Holocene Diatom Flora and Sedimentary Environment of the Echigo Plain, Central Honshu, Japan : Part 1 The Analysis of Fukushima-gata Well Core

Based on ecological spectra, diatoms of the Fukushima-gata well core are grouped into five diatom assemblages, namely marine, marine-brackish, brackish, fresh-brackish and fresh water diatom assemblages. The change of these assemblages is divided into six diatom divisions in ascending order; (1) Fresh water diatom division 1 (FD1) is characterized by a fresh water diatom assemblage.(2) Transitional division 1 (Tr1) is characterized by the alternation of marine and fresh water diatom assemblages. (3) Marine diatom division 1 (MD1) is dominated by a marine diatom assemblage reflecting Holocene transgression. (4) Transitional division 2 (Tr2) which composes of marine and fresh water diatom assemblages indicates a regression after the culmination of the maximum Holocene transgression. (5) Marine diatom division 2 (MD2) indicating a small transgression is dominated by marine diatoms. (6) Fresh water diatom division 2 (FD2) is dominated by fresh water diatoms. It is probably that each diatom division closely related to the development of sedimentation was caused by Holocene sea level and geomorphological changes