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Adapted from an article by Louise Hose, cave specialist and geologist for the Great Basin Institute, in The Midden*, Great Basin National Park's newsletter, Summer 2023.*
Every known cave in Great Basin National Park formed in rock made of the minerals calcite (CaCO₃), dolomite (MgCa(CO₃)₂) or both. But they developed in several different rock units, by three different mechanisms, in two different geologic settings.
The rocks
| Rock unit | Age | What it is | Caves |
|---|---|---|---|
| Pole Canyon Limestone (Middle Cambrian) | about 515 million years | Mostly calcite with little magnesium, laid down in a shallow sea along the western edge of the early North American continent | The oldest and most prolific cave-bearing rock in the Snake Range |
| Pole Canyon Marble | formed about 17 million years ago | Pole Canyon Limestone transformed by intense shearing, and probably heat, along a gigantic fault; only its chemistry survives, mostly calcite with some dolomite bands | Caves of the Baker Creek and Lehman drainages |
| Notch Peak Limestone (Upper Cambrian) | perhaps 500 to 485 million years | Very fine- to fine-grained limestone with chert nodules and stringers, also from a shallow sea | Two caves in the Snake Creek drainage and two high-elevation caves |
| Fish Haven and Laketown Dolomites (Ordovician–Silurian) | about 440 million years | Ledge- and cliff-forming dolomites full of fossils, including branching corals, from a lagoon or shallow shoal | One small cave and spring in the Snake Creek drainage |
| Sevy Dolomite (Devonian) | about 400 million years | Thin- to medium-bedded, very fine-grained, fossil-free laminated dolomite from extensive mud flats | Possibly one small cave in the South Fork of Big Wash |
| Guilmette Formation (Middle and Upper Devonian) | — | Dark gray limestone with some brown dolomite and fossils of corals, gastropods, brachiopods, crinoids and stromatoporoids, from a shallow-sea shelf | At least one significant cave just outside the park; one researcher placed the Big Wash cave here instead |
The small Big Wash cave sits at a junction of three rock units, so which one it lies in is uncertain.
Three ways to make a cave
Many caves in the park, perhaps most, were shaped by more than one of these processes.
Hypogenesis: acid from below. Most caves in limestone and dolomite form by dissolution, as acidic groundwater dissolves the rock and leaves voids. Hypogenic caves form where groundwater rising from depth carries carbon dioxide or hydrogen sulfide, which make carbonic acid (as in soda) or sulfuric acid (as in battery acid) near the water table, hollowing out the rock. Lehman Caves, the park's best-known hypogenic cave, probably formed from both acids rising from deep below.
Epigenesis: acid from above. The textbook explanation of cave formation: rain and snowmelt pick up carbon dioxide from the air and soil, forming carbonic acid that dissolves the rock. The Baker Creek cave system formed, and is still forming, mostly this way.
Tectonic movement. When fissures open in bedrock, a massive block that shifts can leave a large opening behind it. Some high-elevation caves in the park are deep, narrow cracks formed this way.
The Snake Range décollement
About 17 million years ago, a huge, low-angle fault, the Snake Range décollement, stripped away about 5,000 vertical meters of rock from the park area and moved much of it east to today's Confusion Range. That event set up the park's two cave settings:
- Above the fault, the rocks are extremely fractured and contorted; they appear at the park's higher elevations, and at least half a dozen caves have formed in them.
- Just below the fault, shearing turned limestone into the marble that holds Lehman Caves and the other caves of the Lehman and Baker Creek drainages. Many smaller caves formed in the less disturbed rocks farther below.
Sources
- Louise Hose, Great Basin Institute, "Geologic Fundamentals of Great Basin National Park Caves," The Midden, Vol. 23, No. 1 (Summer 2023), National Park Service. https://www.nps.gov/articles/000/grba-geologic-fundamentals.htm
- The source says the caves developed in "five different rock units" but describes six, so no count is given here; it refers to the cave as both "Lehman Caves" and "Lehman Caverns"; and it spells stromatoporoids "stomatoporoids." Photographs credited to an individual photographer and to the Great Basin Institute are not reproduced.
Dans ces publicationsGreat Basin National ParkThe Midden - Great Basin National Park: Vol. 23, No. 1, Summer 2023
Licence : CC0 1.0 (domaine public) · Adapté de www.nps.gov
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