Biostratigraphic and isotopic age of the Huerfanito bentonite bed of the upper Cretaceous Lewis Shale at an outcrop near Regina, New Mexico
— James E. Fassett, William A. Cobban, and John D. Obradovich

The Huerfanito Bentonite Bed of the Upper Cretaceous Lewis Shale has been a useful stratigraphic marker bed throughout the subsurface of the San Juan Basin in New Mexico and Colorado. This bed was defined strictly as a subsurface unit because it was not known to crop out in the basin. The recent discovery of an exposure of the Huerfanito Bed at a road cut near the town of Regina, New Mexico, has provided the first opportunity to directly observe and sample this important bed. This work confirms the original assumption that the Huerfanito is an altered volcanic ash bed. It is about 9 in. thick at the Regina site, is in the uppermost part of the Baculites scotti Western Interior ammonite zone, and it has a radiometric age of 75.76 ± 0.34 Ma.