If you aren't moving, just standing in the quicksand won't make you sink, but the more you move the faster you sink. Holding down control will let you float to reduce the speed you're sinking at (in real life you do this by laying back horizontally, but I didn't manage to add an animation for it..). Each pit is a cone with sloped sides that you can walk up as long as you aren't sinking too fast. So you need to try to head towards the sides once you grab the treasure and take breaks to float and keep yourself from sinking too fast.
EDIT: Published an update to add leaning animation, hoping that makes it more clear what's going on.