Age of the N7/N8 (M4/M5) planktonic foraminifera zone boundary: constraints from the zircon geochronology and magnetostratigraphy of early Miocene sediments in Ichishi, Japan

Abstract

The age assigned to the Miocene N7/N8 (M4/M5) planktonic foraminifera zone boundary is markedly different from the astronomically tuned Neogene time scales published in 2004 (ATNTS2004) and 2012 (ATNTS2012). This difference is primarily because of the lack of reliable assignment of the zone boundary to magnetostratigraphy, which needs to be resolved for the better refinement of the geologic time scale. Our previous biostratigraphic results from an onland sedimentary sequence of the upper Ichishi Group in central Japan allow location of the stratigraphic position of the lowest occurrence datum (LOD) of Praeorbulina sicana, a key planktonic foraminifera species for defining the N7/N8 (M4/M5) zone boundary. In this study, we performed laser ablation-multiple collector-inductively coupled plasma mass spectrometry U–Pb dating on zircons from two felsic tuff beds in the Ichishi sequence. The age of the zone boundary is approximated by a U–Pb age of 17.03±0.11 Ma for a tuff bed ∼2m above the boundary. We also conducted a magnetostratigraphic analysis in which site-mean characteristic remanent magnetization directions were determined for 19 sites and showed a reverse–normal–reverse (R–N–R) stratigraphic change in polarity. The N7/N8 (M4/M5) zone boundary is within the upper reverse polarity zone. The R–N–R sequence can be correlated with a stratigraphic portion corresponding to chrons C5Dr–C5Dn–C5Cr; hence, the age of the zone boundary is within Chron C5Cr. Our results are thus compatible with the age (16.97 Ma) assigned to the boundary in ATNTS2004 rather than ATNTS2012 and should be considered when the Neogene time scale is updated. A normal polarity subzone found in the upper reverse polarity zone can also be dated at ∼17.0 Ma and is probably correlated with a short normal polarity episode in Chron C5Cr. In terms of tectonic rotation, the site-mean magnetization directions determined in this study suggest an ∼35° clockwise rotation in Ichishi with respect to the North China Block in the Asian continent since ∼17.2 Ma (the upper boundary age of Chron C5Dn). Clockwise rotation of Southwest Japan, which occurred in association with backarc opening of the Japan Sea, was probably in progress during deposition of the upper Ichishi Group sediments.