Its made of limestone which rules out volcanic and meteor. It appears to be little understood but one theory is that its rock pushed up by underground gas.
http://en.wikipedia.org/wiki/Patomskiy_crater
I lean toward a strike by something small and dense, breaking up the limestone as it penetrated. This makes for a better material to be pushed back out. High pressure gas seems to me, would find/create cracks and fissures to escape from?
Bttt for tomorrow