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

The middle part of the Omma Formation exposed along the Sea of Japan coast in central Japan comprises eleven sixth-order (41-k.y.) depositional sequences that were deposited in 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 at the following glacial period. The stratigraphic distribution of these microfossils provides datums which allow 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 TST (2-5 m thick) consisting of a basal shell bed (0.3 m thick) (a condensed onlap shellbed) 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 carbonate grains; (4) an HST (2-3 m thick) consisting of fine-grained sandstone and sandy siltstone; and (5) an RST (< 1 m thick) comprising fine-grained sandstone with a coarsening-upward trend.