Detection and correlation of widespread cryptotephras in middle Pleistocene loess in NE Japan using cummingtonite geochemistry

Abstract

We used cummingtonite chemistry to detect strongly weathered widespread cryptotephras within middle Pleistocene loess and fluvial terrace deposits and to correlate them with previously reported tephras in central and NE Japan. We correlated a cummingtonite-bearing cryptotephra widely distributed in NE Japan below the Ichihasama Pumice (108–146ka) and above the Iwadeyama Pumice (IwP, 160–240ka) with the Iizuna-Kamitaru c tephra (Iz-KTc), which erupted from Iizuna volcano, central Japan. Iz-KTc is thus a widespread middle Pleistocene tephra covering both central and NE Japan. Moreover, we detected the Adachi-Medeshima tephra (Ac-Md), which had been previously inferred to be a late Pleistocene tephra (80–100ka), below the Iz-KTc and IwP tephras (125–150ka) and above the Hagimori White Ash tephra (210–250ka). Therefore, Ac-Md must be a middle Pleistocene tephra with an eruptive age older than 125–150ka and younger than 210–250ka. We also detected a cummingtonite-bearing tephra older than 210–250ka and younger than 250–300ka in the Isawa area of NE Japan and correlated it with a previously reported tephra in central Japan. Thus, like Iz-KTc, this tephra is a widespread middle Pleistocene tephra.