Overview of Grand Canyon Geology and Rock Formations
Cenozoic Volcanic Deposits |
The Paleozoic Sedimentary Column |
The Proterozoic Supergroup |
The Metamorphic Crystalline Core |
The rocks of the Grand Canyon reveal an ancient geologic history that is rich and complex. All three basic rock types with sedimentary, metamorphic and igneous origins are represented within its walls, terraces, mesas and buttes. A vertical mile of formations underlying its rim have been exposed by the erosional processes creating it, revealing strata and formations ranging in age from mid Phanerozoic Eon (about 250 million years old) at the rim, to mid Proterozoic Eon (about 1.75 billion years old) along deep cut portions of the inner gorge where the Colorado River runs. The still ongoing erosion carving the Grand Canyon below its rim and exposing these primordial rocks is very young in terms of geologic time and was triggered by an uplift 'pulse' within the Colorado Plateau region that began approximately 6 million years ago.
Genesis V2.0 - God's Grand Work Week
The Eons of geological time and durations of creation are so vast in magnitude they inevitably boggle the human mind when expressed in millions or billions of years. However, by at least one account even God Almighty was acquainted with the more mundane six-day work week. So expressing geological time using an analogy of more familiar units along a more compressed time line seems appropriate and helps lend comprehensibility to the relative sequences and durations of events in the geological history of His Grand Canyon.
So we'll represent 4.6 billion years with 6 days. That translates to roughly 767 million years per day, 32 million years per hour, 532 thousand years per minute, and 9,000 years per second.
For the purposes of our temporal metaphor we'll give God a break and prior work credit for the preceding dozen billion years or so of cosmological time while He was working out light and gravity, populating the Milky Way and other galaxies and what not, and commence our Genesis with the beginning of geological time on Earth. If the 4.6 billion years of geological time were compressed into a six-day work week beginning in the wee hours on Monday morning with the formation of Earth and its Hadean Eon of meteoric bombardment, the oldest rocks at the Grand Canyon's bottom, the Vishnu Group, would form at approximately 5:00 PM on Thursday afternoon.
At this time life on Proterozoic Earth is believed to have been limited to simple microscopic organisms such as archaeans, bacteria, and probably primitive single cell eukaryotes (cells containing a DNA nucleus with chromosomes replicating by mitosis.) The development of eukaryotes was a very significant evolutionary milestone, as all the species in the plant and animal kingdoms, modern and extinct, were evolved from and are constituted by eukaryotic cells.
At right, great dikes and sills of pink Zoroaster granite intrude the nearly black, crystalline Vishnu schist, rising in precipitous, jagged cliffs over thirteen hundred feet above the Colorado River, with a horizontal cap of Tapeats Sandstone. This westerly view is from the Tapeats at an inner canyon overlook of Granite Gorge, just off the Tonto Trail near the west side of the confluence of Cottonwood Creek. The white water is Grapevine Rapids about a mile downstream.
Rocks in the Vishnu Group were initially laid down on the sea floor and were composed of quartz rich sands, silts and clays, which were interspersed with ashes, lava flows and feeder dikes associated with a volcanic chain of offshore islands. The ocean plate these islands rode on was subducted and they were accreted onto Laurentia, the North American craton. God only knows the geologic history of the structural features and preceding rocks from which these parent sediments and deposits originated. The Vishnu Group rocks are contemporaneous with the oldest known rocks throughout Arizona and the evidence and geologic record afforded by older rocks elsewhere on the Earth is so fragmented and incomplete as to render interpretations highly speculative and debatable.
During the Yavapai Orogeny the Vishnu Group was raised above sea level and its sediments and deposits were extensively folded and metamorphosed into schist and gneiss at the roots of an ancient mountain range rivaling the modern Rocky Mountains in scale, and subsequently injected with and further cooked and metamorphosed by immense volumes of magmatic intrusions of granite forming plutons, sills and dikes. Any fossil evidence of Proterozoic life that may have existed in the constituent sediments was obliterated during their metamorphic recrystallization. Vishnu Group rocks were exposed at the erosion surface as the mountainous region and overlying rocks were eroded away and reduced to a hilly plain.
The region eventually subsided and was submerged by an ancient sea which cyclically invaded from and retreated towards the continental margin to the (present day) west. At about 9 AM Friday morning, sediments of primarily marine origin and volcanic extrusives began to build up the Grand Canyon Supergroup, accumulating in horizontal layers over the Vishnu Group rocks to a depth of several miles. The Supergroup rocks were not extensively metamorphosed and altered like the underlying Vishnu Group, and Supergroup remnants are generally well preserved.
At left is Chuar Lava Hill, a heavily eroded exposure of the Cardinas Lavas, a Grand Canyon Supergroup formation in eastern Grand Canyon, between the confluences of Carbon Creek and Lava Creek on the west bank of the Colorado River. Underlying and to the left of Lava Hill is pinkish purple Supergroup Dox Sandstone (note its pronounced tilt), with the banded Supergroup Galeros Formation, cut by Carbon Creek Canyon, forming the background. This northwesterly ~200mm telephoto view is from an overlook in the Palisades north of Desert View, just north of Comanche Point. The Colorado River is just visible behind foreground rocks in the lower right.
