This was found in central South Dakota in Sully County where I live on our farmland. Sully Co lies almost center of the state on the east side of the Oahe Dam. Our farm is located 6 miles north of Onida, SD on hwy 83 and due east another 7 miles.
The bedrock in your area should be Cretaceous, but that coral bearing rock sure looks Paleozoic to me. It may perhaps be a glacial erratic – a piece of rock scraped up by an ice sheet during their advance, which is then left behind when the ice melts and retreats.