I have to wonder about the whole sand slide scenario. I rode dirt bikes and a Banshee in the sand on the Oregon coast and in Idaho for years. Even on really long steep hill climbs when you are really throwing sand, the sand itself doesn't slide very quickly. I have also watch sand shifting off a cliff edge while diving in Cozumel. It also moves very slowly. Its not like liquid mud or snow. He would have had to be trapped and not moving to be buried under any kind of sand fall. Even then, I think it would just push him to the surface of the slide. He should have been close to neutral if he was moving at all. From the pics of his gear, it didn't look like he was carrying a large amount of extra weight. To be buried, he would have had to be pinned in one spot.