Constraints on the timing of systems tract development with respect to sixth-order (41 ka) sea-level changes: an example from the Pleistocene Omma Formation, Sea of Japan

Abstract

Abstract The middle part of the Omma Formation exposed along the Sea of Japan coast in central Japan comprises 11 sixth-order (41 ka) depositional sequences that were deposited in the inner to outer shelf environments and accumulated during oxygen isotope stages 50 to 28. Warm-water planktonic foraminifera intruded into the Sea of Japan during interglacial periods and were extirpated in the following glacial period. The stratigraphic distribution of these microfossils provides datum that allows the timing of systems tract formation and the degree of truncation at the superjacent ravinement surface to be constrained. The disappearance datum of warm-water planktonic foraminifera demonstrates unequivocally that the regressive systems tract was deposited during falling sea-level. Sixth-order cyclothems of the Omma Formation include the following architectural elements in ascending stratigraphic order: (1) a basal sequence boundary that is superposed on the ravinement surface; (2) a transgressive systems tract (2–5 m thick) consisting of a basal shell bed (0.3 m thick) (a condensed onlap shell bed) and overlying fine- to very fine-grained sandstone; (3) a maximum flooding horizon that coincides with the horizon that has the maximum concentration of sand-size biogenic grains; (4) a highstand systems tract (2–3 m thick) consisting of fine-grained sandstone and sandy siltstone; and (5) a resgressive systems tract (