Grand Canyon's Layered Paleozoic Rocks: Ages From the Kaibab to the Tonto Group

Adapted from the Park Service's series on the report Telling Time at Grand Canyon: 2020 Update.

The Supai Group's red sandstones and siltstones along the Hermit Trail. NPS photo by Kristen M. Caldon.

The layers everyone pictures

The Layered Paleozoic Rocks are the horizontal sedimentary layers that form the upper part of the Grand Canyon's walls — the rock sequence most people picture when they think of the canyon's geology. They were laid down on a low coastal plain and in shallow seas on the continental shelf of the ancient proto-Pacific coast, over roughly 530 to 270 million years ago, in the Paleozoic Era.

What each layer records

Each layer is a snapshot of an ancient landscape, compared here with a modern one:

• Sixtymile Formation, Tapeats Sandstone — sandy beaches and river bottoms, like today's Atlantic sandy coasts.
• Bright Angel Formation — a muddy sea floor, like the Gulf of Mexico.
• Muav Formation, Frenchman Mountain Dolostone — a tropical sea, like the Bahama Banks.
• Temple Butte Formation — a shallow sea in the west and tidal channels in the east.
• Redwall Limestone — a tropical sea, like the Java Sea; it forms the distinctive red cliffs partway down the canyon.
• Surprise Canyon Formation — a limestone plateau, like the Yucatán Peninsula.
• Watahomigi Formation — coastal lowlands and shallow sea, like the U.S. Gulf Coast.
• Manakacha, Wescogame, Esplanade — an arid coast with dune fields, like Namibia.
• Hermit Formation — rivers and swamps in a dry land, like the Nile Delta.
• Coconino Sandstone — desert dune fields along a coast, like the Arabian Desert.
• Toroweap and Kaibab — near-shore and shallow sea, like the Persian Gulf.

Tapeats Sandstone ledges above Salt Creek, from the Tonto Trail. NPS photo by Michael Quinn.

The Redwall Limestone's red cliffs. NPS photo by Kristen M. Caldon.

A new Tonto Group

The Tonto Group now has five formations, the first big change to the canyon's Paleozoic names since Edwin McKee — USGS geologist and former park naturalist — defined the Supai Group, and geologists G. H. Billingsley and Stanley Beus identified the Surprise Canyon Formation:

• the Sixtymile Formation was added at the base, once it was shown to be Cambrian (not Proterozoic) and laid down in the same way;
• the Bright Angel Shale became the Bright Angel Formation, and the Muav Limestone the Muav Formation, because each contains several rock types;
• the old "undifferentiated dolomites" became the Frenchman Mountain Dolostone, placed at the top of the group because it continues the Muav's marine setting; its fossils are more than 497 million years old.

The youngest zircon grains in the upper Sixtymile are less than 508 million years old, so the whole Tonto Group above it must be younger than that — younger than once thought. The sea that laid it down spread from Nevada across Arizona into eastern Colorado and on toward the mid-continent in only about 10 million years — remarkably fast.

Dating the upper layers

Numeric ages are hard here: there are no volcanic beds to date directly, and zircon grains don't include enough young ones to pin down deposition. But the rock types, setting and above all index fossils constrain the ages well, calibrated against the 2020 International Chronostratigraphic Chart; better-fossiled units have tighter ages.

• The Hermit, Coconino, Toroweap and Kaibab were laid down almost continuously in the Permian; they're told apart by the rock each environment made.
• The Coconino Sandstone's only fossils are tracks — of invertebrates and of four-limbed animals that walked the dunes before the dinosaurs — and the tetrapod tracks point to the Early Permian, about 280 million years ago.
• The Kaibab forms the rim and is the youngest Paleozoic rock in the canyon, dated to the early Middle Permian by microfossils and invertebrates.

Future zircon dates across key fossil transitions may refine these ages.

The Coconino Sandstone along the Bright Angel Trail. NPS photo by Kristen M. Caldon.

More: K. Karlstrom, L. Crossey, A. Mathis and C. Bowman, Telling Time at Grand Canyon National Park: 2020 Update, Natural Resource Report NPS/GRCA/NRR—2021/2246; the park's Geodiversity Atlas page; and the NPS geologic time scale.

The report the series is adapted from. NPS.

Sources

Based on "Layered Paleozoic Rocks," Telling Time at Grand Canyon National Park series, National Park Service, adapted from the report by Karl Karlstrom and Laurie Crossey (University of New Mexico), Allyson Mathis and Carl Bowman; rewritten in hubnx's own words. The report gives the Redwall Limestone a best age of 340 million years outside its own range of 335–338, and dates the Supai Group differently in a caption than in its table; this page gives no Redwall age and follows the table. A photograph by Laurie Crossey and a published trackway figure are not reproduced.