What lies below the Tapeats Sandstone will be different depending on where you are in the Canyon. On the Kaibab Trail, you will step from the Tapeats Sandstone down into more sedimentary layers of red, gold, brown and purple. These layers are much older than the layers above – they are over 1 billion years old! These are part of a stack of layers collectively known as the Grand Canyon Supergroup. In most parts of the canyon the Supergroup has eroded away and no longer exists, but in a few pockets here and there these older layers are preserved to tell us some stories from mysterious ancient worlds.
In total, if you could stack all the layers of the Supergroup back up on top of each other, they would total about 13,000 feet. Thirteen thousand feet! That’s over two miles of sediments, which for the most part have eroded back down to the oceans of the past. Now all we have left are bits and pieces to put back together almost like a jigsaw puzzle. The Supergroup is made up of two sequences of layers. On top is the Chuar Group, which is between 700 and 900 million years old, and only exists in one very remote part of the eastern Grand Canyon (in fact, the Chuar Group is thought only to have existed in Grand Canyon, never anywhere else). Underneath this stack of sediments is the Unkar Group, which is more common throughout the Grand Canyon. It’s very easy to see some of these layers in the Phantom Ranch area. The layers of the Unkar Group are between 1 billion and 1.2 billion years old, and they tell us the story of a time when this region was assembling with other continents into a much larger supercontinent called Rodinia (this is before Pangea, which was only 240 million years old). The stresses of this ancient supercontinent coming together caused large faults to form and big chunks of land were sinking down along these faults. As the land sank, the sea invaded, tidal flats and deltas formed, and rivers flowed into these oceans. All these environments were preserved in the sediments of the Unkar Group. Along the Kaibab Trail below Tipoff, you walk through the huge, purple-brown cliff of the Shinumo Quartzite, an ancient delta. Then the trail changes as it contacts the bright, brick-red Hakatai Shale, a soft slope filled with ripple-marked and mud-cracked shale from an ancient tidal flat. Below this is the thin, blocky brown cliff of the Bass Formation, a limestone and dolomite (magnesium-rich limestone) formed in the waters just offshore.
Some of the Grand Canyon Supergroup layers contain evidence of the earliest life in Grand Canyon. Mostly this consisted of photosynthesizing bacteria that created mats on the sea floor, trapped sediments and kept growing through this sediment to create a layered structure known as a stromatolite. Life at this time was all single-celled plants, animals and bacteria in the oceans. Multi-celled organisms didn’t appear until a few hundred million years later.
The Grand Canyon Supergroup layers are easy to tell from a distance, because they are tilted at an angle to all the layers above them. While everything from the Tapeats on up to the rim is flat, the Supergroup layers are tilted so they are higher on the west and lower on the east. Sometime after these layers were deposited and turned to stone, and before the ocean bringing the Tapeats beach invaded the land, the region was stretched and cracked along great fault lines which all ran parallel to each other. Chunks of land sank and tilted like shingles along these faults, and created a landscape of up-tilted mountain ranges, with low valley between them. Erosion began to wear down the high places and ultimately took away most of the landscape before the Tapeats sea washed over the land. Only in a few places was the Supergroup preserved, and then covered by the sands of the advancing ocean. In a few amazing instances, a ridge of harder Supergroup rock stuck up high enough on the land that the Tapeats sands could not cover it. The Tapeats beach merely surrounded the obstacle and created an island, and as the sea advanced and grew deeper, finally the island was covered with the muds from the Bright Angel Shale, or the muddy limestone of the Muav. You can see one of these really easily from the Bright Angel Trail. Look across the river from Plateau Point and you will see the tilted purple cliff of the Shinumo Quartzite. If you look around you will see that there is Tapeats all around this cliff, but there is no Tapeats on top of it. This was an island 500 million years ago, and if you stand at Plateau Point, you are standing on the shore of that island. Somewhere in between you and the other side of the canyon, the river carved right through that shoreline!
