Fascinating. It's the decayed concrete stone! I've come across some of that before but if context didn't give me some clues I was never quite sure if it was stone or decayed concrete. It usually had a few shells mixed in though, interesting to see one without that.

Do you know the name of that impact event? It sounds like a good candidate for a wikipedia rabbit hole. Saw this impact crater a while back and have been slightly obsessed since.