Tapeats Sandstone
The brown Cambrian sandstone at the base of the Grand Canyon's Paleozoic — the cliff that rests on the Great Unconformity above the basement.
Also known as: Tapeats
Cambrian — the first great burst of sea life
The world back then
Early Cambrian, about 508 million years ago. The first sea in ages is washing across the bare basement — a brown, sandy beach and tidal flat riding the leading edge of a great transgression, waves reworking coarse sand over rock a billion years older. Shallow, restless, and new. The very bottom of the whole Paleozoic stack, just getting started.
That old beach is the brown cliff at the very bottom of the flat layers, and you sit above all of it in a boat. It rests on the Great Unconformity — a billion missing years; you rest on a dry bag. Being the comfortable animal perched atop that much lost time is a strange, enormous piece of luck.
Recognize it in the field
- Typical color
- tan to reddish-brown (weathered)
- Weathers as
- Cliff-former — brown vertical cliffs above the basement; the transitional upper beds grade into the surface of the Tonto Platform.
- Grain
- medium- to coarse-grained, thin-bedded
Look for
- basal Cambrian sandstone on the Great Unconformity
- conglomeratic near the base
- abundant vertical burrows (Skolithos)
- interfingers upward with the Bright Angel Shale
- thickness controlled by paleotopography on the unconformity
How this rock was made
The leading edge of the Cambrian (Sauk) marine rising sea drowning the land (transgression) across an eastward-advancing shoreline.
The setting: shallow-marine / between the tides (intertidal) / coastal-plain (transgressive)
~230 ft typical, to ~300 ft in lows, thin-to-absent over paleohighs.
In the rock record
Part of the Tonto Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Grand Canyon / Colorado Plateau
- Granite Gorge rims / Colorado River corridor
- Tapeats Creek (type area)
Float past it: Grand Canyon