The Geology of Oregon Caves: A Marble Cave, Ancient Seafloor and Ice Age Fossils

The park

Oregon Caves National Monument and Preserve lies in the northern Klamath Mountains of Josephine County, southwestern Oregon, about 10 km (6 mi) north of the California border. Proclaimed on July 12, 1909, it was run by the U.S. Forest Service until the Park Service took it over on August 10, 1933. Its roughly 1,843 hectares (4,554 acres) protect remnant old-growth Douglas fir, many glacial features, and the longest marble solution cave in the Siskiyou Mountains.

Despite the plural name, Oregon Caves is one cave, with 4.8 km (3 mi) of connected passages. Its entrance is on Mount Elijah, named for Elijah Davidson, a 24-year-old hunter who stumbled on the cave in 1874 — its first recorded discovery.

A mosaic of rocks from elsewhere

The cave formed over millions of years as acidic water slowly dissolved marble, inside an unusually varied assembly of rocks that were carried here by plate tectonics:

• Rattlesnake Creek terrane (Triassic–Jurassic, 207–193 million years old) underlies most of the monument. The Klamath Mountains, on the edge of the North American plate, have repeatedly collected such terranes — fragments of other plates — which have gradually built the continent westward.
• Josephine ophiolite (Jurassic, 157 million years old), a slab of seafloor, underlies the Cave Junction part of the monument. It is one of the largest and most complete ophiolites anywhere, famous for sulfide deposits yielding gold, silver, copper, zinc and cobalt.
• Grayback pluton (Jurassic, 160 million years old): molten rock that intruded the terrane, welding the pieces together. Its dikes cut repeatedly through the cave's marble block.

Together they include complete ophiolite sequences, broken formations (layers disrupted by faults), and mélange (jumbled fragments), with rocks from the upper mantle, submarine lavas, land sediments, plutons — and marble.

The marble belongs to the Applegate Group within the Rattlesnake Creek terrane: gray to white, mostly bedded, made of calcite crystals. It began as limestone on a seamount and was turned to marble by the terrane's accretion (about 170 million years ago) and by igneous intrusion (about 160 million years ago). Argillite and metabasalt surround most of it.

Overview of the digital geologic data for Oregon Caves, from the Geologic Resources Inventory. NPS figure.

The caves

The monument's main feature is a solution cave in marble, fed by several water inputs that join like a stream, and rich in speleothems formed mostly by dripping water. Protecting, restoring and interpreting it is the monument's first priority.

• 12 known caves, including Oregon Caves, the largest known in the Siskiyou Mountains.
• About 10 endemic cave species — a fly, a beetle, a millipede, a grylloblattid, a springtail and a water mite.
• Ten species of bats use the monument.
• All Park Service caves are protected by the Federal Cave Resources Protection Act of 1988.

Mining country

Mining in southern Oregon began in 1850 with the discovery of gold in what is now Josephine County. The region's metals — gold, silver, copper, nickel and chromium — come mainly from mineralization during ancient seafloor spreading, and also from ores formed as plutons intruded. Marble has also been quarried as building stone.

Fossils in the cave

The bedrock holds few fossils, but the cave sediments — from under 10,000 to at least 120,000 years old, and possibly 1.5 million — hold an impressive collection. More than 50 fossil localities have been documented, with more likely.

• 1995: Park Service paleontologist Greg McDonald identified a jaguar, a black bear and a grizzly bear. The grizzly's limb bone was dated to more than 50,000 years — at the time the oldest known grizzly remains in North America. The 38,600-year-old jaguar is the most complete North American jaguar outside Appalachia and the northernmost ever found.
• Trace fossils: a well-preserved claw print, likely a bear's or a cat's, with five claw points pressed into mud, and at least 20 distinct claw scratch marks.
• 1997–2000: Northern Arizona University researchers sieved about 270 kg (600 lbs.) of sediment. Salamanders — mostly lungless plethodontids — were the most common fossils, alongside lizards, a rubber boa, shrews, moles, mice, a packrat, mountain beaver, pocket gophers, voles, rabbits, bear, skunk, deer, bats, snails, birds and a rare frog or toad.

All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

A rock outcrop at Oregon Caves. Image from the NPS's page.

Learn more

Oregon Caves belongs to the Pacific Border physiographic province. The Park Service's Geologic Resources Inventory has produced geologic maps, a report and scoping summaries for the park (NPS DataStore), and a Soil Resources Inventory is complete. See also the park's cave and karst systems.

Sources

Based on "NPS Geodiversity Atlas—Oregon Caves National Monument & Preserve, Oregon," National Park Service, drawing on KellerLynn (2011), Santucci and others (2001, 2009), Mead and others (2000) and other works it cites; a work of the United States government in the public domain. The source dates the Rattlesnake Creek terrane both 207–193 million years old and "Late Triassic (210 million years ago)"; this page gives the first. Its "grylloblatid" is corrected to grylloblattid.