New Minimum Age for Lehman Caves

This article was originally published in The Midden – Great Basin National Park: Vol. 25, No. 2, Winter 2025.

Cave scientist prepares to sample a thick mammillary deposit near the Entrance Tunnel.

Louise Hose

By Louise D. Hose, Harvey R. DuChene, Victor J. Polyak, and Yemane Asmerom, researchers

It is tough to determine the age of nothing. How can we tell the age of a cave? Specifically, how can we know the age of Lehman Caves? A few caves elsewhere have special mineral deposits that formed while the cave was dissolving and those minerals have been dated (Polyak et al., 1998), but we haven’t found these special minerals in Lehman Caves. So, the best we can do is find dates for the things that fill the cave, understanding that the cave had to form before the decorations filled the cave. Until late last summer, the date of the oldest analyzed deposit in the cave, a stalagmite, was 2.2 million years old (Lachniet and Crotty, 2017).

Isochron showing the age of a mammillary deposit in the Entrance Tunnel of Lehman Caves to be about 4 million years old. This represents a minimum age for the cave, which may be much older.

That proven minimum age for Lehman was doubled when Victor Polyak of the University of New Mexico analyzed a piece of mammillary (a calcite deposit formed under water but near the top of the water table as the aquifer dropped lower) and learned that its age is 4.136+0.304 million years old. The analysis was done by comparing the amount of uranium isotopes with a mass of 238-amu to the presence of daughter isotope lead-206 (U-Pb dating).

The date is surprisingly young in the eyes of the geologists who have been studying the cave as strong circumstantial evidence suggests that the cave is more than ten million years old. In the Grand Canyon, Arizona, and Carlsbad Cavern, New Mexico, the ages of similar deposits at higher elevations are older than the samples taken lower. Yet here in the Great Basin we have a mammillary date from a cave lower than Lehman Caves that is 13 million years old (Hose et al., 2024). In the North Snake Range, we also have a non-linear set of dating results. The history of water table fluctuations in Lehman Caves and the Snake Range is clearly more complex than at the Arizona and New Mexico sites.

Stars represent the elevation (in meters) of eight Snake Range caves with dated mammillary deposits. The bar of the same color shows the age with error bars of the U-Pb dates for those deposits.

We now have a much older proven, minimum date for Lehman Caves. But, we believe that the cave is much older than this deposit. If we are correct, Lehman Caves had a complex history involving rising and falling groundwater, possibly more than one episode of cave dissolution, and likely at least two periods of mountain uplift and/or valley floor dropping.

References cited
Hose, LD, Polyak, VJ, DuChene, HR, Powell, JD, Melim, LA, Baker, GM, Davis, DG, Asmerom, Y, 2024, Mid- to Late-Miocene hypogene speleogenesis tied to the tectonic history of the central Basin and Range Province, USA. International Journal of Speleology, 53(2), 129-146. https://doi.org/10.5038/1827-806X.53.2.2507

Lachniet, M, and Crotty, C., 2017, Lehman Caves, Nevada are older than 2.2 million years. https://lachnietblog.wordpress.com/2017/02/01/lehman-caves-nevada-are-older-than-2-2-million-years/

Polyak, V.J., Mcntosh, W.C., Guven, N., and Provencio, P., 1998, Age and origin of Carlsbad Cavern and related caves from ⁴⁰Ar/³⁹Ar of alunite, Science, 279(5358), 1919-1922. https://www.science.org/doi/abs/10.1126/science.279.5358.1919