Mancos Shale
The thick gray marine shale of the Cretaceous Western Interior Seaway — the badland 'blue desert' that floors canyon country's broad valleys.
Also known as: Mancos
Cretaceous — the last of the dinosaurs, ended by an asteroid
The world back then
Late Cretaceous, about 90 million years ago. You're way offshore now, in a deep, muddy inland sea that split the continent in two — cold, dark, quiet water, fine mud raining down for millions of years onto a lightless bottom. No shore in sight. Just gray water above and gray ooze below, thicker and thicker.
That lightless ooze is the gray badland your valley is floored with, and you drift over it in the sun, breathing. The sea was a thousand feet deep and black as ink; you get the bright, airy version. It's worth remembering, at the take-out, how rarely a living thing draws that seat.
Recognize it in the field
- Typical color
- dark gray to lead-gray / black
- Weathers as
- Recessive badland / smooth-slope former ('blue desert' badlands); interbedded sandstone members (Ferron, Muley Canyon) form cliffs.
- Grain
- clay to silt; often calcareous
Look for
- recessive gray marine badlands with a popcorn/bentonitic surface
- intertongues with deltaic sandstone members (Ferron, Emery)
- fossiliferous concretionary limestone beds
- thickens dramatically westward toward the seaway axis
How this rock was made
Offshore muds of the Cretaceous Western Interior Seaway during rising sea drowning the land (transgression), with river-delta (deltaic) sandstone tongues prograding from the Sevier orogen to the west.
The setting: offshore / open-marine (Western Interior Seaway)
~1,200 ft (type, near Mancos CO); ~2,000 ft (San Juan Basin); to 4,000+ ft westward toward the seaway axis. Quote with locality.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
- Capitol Reef (Blue Gate badlands / 'Blue Desert')
- San Rafael Swell east flank
- Book Cliffs
- the Cisco / Ruby–Horsethief corridor (broad gray valley)
Float past it: Cisco to Dewey Bridge · Ruby–Horsethief Canyon