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.
Start here and then my part of the crater is Johannesburg, called the Witwatersrand where the inland sea part that formed those rocks is described
Your crater is interestingly different - the ones I know are clearly holes. This is also near my home