Can someone help me identify this cloud formation?
Well, after a bit of research, I stumbled upon the following image:



This is from a blog of two storm chasers Roger and Elke who I believe witnessed the same cloud formation as you. In the following link:

http://stormeyes.org/latest/tag/left-moving-supercell/


They refer to the formation as a 'rear flank arc cloud'.

So in conclusion I believe, the formation is a kind of arcus cloud. You can learn more about arcus clouds on wikipedia by following the link: 

https://en.wikipedia.org/wiki/Arcus_cloud


I hope I have been of help.