2026 Volume 132 Issue 1 Pages 79-93
The Middle Pleistocene Tanabu Formation crops out along the sea cliffs in the northern part of the Shimokita Peninsula, northern Japan. The formation consists of the Shiosaki and Ishimochinaya members, (in ascending stratigraphic order) and the present study investigated its depositional environments, formation processes, and sequence stratigraphy. Six major depositional facies were identified in the formation, which comprise deposits formed in coastal to shallow-marine environments. On the basis of lateral and vertical variations in the properties of the six depositional facies, along with identification of some distinct erosional surfaces, three depositional sequences (DS1–3; in ascending order) were identified. The depositional sequences are characterized primarily by lowstand incised valley fill and transgressive three-dimensional dune and gravelly spit deposits formed in coastal and shallow-marine environments. On the basis of the age framework of the formation and its correlation with marine O isotopic stages (MISs), DS1 was deposited in MIS12–11, and DS2 and DS3 were deposited in MIS10–9 and MIS8–7, respectively. The maximum water depths for DS1, DS2, and DS3 during their highstand stages are estimated to have been 40–130, 20–70, and <10 m, respectively, based on the set thickness of the trough cross-stratified beds of the three-dimensional dune deposits. Paleocurrent directions obtained from the three-dimensional dune and gravelly spit deposits, along with the correlation of DS1–3 with MISs 12–7, suggest that a strait may have developed in an area between the present-day Osorezan Mountains and Shiriyazaki. During highstand stages, oceanic currents flowed northwestward to northward through this strait. These currents are thought to have originated from the Tsugaru Warm Current and likely flowed from the present-day Mutsu Bay area to the Tsugaru Strait region.
