Obsidian is brittle enough that it doesn't make decent axes/celts like some types of flint or chert can. I've seen a lot of bifaces like that near quarries along the Nevada/California border, the tan spots look like cortex from the outside of the obsidian chunk. If you found it isolated from a quarry, it was probably a combo tool/core. It could handle some cutting & scraping, and the flakes driven from the piece would have been used for cutting or flaked into points.
Nice find either way.
Was there lots and lots of obsidian there?