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Geology and age model of the L ower P leistocene N ojima, O funa, and K oshiba Formations of the middle K azusa G roup, a forearc basin‐fill sequence on the M iura P eninsula, the P acific side of central J apan
Author(s) -
Nozaki Atsushi,
Majima Ryuichi,
Kameo Koji,
Sakai Saburo,
Kouda Atsuro,
Kawagata Shungo,
Wada Hideki,
Kitazato Hiroshi
Publication year - 2014
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12066
Subject(s) - geology , forearc , isotopes of oxygen , lithology , group (periodic table) , paleontology , geochemistry , tectonics , subduction , chemistry , organic chemistry
We present field and core observations, nannofossil biostratigraphy, and stable oxygen isotope fluctuations in foraminiferal tests to describe the geology and to construct an age model of the L ower P leistocene N ojima, O funa, and K oshiba Formations (in ascending order) of the middle Kazusa Group, a forearc basin‐fill succession, exposed on the northern Miura Peninsula on the Pacific side of central J apan. In the study area, the Nojima Formation is composed of sandy mudstone and alternating sandy mudstone and mudstone, the Ofuna Formation of massive mudstone, and the Koshiba Formation of sandy mudstone, muddy sandstone, and sandstone. The Kazusa Group contains many tuff beds that are characteristic of forearc deposits. Thirty‐six of those tuff beds have characteristic lithologies and stratigraphic positions that allow them to be traced over considerable distances. Examination of calcareous nannofossils revealed three nannofossil datum planes in the sequences: datum 10 (first appearance of large G ephyrocapsa ), datum 11 (first appearance of G ephyrocapsa oceanica ), and datum 12 (first appearance of G ephyrocapsa caribbeanica ). Stable oxygen isotope data from the tests of the planktonic foraminifer G loborotalia inflata extracted from cores were measured to identify the stratigraphic fluctuations of oxygen isotope ratios that are controlled by glacial–interglacial cycles. The observed fluctuations were assigned to marine isotope stages ( MISs ) 49–61 on the basis of correlations of the fluctuations with nannofossil datum planes. Using the age model obtained, we estimated the ages of 24 tuff beds. Among these, the SKT ‐11 and SKT ‐12 tuff beds have been correlated with the Kd25 and Kd24 tuff beds, respectively, of the Kiwada Formation on the Boso Peninsula. The Kd25 and Kd24 tuff beds are widely recognized in Pleistocene strata in J apan. We used our age model to date SKT ‐11 at 1573 ka and SKT ‐12 at 1543 ka.