Mastodon isn't sludge in any way, mate... sludge is hardcore punk + (proto) doom metal. It's what the Melvins, the B-side of Black Flags's My War and early Flippers spawned, so mostly the NOLA scene (Eyehategod, Crowbar, Down, Acid Bath, Buzzoven, etc...) and "others" (Grief, Floor, 16).

Perhaps we need a word to disambiguate "atmosludge" from actual sludge, for the same reason "skramz" was invented.