Some excerpts from K. Kris Hirst's article on the subject give the basics:
Clovis, Black Mats, and Extra-Terrestrials
By K. Kris Hirst
None of the black mats investigated at these Clovis sites contained any Clovis nor any evidence of any of the Pleistocene fauna, although beneath it, and occasionally immediately beneath it, can be found Clovis mammoth kills.
In 2007 at the American Geophysical Union meetings, a session was given explaining the black mat as having followed the explosive destruction of a comet which was postulated to have broken into pieces over the Laurentide ice shield.
A formal paper, published in the Proceedings of the National Academy of Sciences in September of that year, described a thin sedimentary deposit immediately beneath the black mat, which contained high concentrations of magnetic grains with iridium, magnetic microspherules, glass-like carbon containing nanodiamonds, and fullerenes with extra-terrestrial helium. Firestone et al. argue that the stuff underneath the black mat represents the detritus of an explosive low-density object--a comet--which destablized the Laurentide Ice Sheet. Widespread fires ensued, followed by an accelerated melting of the ice sheet and then a cooling period (the YD), brought on perhaps by persistent cloudiness. This combination, they claim, led to the megafaunal extinctions and the end of the Clovis big game adaptation.
Most recently, in fact late last week, C. Vance Haynes reported in PNAS that he had increased the database of Clovis sites with black mats to 70.
The evidence seems to be very strong. In a (geological) instant, the extinction of horses, camels, mammoths, mastodons, dire wolves, American lions, and tapirs occurred. At the same time, Clovis patterns of big game hunting ends, and clear evidence of a surge in spring discharge at former Clovis sites occurs.
The black-mat boundary (BMB)
A much weaker version of the KTB may have occurred about 13,000 years ago centered in North America. The geological markers in this case are carbonaceous "black mats," which cap many sites occupied by the Clovis people and may have ended their culture. We will call this event the BMB (Black Mat Boundary). The KTB and BMB parallels are interesting.
What the black mats represent isn't entirely clear. Although some or all of them might have been caused by algae blooms, other evidence suggests that the carbon enrichment came from extensive biomass burning in very hot fires such as might be ignited by both the thermal pulse and burning ejecta of a cometary strike.
The BMB is also associated with high levels of iridium, nickel, chromium, and magnetic microspheres. These inclusions strengthen the comet theory.
Sounds pretty conclusive to me. I'm having a hard time imagining how the Paleo-Indians' excessive hunting could've triggered all these geological phenomena. And a hard time believing the megafauna extinctions and geological phenomena happened simultaneously but were just an amazing coincidence.
12,900 Years Ago: North American Comet Impact Theory Disproved
Dr Sandy Harrison from the University of Bristol and colleagues tested the theory by examining charcoal and pollen records to assess how fire regimes in North America changed between 15 and 10,000 years ago, a time of large and rapid climate changes.
Their results provide no evidence for continental-scale fires.
Fires seem to be required only for one theory of what caused the black mats. Since no one is sure what caused them, the lack of fires disproves only one explanation for one part of the comet theory. It doesn't disprove the whole theory.
Recall that the animals presumably died because of extensive changes to the climate, which would've disrupted the food cycle by killing the vegetation. If enough dust was thrown into the air, it would've triggered a "little Ice Age" or "nuclear winter." Fires are not required to explain the massive die-off.
According to Hirst, Haynes isn't sure of his own theory:
It's still common to hear people blame Paleo-Indians for the megafauna extinction. Shepard H. Krech devoted a whole chapter to the subject in his book The Ecological Indian.
Notably, Krech didn't seriously consider any other theories. Nor do his fellow Indian haters. They're convinced they've proved Indians really are murderous savages at heart.
Has there ever been a case of a pre-contact indigenous tribe's exterminating a species? Not that I know of. If there is one, it's so obscure that I haven't heard of it.
Yet we're supposed to believe that it happened once in history: when the Paleo-Indians first arrived in the Americas. Out of thousands of indigenous tribes around the world, these people were the only ones stupid and savage enough to eliminate their primary food sources. And this just happened to occur at the same time something created a strange "black mat" at sites across the continent.
I'm sure there are tons of evidence for and against the comet theory--much more than I can find in a quick Google search. But let's get real here. Scientists have found a geological layer with extraterrestial indicators at 70 of 97 Clovis sites. And critics don't think it's relevant?! They can say they're not convinced, but if they won't even consider the comet theory--as Krech didn't--they must be anti-Indian ideologues.
For more on Indians' regard for nature, see Ecological Indian Talk.