in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Long-period Astronomical Cycles from the Upper Triassic to Lower Jurassic Bedded Chert Sequence: Implications for Jurassic Cyclostratigraphy 112 M. Ikeda * , R. Tada, N. Karasuda, H. Sakuma Department of Earth and Planetary Science, University of Tokyo, Tokyo, 113-0033, Japan (E-mail: m_ikeda@eps.s.u-tokyo.ac.jp) Astronomical timescales provide the highest resolution robust timescales over the tens of thousands to millions of year timescales. Milankovitch forcing is one of the main drivers of climate change, and Milan- kovitch cyclicitiy recorded in sediments provides a high-resolution and high-precision age model that is critical to the understanding of dynamics of global geological events. Bedded cherts consist of rhythmical alternations of chert and shale beds and Milankovitch cycle origin of bedded chert was demonstrated based on the hierarchal cyclicity of chert bed thickness variations in Triassic bedded chert sequence (Ikeda et al., in review). However, the chaotic evolution of the Solar system prevents a precise determination of the Earth’s motion over Mesozoic (Laskar et al., 2004). Cyclo- stratigraphic studies of middle Triassic bedded chert sequence and upper Triassic lacustrine sequence suggested that Earth-Mars secular resonance during Triassic had been in a different librational state from the present state (Olsen and Kent, 1999; Ikeda et al., in review). To examine the Jurassic astronomical timescale and the chaotic behavior of Earth-Mars secular resonance, we extend cyclostratigraphy for the upper Triassic (Rhaetian) to lower Jurassic (Toarcian) bedded chert in the Inuyam area, central Japan (e.g. Hori, 1990). The section is comprised bedded chert sequence in the Jurassic accretionary complex of Mino-Tamba terrane (Kimura and Hori, 1993), which is representative of deep-sea sedimentary rocks deposited below carbonate compensation depth (CCD; Matsuda and Isozaki, 1991). The bedded chert is mostly red-brown to green in color, except for the dusty red bedded chert near the Triassic/Jurassic boundary and black bedded chert near the Toarcian oceanic anoxic event interval (Carter and Hori, 2005; Hori, 1997). We measured thickness of approximately 800 chert and shale beds, respectively. The estimated average duration of a chert-shale couplet based on the radiolarian biostratigraphy (Carter and Hori, 2005; Hori, 1990) were about 20-kyr, confirming the possibility of its precession cycle origin. Spectral analysis of bed number series of chert bed thickness variation was conducted assuming one chert-shale couplet as representing a 20-kyr precession cycle. The result revealed approximately 100, 20, and 5 beds cycles, which correspond to approximately 2000-, 400-, and 100-kyr periodicities above 95% confidence level, respectively. By further assuming approximately 20 beds cycle as representing 405-kyr eccentricity cycle of constant and stable periodicity, we converted the bed number series to the time series. Spectral analysis of the time series detected distinct periodicities of ca. 2400-, and 105-kyr in addition to 405-kyr above 99% confidence level, which agree well with the periodicities for eccentricity cycles. Moreover, we detected amplitude modulation of 405-kyr cycle of time series with approximately 2400-kyr periodicity, which would correspond to the amplitude modulation of 405-kyr eccentricity cycle by 2400-kyr eccentricity cycle. 2400-kyr cycles would be related to the present librational state of Earth-Mars secular resonance. Our results suggested that the chaotic transition of Earth-Mars secular resonance would have occurred near Triassic/Jurassic boundary. Further investigation in continuous late Mesozoic cyclostratigraphy by continuous pelagic bedded chert sequence will be necessary to reconstruct the chaotic evolution of Solar system and astronomical timescale for Mesozoic era. Key words: Cyclostratigraphy; Chaos; Toarcian; Raetian References: Carter E.S., Hori R.S. Global correlation of the radiolarian faunal change across the Triassic– Jurassic boundary. Canadian Journal of Earth Sciences, 2005, 42: 777-790. Kimura K., Hori R.S. Offscraping accretion of Jurassic chert-clastic complexes in the Mino and Tamba Belt, central Japan. Journal of Structural Geology, 1993, 15: 145-154. Hori R.S. Lower Jurassic radiolarian zones of SW Japan. Transactions and Proceedings of the Paleontological Society of Japan, New Series, 1990, 159: 562-586. Hori R.S. The Toarcian radiolarian event in bedded chert from southwestern Japan. Marine Micro- paleontology, 1997, 30: 159-169. Ikeda M., Tada R., Sakuma H. Milankovitch cycle origin of bedded chert. A middle Triassic bedded chert sequence, Inuyama, Japan. Earth and Plane- tary Science Letters (in review). Laskar J., Robutel P., Joutel F., et al. A long-term numerical solution for the insolation quantities of
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 the Earth. Astronomy & Astrophysics, 2004, 428: 261-285. Matsuda T., Isozaki Y. Well-documented travel history of Mesozoic pelagic chert in Japan: From remote ocean to subduction zone. Tectonics, 1991, 10: 475-499. Olsen P.E., Kent D.V. Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the early Mesozoic timescale and the long-term behavior of the planets. Phil. Trans. Royal Soc. London ser. A, 1999, 357: 1761-1786. 113
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