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...
Not to mention with your solar mass ejection or gamma ray burst you would create a likely large scale life sanitizing. With a large scale meteor strike you could get glass projectiles extends hundreds if not thousands of miles while being small enough to maintain life globally. Not to mention the meteor strikes could be simultaneous across the world and the resultant pollution could have created a short ice age as well.