6:15 PM - 7:30 PM
[HQR23-P04] Identification and correlation of tephras in aeolian deposits covering marine terraces on the northern Sanriku Coast
Keywords:the Sanriku Coast, marine terrace, tephra, Toya tephra
In this study, we first mapped the LIG marine terraces by aerial photograph interpretation, and then conducted field surveys and collected tephra samples. In tephra analysis, we observed grains in tephra sample under a polarizing microscope and clarified their grain composition and shape of their volcanic glass shards. In addition, for some tephra samples refractive index of their volcanic glass shards were measured by using RIMS2000.
In Kamikita Plain, the Takadate surface (Miyauchi, 1985), one of the LIG marine terraces, is well developed. We collected tephra samples (To-H, To-BP1, To-G, To-Kb, To-Ok2, To-AP, Toya, ZP2) at Mitateyama reported as a type locality of tephras covering the Takadate surface by Miyauchi (2001). To-H is composed of several fall units. Its volcanic glass shard content is high and its glass type is mainly pumice type. To-Ok2 is composed mainly of coarse-grained pumice. To-AP contains blue-gray lithic fragments in the upper part of the layer. Toya appears as a gray-white ash layer. Its volcanic glass shard content is high and its glass type is characteristically bubble-wall type. We measured refractive indices of its volcanic glass shards at n=1.493-1.497. These characteristics are similar to the ones in the previous reports (Machida and Arai, 2003; Kuwabara, 2010).
On the northern Sanriku Coast, the Taneichi surface, reported as a LIG marine terrace by Yonekura (1966), are developed at 20-30 m a.s.l.. From field surveys, it is presumed that terrace deposits of the Taneichi surface are marine beds because they consist of well-sorted sand layers and rounded gravel layers, which are similar to modern beach deposits. In addition, we identified four tephra layers (Tephra 1-4 in descending order) covering the Taneichi surface. The Tephra 1 is interbedded with the upper part of aeolian deposits and consist of a few fall units. Its glass type is mainly pumice type, and refractive index of its volcanic glass shard is n=1.503-1.513. These characteristics are similar to To-H and thus we correlated this tephra with To-H. The Tephra 3 is probably correlated with To-AP because it contains blue-gray lithic fragments in the upper part of the layer. The Tephra 2 is probably correlated with To-Ok2 because of its stratigraphy and consistency with the thickness from an isopach map in Miyauchi (1985). The Tephra 4 is correlated with Toya because its volcanic glass shard content is high, and its glass type is mainly bubble-wall type.
In this study, we found four tephra layers covering the Taneichi surface on the northern Sanriku Coast and preliminarily correlated them with To-H, To-Ok2, To-AP and Toya. Consequently, we estimated that the Taneichi surface was a LIG marine terrace because Toya covered immediately above marine beds. In the meeting, we will report more detailed petrographic characteristics of the tephras and results of correlation and chronology of LIG marine terraces on the northern Sanriku Coast.