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Zircon U-Pb Dating of a Tuff Layer from the Miocene Onnagawa Formation in Northern Japan

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Zircon U-Pb Dating of a Tuff Layer from the Miocene Onnagawa Formation in Northern Japan Geochemical Journal, Vol. 55, pp. 185 to 191, 2021 doi:10.2343/geochemj.2.0622 NOTE Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Formation in Northern Japan JUMPEI YOSHIOKA,1,2* JUNICHIRO KURODA,1 NAOTO TAKAHATA,1 YUJI SANO,1,3 KENJI M. MATSUZAKI,1 HIDETOSHI HARA,4 GERALD AUER,5 SHUN CHIYONOBU6 and RYUJI TADA2,7,8 1Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan 2Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 3Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China 4Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 5Institute of Earth Sciences, University of Graz, NAWI Graz Geocenter, Heinrichstrasse 26, 8010 Graz, Austria 6Faculty of International Resource Sciences, Akita University, 1-1 Tegatagakuenmachi, Akita, Akita 010-8502, Japan 7The Research Center of Earth System Science, Yunnan University, Chenggong District, Kunming, Yunnan 650500, China 8Institute for Geo-Cosmology, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan (Received December 10, 2020; Accepted March 3, 2021) During the Middle-to-Late Miocene, diatomaceous sediments were deposited in the North Pacific margin and mar- ginal basins. The Onnagawa Formation is one of such deposits, which shows cyclic sedimentary rhythms reflecting oscil- lations of the marine environment in the Japan Sea. However, the age of the Onnagawa Formation is still poorly con- strained due to the poor preservation of siliceous microfossils. To better constrain its age, we performed U-Pb dating of zircon grains from a tuff layer from the middle part of the Onnagawa Formation, and obtained an age of 11.18 ± 0.37 Ma. By combining our data with previously reported radiolarian biostratigraphy from the same section, we improved the age model of the diagenetically altered and lithified sediments of the Onnagawa Formation. Keywords: U-Pb dating, tuffaceous zircon, the Onnagawa Formation, the Middle-to-Late Miocene, NanoSIMS alternation rhythms have been interpreted as reflecting INTRODUCTION orbital-forced climate changes (Tada, 1991), timescales During the Middle-to-Late Miocene, diatomaceous of the sedimentary cycles have not been precisely con- sediments were deposited in the North Pacific margin and strained. Knowledge of the mechanism controlling the marginal basins including the Japan Sea (e.g., Koizumi sediment cycles of the Onnagawa Formation is critical to and Yamamoto, 2018; Vincent and Berger, 1985). Depo- understand the evolution of the Japan Sea basin and its sition of these diatomaceous sediments are thought to have associated changes in the environment during the Mid- been related to the global cooling prevailing during that dle-to-Late Miocene, when the global climate faced a sig- time, and they occasionally display significant lithological nificant cooling phase (e.g., Holbourn et al., 2013). changes such as a shift from calcareous to siliceous de- Chronostratigraphy of the Onnagawa Formation has posits reflecting the cooling of the surface water (Koizumi been developed by diatom biostratigraphy of and Yamamoto, 2018; Tada, 1994). The Onnagawa For- diatomaceous sediment in the Oga Peninsula (Koizumi mation, which is widely exposed in Akita Prefecture, et al., 2009). However, many outcrops of the Onnagawa Northern Japan, is one of these diatomaceous sediments Formation have been suffered from silica diagenesis, deposited along the Northwestern Pacific margin (Fig. 1a). which dissolved most diatom frustules composed of opal- The Onnagawa Formation is characterized by cyclic A and reprecipitated as opal-CT (e.g., Koizumi et al., changes in alternation rhythms of biosiliceous and detri- 2009; Tada and Iijima, 1983). Consequently, preserva- tus-rich beds (Tada, 1991). Although cyclic changes in tion of diatom frustules became very poor and diatom biostratigraphy could be barely applicable. Although the Onnagawa Formation in the studied area is suffered from *Corresponding author (e-mail: [email protected]) silica diagenesis, it is well exposed, and its continuous Copyright © 2021 by The Geochemical Society of Japan. sequence can be obtained by splicing several sections. 