Grand Canyon Stratigraphy
(Rocks of the Grand Canyon)
You'll see photos of each layer in the Grand Canyon posts, where they are labeled and further described.
To me, the most fascinating thing about sedimentary rocks is the paleogeography they reveal -- that is, they tell us what the geography of this place was when they were deposited long ago. Here is a quick run-down of the Grand Canyon stratigraphy depositional environments, starting at the top of the Canyon.
All of the rocks in the Grand Canyon are older than the dinosaurs. To find dino fossils, you have to go northward into the younger strata in Utah and Colorado.
The Kaibab Formation is a white cliff-forming limestone at the rims of the canyon. It was deposited about 270 Ma (Permian) in a shallow sea that deepened toward the west. It contains chert nodules, which are silica deposits that stick out of the rock because they are more resistant to erosion. The Kaibab contains fossils near the top of the Bright Angel trail.
The Toroweap Formation is composed of thin layers of gray to red sandstone and limestone that form thin cliffs and slopes between the thick Kaibab cliffs above and Coconino cliffs below. The limestone weathers dark gray. It is about 273 million years old (Permian), and formed in fluctuating shallow marine, intertidal, and coastal sand dune environments.
The Coconino Sandstone formed in coastal sand dunes about 275 million years ago (Permian). It forms a prominent nearly white cliff around the canyon. Along the Bright Angel trail, you can see the cross-bedding inside the ancient dunes.
The Hermit Formation is a thin layer of red shale and sandstone that forms the slope below the Coconino cliffs. These rocks were deposited in a coastal environment about 280 Ma (late Pennsylvanian to early Permian).
The Supai Group is a thick interval of mostly reddish sandstone and siltstone deposited along the coast while sea level fluctuated repeatedly. That fluctuation is seen today as the alternating cliffs and slopes -- cliffs are sandstones deposited in higher energy environments like coastlines, and slopes are siltstones deposited in lower energy places like lagoons and tidal flats. The source of these sediments was iron-rich rocks like schist or basalt that provided the bright rusty reds that characterize the Grand Canyon. Age is Pennsylvanian, about 315 Ma.
The Redwall Limestone forms the most prominent and continuous thick cliff around the canyon. It was deposited in a shallow ocean about 340 Ma (Mississippian). It is stained red on the outside by water running down from the red Supai Group above -- the rock is actually gray inside! Like many limestones, the Redwall contains many caves eroded by dissolving of the limestone (which contains calcium carbonate, like antacids!).
The Muav Limestone forms thin- to medium-thickness cliffs with thin breaks between them. It is gray or brown on the inside, but stained reddish from the rocks above. It was deposited in a shallow marine and intertidal environment about 505 Ma (Cambrian). Notice the huge time interval between the Muav and the Redwall -- this boundary is called an unconformity, and in a few places it has shallow channels eroded into it and filled with the Temple Butte Limestone.
The Bright Angel Shale forms the broad terrace in the bottom of the canyon. It was deposited about 515 Ma (Cambrian) in a calm shallow marine environment. It forms slopes because it is so easily eroded, but you can see a few outcrops next to the Bright Angel trail in gullies. Take a close look there -- it's quite a fascinating rock!
The Tapeats Sandstone marks the lowest of the Cambrian period about 525 Ma, which is significant because it coincides with the blossoming of Life around the world and flooding of the continent as the sea progressed from west to east. It is the brown sandstone cliffs above the river and at the end of the Bright Angel Trail, and was deposited by rivers at the coastline. Look for coarse-grained intervals that were deposited by faster-moving water.
The GREAT UNCONFORMITY is a boundary across which about 1.2 Billion years of Earth's history is missing from the rock record (where the Tapeats overlies the Vishnu and Zoaraster at the end of the Bright Angel Trail). Think of it! Nearly 1/4 of all of Geologic Time is gone across this inch-thick boundary! This unconformity is one of the most profound things that Geologists ever see. It causes us to ponder Eternity. It makes us contemplate our puny position in the Universe. It makes us feel our mortgages and car payments are tiny in The Scope Of Things. It is an angular unconformity, meaning that the 12,000 feet (3.65 km) thick sedimentary rocks AND the tens of thousands of feet of metamorphic and igneous rocks below it were uplifted, faulted, tilted, and eroded flat before the Tapeats was deposited. Ponder the time that took!
The Grand Canyon Supergroup is the mostly bright reddish, orange, purplish, and brown layered sedimentary rock layers near the river in the eastern part of the canyon. They are the only tilted layers in the canyon, and so can be easy to spot. Just the rocks we can see in the canyon are at least 12,000 feet (3.65 km) thick, and tectonic activity tilted them all about 15 degrees to the east. they were then eroded nearly flat before the Tapeats Sandstone was deposited. Geologic Islands: While the Grand Canyon Supergroup was being eroded down, a very hard, resistant layer was not eroded and stood up as hills, or islands in the rising sea -- the Shinumo Quartzite near Phantom Ranch and the B.A. trail. The Tapeats Sandstone and Bright Angel Shale were deposited around the islands. You'll see this from Plateau Point.
The Vishnu metamorphic and Zoaraster igneous rocks include schist and granite about 1.7 Billion years old. From a distance they are the non-descript brown to gray to pinkish mass in the inner gorge. The contact between them and the sedimentary Grand Canyon Supergroup is called the "lesser unconformity" because it represents a mere 600 million year gap in time.
Here's another diagram of the Grand Canyon strata from the great scientists at the USGS:
