Abstract
A time-diagnostic positive excursion (∼1‰) of carbon isotope ratio (δ13C) recorded in organic matter from the Middle Turonian (Late Cretaceous) clastic sequences in Japan and Russian Far East provides inter-regionally recognizable chemostratigraphic keybed at 90.7 Ma. Compilation of recent studies on carbon-isotope stratigraphy for this region revealed that the δ13C event is located in the middle of gradual∼1.5‰ negative shift above the prominent Cenomanian/Turonian boundary δ13C “spike”. Limited occurrences of cosmopolitan age-indicative ammonoid species, Collignoniceras woollgari revealed that the range of this species encompasses the δ13C event horizon confirming its utility as an international chemostratigraphic keybed, whereas the common occurrence of Inoceramus hobetsensis across the δ13C event benefits identifying this important marker horizon.
(Communicated by Tatsuro MATSUMOTO, M. J. A., June 10, 2003)
