Glaucony and carbonate grains as indicators of the condensed section: Omma Formation, Japan

Abstract

The fifth-order depositional sequences of the Early Pleistocene Omma Formation exposed along the Japan Sea coast of central Japan were formed by glacial-eustasy during oxygen isotope stages 50 to 28. In each depositional sequence, two ecostratigraphic datums are always present: the appearance and disappearance datums of warm-water molluscan species. These datums are independent of sequence stratigraphic concepts, because the establishment of them is based on immigration events of molluscan species associated with glacio-eustatic sea-level changes. Determination of time planes shows that the appearance datum seems to occur near the midpoint of sea-level rise on the glacial to interglacial shift in deep-sea d 18 O records. In order to evaluate the significance of the condensed section in sequence stratigraphy and also to facilitate its recognition, this study examines the stratigraphic relationship of the condensed section indicators, glaucony and carbonate grains, with respect to the position of the ecostratigraphic datums in depositional sequences of the Omma Formation. The results show that the maximum concentration of carbonate grains is a more reliable maximum flooding surface indicator than the concentration of glaucony. The combination of indicators of condensed section and ecostratigraphic datums represented by incursion epiboles enables the boundary between transgressive and highstand systems tracts to be recognized in the inner shelf parts of depositional sequences. Moreover, truncation of ecostratigraphic datums during sea-level falls demonstrates significant erosion at the sequence boundaries. Integration of climatic palaeoecology and sequence stratigraphy permits a level of correlational precision of the order of a few thousands of years. © 1998 Elsevier Science B.V. All rights reserved.