I think the obsidian cliff in Yellowstone is black obsidian...and plenty of it.  It is a bit closer than eastern Oregon.  Obviously it walked to the place you found it.  They have tracked trade routes across the North American continent with that kind of information.  Apparently it was a more extensive network of trade than most of us would recognize.

Why would you bury something like that, Reallife?
You found it, I guess you can do with it what you please, but why bury it?