I see at least three distinct populations here. First are the Mormons of Utah, south Idaho, and adjacent areas, which you missed.
The big one is the Bible Belt minus the Black Belt, and also seems to be missing the Carolinas, maybe because they've had so much internal migration from the Northeast recently.
El Paso and surrounding area is surely associated more with the Rio Grande Valley of far south Texas than either of these two areas -- a culturally distinct area of Tejanos and more recent Hispanic immigrants.
I don't know what to attribute the area of lower-age married fertility in eastern CO + western KS to, or other scattered areas.