The Journal of the Geological Society of Japan

Articles

Verification of the “Yayoi regression” in the Tonegawa Lowland, central Japan

Susumu Tanabe, Kazuaki Hori, Arata Momohara, Rei Nakashima

2016 Volume 122 Issue 4 Pages 135-153
Published: April 15, 2016
Released on J-STAGE: June 21, 2016

DOIhttps://doi.org/10.5575/geosoc.2016.0007

JOURNAL FREE ACCESS
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The so-called “Yayoi regression” in Japan is known as a relative sea-level lowering to 2 m below T.P. (Tokyo Peil; Mean Sea Level of Tokyo Bay) during 3-2 cal kyr BP. However, the regions where this event occurred in the Japanese Islands and its continuous sea-level index points from verified regions have yet to be clarified. Lake sediments beneath the Innermost Tonegawa Lowland, deposited in response to a sea-level rise during 3-2 cal kyr BP, indicate a paleo-water depth of ca. 1-2 m. The sea-level index points of the lake sediments indicate that the sea level dropped to −2.2 m T.P. at 3.0 cal kyr BP, while the paleo-water depth estimated from plant macrofossils in the sediments is 2 m. The amount of sea-level lowering is greater than the sediment compaction effect; moreover, no large tectonic movements have affected the region over the past 3000 years. These results demonstrate that the “Yayoi regression” occurred in the Innermost Tonegawa Lowland and that isostatic movements triggered by sediment loading might have brought about the observed lowering of sea level.

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A Boreal-type brachiopod species, Waagenoconcha irginae (Stuckenberg), from the middle Permian (Wordian) of the South Kitakami Belt, Japan

Jun-ichi Tazawa, Hideo Araki

2016 Volume 122 Issue 4 Pages 155-161
Published: April 15, 2016
Released on J-STAGE: June 21, 2016

DOIhttps://doi.org/10.5575/geosoc.2016.0005

JOURNAL FREE ACCESS

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This study describes a Boreal-type brachiopod species, Waagenoconcha irginae (Stuckenberg), from the lower part of the Kamiyasse Formation (Wordian) in the Kamiyasse‒Imo area, South Kitakami Belt, northeastern Japan. The occurrence of W. irginae in the South Kitakami Belt provides additional evidence for the palaeoposition of South Kitakami in the Wordian, located near North China.

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Age of the Isomatsu Formation in the Tsugaru Peninsula, northeastern Japan: zircon U-Pb dating of a Miocene green tuff bed

Hiroyuki Hoshi, Hideki Iwano, Tohru Danhara

2016 Volume 122 Issue 4 Pages 163-170
Published: April 15, 2016
Released on J-STAGE: June 21, 2016

DOIhttps://doi.org/10.5575/geosoc.2016.0006

JOURNAL FREE ACCESS

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The Isomatsu Formation in northeastern Japan comprises Miocene terrestrial to shallow-marine deposits displaying a typical lithofacies of so-called ‘green tuff’. Zircon fission track ages of the formation (~14-16 Ma) are significantly younger than those expected from stratigraphic inferences. Here, we present the results of U-Pb geochronological analysis using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), performed on the same suite of zircon grains that yielded the young fission track age. The LA-ICP-MS analysis yielded a mean ²³⁸U-²⁰⁶Pb age of ~17.7 Ma for a dacitic pyroclastic deposit in a relatively low stratigraphic horizon of the Isomatsu Formation, suggesting deposition at or later than this time. This result is used to revise the chronostratigraphy of the lower Miocene sequence in the study area, and it constrains the depositional age of the Isomatsu Formation to the period from ~17.7 Ma to ~16 Ma. These ages further constrain the timing of: (1) felsic magmatic activity in and adjacent to the study area, (2) the marine transgression that caused the deposition of the Isomatsu Formation, and (3) the possible formation of a half-graben that may also have influenced deposition.

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