Iowa huskerhunter, that's a really nice Archaic butcher knife you found there! My son found one very similar, it's not just the type but even more the material that yours is made of that interests me. The one my son found is medium-dark brown with creamy white inclusions like yours, not quite as dark brown as yours probably, however the particular lighting when a picture is taken can effect the apparent color a lot.
If you know much about the Nehawka chert you mentioned I'd like to put pictures of my sons find on here to see if you think it's made of the same stuff. So far most collectors and a local archeologist have opined that all of the "brown" on my sons find is creek patina.
Also I just got permission to hunt a bunch of fields in the Washington, Iowa area, a bit east of you but some good hunting I'm guessing.
If you'd give me your opinion just me know and I'll post a couple of pics. Happy hunting!