Presentation Information
[O12-P54]Estimation of the Zircon Deposition Period in the Tenryu River Basin in the Western Part of Shizuoka Prefecture
*Minato TAKEYAMA1, *Nobuaki HIDEHIRA1, *Masaaki AOSHIMA1 (1. Shizuoka Prefectural Iwata Minami High School)
Keywords:
Zircon,Tenryu River,Age estimation,Samejima Coast,Color analysis
Background and Objectives
Our school has studied garnet sand rich in heavy minerals along the Tenru-river Samejima coast. Zircon can indicate its source by its color and shape. Last year’s study estimated that zircon at Samejima Beach is mainly supplied from the Ryoke Belt’s Ikuta–Tenryu Gorge granites. However, the geological age when zircon began to be supplied is unclear.
This study aims to estimate the depositional ages of zircon. Since U–Pb dating requires expensive equipment and is difficult for high school students, So we used a previously established color-and-shape origin method: we collected zircon from sedimentary layers and used them to infer the Tenryu River’s influence by geological age.
Methods
Geological maps indicate that around the Tenryu River, lower (deeper) strata are older. We collected sandy sediments from 11 sites along the river, from downstream to upstream.
Each sample was crushed in a mortar to <1 mm, and 25 g of the powder was taken.
We concentrated the heavy minerals and picked out zircon grains.
Samples were labeled ①–⑪ from newest to oldest.
Depositional ages of samples ①–⑪ are:
① present–1,17 years ago (Tenryu River lowland deposits, Quaternary Holocene).
②–③ 1,17–2.58 million years ago (Iwata Gravels, Ogasa Group; Quaternary Pleistocene).
④–⑥ 2.58–5 million years ago (Soga Group and Kakegawa Group; Neogene Pliocene).
⑦–⑨ 5–23 million years ago (Saigo Group; Neogene Miocene).
⑩ 23–66 million years ago (Kurami Group; Paleogene).
⑪ 66–260 million years ago (Mikura Group; Cretaceous).
Zircon color was analyzed using the “Makarii” software, with results plotted on an RGB triangular diagram.
Results
Zircon was recovered from samples ①–③ and ⑤–⑧, but not from ④ or ⑨–⑪.
This indicates zircon supply began in the age of sample ⑧ and continued through ⑤, paused at ④, and then resumed from ③ to ①.
Discussion
First, we confirmed that the collected zircon share the Samejima Beach origin. Using “Makarii”, all sample sites showed the same color trends as Samejima Beach zircon, confirming a common Tenryu origin and validating our use of these grains for age inference.
According to Shiba et al. (2024), the Samejima area was under the Tenryu-river sea during ages ⑨–⑪. After uplift and Tenryu River inflow, sediments were supplied and the area became land. This matches our finding that zircon supply started around sample ⑧ (23–5 Ma), implying Tenryu River zircon was carried by coastal currents from that time. During the time of sample ④ (5–2.58 Ma), a sandbar created a lagoon, isolating the bay and cutting off oceanic input (Shiba et al., 2024). This likely caused a temporary absence of zircon supply.
Summary
Zircon supply began around 23 million years ago and continued until about 5 million years ago.
Between 5 and 2.58 million years ago, the bay became an isolated lagoon, halting zircon supply.
From 2.58 million years ago to the present, zircon supply resumed.
Acknowledgments
We thank our advisor Hiroyuki Kurematsu, associate advisors Noriko Tateishi and Haruka Yamaguchi, research fellow Akira Aoshima (Fujinokuni Earth Environmental History Museum), and all geology club alumni who cooperated with this study.
References
Shiba et al. (2010). Sedimentary processes of the lower Ogasa Group in Kakegawa City, Shizuoka Prefecture.
Kakegawa City official note (2019). “The marine world over a million years ago: The Kakegawa Group.”
Shiba et al. (2020). Stratigraphy and depositional processes of the Kurama and Saigo Groups (Miocene) in Shizuoka Prefecture.
