Russian Journal of Earth Sciences
Vol. 6, No. 4, August 2004
On events of the Terminal Cenomanian in the eastern Central Asia
O. A. Korchagin
Abstract
Changes of the foraminiferal assemblages in eastern Central Asia during a
significant Mesozoic biotic crisis at the Cenomanian-Turonian boundary are discussed.
The reported records are based on the study of the most complete section of the
boundary Cenomanian-Turonian sediments in Asia that is located in southern Tajikistan,
in the Koikitau Range. The ammonite Sciponoceras gracile Zone in
this section contains
extremely diverse and numerous benthic and planktonic foraminiferal assemblages.
The
flourishing of benthic foraminifers in this zone indicates the normal marine basin
conditions. The changes in foraminiferal assemblages were not catastrophic and not
accompanied by mass extinction of fauna, but resulted from the strengthened migration
of foraminifers. The benthic and planktonic foraminifer flourishing in this interval
was a
global phenomenon manifested on shallow shelves not only in eastern Central Asia
but
also in the Western Interior Basin of North America and in Western Europe. At the
same
time in the oceans, on the contrary, anoxic conditions (AOE-II) were widespread.
The
reported materials also indicate the strengthening of lateral water mass circulation
in the
terminal Cenomanian, when moderately cold intermediate waters from the West
European marine basins and warm surface waters from the southern tropical realm
concurrently penetrated to Central Asia. It is suggested that one of the reasons
for
extinction of the characteristic planktonic foraminifer genus Rotalipora
immediately prior
to the events of the Sciponoceras gracile time, was a sharp restoration
of normal marine
basin conditions that occurred on the mid-latitude shelves in the Northern Hemisphere.