Your very fine image has got me curious about what is going on in this dynamic area of our Galaxy!
The supergiant shell nebula is likely to be called RCW 113 , also known as Gum 55, but I have to check this. I have reliable Idents for many of these nebulae, hidden somewhere in my tall piles of scientific papers....but I must dig for it!
The cluster itself forms the core of Scorpius OB1, which is an extended association of O and B stars. (= Sco OB1)
N6231 has been reported as having 9 stars of spectral type O, which means that the cluster is the source an enormous amount of ionizing Ultraviolet Light output into the surrounding interstellar medium. Wouldn't surprise me if the cluster itself was the energy source for the supergiant shell.
Johannes Schedler has done an extremely wide field shot of this region, see:
http://panther-observatory.com
(It is necessary to scroll a long way down his page of thumbnails)