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Sequence-stratigraphic signatures of hemipelagic siltstones in deep-water successions: the Lower Pleistocene Kiwada and Otadai Formations, Boso Peninsula, Japan |
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| Authors: | Sotaro Takano Makoto Ito Takanori Nakano Keiji Horikawa Yuzuru Nakamura Takahiro Saito |
| Institution: | aGraduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan bDepartment of Earth Sciences, Chiba University, Chiba 263-8522, Japan cInstitute of Geoscience, University of Tsukuba, Tsukuba 305-8571, Japan |
| Abstract: | Sequence-stratigraphic signatures of hemipelagic siltstones were investigated using profiles of the magnetic susceptibility and selected chemical composition of the Early Pleistocene deep-water successions of the Kiwada and Otadai Formations on the Boso Peninsula, Japan. In the context of an independently developed sequence-stratigraphic framework for the submarine-fan deposits of the Otadai Formation, the magnetic susceptibility and chemical composition, such as the concentrations of TiO2, MgO and Fe2O3, show that the lowstand systems tract deposits have higher values of these parameters than the transgressive and highstand systems tract deposits. In contrast, the CaO contents have inverse relationships with the magnetic susceptibility and are higher in the transgressive and highstand systems tract deposits. The positions of sequence boundaries largely coincide with the horizons from which the magnetic susceptibility and the contents of mafic component increase abruptly. The sequence-stratigraphic variations in the magnetic susceptibility and chemical composition of the submarine-fan hemipelagic siltstones are due to increases in the input of fine-grained, terrigenous clastic sediments from midwater flow suspension, in addition to the direct fluvial supply of relatively unmodified terrigenous clastic sediments during relative sea-level lowstands, although grain size of hemipelagic siltstones does not exhibit any distinct variation through depositional sequences. The Kiwada Formation is characterized by siltstone-dominated basin-plain deposits and its sequence-stratigraphic classification has been difficult when using just lithofacies features. Nevertheless, the profiles of the magnetic susceptibility and chemical composition of the basin-plain deposits are similar to those of the submarine-fan deposits with duration largely equivalent to the 41,000-years obliquity cycle of the Early Pleistocene oxygen isotope sea-level index. This finding indicates that the profiles of the magnetic susceptibility and chemical composition of hemipelagic siltstones reflect sequence-stratigraphic variation in the input of fine-grained terrigenous clastic sediments to the deep-water environments and are crucial for the recognition of cryptic sequence boundaries in hemipelagic successions. |
| Keywords: | Hemipelagic siltstones Submarine-fan deposits Basin-plain deposits Magnetic susceptibility Chemical composition Sequence stratigraphy |
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