Toroweap Formation
The mixed limestone-sandstone-gypsum Permian unit between the Coconino and Kaibab — a shallow sea over an evaporating coast.
Also known as: Toroweap
Permian — vast deserts, ending in the worst extinction ever
The world back then
Permian, about 279 million years ago. A shallow, evaporating sea washes on and off a low shelf — carbonate one place, salty gypsum flats the next, changing character from west to east. Hot, briny, and shallow, the coast neither fully sea nor fully land. A layer that can't decide what it is, baking under the Permian sun.
Now it's the mixed red-and-gray band between the white Coconino and the gray Kaibab, and you drift past it, cold drink in hand. That old shelf never could decide what it was; your afternoon has no such trouble. Comfortable, unhurried, and 279 million years too late to care.
Recognize it in the field
- Typical color
- red, yellow, and gray
- Weathers as
- Mixed cliff and slope — the Brady Canyon carbonate forms a cliff; the gypsum-rich Woods Ranch forms a recessive slope just below the rim.
Look for
- three members (Seligman, Brady Canyon, Woods Ranch)
- striking west-to-east facies change (carbonate/evaporite → sandstone)
- evaporitic; the members merge and disappear eastward
- semi-arid coastal marine
How this rock was made
Repeated marine rising sea drowning the land (transgression) and regression across a shallow, evaporating Permian shelf.
The setting: shallow evaporating-sea (evaporitic) marine shelf and salty coastal flat (coastal sabkha)
~50 ft (east) to ~525 ft (west, summing members); ~200 ft typical.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Grand Canyon / Colorado Plateau
- Toroweap Valley (type section), western Grand Canyon
- the slope between the Coconino and Kaibab across the plateau
Float past it: Grand Canyon