The Geology of Zion National Park, Utah

Zion National Park sits in semi-arid desert on the edge of the Colorado Plateau in Iron, Kane and Washington counties, southwestern Utah. Established as a national park on November 19, 1919, it covers about 147,237 acres of plateaus, terraces, benches, monoliths and canyons. Zion Canyon, cut by the Virgin River, has sheer walls rising 2,000 feet above its floor, along with blind arches, alcoves, hanging valleys and waterfalls. Kolob Arch, spanning 287 feet, is the second-largest arch in the United States.

Zion Canyon. Image from the National Park Service’s page.

The NPS Geodiversity Atlas, from the Park Service’s Geologic Resources Inventory, describes the full range of a park’s rocks, fossils, landforms, soils and the processes shaping them. Zion’s geology is a fundamental resource of the park and a primary reason it was protected, and it has been studied for more than 150 years — since John Wesley Powell’s work in 1871 and Clarence Dutton’s Tertiary History of the Grand Canyon District in 1882 — in more than 130 scientific articles.

275 million years in stone

Zion’s colorful rock layers record changing environments over 275 million years, mostly from the Mesozoic era but ranging from the Permian to the Quaternary: the Permian Toroweap and Kaibab formations; Mesozoic units including the Moenkopi, Chinle, Moenave and Kayenta formations, Navajo Sandstone, Temple Cap Sandstone, and the Carmel, Cedar Mountain, Naturita, Tropic Shale and Straight Cliffs formations; and, on top, the Cenozoic Claron Formation and younger lava flows. Sandstone is the most abundant rock, forming most of the benches, terraces and canyon walls. In the park, the sedimentary stack is more than 7,000 feet thick, from the Kaibab limestone up to the Carmel formation.

The Navajo Sandstone

The main cliff-former is the Navajo Sandstone, remnant of the largest sand-dune desert known on Earth, which held 14,000 to 34,000 cubic miles of sand. Near Zion it is thickest, more than 2,000 feet, and its cross-bedding preserves the shapes of Jurassic dunes. Once a uniform deep salmon red, it has been leached in places to pink, yellow and near-white toward the top, and streaked with dark desert varnish — the source of Zion’s famous colors.

Fast-cutting canyons

Zion is eroding at a spectacular rate: its rivers have cut down 1,300 feet in the last million years. The plateau here is rising relative to the Basin and Range province just to the west, and the Colorado River system reached the sea only about 4.5 million years ago, giving rivers the energy to carve deep, narrow slot canyons and keep the cliffs high. In weak, deeply cut rock, this also causes landslides that have dammed canyons.

Many canyons follow joints — straight, evenly spaced cracks in the Navajo Sandstone formed as the rock relaxed from old compression — which is why they run in straight, parallel lines.

Fossils

Zion’s fossils are rare and scientifically important; the 1918 presidential proclamation for the area cited its “fossiliferous deposits of unusual nature.” They range from marine invertebrates in the Permian Kaibab limestone through marine and land deposits of the Mesozoic. Tracks in the Moenkopi Formation are among the oldest Mesozoic vertebrate tracks in North America, and trackways of early dinosaurs are abundant and well preserved. All park fossils are protected by the Paleontological Resources Preservation Act of 2009.

Lava and inverted topography

Twelve times in the last 1.5 million years, basalt lava flowed down the Virgin River and its tributaries, blocking them. Because the hard basalt resists erosion while the soft rock around it wears away, some old lava-filled valleys now stand higher than their surroundings — textbook inverted topography. The Crater Hill lava flow, from a cinder cone in the park’s southwest corner, dammed the Virgin River and Coalpits Wash about 300,000 years ago; parts of six flows from two sources lie in the park.

Soils

Zion’s soils are young and poorly developed. Some swell or collapse, and others are prone to piping, erosion or liquefaction — problems for buildings. Former farm tracts in lower Zion Canyon, disturbed before the park acquired them, are overrun with invasive weeds.

Where two provinces meet

Zion lies where the Colorado Plateau, with its flat, intact layers, meets the Basin and Range, where faults break the rock into steeply tilted blocks. The Hurricane Fault, the clearest boundary between them, runs through the park’s Kolob Canyons district, marked by the Hurricane Cliffs, where the rock is offset by 4,000 to 5,000 feet. The fault has divided the provinces since the late Cretaceous and is still moving.

Sources

Based on "NPS Geodiversity Atlas—Zion National Park, Utah," Geologic Resources Division, National Park Service; a work of the United States government in the public domain. The source misspells Clarence Dutton’s first name.