[blam]

"And the impact at the end of the Cretateous down in Yucatan did not create a plume,..."

I read that the Chicxulub meteorite came in at a 35 degree angle, may be that's why it didn't 'punch' through.

[Mike Darancette]

I read that the Chicxulub meteorite came in at a 35 degree angle, may be that's why it didn't 'punch' through.

Exactly so, even a smaller object hitting at higher angle would be more likely to punch through. Also an object hitting mid-ocean (where the crust is thin) would be more apt to create a plume.