Where Did the Dirt in Lehman Caves Come From?

First published in The Midden, the Great Basin National Park newsletter, Vol. 20, No. 1, Summer 2020, by cave geologists Louise D. Hose and Harvey R. DuChene.

Shale exposed in a short side passage of the Gypsum Annex. Image from the Park Service article.

A puzzle

Lehman Caves' passages and rooms formed through hypogenic processes — groundwater rising from below — and there's no evidence that surface streams ever ran through the cave. Surface water has seeped in through the natural entrance, but no streams. The "scallops" on the walls, once taken as signs of old rivers, are now recognized as pseudo-scallops, made by sulfuric acid condensation in a hypogenic setting.

Yet clastic sediment — dirt, made of broken fragments of older rock and minerals — lies in several places in the cave. Where did it come from? The authors see four possible sources.

1. Washed in from outside

The most obvious: debris washed in from the surface. The material under the natural entrance, in the Gothic Palace and nearby passages, arrived this way — but this source seems limited to the area right below and beside the entrance.

2. Leftovers from the marble

Probably very minor: insoluble bits in the marble bedrock itself. The Pole Canyon Limestone is quite pure limestone and dolostone, so it likely contributes only a trace (table 1, p. 69).

3. Filled cracks — clastic dikes

Shale-siltstone shows in the walls in several places in the Gypsum Annex, the cave's northwestern end, and similar material fills fractures in the ceilings there.

• Clastic dikes form when less dense, plastic rock from below flows up into cracks in denser overlying rock, moving from higher pressure to lower.
• Hose had noticed pods of shale on the surface near the northwest part of the cave, and first thought they were pieces of Pioche Shale torn off and caught up in the Pole Canyon marble during the Snake Range Décollement.
• After looking this spring in Lehman Caves and nearby Little Muddy Cave, the authors now think these outcrops are more likely clastic dikes of the lower-lying Pioche Shale, injected after the marble turned brittle — during or after it fractured.

Could dikes like these explain the unusual amount of dirt on the floor of the Inscription Room? It's out of character for the rest of the cave away from the entrance. The authors suggest a fairly simple study — perhaps an undergraduate senior thesis — comparing the mineralogy of the Inscription Room dirt with the fracture fillings, and mapping the clastic dikes in Lehman and Little Muddy caves to see what they reveal about how the caves formed. Any geology students interested?

4. Carried in by people

Finally, some of the Inscription Room dirt may have been hauled in by people in the distant, unrecorded past. Only more study of that dirt will solve the mystery.

More from this issue of The Midden: evidence of a sulfuric acid origin for Lehman Caves · a brief history of the park's Green Team.

Sources

Based on "Where Did Lehman Caves Dirt Come From?" by Louise D. Hose and Harvey R. DuChene, The Midden — Great Basin National Park, Vol. 20, No. 1, Summer 2020; published by the National Park Service and in the public domain; rewritten in hubnx's own words. A diagram and photograph of clastic dikes from a 2015 conference paper by Weinberger and others are not reproduced.