One other comment; the fracturing and angularity may be due to post-exposure weathering. I.e., the concretions all start out fairly rounded and spherical, but as they are exposed and dry out or simply crack, you'll get more angular grains.
If you suppose that concretions are "rounded" then when you break a concretion you will get angular grains. So far so good. But we also have sub-angular and sub-rounded grains. It is hard to get those in the absence of erosion. In fact, it appears we have the entire continuum from very angular to well rounded.
My field experience is somewhat limited to the American Southwest, particularly the deserts and mountains of California, Nevada, and Arizona. So my observations are made with those parochial biases. (I have very little experience with carbonates, for instance.) However, I think some of the "desert pavements" I have seen bear a striking resemblance to Martian pictures.