Abstract
The Mukaiyama Formation, one of four formations of the Sendai Group located along the Pacific Ocean side of southern Iwate, Miyagi and Fukushima Prefectures, Japan, is a Pliocene fluvial and lagoonal deposits. A sample from black mudstone of the upper Mukaiyama Formation was collected from the upper stream tributary site of Tatsunokuchi Gorge, Sendai City, Miyagi Prefecture. The diatom assemblage of the sample is composed of marine (including brackish) diatoms with a frequency of 52% and freshwater diatoms with a frequency of 48% (Table 1). Among all species, the most abundant was marine species Paralia aff. sulcata (26%), followed by two marine species P. aff. elliptica (9%) and Lancineis fatula (3%). Freshwater species are characterized by many taxa of two genera Pinnularia (16 taxa) and Eunotia (11 taxa). The most abundant species among the freshwater taxa was Eunotia minor (16%), followed by Gomphonema parvulum var. parvulum (5%) and Aulacoseira crassipunctata, Eunotia bidens, E. hexaglyphis, Pinnularia subcapitata var. elongata (all 3%). The assemblage suggests that the sampling site at the Mukaiyama Formation was once a bay or lagoonal environment influenced by freshwater.
