The Grand Canyon Supergroup

The Grand Canyon Supergroup is a thick sequence of tilted, mostly sedimentary rocks, with some igneous layers, exposed in places in the eastern Grand Canyon. Laid down in the Meso- and Neoproterozoic eras of the Precambrian, from 1,255 to 729 million years ago (Ma), they formed in fault-bounded rift basins, in rivers and shallow seas far from the active edge of the continent, while the supercontinent Rodinia came together and then broke apart. The rocks are split into the older Unkar Group and the younger Chuar Group, separated by a slight angular unconformity. No single viewpoint shows the whole supergroup.

This page draws on Telling Time at Grand Canyon: 2020 Update, a National Park Service report on the ages of the canyon's rocks.

Rock and time: Grand Canyon has one of the world's most complete geologic records, yet more time is missing along its unconformities than is preserved. From the NPS report.

The canyon's three sets of rocks and its major unconformities — the Great Nonconformity, the Great Angular Unconformity and the Great Unconformity. From the NPS report.

The Unkar Group, 1,255–1,082 Ma

The Unkar Group is made of sediment shed from mountains, in what is now west Texas, raised as Rodinia assembled. Continental collisions far to the south (in today's coordinates) squeezed southwestern North America into northeast-trending folds whose basins flooded with shallow seas; later northwest–southeast compression split the continent along northwest-trending basins that filled with river and floodplain sediment.

A gap of about 100 million years separates the Hakatai Shale from the Shinumo Sandstone. The stacked lava flows of the Cardenas Basalt, fed by dikes and sills of diabase, are the youngest Unkar rocks. They erupted during an early, failed attempt to rift ancient North America apart — the rifting that tilted the Unkar Group.

The Unkar Group exposed near Unkar Rapid. National Park Service photo.

The Chuar Group

The Chuar Group is mudstone interbedded with sandstone and carbonate. It records Rodinia's breakup and the opening of the proto-Pacific Ocean, in shallow inland seaways that reached as far as Death Valley and northern Utah. Its fossils show a rich variety of single-celled life, including the first heterotrophic predators — organisms that ate one another — and stromatolites built by colonies of cyanobacteria. Its chemistry swings wildly as the growing biosphere interacted with the atmosphere and oceans in the run-up to the Snowball Earth glaciations of 717 to 635 million years ago, a time not recorded in the canyon's rocks.

It has three formations:

• Nankoweap Formation (770–775 Ma): sandstones laid down near the shore of an inland seaway.
• Galeros Formation (755–770 Ma): mud and lime deposited as the basin sank, in a seaway that repeatedly dried up and flooded again. Its members are the Tanner (770 Ma), Jupiter (765 Ma), Carbon Canyon (757 Ma, from a rhenium-osmium date of 757 ± 7 Ma on organic-rich carbonate) and Duppa (755 Ma).
• Kwagunt Formation (729–755 Ma): sediments filling rift basins as Rodinia broke up. Its members are the Carbon Butte (753 Ma), Awatubi (dated by rhenium-osmium on marcasite nodules at its base) and Walcott (729 ± 1 Ma from zircons, refining an earlier age of 742 ± 6 Ma).

Because the order of the layers is so clear, most Chuar members are thought to span only 3–5 million years each, and geologists study each member as closely as they would a whole formation.

Revisions in 2017

New research in 2017 reshaped the supergroup:

• New zircon data showed the Nankoweap Formation is no older than 775 Ma, not 900 Ma as once thought, and with its similar history it was moved into the Chuar Group.
• The Sixtymile Formation had been placed in the supergroup because it is folded with the Chuar syncline, and given an age of 650 Ma on the mistaken idea that it formed as sea level dropped during Snowball Earth glaciations. Detrital zircons as young as 530 Ma at its base and 508 Ma at its top moved it to the base of the Cambrian Tonto Group, among the Layered Paleozoic Rocks.

Several formations were also renamed to reflect their makeup: the Bass Limestone is now the Bass Formation, the Shinumo Quartzite the Shinumo Sandstone (it is not metamorphic), and the Dox Sandstone the Dox Formation.

Sources

• National Park Service, Grand Canyon National Park: "Grand Canyon Supergroup," part of the series Telling Time at Grand Canyon, adapted from Karlstrom, K., L. Crossey, A. Mathis and C. Bowman, 2021, Telling time at Grand Canyon National Park: 2020 update, Natural Resource Report NPS/GRCA/NRR—2021/2246.
• The source gives the Awatubi Member's age as both 750 ± 8 Ma and 751 ± 8 Ma, so neither is given here, and lists a "Juniper Member" once for the Jupiter Member. Its photographs credited to Laurie Crossey and its image of the report's cover are not reproduced.