I would highly recommend Utah, the geology out there is insane. I just came back from field work a few weeks ago out west. The cross bedding in Zion is just stunning. You don't even have to do boots on the ground exploring, the national parks out in Utah are incredible. Though if you want to collect samples, just stop at any blasted out side of the highway, you'll also see a shocking amount of igneous formations (I saw mostly basalt with colummunar jointing) for a place associated with sand and red rock lol
~2 hours outside of Vegas there is a tiny tiny trail and "park"with incredibly well preserved dino tracks if you're into fossils
also i think this borders Utah and Arizona, but Goblin state park is sick as hell you can climb all over the goblin formation.