Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.75 No.4 (2024)

Cover photograph | Table of Contents | Abstract

Cover photograph

Precise age of Neogene lacustrine centric diatoms in Japan

　The ages of lacustrine centric diatoms of the Neogene have been roughly estimated in studies of diatom fossils in lake sediments in Japan. In contrast, recent research on freshwater diatom fossils in marine sediment and volcanic ash layers (e.g., Yanagisawa et al., this issue) has yielded more-precise ages. By considering the information from these studies, precise ages of freshwater diatoms were determined in this study.

(Figure and Caption by YANAGISAWA Yukio)

Table of Contents

All the pages PDF ： 75_04_full.pdf ［8.6MB］

Title	Author	PDF
Article
Diatom biochronology and U–Pb age of the Miocene strata in the Satomi area along eastern margin of the Tanakura sedimentary basin, Japan	YANAGISAWA Yukio and HOSOI Jun (p.103-120)	75_04_01.pdf [3MB]
Stratigraphy and diatoms of the Neogene Taga and Hitachi groups in the Hanareyama district, Kuji Town, Hitachi City, Ibaraki Prefecture, Japan	YANAGISAWA Yukio, ANDO Hisao and KUSHIBIKI Sora (p.121-159)	75_04_02.pdf［5.5MB］

Abstract

Diatom biochronology and U–Pb age of the Miocene strata in the Satomi area along eastern margin of the Tanakura sedimentary basin, Japan

YANAGISAWA Yukio and HOSOI Jun

Diatom biostratigraphic analysis and U–Pb dating were conducted for the Miocene deposits in the Satomi area and its environs of the eastern margin of the Tanakura sedimentary basin, where few age data have been available. The Miocene sequence in this area is composed of the Akasaka and Hase formations, and the Higashikanasayama Formation including the Tatsukuroiso Mudstone Member is distributed in the adjacent Central Shear Zone. Zircon U–Pb age of a tuff bed intercalated in the middle part of the Akasaka Formation was dated at 13.0 ± 0.4 Ma. The Hase Formation includes diatom assemblages correlative with the middle to upper part of the zone NPD5B of the North Pacific diatom zonation, and this formation was deposited during the period between 12.4 Ma and 11.6 Ma. Furthermore, diatom zones of uppermost NPD3A, NPD3B and lower most NPD4A were recognized in the Tatsukuroiso Mudstone Member, indicating that this member was deposited between 16.7 Ma and 15.6 Ma. The age data revealed in this study allows accurate correlation between the Miocene deposits of the Satomi area and those of the Daigo, Hitachiota and eastern Tanagura areas in the Tanakura sedimentary basin.

Stratigraphy and diatoms of the Neogene Taga and Hitachi groups in the Hanareyama district, Kuji Town, Hitachi City, Ibaraki Prefecture, Japan

YANAGISAWA Yukio, ANDO Hisao and KUSHIBIKI Sora

The marine diatom biostratigraphy has been established for Neogene sediments in the Hanareyama district, Kuji Town in the southern part of Hitachi City, Ibaraki Prefecture, Japan. The Neogene sediments in this district are tentatively divided into four units (Unit 1–4, from bottom to top) separated by hiatuses. Unit 1 consists of diatomaceous mudstone, corresponding to the lowest part of diatom interval NPD5C3 to 5D1 (10.2–9.9 Ma). Unit 2 is composed of sandy mudstone and mudstone with slump and submarine debris-flow deposits, and belongs to diatom zone NPD6A (9.3–8.7 Ma). Unit 3 consists of tuffaceous muddy sandstone interbedded with numerous tuff beds and contains diatom fossils of the interval NPD6B1 (8.7–8.5 Ma). Unit 4 consists of tuffaceous muddy sandstone and corresponds to diatom subzone NPD7Ba (6.5–5.6 Ma). Unit 1 is the southern extension of the Kokubu Formation, which comprises shelf and shelf slope deposits in the Taga Group. Units 2 and 3 are presumed to be units of the Hase Formation, which is a slope channel-fill complex corresponding to the Taga Group. Unit 4 may be a unit in the slope channel fill of the Kume Formation of the Hitachi Group. Lacustrine centric diatom fossils in tuff layers or muddy sedimentary rocks in the Hanareyama district are invaluable in reconstructing the long-term change in lacustrine diatom assemblages in Japan. Two elephant molar fossils in Unit 3 fill a gap in the proboscidean record in Japan that previously extended from the middle Miocene to the end Miocene.