Some etching of the rotor is not uncommon, but it sounds like a couple pieces of debris may have gotten lodged between the brake pad and the rotor and scored it pretty good. This may be something the dealer can rememdy just by turning the rotors and possibly sanding the pads if necessary. I'm not real sure if the warranty would cover this if they think road debris may have caused it, but it wouldn't hurt to talk to your dealer just to see.
I would take it in to the dealer. The front pads on my silverado did this as well and it turned out there was a batch of bad pads that came through the factory. The pads actually had pieces of metal in them that were wearing grooves in the rotor. This may be what is going on with your rotor. If it is a bad pad then it will be fixed under warranty.
This can also be caused by the the pads and the retainers not being lubed properly for them to slide when the brakes are engaged. I have done many brake jobs in the past and this is the most common problem I see. the pads get stuck at times and they don't grip the rotor properly causing uneven braking and either grooving the rotor.