Inferred glacio-eustatic control for high-frequency depositional sequences of the Plio-Pleistocene Kazusa Group, a forearc basin fill in Boso Peninsula, Japan

Abstract

The Plio—Pleistocene forearc basin fill of the Kazusa Group, as much as 3000 m thick, is represented by a series of marine environments ranging from deep-sea basin plain to shallow sea. It developed during about 2.4 to 0.45 Ma, and is well exposed in the Boso Peninsula, Japan. Chronostratigraphic resolution of the Kazusa Group is partly within about 0.05 m.y., based on the combination of fission track dating of volcanic ash layers, and bio- and magneto-stratigraphy. Many volcanic ash layers are intercalated in the Kazusa Group and provide distinct chronostratigraphic surfaces within the heterogeneous lithologies. Detailed outcrop studies on depositional facies discontinuities and lateral terminations of volcanic ash layers allow high-resolution sequence stratigraphic interpretation of the Kazusa Group. Seventeen depositional sequences were identified in the Kazusa Group. Each depositional sequence was tentatively correlated with the stable oxygen isotope sea-level index, integrated with the age framework of the Kazusa Group. This correlation indicates that frequencies of the depositional sequences can be estimated to be in the range of 0.2-0.02 m.y. and are within fourth- and fifth-order stratigraphic cyclicities. Depositional sequences constitute third-order transgressive and highstand sequence sets and the Kazusa Group is interpreted to represent a third-order composite sequence with a duration of about 1.95 m.y. Development of the third-order composite sequence was also controlled by an overall rise and fall of glacio-eustatic sea levels, overprinted by tectonic events related to fluctuations in oceanic plate motion which occurred at about 2.6 Ma and 0.5 Ma, respectively.