185 Fig. 1. (a) The distribution of the Onnagawa and Funakawa Formations in Akita Prefecture after Ozawa and Suda (1978, 1980) and Ozawa et al. (1987). (b) A geological map of the studied area in Yurihonjo. (c) A photograph showing field occurrence of examined tuff layer. (d) A stereo-microscopic photograph of grains of the tuff layer after sieved with 44 µm mesh. Abbreviations; Fm. = Formation, Pref. = Prefecture. Through detailed field observation, we found several in- GEOLOGICAL BACKGROUND tercalated tuff layers, some of which contain zircon grains. In this study, we performed U-Pb dating of zircon grains According to Ozawa et al. (1988), the Onnagawa For- separated from one of the tuff layers in the middle part of mation conformably overlies basaltic lava and pyroclastic the section to better constrain the chronostratigraphy of rocks of the Aosawa Formation and pebbly sandstone of the Onnagawa Formation. the Sugota Formation, and is conformably overlain by 186 J. Yoshioka et al. dark gray mudstone of the Funakawa Formation (Fig. 1a). The Sugota and the upper part of the Aosawa Formations were formed contemporaneously with littoral and neritic marine deposits of the Nishikurosawa Formation in the Oga Peninsula, whereas the Onnagawa and Funakawa Formations show wide-spreading deep-sea facies (Ozawa et al., 1988; Koizumi et al., 2009). The Onnagawa For- mation is characterized by the alternation of hard sili- ceous mudstone and softer mudstone deposited during the Middle-to-Late Miocene (e.g., Tada, 1991; Ozawa et al., 1988). In the Oga peninsula, diatomite that is considered to be equivalent to the Onnagawa Formation deposited between ca. 13.0 and 6.5 Ma according to diatom biostratigraphy of Koizumi et al. (2009). The studied sections are located in the Yurihonjo area in the southern part of Akita Prefecture, and four lithological units (Units 1 to 4) are identified (Fig. 1b). The lowest unit (Unit 1) is characterized by the several- meter-scale cyclic alternation of marl and mudstone, which is conformably covered by the second unit (Unit 2) characterized by the several-meter-scale alternation of bedded siliceous mudstone and mudstone. Unit 2 is con- formably overlain by a glauconite-rich sandstone bed at the base of the third unit (Unit 3), whose base was de- fined as 0 m level in the columnar section of Fig. 3b. The lithology of Unit 3 above the glauconitic sandstone is the several-meter-scale alternation of distinctly bedded muddy porcellanite and bedded siliceous mudstone. Fi- nally, weakly bedded to massive siliceous mudstone of Unit 4 overlies Unit 3 and consists of the uppermost part of the sections. There is no significant fault nor unconformity from Units 1 to 4 in the studied sections. Lithologically, Unit 1 might be correlated to the Nishikurosawa Formation, Units 2 and 3 to the Onnagawa Formation, and Unit 4 to the Funakawa Formation (see Section “Results and Discussion”). MATERIAL The tuff layer from which we separated zircon grains for dating is intercalated in Unit 2 (the star point in Fig. Fig. 2. (a) CL images of dated zircon grains. Numbers are 1b) at –5.6 m level in the columnar section of Fig. 3b. grain No., and circles are measured points by NanoSIMS. (b) 204 206 238 206 The tuff layer is 7 cm thick and composed of very coarse An isochron diagram between Pb/ Pb and U/ Pb ra- sand-size felsic grains with a bluish-gray fine matrix (Fig. tios (N = 15). Errors are portrayed as the 2σ level. The isochron age was calculated by using Isoplot (Ludwig, 2008). 1c). The tuff layer shows a normally graded bedding struc- ture in the upper part. The tuff layer has a distinct contact with siliceous mudstone above and below, and is later- ally continuous. Grains sieved with 44 µm mesh are com- posed mainly of quartz and feldspars with a minor amount mostly present as colorless to pale pink primary euhedral of dark gray lithic fragments, pyrite, altered volcanic grains smaller than 300 µm. The lack of detrital (rounded) glasses, and zircon (Fig. 1d). Quartz and feldspars grains grains and flow structures (cross laminations), and the are smaller than 2 mm, and show crystal faces or angular lateral continuity of the tuff layer strongly suggest that fraction surfaces. Pyrite is found as small crystals in clus- the tuff layer is not reworked deposit such as debris flow ters or on the surface of quartz and feldspars. Zircon is deposit and turbidite. Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Fm. in North. Japan 187 METHODS Table 1. NanoSIMS U-Pb results of zircon grains Zircon grains were separated from the volcanic tuff Grain No. †U content 204Pb/206Pb 238U/206Pb layer by using heavy-liquid techniques and acid treatment [ppm] [×10–4] (operated by Kyoto Fission Track, Co. Ltd.). The sepa- 9-1 868 123 (12) 523 (19) rated zircon grains were mounted in an epoxy resin disc. 13-1 2141 21.4 (9.9) 559 (17) The zircons were polished until the midsections of the 34-1 597 42.0 (10.2) 607 (22) 36-2 650 45.9 (10.0) 578 (25) grains were exposed. Final polishing was done with a 0.3- 36-4 676 11.7 (6.7) 598 (23) µm plastic abrasive sheet. Cathodoluminescence (CL) imaging was performed using SEM-EDS (JEOL JSM- 36-5 595 38.4 (21.5) 579 (24) 6610 LV) equipped with a CL system (Gatan Mini CL) at 47-1 426 29.2 (16.5) 530 (24) the Geological Survey of Japan to observe the internal 47-2 333 44.7 (29.6) 533 (29) 62-1 1044 23.8 (7.9) 574 (20) structure and zonation of the separated zircon grains, and 62-2 482 38.9 (16.0) 565 (25) to select suitable sites for the U-Pb dating.
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