Diatoms from the Shirasawa Formation (Late Miocene) distributed in western Sendai, Japan

Abstract

The Shirasawa Formation, one of three formations of the Akyu Group located in western Sendai City, Miyagi Prefecture, Japan, is a Late Miocene freshwater deposit. Samples from tuffacious mudstone of the Shirasawa Formation were collected from the northern Akyu site and the Imosawa River of the Okubushi site. We observed 39 diatom taxa belonging to 26 genera which were composed of 88–100% of centric diatoms and 0–12% pennate ones. The most abundant diatoms of the Akyu site were Aulacoseira distans (86–89%), followed by Mesodictyon yanagisawae (6–9%) and Aulacoseira praegranulata var. praeislandica f. praeislandica (2–4%). At the Okubushi site, Mesodictyon yanagisawae (1–88%), Aulacoseira subarctica (1–85%), A. distans (2–31%), A. praegranulata var. praeislandica f. praeislandica (0–26%), Staurosilla lapponica (1–7%), Aulacoseira pusilla (0–2%) and Gomphosphenia grovei (0–2%) were abundant or frequent. The five aboundant taxa from both sites are reported herein with their morphology and related information.