It has been our good fortune to row 280 miles of the Colorado River through the Grand Canyon twice in the last seven months. This is rare for private boaters limited to one trip per calendar year and subject to a lottery system where a decade can pass between successful applications.
Carved very recently, by many accounts within the last six million years, the river has exposed a section of the earth’s crust that provides a rare opportunity to study the ways our planet worked in the past. The clear message of this section through the crust is change. Constant and unremitting change; change working simultaneously on different time scales. Change working differently in different parts of the Canyon.
A boat trip down the Grand Canyon begins a few miles below Glen Canyon Dam and Lake Powell, at a place called Lee’s Ferry. Faulting has resulted in the canyon being eroded back here, allowing access by road. The next road access is 226 miles downstream. The red formation in the middle background is the Moenkopi formation, deposited above sea level in a low energy delta and braided stream setting. Moenkopi is of earliest Mesozoic age, the age of dinosaurs, but dinosaurs had not yet evolved. Tracks of Therapsids, the reptilian branch that would eventually lead to mammals can be found in the Moenkopi mud. Below Lee’s Ferry, you bid farewell to the Mesozoic, and enter a realm where older Paleozoic rocks form the canyon rim.
Conformity in geology is when sediments are deposited in an uninterrupted sequence. The most fundamental changes in the Grand Canyon are the unconformities, where a time interval is missing. The first such unconformity is between the Mesozoic Moenkopi and the underlying Paleozoic Kaibab limestone. It is difficult to know if sediments were simply not laid down during the interval of unconformity, or if they were laid down and later eroded away.
The missing time between the Moenkopi and the Kaibab means that the Grand Canyon section does not speak to the greatest extinction in the history of life. This extinction marks the end of the Paleozoic and the beginning of the Mesozoic eras. Fortunately for us, our Moenkopi Therapsid relatives survived this extinction. Unconformities, missing time intervals, mark most layers, and all layer groups in the Grand Canyon.
Above is the configuration of continents 250 million years ago at the Mesozoic/Paleozoic boundary according to Christopher Scotese’s Paleomap Project. The supercontinent Pangea was beginning to break up, and a seaway had formed between North America and Africa.
Above is the configuration 260mya during the formation of the Kaibab Limestone that forms the rim of the Grand Canyon. The 250mya continents remain in tan beneath and it can be seen that Pangea tightened up and rotated as we progress backward in time. The Kiabab was very much an inland sea, and as the river cuts quickly through the upwarp visible in the first photo, the Toroweap formation beneath the Kaibab formed in an arid inland lake like the current Great Salt Lake. By mile 4 the river has already cut to the underlying Coconino sandstone, laid down as vast dunes like the current Sahara Desert covered large areas of western North America. You may have guessed that the Coconino and Kaibab are separated by an unconformity.
By mile 8 the river has cut through to the Coconino to the Hermit shale, separated by an unconformity. The Hermit was laid down in low angle braided streams on land. By mile 12, separated by another unconformity, the Supai group is exposed. The Supai group is characterized by alternating marine limestone cliff forming units and stream deposits laid down on land that are less resistant to erosion and form slopes.
A typical river trip makes its first camp in the Supai. It has been a busy day cutting through 50 million years!
At 300mya the continents are shown in purple with 250mya as tan background. Remember that the tan areas were ocean 300mya.
Entropy is a fundamental force of nature that acts to minimize all gradients. It is universal, working from molecular scale to the forces that move continents. Rocks fall from the canyon walls to the river because the gradient of altitude is reduced. The river flows to the Sea of Cortez because the gradient of altitude is reduced. “Ah”, you say, “this is just gravity.” The river shows us it is not just gravity. When the water is forced to separate around an obstruction, usually a rock, the water curls back upstream, defying gravity, to fill the void and reduce the gradient in its own cohesion. We call these “holes” and try to avoid the big ones as they can turn your boat upside down when the water curls back upstream.
The Supai has an unconformity within it that separates the Permian, the youngest period of the Paleozoic era, from the older Pennsylvanian. Besides the “icehouse” period we currently live in, the Pennsylvanian/Permian transition about 300mya marks the center of the mist recent “icehouse”, a long period of glaciation that extended well into the Permian. The Pennsylvanian is notable for the highest atmospheric Oxygen levels in our planet’s history.
It takes the river 11 miles to chew through the Supai, and at mile 23 the Redwall limestone that defines the older Mississippian period in the canyon is first exposed at river level. Of course, an unconformity separates it from the overlying Supai. The Mississippian period saw the first amphibians crawl on to land to live, but in the Grand Canyon the Redwall Sea resulted in the deposition of carbonate shells of foraminifera, corals, et al 800 feet deep. The resulting limestone is resistant to erosion and forms imposing cliffs above the river.
We can see above that even the massive Redwall has striations. Despite its name, the Redwall limestone is not naturally red. It is painted red by color from above. The lighter bands have higher carbonate concentration, are more resistant to erosion, and stick out from the wall further than the softer intervening bands. The softer bands are more porous and hold the paint better.
More Resistant Lighter Layers Form Cavern “Cieling”
Above is detail of Redwall Layering from the near the bottom of the Redwall section at the famous Redwall Cavern.
The layers get tighter toward the bottom of the Redwall sequence, and some folding aided the river in undercutting this cavern.
