Well, from the article:
"Morrison posed several questions: "What size impact does it take to produce diamonds? What size crater would that be? Where is it? If it hit in the ocean, would it have had the same effect? These are all questions one can ask." "

Let guess, 1/2 to 1 km diameter rock.
Simulation of such impact was done if it hit the ocean.

Ah, here:
"The 60,000-megaton blast of the impact vaporizes the asteroid and blows a cavity in the ocean 11 miles across and all the way down to the seafloor, which is about 3 miles deep at that point. The blast even excavates some of the seafloor."
http://www.ucsc.edu/news_events/press_releases/text.asp?pid=355

So if it hit America at that time there is a mile or two ice covering large areas.

Say at the present if such impactor hit the middle of Greenland, it seems to me it's likely there won't be any crater- less crater then you find in the ocean.

So, if one had a mile glacier ice, it seem if you have impactor 1 km or less in diameter the signs of it would hard to find- because most what we see of crater impact involves the surface terrain. Second the impactor is going melt a large area of ice. It vaporize 10 mile diameter an mile thick of ice as it would with the ocean, and the energy from the shock should liquefy ice beyond this. Which at some point flows back in- and not just water, but water mixed dirt and rock.

Now if ocean impact causes 400' waves to travel 2 hours at speed of sound, why would it do something similar with the glacial ice.