Hermosa Group
The Pennsylvanian marine-and-evaporite package beneath canyon country — Honaker Trail limestone over Paradox salt over Pinkerton Trail limestone, walling lower Cataract Canyon.
Also known as: Hermosa
Pennsylvanian — the great coal swamps
The world back then
Pennsylvanian, about 308 million years ago. You're in a strange basin beside a rising mountain range — a shallow sea that keeps getting cut off, evaporating down to brine and salt, then flooding back to drop black mud and lime. It cycles like that, over and over: sea, salt, sea. Hot, restricted, and unstable, the whole basin breathing salt.
That restless salt basin is the deep stuff at the bottom of Cataract country, and the buried layer is still working while you just float. It spent millions of years drying to a crust and cracking open; you spend an afternoon riding over it. The rock does the labor; you do the sightseeing.
Recognize it in the field
- Typical color
- gray to black (marine); white-to-gray (evaporite)
- Weathers as
- Cliff-and-slope 'canyon country' — resistant limestones (Honaker Trail, Pinkerton Trail) form ledges and cliffs, while the Paradox evaporite is recessive and drives salt tectonics.
Look for
- three-part vertical order: carbonate/clastic over evaporite over carbonate
- Pennsylvanian cyclothems
- grades from an evaporite basin center to a carbonate-only shelf margin (the undivided Hermosa Formation)
- deposited flanking the Ancestral Rockies (Uncompahgre uplift)
- a major oil-and-gas-producing interval
How this rock was made
The Paradox Basin — a fault-bounded foreland basin beside the Uncompahgre highland of the Ancestral Rocky Mountains.
The setting: cyclic marine shelf and restricted evaporating-sea deposit (evaporite) basin
~400–2,800 ft on/near the Navajo Reservation, up to ~3,200 ft to the NW; type-area Hermosa ~1,800–2,150 ft. The Paradox member alone reaches thousands of feet (salt flowage).
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Formations in this group
Where to see it
Regions: Colorado Plateau (Paradox Basin)
- Hermosa Creek / Animas Valley near Durango (type area)
- walls of Cataract Canyon (Honaker Trail + Paradox portions)
Float past it: Cataract Canyon