In Badlands National Park there was one area with yellow mounds mixed with the typical red/orange mounds. "On top of the Pierre Shale sit the Yellow Mounds, which are just an altered version of the Pierre Shale despite their striking differences in appearance. After the Western Interior Seaway drained North into the Arctic Ocean, the leftover shales weathered into soils. Those soils are now preserved as the Yellow Mounds, which are what geologists call a paleosol. Paleosols are ancient fossilized soils preserved in the rock record, and they often appear as brightly colored layers like the Yellow Mounds, which gets its mustardy color from a mineral called Goethite." ( https://www.nps.gov/articles/000/badl-geologic-formations.htm)