Above is the current distribution of Pennsylvanian (lower Supai) and Mississippian (Redwall) surface rocks in the western United States. These are broad categories chosen by individual State Geologists, and some Permian rocks are included, but it can be seen that shallow ocean covered large areas. These are all sedimentary rocks except some minor volcanics in central Nevada. These volcanics and the folding at the bottom of the Redwall are probably related to the Antler overthrust in Nevada where sediments were pushed over what was then a continental shelf.
Above are the continents 360mya, the transition between the Mississippian Redwall and the older Devonian Temple Butte Formation. Dominant ocean floor spreading was sweeping the continents into a pile we call Pangea. The Temple Butte is really not encountered at river level by boaters because it thins out against higher terrain at the time in the eastern Grand Canyon. There are a few lenses of Temple Butte between miles 38 and 48. The Temple Butte thickens to the west and becomes a significant layer in the western Grand Canyon, but above river level.
For boaters, the Redwall sits atop the Cambrian Muav Limestone, first seen at mile 36. The unconformity between these two spans three periods, the Ordovician, Silurian, and Devonian. The missing 120 million years between the Redwall and the Muav is an awfully long time, considering that complex multi-cellular life forms have only been around for 600 million years. Even the 100 million years between the Muav and Temple Butte, where it occurs further west, is a very long time. The magnitude of this unconformity can be appreciated by comparing the continents at 360mya above and 500mya below.
It is not really clear why The Grand Canyon region would have remained elevated and not received sediments; or why these sediments could have been laid down and later eroded away. From the Mid-Cambrian above to the Mississippian Redwall at 260mya, North America moved from west to east, and not very much compared to the other continents.
Just for fun and to recap the quarter billion years from the Kaibab to the Muav we have traversed in ~50 river miles, the continents are shown above at 250mya in tan and 500mya in violet. It can be seen that North America moved comparatively little as Pangea formed, and the other continents basically self assembled around North America, the oldest continental craton.
Above we show the current surface rocks deposited during the time between the Muav and Redwall in the western US. There seems to have been a broad seaway to the west in Idaho, Utah, Nevada, and California. Not just the Grand Canyon, but the entire Colorado Plateau seems to have missed out on deposition during this time. Perhaps it was elevated like it is today.
The Muav is the youngest member of the Tonto group, a group of three periods taken to represent a classic marine transgression.
S=KlogW is Boltzmann’s formula for entropy. S is entropy, K is Boltzmann’s constant of nature, and W is the number of possible ways invisible components can be arranged without changing the appearance of what we see. W is the tricky one. Unless we can somehow count the invisible things, we are out of luck. What we can’t see are the physical interactions that give rise to the impression that there is a giant beast breathing deep within our planet.
At mile 51 we encounter the Bright Angel shale at river level. The Bright Angel is the middle member of the Tonto group, and represents a transition from the sand dunes of the underlying Tapeats (mile 58) sandstone to near shore sediments. The Bright Angel is like the Moenkopi or the Hermit shale, except that it interfingers and grades into the Muav from west to east, following the marine transgression. As encountered by river runners, The Muav, laid down in marine conditions, begins younger.
Wouldn’t it be too simple if the Grand Canyon could just be read like a book with a few pages torn out? The Grand Canyon Supergroup is first encountered just before mile 69. The unconformity between the Tapeats and the Supergroup, as first encountered by boaters as the Dox formation is half a billion years! This is basically the amount of time from the Muav to the present day.
The Supergroup strata are inclined at about 15 degrees compared with the essentially level Paleozoic section above, and the Supergroup pinches out against even more steeply inclined Vishnu basement below, so the Supergroup does not even exist many places in the Grand Canyon. The Dox, the first unit encountered on the river, is not even the youngest member of the Supergroup, the younger strata having pinched out.
If we think of books, The Grand Canyon is really three books. The first book is the Paleozoic section. It is relatively level, has a few pages missing, and basically extends from the “Cambrian Explosion” of multicellular life forms to the Permian Therapsids (funky lizards)–from the Tapeats sandstone to the Kaibab limestone. This first book encompasses ~300 million years from 544 to 245mya.
The second book is the Supergroup. It is twice as thick, both physically and in time, but it is inclined and does not extend to much of the Grand Canyon. In the history of life, the Supergroup extends from the evolution of sexual reproduction in single celled creatures through the Marinoan glacial period or “Cryogenian”. This second book encompasses ~600 million years from 1250 to 650mya.
The third book is nearly illegible. The Vishnu metamorphic complex has been squished, stomped, and injected. Many of what were once level strata are now vertical in the inner gorge. Many areas were never sedimentary pages at all, but blobs of ascending magma called plutons. The isotopic information we use to date rocks gets overwritten each time the rock is melted or metamorphosed, so we can’t really know how old the sediments were. The dates of last crystallization range from 1.85 to 1.7 billion years ago. This is about the center of the period when the most advanced life forms were single cells that had learned to keep their DNA in a nucleus (Eukaryotes).
The Vishnu is first encountered at mile 78. There are still over 200 miles yet to run, but the entire geological sequence has been revealed. Due to undulations in the strata, the river will actually “climb” out of the Vishnu gorge into younger strata again, before cutting a second Vishnu “lower gorge” further west.