[aft_lizard]

Or simply a meteor shower.

[piytar]

Would have to be on large meteor shower to create that wide an expanse of fairly uniform glass.

Then again, a solar mass ejection or gamma ray burst that did so would also probably have done some other noticable things. Like melt a mountain (range) or two and boil off a chunk of sea and ocean causing massive climate damage that would be apparent in the geological record worldwide. So yeah, meteor shower sounds just as likely...