This is an interesting article (click on link) which attempts to tie up some loose ends about North America prehistory....

Nano-diamonds and other exotic impact materials have been unearthed in thin sediments, Science magazine reports.

The age of these materials coincides with the start of a millennium-long climate cooling event known as the Younger Dryas - some 13,000 years ago.

Many large animals vanish from the archaeological record at this time.
It is also the period in Earth history that sees the demise of Clovis culture - the prehistoric civilisation that many regard as the first human occupation of North America.
Taken together, it all makes for a compelling story, claims the team behind the latest research.

http://news.bbc.co.uk/2/hi/science/nature/7808171.stm

Fire trace
Impact theorists maintain that the diamonds peak in abundance in the impact stratum.
The thumb width layer appears in a number of sites across North America including Murray Springs in Arizona and the Channel Islands off the coast of Southern California.
It lies beneath a black mat of biomass formed during the Younger Dryas.

Diamonds occur at the base of this black layer at Murray Springs, directly above extinct animals remains

The bottom-most film contains charcoal and soot, thought to be associated with impact fires, said University of Oregon geo-archaeologist Doug Kennett, son of James Kennett and another author on the Science paper, who has studied sedimentary vegetation and charcoal records.

No mega-fauna skeleton or Clovis artefact has been found above the impact layer or the black mat, he said.

"The black mat covers them like a blanket," said Dr Kennett.
Before they disappeared, woolly mammoths and other massive beasts such as sabre-toothed cats, giant sloth, camels, and teratorns (predatory birds with a nearly four-metre wingspan) roamed North America.