Friday, July 13, 2012
The Abandoned Lands...A Journey Through the Colorado Plateau: Three Grand Canyons in one
And there are even huge chunks of time missing from the Paleozoic layers that are present in the Canyon. The Cambrian is represented by three formations (the Tapeats, Bright Angel and Muav formations), but rocks of the Ordovician and Silurian periods are missing entirely. The Devonian period is represented by a single discontinuous layer, the Temple Butte, as is the Mississippian (by the prominent Redwall Limestone). The Pennsylvanian and Permian periods are recalled by the Supai Group, the Hermit formation, the Coconino Sandstone, the Toroweap formation, and the Kaibab formation, which forms the rimrock of the Grand Canyon. All told, there are about 4,000 feet of Paleozoic rocks in the Grand Canyon. A patchy record, albeit very scenic and dramatic.
Of course, the horizontal layers aren't the whole story. I wrote an earlier post about the ancient rocks of the Inner Canyon, the gneiss, schist and granite of a long-gone and never seen mountain range that formed 1.7 billion years ago. We were able to see these rocks on our journey down Peach Springs Canyon and Diamond Creek and we were able to lay our hands on the Great Unconformity that divides the older rocks from the Cambrian sediments. But the Grand Canyon has other secrets hidden in its depths, and they aren't as easy to access. But you can spot them from the rim in the right places.
500 million years is a long time. The last 500 million years or so of history include all the Paleozoic events mentioned above, but also the rise and fall of the dinosaurs, and the rise of the mammals, the birds, and of course at the very end, humans. The rocks in the Grand Canyon Supergroup encompass the period from 1.2 billion years to around 700 million years ago.
like the earlier range was never seen by sentient beings. Eyes had not yet evolved in the world, and neither had life emerged on the land (and the seas held only unicellular life forms).
The story goes something like this: after the Mazatzal Mountains of 1.7 billion years ago wore away, they left behind an essentially flat surface near sea level. As a result of lands sinking, or sea levels rising, this surface was inundated under shallow seas for a time, and covered by river floodplains at other times. Intrusions of basaltic rock sometimes erupted at the surface, producing thick lava flows (the Cardenas Lavas, the black layer in the picture above). In late Proterozoic time, many of the continents were massed together in a percursor supercontinent to the better-known Pangaea: Rodinia. Rodinia was beginning to fall apart at the seams, and faults formed around rift valleys. The rocks of the supergroup rose and were tilted into a series of mountains and valleys not unlike the Basin and Range province of today. The mountains rose thousands of feet into the air, and erosion tore at their flanks. Eventually nothing was left but low ridges a few hundred feet high. The continent split completely and a large landmass that included Australia and Antarctica rifted away from western North America. The continental edges subsided and the sea started to transgress across the region in Cambrian time. The flatlands were covered with beach sands and the ridges persisted as islands for a time until they too were submerged (one of those islands is visible in the picture below where the cliff of purplish Shinumo Quartzite and magenta Hakatai Shale pierce the overlying Bright Angel Shale).