Geologic Formations

Artist's rendition of rock layers at Timpanogos Cave National Monument, American Fork Canyon

Timpanogos Cave National Monument resides at the boundary between the Rocky Mountains and the Great Basin. These are two of the huge physiographic provinces that make up the American West. Locally, the Wasatch Range is the westernmost expression of the Rocky Mtns, while Utah Valley is the beginning of the Great Basin. Both the Wasatch Mountains and the Great Basin are relatively new landforms, geologically speaking, and are still developing today.

Local sequence of relevant historical geologic events leading up to the speleogenesis (creation) of Timpanogos Cave:

• 340 mya Deposition of Deseret Limestone
• 100 mya Sevier Orogeny and overthrust
• 70 mya Laramide Orogeny
• 17 mya Basin and Range extension – initiation of Wasatch Fault
• 20,000 ya Lake Bonneville

Deposition of Deseret Limestone 340 mya

During the Mississippian Period, Utah was near the equator and mostly covered with a warm, shallow sea. Out in the ocean basin, away from terriginous (land-derived) sediments, conditions were perfect for the deposition of calcium carbonate, the main ingredient of limestone.

In these clear, warm, shallow seas, calcium carbonate accumulates in two major ways. The first is by the accumulation of animal skeletal remains. Many reef and shallow ocean animals like coral, mollusks, crinoids, brachiopods, etc. make their hard parts out of calcium carbonate. When these animals die, wave action breaks apart their skeletons and the pieces accumulate on the ocean floor. In other cases, calcium carbonate can precipitate directly out of seawater, accumulating as a limey ooze on the ocean bottom.

In either case, as these carbonate sediments are covered by more and more on top, they slowly lithify (become rock) through the processes of compaction and cementation. Before eventually being exposed at the surface, the Deseret Limestone was buried under 10 miles of sediments above. This has caused a high degree of compaction and concentration of carbonate in the Deseret.

Modern Significance: The Deseret is a 500 foot thick layer of dolomitic limestone that is host rock for many Utah caves.

Rock Layers along the Cave Trail

The layers you hike past as you climb to the caves are, in sequence from the bottom:

• 600 mya Mutual Quartzite
• 540 mya Tintic Quartzite
• 535 mya Ophir Formation
• 530 mya Maxfield Limestone
• 360 mya Fitchville Formation
• 350 mya Gardison Limestone
• 340 mya Deseret Limestone

 

Geology layers sign found along the cave trail

NPS / A. Shirey

Window Into the Past

Much of the early geologic history of Utah is exposed within the rocks of Timpanogos Cave National Monument. Along the trail you will see seven types of rocks that were deposited in shallow seas 600 million to 340 million years ago. These shallow seas existed at sea level near the equator. After the deposits were turned to rock, they rode on a continental plate to their current location and then were uplifted to their present elevation.

Deseret Limestone - As the sea continued to advance to the east about 340 million years ago, this limestone was deposited at the bottom of an ever deepening sea. Mostly broken pieces of fossils are found here, however solitary horn corals and colonial corals are locally common. This formation is 400-feet thick, is composed mostly of dolomite with some limestone, has numerous nodules and lenses of tan to black chert, and has a unit of shale near the base.

Gardison Limestone - A warm shallow sea was created as the shoreline once again advanced to the east over the continental shelf about 350 million years ago. Numerous fossils are found here, including several types of corals, brachiopods, crinoids, and gastropods. This formation is 360-feet thick, is composed of both dolomite and limestone, is thin-bedded, shows some cross-bedding, and contains some calcite filled vugs and lenses of gray chert.

Fitchville Dolomite - This dolomite was deposited about 360 million years ago in a shallow sea on a continental shelf. This formation also contains fine sand and silt eroded from mountains that once existed to the west. This formation is 330-feet thick, is thin bedded, and contains lots of cross-bedding and fossil hash.

Maxfield Limestone - About 530 million years ago, the sea continued to deepen as the shoreline advanced to the east. This limestone was deposited by algae on a platform in this shallow sea, far from the shoreline. Fossils are uncommon here. This formation is 90-feet thick and is composed of massive-bedded limestone with some siltstone partings.

Ophir Formation - About 535 million years ago, the sea deepened as the shoreline continued advancing to the east. These thin clay and silt deposits were laid down some distance from the shoreline. Inarticulate brachiopods, trilobites, and animal burrows and tracks have been found here. This formation is 180-feet thick and is composed of siltstones and shale.

Tintic Quartzite - About 540 million years ago, sand was deposited along the shoreline of a shallow sea as it started advancing to the east. These sands were continually moved around by wave action, forming the cross-bedding visible in these rocks. Fossils are very rare in these deposits. Much later, these grains of sand were cemented together, forming quartzite. This formation is composed of light pink and white quartzites. Although this formation is nearly 1,200-feet thick, the total thickness is not visible within the Monument due to faulting.

Mutual Formation - About 600 million years ago, sand and clays were deposited in a deep trough of an ancient sea. These deposits were later turned into sandstones and shales. Much later, the sandstones were altered to purple and gray quartzites and the shales were changed to purple and green phyllites and argillites. Only 200 feet of this formation is exposed within the Monument.