I could not find any further information on it, but judging by the waterproof/breatheability ratings and the use of the DWR finish, I would assume that it's a nylon outer layer and a poly/synthetic inner layer.
It comes in 2- and 3-layer finishes, depending on the waterproof and durability desired.
for 13/14 season Mega Rex only comes in 10k and 15k I think. I have some 20k Overshoot pants from two seasons ago and they are super sick but they do tear easily. This year the 15k has a stretch to it which is something you will have to feel in person. The 10k is non stretch. I don't know why they stopped making 20k I guess because they have the Gore Tex stuff now. Overall the Mega Rex fabric feels pretty similar to a lot of other materials I have seen