The Great Unconformity near Blacktail Canyon -
click on any pic to enlargenate
We have been floating for several days through the magnificent rocks of the Upper Granite Gorge. For over 40 sinuous river miles, from mile 78 near Mineral Canyon to mile 120 at Blacktail Canyon, the river carves its way through the dense, massive cliffs of Precambrian metamorphic rocks.
Geologic evidence suggests that, around 1.75 billion years ago, a chain of volcanic islands was perched on an unnamed oceanic tectonic plate. Now called the Yavapai Arc and resembling Alaska’s Aleutian Islands, this thousand–mile long chain of islands was slowly on the move. Over millions of years it drifted toward an ancient North American continent, an edge of which stretched from modern–day southern California, up through Utah and Wyoming, and into central Canada. Sandstone, mud, lava, and ash weathered and eroded off the volcanic islands and were deposited and buried in deep ocean basins. For 150 million years, as the island arc glided closer and closer to the ancient continent, these buried sediments were squished, folded, and deformed.
When the Yavapai Arc finally collided with and accreted, or sutured, itself to the continent, it was pushed up and over the edge of our ancient North America, forming a new continental mountain range. The basin deposits were buried even more deeply, up to 10 or 15 miles below the surface. At the pressures and temperatures found at these depths, the sediments metamorphosed into schists, gneisses, and amphibolites. Still they were not deep enough nor hot enough to melt. Deeper in the crust, though, melting did occur due to the higher temperatures and pressures. These melted rocks eventually forced their way up through fractures and fissures into the solid rocks above. It is these dark–colored schists intruded by lighter–colored bands of igneous rocks that so enrapture any geologist (especially this one) who floats this river.
Dated at 1.84 billion years old, rocks of the Elves Chasm Pluton are the oldest known rocks in the southwest US. They are the basement, or foundation, upon which the rocks of the younger Grand Canyon Metamorphic Suite were deposited. The Elves Chasm Pluton is particularly well exposed between river miles 112 and 118, through which we have just passed.
As we float along the three–mile stretch of Stephen Aisle, the panorama of Precambrian igneous and metamorphic rocks slowly but steadily dips beneath the water. The familiar 540–million–year–old Tapeats Sandstone takes its place at river level, reappearing like a long–lost friend. We soon reach Blacktail Canyon and disembark for lunch.
|This is how we do lunch on the river|
|Small-scale cross-bedding in the Tapeats Sandstone|
Because there is a regional westward dip of the rock strata in this vicinity, the Tapeats Sandstone descends to river level. From a slightly northeast direction, Blacktail Canyon drains a notch along the Great Unconformity, exposing the contact between the Tapeats and the underlying Vishnu Schist. There is lots to see here: large quartz boulders and cobbles in the bottom of the otherwise fine–grained sandstone, along with garnets in the schist. The garnets give an indication of the pressures and temperatures that this schist underwent (remember – rocks don’t suffer metamorphism, they enjoy it!) while the cobbles and boulders tell a story of a 540–million–year–old Cambrian Sea advancing across the barren, low–relief landscape, relentlessly breaking and churning the resistant schists into boulders, cobbles, and sand.
|Strolling into Blacktail Canyon|
|The Principle of Inclusions is alive and well!|
|Garnets with plagioclase rims indicate decompression from depth|
|Someone is caressing the Great Unconformity|
|Kris enjoying a beautiful day on the river|
We wander slowly back to the boats. The sky is clear and the air is warm. It is a fine day to be on the river. Beyond Blacktail Canyon we will be in the Middle Granite Gorge.
|Beyond Blacktail Canyon|
Somewhere past Conquistador Aisle, past Forster Rapid and Fossil Rapid and 128–Mile Rapid, the sublime palette of dark schists interwoven with pink igneous intrusions appears once again. As with everything in this canyon, the palette mesmerizes the mind.
|The canyon widens as we exit the metamorphic rocks|
At our campsite near 135–Mile Rapid, the tilted layers of the Grand Canyon Supergroup reappear in the downstream distance. Our tents seem to be tilted almost as much as those layers. I hope I don’t wake up in the river.
Abbott, L., and Cook, T., 2004, Hiking the Grand Canyon’s Geology, The Mountaineer Books
Blakey, R. and Ranney, W., 2008, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association
Collier, M., 1980, An Introduction to Grand Canyon Geology, Grand Canyon Natural History Association
Karlstrom, K.E., Ilg, B.R., Williams, M.L., Hawkins, D.P., Bowring, S.A., and Seaman, S.J., 2003, Paleoproterozoic Rocks of the Granite Gorges, in Beus, S. and Morales, M., eds., Grand Canyon Geology, 2nd edition, Oxford University Press
Potochnik, A.R., and Reynolds, S.J., 2003, Side Canyons of the Colorado River in Grand Canyon, in Beus, S. and Morales, M., eds., Grand Canyon Geology, 2nd edition, Oxford University Press
Stevens, L., 2013, The Colorado River in Grand Canyon – River Map & Guide, Grand Canyon Wildlands Council