Among Bivalves of high stratigraphical significance Boreal Buchia flourished from at least from Oxfordian to Hauterivian.
Buchia-zones near to J/K boundary corresponds usually to 1-2 ammonite zones. They are based on the phylogenetic succession of evrybiont bivalves and thus has a great correlational potential.
The same or very close zonal succession could be traced from Russian Platform and Svalbard to N.Siberia, NE Russia, Russian Far East and even California, while rare buchiids are also known from S.Turkmenia, Azerbaijan and Mexico.
Best correlatable level which lies near to J/K boundary is a base of the Buchia okensis Zone. It based on the remarkable short-ranged Buchia species, easily determinable from Greenland to California (californian B.aff.okensis are really belongs to true B.okensis) 
Succession of the Buchia-Zones near to J/K boundary (after Zakharov, 1987, Cret.res., V.8)
Correlation of boreal ammonite and Buchia zones (Zakharov, 1990)
Other level which is perhaps lies slightly below from the base of Jacobi ammonite zone, i.e. base of the Unschensis Zone, could be traced in some regions only
