Sulfuric acid speleogenesis at the new section level of Carlsbad Cavern roughly 6 Ma
— Victor Polyak, Paula Provencio, and Bill McIntosh

Alunite is a byproduct of the sulfuric acid speleogenesis of Carlsbad Cavern and other caves in the Guadalupe Mountains. It forms by the sulfuric acid alteration of clays of the illite and smectite groups hosted in the bedrock. Therefore, dating the alunite using 40Ar/39Ar determines the timing of speleogenesis and the position of the water table where, by our interpretation, speleogenesis largely takes place. In the Carlsbad Cavern–Lechuguilla Cave region, three “alunite levels” are known at 1230 m (6 Ma), 1180 m (5 Ma), and 1120 m (4 Ma). Even though Carlsbad Cavern has a large vertical extent, until now, only one “alunite level” has been documented (the Big Room level; 4 Ma). Here we report an age of natroalunite from the New Section of 5.7±0.1 Ma at 1210 m, which is largely consistent with the 6 Ma level. With a relatively flat water table setting back to 6 Ma within the Capitan aquifer and mass wasting rates for the Guadalupe Mountains, we interpret an incision rate of the Pecos River between 48 and 18 m/Ma in this region east of the mountain range, given that the river existed that far back.