Surprise Canyon Formation
The discontinuous Late Mississippian fill of ancient rivers and caves cut into the Redwall top — a buried paleodrainage, not a continuous layer.
Also known as: Surprise Canyon
Mississippian — warm, shallow tropical seas
The world back then
Late Mississippian, about 327 million years ago. The sea has dropped and the Redwall is exposed as land — rivers carve valleys into it and caves dissolve out beneath, then the sea creeps back and fills the holes with mud and lime. It only exists where those old valleys and caverns were. A drowned, patched-up landscape, here and gone.
Now it's a scattering of purplish lenses tucked into the Redwall, and you drift past the tidy fossil of a very messy episode. That whole formation is just the fill of ancient holes; you get to read the wreckage instead of live it. Pure luck, wearing a lifejacket.
Recognize it in the field
- Typical color
- purplish-red
- Weathers as
- Discontinuous — isolated lens-shaped bodies, not a continuous band; internally the middle limestone forms a cliff and the upper siltstone a slope.
- Grain
- coarse (basal conglomerate) to fine (upper estuarine)
Look for
- local paleovalley and karst-cavern fill only
- three-part: basal fluvial conglomerate, middle marine limestone, upper estuarine siltstone
- maps a buried dendritic paleodrainage
- occurs even inside Redwall caves
How this rock was made
Infill of valleys and caves eroded into the Redwall during a Late Mississippian sea-level lowstand.
The setting: river (fluvial) → marine → estuarine infill of river valleys and karst caverns
Paleovalleys as much as ~400 ft deep; thickest at Quartermaster Canyon; absent across most of the canyon.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Grand Canyon
- western and central Grand Canyon paleovalleys
- type section near Bat Tower (~River Mile 265)
Float past it: Grand Canyon