what has changed in the atmosphere that give the rock strata the appearance of rust?(red/orange color).

Explained below.

Explanation:

What has changed in the atmosphere is that oxidation has occurred where an oxidizing chemical specie has reacted with an oxygen containing compound.

Now these chemicals when they react with rain and fall on rocks react with the rocks to weaken them in a process known as chemical weathering.

Now when this weathering from the oxidizing chemicals from the rain react with rocks that contain iron, it leads to the rock having an appearance of rust.