Fourth‐ to third‐order cycles in the Hakobuchi Formation: Shallow‐marine Campanian final deposition of the Yezo Group,Nakagawa area,northern Hokkaido,Japan |
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| Authors: | Hisao Ando Yoshitaka Tamura Daisuke Takamatsu |
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| Institution: | 1. Department of Environmental Sciences, Faculty of Science, Ibaraki University, Mito 310‐8512, Japan (email: ando@mx.ibarki.ac.jp), and;2. Graduate School of Science and Technology, Ibaraki University, Mito 310‐8512, Japan;3. Present address: Universe Co. Ltd., Clover Nishi‐Shinjuku Bldg., 3‐23‐4 Yoyogi, Shibuya, Tokyo 151‐0053, Japan;4. Present address: Oyama Process Co. Ltd., 13‐10 Kumano, Itabashi, Tokyo 173‐0025, Japan |
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| Abstract: | The Yezo Group has a wide longitudinal distribution across Hokkaido, northern Japan. It represents a Cretaceous (Early Aptian–Late Maastrichtian) and Late Paleocene forearc basin‐fill along the eastern margin of the paleo‐Asian continent. In the Nakagawa area of northern Hokkaido, the uppermost part of the Yezo Group consists of the Hakobuchi Formation. Along the western margin of the Yezo basin, 24 sedimentary facies (F) represent 6 facies associations (FA), suggesting prevailing storm‐dominated inner shelf to shoreface environments, subordinately associated with shoreface sand ridges, outer shelf, estuary and fluvial environments. The stacking patterns, thickness and facies trends of these associations allow the discrimination of six depositional sequences (DS). Inoceramids Sphenoceramus schmidti and Inoceramus balticus, and the ammonite Metaplacenticeras subtilistriatum, provide late Early to Late Campanian age constraints to this approximately 370‐m thick final stage of deposition and uplift of the Yezo forearc basin. Six shallow‐marine to subordinately non‐marine sandstone‐dominated depositional sequences include four 10 to 110‐m thick upward‐coarsening regressive successions (FS1), occasionally associated with thin, less than 10‐m thick, upward‐fining transgressive successions (FS2). The lower DS1–3, middle DS4–5 and upper DS6 represent three depositional sequential sets (DSS1–3). These eastward prograding and westward retrograding recurring shallow‐marine depositional systems may reflect third‐ and fourth‐order relative sealevel changes, in terms of sequence stratigraphy. |
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| Keywords: | Campanian Cretaceous depositional sequence Hokkaido sequence stratigraphy shallow‐marine sediments Yezo forearc basin Yezo Group |
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