Source: http://en.wikipedia.org/wiki/Boltysh_crater
Overview
The Boltysh Crater is located in central Ukraine,
in the basin of the Tyasmin River, a tributary of
the Dnieper River. It is 24 km in diameter, and is
surrounded by an ejecta blanket of breccia covering
an area of 6500 km². It is estimated that immediately
after the impact, ejecta covered an area of 25,000
km² to a depth of 1 m or greater, and was some
600 m deep at the crater rim.
The crater contains a central uplift about 6 km in
diameter, rising about 550 m above the base level
of the crater. It currently lies beneath about 500
m of sediment deposited since the impact, and was
discovered in the 1960s during prospecting for oil
in the region.
Age
When it was first identified, the age of the crater
could only be roughly established. Sediments from
the Cenomanian (98.9 to 93.5 million years ago) and
Turonian (93.5 to 89 million years ago) epochs were
found beneath ejecta from the impact, thus setting
an upper limit of about 89 million years on the age.
Bore samples taken from sediments overlying the crater
contain many fossils, and analysis of these dates
the sediments at the Paleocene epoch, 65 to 54.8 million
years ago. The age of the crater was thus constrained
to between 54.8 and 98.9 million years.
Later radiometric dating constrained the age further.
The concentration of Uranium-238 decay products in
impact glasses from the crater was used to derive
an age of 65.04±1.10 million years, the first
indication that it was of similar age to the Chicxulub
Crater. Analysis of Argon radioactive decay products
yielded an age of 65.17±0.64 million years.
Likelihood of multiple impact
Although the ages derived for Chicxulub and Boltysh
are the same to within the statistical errors, it
does not necessarily follow that they formed at exactly
the same time. At the estimated rate of impacts on
the Earth, it would not be extremely unusual for a
Boltysh-sized crater to be formed within half a million
years of Chicxulub.
However, the subsequent discovery of the Silverpit
crater and its dating to approximately the same epoch
gives greater weight to the theory that the Earth
was struck by multiple impactors at this time. The
dating of these impact craters is not yet accurate
enough to establish whether the multiple impactors
arrived over several thousand years, as part of a
generally elevated rate of impacts at that time, or
were almost simultaneous, like the impacts of the
fragments of Comet Shoemaker-Levy 9 on Jupiter in
1994.
Another crater to form at the same time was the Shiva
crater.
References
1. Grieve R.A.F., Reny G., Gurov, E.P., Ryabenko
V. A. (1985), Impact Melt Rocks of the Boltysh Crater,
Meteoritics, v. 20, p.655
2. Gurov E.P., Gurova H.P. (1985), Boltysh Astrobleme:
Impact Crater Pattern with a Central Uplift, Lunar
& Planetary Science XVI, pp. 310-311
3. Kashkarov L.L., Nazarov M.A., Lorents K.A., Kalinina
G.V., Kononkova N.N. (1999), The Track Age of the
Boltysh Impact Structure, Astronomicheskii Vestnik,
v. 33, p. 253
4. Kelley S.P., Gurov E. (2002), The Boltysh, another
end-Cretaceous impact, Meteoritics & Planetary
Science, v. 37, pp. 1031-1043