Tonto Group
The Cambrian base of the Grand Canyon's Paleozoic — Tapeats, Bright Angel, and Muav, resting on the Great Unconformity above the basement.
Also known as: Tonto
Cambrian — the first great burst of sea life
The world back then
Middle Cambrian, around 503 million years ago. The great sea drowns the bare, ancient continent — first a sandy beach, then offshore mud, then open-water lime, the shoreline marching east for ages. Shallow, warm, and rising, trilobites the height of technology. A world that is basically all coastline, deepening by the century.
That drowning coast is the cliff-bench-cliff of the Tonto Group, and you read the whole rising sea in a week. It took the continent a half-billion years to sink; you take the canyon between two Fridays. The warm creature with the good seat wins again, at the far end of it all.
Recognize it in the field
- Typical color
- brown (Tapeats), green-and-red (Bright Angel), gray (Muav)
- Weathers as
- A cliff–bench–cliff sequence: the Tapeats cliff, the Bright Angel slope (the Tonto Platform), and the Muav cliff.
Look for
- the Cambrian base of the Grand Canyon's Paleozoic
- rests on the Great Unconformity
- the Bright Angel forms the Tonto Platform terrace
- records the Sauk marine transgression
How this rock was made
The great Cambrian marine rising sea drowning the land (transgression) burying the eroded Precambrian basement, its shoreline advancing eastward.
The setting: shallow-marine rising-sea (transgressive) sequence (Sauk transgression)
~1,250 ft total (stated). A composite of its member formations.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Formations in this group
Where to see it
Regions: Grand Canyon / Colorado Plateau
- the inner Grand Canyon above the Granite Gorge
- Blacktail Canyon (reference locality)
Float past it: Grand Canyon