Geological Survey of Japan , Facies: Tenryu basin.
Geological Survey of Japan . Mitsuke geological map sheet explanation.
Yoshioka et al. (2024). “Estimation of zircon source rocks at the Enryu-nada Samejima Coast.”.
Our school has studied garnet sand rich in heavy minerals along the Tenru-river Samejima coast. Zircon can indicate its source by its color and shape. Last year’s study estimated that zircon at Samejima Beach is mainly supplied from the Ryoke Belt’s Ikuta–Tenryu Gorge granites. However, the geological age when zircon began to be supplied is unclear.
This study aims to estimate the depositional ages of zircon. Since U–Pb dating requires expensive equipment and is difficult for high school students, So we used a previously established color-and-shape origin method: we collected zircon from sedimentary layers and used them to infer the Tenryu River’s influence by geological age.
Methods
Geological maps indicate that around the Tenryu River, lower (deeper) strata are older. We collected sandy sediments from 11 sites along the river, from downstream to upstream.
Each sample was crushed in a mortar to <1 mm, and 25 g of the powder was taken.
We concentrated the heavy minerals and picked out zircon grains.
Samples were labeled ①–⑪ from newest to oldest.
Depositional ages of samples ①–⑪ are:
① present–1,17 years ago (Tenryu River lowland deposits, Quaternary Holocene).
②–③ 1,17–2.58 million years ago (Iwata Gravels, Ogasa Group; Quaternary Pleistocene).
④–⑥ 2.58–5 million years ago (Soga Group and Kakegawa Group; Neogene Pliocene).
⑦–⑨ 5–23 million years ago (Saigo Group; Neogene Miocene).
⑩ 23–66 million years ago (Kurami Group; Paleogene).
⑪ 66–260 million years ago (Mikura Group; Cretaceous).
Zircon color was analyzed using the “Makarii” software, with results plotted on an RGB triangular diagram.
Results
Zircon was recovered from samples ①–③ and ⑤–⑧, but not from ④ or ⑨–⑪.
This indicates zircon supply began in the age of sample ⑧ and continued through ⑤, paused at ④, and then resumed from ③ to ①.
Discussion
First, we confirmed that the collected zircon share the Samejima Beach origin. Using “Makarii”, all sample sites showed the same color trends as Samejima Beach zircon, confirming a common Tenryu origin and validating our use of these grains for age inference.
According to Shiba et al. (2024), the Samejima area was under the Tenryu-river sea during ages ⑨–⑪. After uplift and Tenryu River inflow, sediments were supplied and the area became land. This matches our finding that zircon supply started around sample ⑧ (23–5 Ma), implying Tenryu River zircon was carried by coastal currents from that time. During the time of sample ④ (5–2.58 Ma), a sandbar created a lagoon, isolating the bay and cutting off oceanic input (Shiba et al., 2024). This likely caused a temporary absence of zircon supply.
Summary
Zircon supply began around 23 million years ago and continued until about 5 million years ago.
Between 5 and 2.58 million years ago, the bay became an isolated lagoon, halting zircon supply.
From 2.58 million years ago to the present, zircon supply resumed.
Acknowledgments
We thank our advisor Hiroyuki Kurematsu, associate advisors Noriko Tateishi and Haruka Yamaguchi, research fellow Akira Aoshima (Fujinokuni Earth Environmental History Museum), and all geology club alumni who cooperated with this study.
References
Shiba et al. (2010). Sedimentary processes of the lower Ogasa Group in Kakegawa City, Shizuoka Prefecture.
Kakegawa City official note (2019). “The marine world over a million years ago: The Kakegawa Group.”
Shiba et al. (2020). Stratigraphy and depositional processes of the Kurama and Saigo Groups (Miocene) in Shizuoka Prefecture.
Geological Survey of Japan , Facies: Tenryu basin.
Geological Survey of Japan . Mitsuke geological map sheet explanation.
Yoshioka et al. (2024). “Estimation of zircon source rocks at the Enryu-nada Samejima Coast.”.