The cenomanian/turonian boundary in sakhalin, far East Russia: Ammonites, inoceramids, foraminifera, and radiolarians

Abstract

The Cenomanian-Turonian succession of faunal assemblages identified in Sakhalin has enabled the establishment of 10 ammonite, 7 inoceramid, 4 radiolarian, and 2 foraminiferal zones, which correlate relatively well with those recorded for the northeastern region of Russia (Kamchatka and Koryakia) and for Japan. The problems surrounding placement of the Cenomanian/Turonian boundary in Sakhalin and in adjacent areas are reviewed. Significant macrofaunal turnover and radiation have been identified across the (locally defined) Cenomanian/Turonian boundary, as well as in the middle Turonian for ammonites and in the upper Turonian for inoceramids. The first occurrences of the widely distributed Pacific ammonite Jimboiceras planulatiforme (Jimbo), the cosmopolitan ammonite Fagesia, and the inoceramid Mytiloides aff. labiatus (Schlötheim), define the base of the Turonian Stage. The succession of foraminiferal assemblages does not exhibit any major extinction at the Cenomanian/Turonian boundary; however, a temporary faunal restructuring occurred at that time. The radiolarian fauna appears to have survived this interval without marked taxonomic change; a diversity decrease took place later, near the middle/late Turonian boundary. © 2004 Taylor & Francis Group, LLC.