in addition to tephra layers that are older and younger than the Ag-KP.The older layer occurs in the Ibaraki Formation and is presumed to be the Akagi-Mizunuma 6 tephra (Ag-MzP6), based on refractive index values and surrounding geology.The younger layer is most likely the Aira-Tanzawa tephra (AT).The Ag-KP deposit around the study area is charactered by a basal unit of fine-grained ash containing orthopyroxene with a lower refractive index than that of orthopyroxene in the rest of the deposit.Previous studies have reported similar basal units in areas to the ESE and SE of Akagi volcano.This distribution of the basal unit is generally consistent with that of the thicker part of the Ag-KP.The spatial distribution of refractive index values indicates that a precursory eruption might have occurred before the main eruption.This hypothesis should be evaluated by further studies, especially geochemical analyses that provide insight into magma mixing during the eruption event.