Thank you VERY much. OOPArts (out of place artifacts) is a very popular subject among many. I would offer one caution, though, and that is that we must ask that the artifacts be photographs 'in situ' and verified as being part of that formation or place. There have been too many fakes. This particular website, however, seems to have some pretty good photographs. I have not looked at them all, but definitely the oysters are not faked!
However, they are probably also not from Noah's Flood! Yes, they were buried catastrophically, but it would have been after the Flood and before the mountain ranges like the Andes were shoved up by the crustal plate movement we find mentioned in Genesis 10:25. The catastrophe that came between those two was at the time of Babel. It is very possible that the geologic activity following the Flood could have buried these giant oysters. But my suspician would be that the oysters were buried during the geologic activity following the asteroid hit and rampant volcanism that marked the Babel catastrophe. This would have put them in a position not only to be raised by the uplift that occurred later, but also in a position to be uncovered by that uplift. If they had been buried by too many sediments (they are on the continental edge where the majority of geologic activity took place), then they would not be on the surface now.