February 16. Cambrian rocks of the Grand Canyon

White line marks Great Unconformity,
with Tapeats Sandstone above.

The Grand Canyon of the Colorado River is a geologist’s dream. The rocks scream out their relationships, and as you descend into the canyon, the rocks are older and older. In the inner gorge, the dark-colored rocks are Precambrian in age. They are metamorphic rocks, altered during their long lives by heat and pressure. And the top of the Precambrian rocks is a surface called an unconformity. That means a break in the rock record – a gap in time when sediments were not laid down, or they were eroded away, or sometimes a combination of both. The unconformity in the Grand Canyon is called an angular unconformity, because the layers below it are at an angle to the layers above it – a clear violation of the rule of original horizontality that we talked about a few days ago. Not only were the lower rocks cooked and changed, they were tilted – all before they were eroded off to create that unconformity surface.

The Great Unconformity in the Grand Canyon is part of a nearly continent-wide break. The amount of time it represents varies, even within the Grand Canyon area, from as little as 175 million years to possibly as much as a billion years or more, depending on the age of the rocks beneath the erosion surface.

In the Tapeats Sandstone

But it’s February, and we’re in the Cambrian now. Let’s talk about the rocks above the unconformity – the Cambrian strata. There are three distinct packages of rocks, called formations, in the Cambrian of the Grand Canyon. The lowest, the oldest, is called the Tapeats sandstone. When you look into the Canyon, if you can see the inner gorge, the Tapeats is the relatively thin, resistant lip on the rim of the gorge. It’s probably around 525 million years old, which puts it in the Middle Cambrian, and it averages something like 200 feet thick, pretty thin for the Grand Canyon.

Above, and younger than the Tapeats we find the Bright Angel Shale. Shale is a fine-grained rock that solidified from mud, and it often has really thin beds, sometimes microscopic. All of that adds up to a rock unit that may be a lot less resistant to erosion than something like sandstone, and that’s the case in the Grand Canyon. Consequently, the top of the Tapeats Sandstone is marked by a wide, flattish expanse called the Tonto Platform. It’s the place where the Bright Angel Shale would have been but it’s been eroded away – at least eroded back, pretty far from the rim of the inner gorge. When it’s still present, it tends to form slopes rather than cliffs because it’s more easily eroded. The Bright Angel is reddish and greenish in color because of variable iron content, and it contributes to the beautiful colors deep in the canyon. It’s around 500 feet thick, which gives plenty of room for lots of erosional variety and interesting landforms.

The upper, youngest part of the Cambrian in the Grand Canyon is the Muav Formation. It’s a multi-colored limestone interbedded with mudstone and some other rocks. It’s as much as 600 feet thick, and it’s a resistant cliff-former, making some of the first steep cliffs above the inner gorge and the Tapeats Sandstone.

Traditionally, geologists interpreted a change in rock type from sandstone that might have been deposited on a beach, to shale, which would be the finer sediment carried out into deeper water, to limestone, which could form in very deep water – all that would have been seen as evidence of the Cambrian Transgression that we talked about on February 5, with the seas encroaching and getting deeper and deeper across North America. That’s generally the way it worked, but it’s also possible for things like limestone to form in fairly shallow water – think of the calcareous white sand beaches on the west coast of Florida – so don’t look at it as entirely smooth and continuous. Stuff happened.

Geologists name rock formations, like they name periods of geologic time, to make it easier to refer to them, but it’s not arbitrary – there are distinct characteristics in each formation that make each one relatively easy to identify. Names come from a lot of sources, but all the Cambrian formations, Tapeats, Bright Angel, and Muav, were named for creeks and canyons in the Grand Canyon area.

—Richard I. Gibson