Moenkopi Formation
The chocolate-red Triassic tidal-flat siltstone below the Chinle — ripple-marked red beds with marine limestone tongues and reptile trackways.
Also known as: Moenkopi
Triassic — recovery, and the dawn of the dinosaurs
The world back then
Early Triassic, about 250 million years ago. The great Permian sea has pulled out, leaving a low, muddy coastal flat — ripple marks, mudcracks, sluggish rivers, and the occasional salty tongue of returning sea. It's flat, hot, chocolate-brown, and not much to look at. The world is recovering from the worst extinction it ever had.
That grim flat is the chocolate slope below the colorful Chinle, and you drift past it doing nothing at all. The planet spent that whole era barely hanging on after the worst extinction ever; you showed up thriving, with snacks. Sometimes the luckiest thing you can be is late.
Recognize it in the field
- Typical color
- deep red-brown / chocolate
- Weathers as
- Slope- and bench-former (soft red beds); the marine limestone tongues and upper members form ledges and small cliffs.
- Grain
- silt to fine sand
Look for
- chocolate-red slope-forming siltstones with ripple marks and mudcracks
- marine limestone tongues (Sinbad, Virgin, Timpoweap)
- scattered gypsum
- reptile and amphibian trackways (Rotodactylus, Chirotherium)
How this rock was made
Low-relief muddy coastal flats draining to a sea on the western Pangaean margin in the Early Triassic.
The setting: low coastal plain, tidal flats, and sluggish rivers, with brief marine incursions
Feather-edge (0) on paleohighs; ~500–700 ft in northern Arizona; to ~1,800–2,000 ft toward SW Utah. Highly regional — quote with locality.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
- San Rafael Swell
- Capitol Reef (Waterpocket Fold)
- Canyonlands / Paradox salt anticlines
- Little Colorado River (type area)
Float past it: Cataract Canyon · Meander Canyon