Stratigraphy of the Sorachi Group of Hokkaido, Japan - Another oceanic plate in the Mesozoic NW Pacific.

The Sorachi-Yezo belt in central Hokkaido, northern Japan, is characterized by the occurrence of Jurassic oceanic basalts and Early Cretaceous siliceous and volcanogenic deposits (Sorachi Group) and overlying Cretaceous terrigeneous sediments (Yezo Group). The age of the Sorachi oceanic crust is inconsistent with the age of the Izanagi plate reconstructed both by paleomagnetic anomalies and by accreted oceanic sediments. Therefore, it is likely that the Sorachi oceanic crust was a distinct oceanic plate from the Izanagi plate. The Sorachi Group could thus be a key to reconstruct the paleogeography of oceanic plates in the Mesozoic NW Pacific. This study focuses on stratigraphy and clastic composition of the Sorachi Group to clarify the origin and nature of the Sorachi oceanic plate which could have existed in the Mesozoic NW Pacific.

The Sorachi group consists of two stratigraphic units (S1: basalt lavas, and S2: mainly sedimentary rocks), and the S2 unit is subdivided into four units (S2a: basaltic conglomerate, S2b: andesitic volcaniclastic sandstone with mudstone, S2c: siliceous mudstone with tuff, S2d: siliceous tuffaceous mudstone with basalts). The S2 unit is conformably overlain by quartzo-feldspathic sediments of the Yezo Group, whose lower sections are divided into two unis (Ly 1: sandy, and Ly 2: muddy turbidites, respectively).

Bulk-rock and detrital clinopyroxene chemistry indicates that S1 unit basalts and S2a unit basaltic clasts have MORB composition, whereas detrital clinopyroxenes in the S2b unit sandstones were supplied from a mafic arc. U-Pb ages and trace element compositions of detrital zircons suggest that the S2b sandstone does not contain continental materials, whereas felsic tuff in the S2c and S2d units and Ly1 unit sandstone were supplied from the Eurasian continent as represented by Precambrian zircons.

Combining radiolarian and zircon U-Pb ages with geochemical results, deposition of the Sorachi and Yezo groups are considered to have proceeded as the following sequence. The MORB oceanic crust of the Late Jurassic S1 unit was partly eroded and redeposited to comprise the S2a unit in the Tithonian-Berriasian time at a deep-sea environment. Coarse-grained clastics supplied from an intraoceanic arc were deposited in a pelagic basin floor since 139 Ma as represented by the S2b sandstones. In 129 Ma, felsic volcanic ash from the Eurasian active continental margin began to fall as the basin transferred to be hemipelagic (S2c unit). Tuffaceous materials progressively increased toward the overlying S2d unit sediments. The Ly1 unit represents the onset of turbidite supply from the Eurasian continental margin at 122 Ma. The entire sequence suggests that the Sorachi oceanic basin was born in a pelagic environment adjacent to an intraoceanic arc, and the basin then approached and finally attached to the Eurasian continent. Therefore, we interpret that the Sorachi oceanic basin, which existed in the Late Jurassic to Early Cretaceous epochs, was a marginal basin plate like the present-day Philippine Sea plate.