Lehman Caves Geology: Marble, Not Limestone, and a Cap of Quartzite

First published in The Midden*, Great Basin National Park's newsletter, Vol. 19, No. 2, Winter 2019. By Louise Hose, University of Nevada, Reno.*

Much is being learned about the geology of Lehman Caves in Great Basin National Park. Geologists and students have long studied the Snake Range, but not the ground right around the cave — perhaps partly because good topographic maps of this part of the range did not exist until late in the 20th century. Into the 1970s and '80s, researchers mapped where the base maps were better, and by the time decent maps of the land over the cave appeared, attention had moved to the region's remarkable Snake Range Décollement and to areas with economic minerals.

Marble, not limestone

Park literature and interpretive material have always described the cave and its hillside as a simple block of steeply tilted beds of Pole Canyon Limestone, and the professional geologic literature agrees. So it was a surprise to find that the cave formed entirely in high-grade marble.

• Marble is limestone — a sedimentary rock — altered by heat and pressure; here probably the Pole Canyon Limestone.
• Its layering is not bedding, the original horizontal layers laid down as sediment. It is foliation, which records the direction of the stresses the rock has been through. In the cave, the layers pinch, swell and swirl.

U.S. Geological Survey geologists recognised the marble as early as 1960, but the finding seems to have been lost.

Why some passages look so different

Parts of the cave are strikingly unlike the tour route:

• The Talus Room and Royal Gorge have few secondary calcite deposits — stalactites, stalagmites, flowstone, shields. The Talus Room floor is a deep pile of broken rock rather than flowstone, and bedrock shows on the walls and ceiling; on the tour route bedrock is almost impossible to see except in tunnels and where trails were blasted.
• The Gypsum Annex, in the northwest, has walls, ceilings and floors coated with gypsum.

Gypsum dissolves easily. Had it formed before the calcite speleothems — the general name for secondary mineral deposits in caves — it would have been dissolved away when much of the cave's calcite was deposited during the Pleistocene. That suggested a nearly impermeable layer of rock above the Gypsum Annex that kept surface water out — and probably above the Talus Room and Royal Gorge too. Something other than marble had to lie over much of the cave.

What mapping found

Mapping the hillside in summer 2019 confirmed it:

• Quartzite — metamorphosed quartz sandstone, probably Prospect Mountain Quartzite — is at the surface over most of the Royal Gorge, Talus Room and Gypsum Annex. A large outcrop sits just west of the Picnic Area road.
• Shale, probably Pioche Shale, lies on the surface west of the Lehman Caves Visitor Center. It does not seem to be directly over the cave, but the last 100 feet or so of the Gypsum Annex's southwest passage end where shale forms the walls.

A more complex hill

The hill is a jumble of highly fractured and faulted marble, shale and quartzite — far more complex than long understood. The limestone seems to have been metamorphosed by extensive faulting, probably tied to the Snake Range Décollement, since the cave lies in mylonitic marble — marble metamorphosed along a fault zone. The scattered blocks of quartzite and shale may also be fragments torn off and moved during that great tectonic event. Detailed mapping will continue.

Sources

• National Park Service, Great Basin National Park, "Lehman Caves Geology Reveals New Discoveries," The Midden, Vol. 19, No. 2, Winter 2019. https://www.nps.gov/articles/000/lehman-caves-geology-reveals-new-discoveries.htm
• The article cites Moore (1960), Summary report on the geology of Lehman Caves National Monument, Nevada; NPS (2015), "The formation of Lehman Caves"; and Rowley and others (2017), Nevada Bureau of Mines and Geology Report 56.
• The article's photographs (by Dave Bunnell) and the author's sketch map of the hill are not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.