The earliest fossil record of life in Grand Canyon rocks are stromatolites found in the Bass Limestone, the lowermost Supergroup formation. Stromatolites are structures composed of laminated masses of calcium carbonate that are believed to originate from colonies of photosynthesizing cyanobacteria.
At right is a remarkable outcrop of Stromatolite Fossils in the Hakatai Shale, the Supergroup Formation immediately overlying the Bass Limestone, on a mesa just west of Phantom Ranch. Note the 55 mm lens cap for scale.
Sometime around 10 PM on Friday evening the Supergroup formations and underlying rocks were significantly offset and tilted by block faulting during the Grand Canyon Disturbance, and then subsequently eroded. Upthrust blocks of the Supergroup formations were stripped completely away from the underlying foundation of Vishnu Group rocks. Remnants of the Supergroup were preserved in isolated grabens, which are fault bounded blocks of crust that drop down as a result of crustal extension or stretching.
The region subsided again and around 7 AM Saturday morning the Tapeats Sandstone was deposited over the erosion surface of the faulted Vishnu Group and wedge shaped remnants of the Supergroup formations, where preserved. The Tapeats is contemporaneous with the early Cambrian Period and records the beginning of the Paleozoic Era and the Phanerozoic Eon, at which time Earth was devoid of life above its seas. Trilobites, brachiopods, sponges, worms, cystoids, snails and cephalopods were the most advanced marine life, with vetebrate forms yet to appear. A Great Unconformity representing a gap in geological history of about a day and a half, or a quarter of all geological time, is missing from the geologic column where the Tapeats directly overlays Vishnu rocks. During this interval our solar system made four or five complete galactic orbits about the center of the Milky Way. Even where surviving remnants of the of Grand Canyon Supergroup intervene between the Tapeats Sandstone and the Vishnu Group, gaps with durations exceeding the interval of a galactic rotation are missing from above and below the Supergroup formations.
At Left, several hikers nearly to the top of the ascent through the Vishnu by way of the Bright Angel Trail are about to cross the Great Unconformity, occurring along the undulating contact (indicated with the yellow arrow) above the folded metamorphic Vishnu Group rocks and under the overlying, horizontal sedimentary layers of the Tapeats Sandstone.
The depositional environment of the Tapeats Sandstone and formations above it ebbed back and forth from coastal terrestrial to shallow, offshore marine environments as Paleozoic Era seas cyclically invaded from and retreated towards the continental margin to the west.
Indicated below are the Vishnu Group at the deepest exposure within the Grand Canyon, and above it, exposures of each of the fourteen Paleozoic Era spanning formations from the Tapeats Sandstone to the Kaibab formation at the rim. This ~200mm telephoto view is to the northwest from Yavapai Point, and takes in Osiris Temple. Its summit is the cone shaped feature with a base of Hermit Shale and a cap of Coconino Sandstone, to the right and above center.
|KF = Kaibab Formation
TF = Toroweap Formation
CS = Coconino Sandstone
HS = Hermit Shale
ES = Esplanade Sandstone, Wes = Wescogame Formation, Man = Manakacha Formation, Wat = Watahomigi Formation
SCF = Surprise Canyon Formation
RF = Redwall Formation
TBF = Temple Butte Formation
ML = Muav Limestone, BAS = Bright Angel Shale, TS = Tapeats Sandstone
VG = Vishnu Group
The majority, but not all fourteen of the Paleozoic formations above the Vishnu were deposited in open, relatively shallow seas or coastal marine environments such as reefs, shoals, estuaries, tidal channels and flats, and contain primarily marine fossils as a result. A notable exception is the Coconino Sandstone, a cream colored, eolian formation recording immense, Sahara-like dunes of a great desert that once extended from south of the Grand Canyon region into present day Montana during the early Permian, the last age of the Paleozoic Era. During the Permian, amphibians were thriving in rivers and lakes, and reptiles had become the dominant animals on land.
At right is a bed of fossilized reptile footprints in the Coconino Sandstone, just off the Hermit Trail, which preserves the different sized tracks of several reptile species made in a wet sand dune after a rainfall. The largest tracks are equal in size to a cow's, note the 55mm lens cap at the bottom of the picture for scale. The sand humps preserved just behind the large tracks indicate that the animal pushed back loose sand behind its feet as it climbed its way up the dune.
At the Grand Canyon's rim, thirteen formations and over 3000 feet above the Tapeats Sandstone, is the youngest and uppermost strata, the Kaibab Formation. The Kaibab is contemporaneous with the end of the Paleozoic Era and would deposit at about 5 PM on Saturday, quitting time for most of us. But God still had some unfinished details to attend to. By this time ancient life was well developed and represented on land with plants, insects, amphibians and both herbivorous and carnivorous reptiles, but the dinosaurs had yet to appear. "Arizona" and the rest of the North American crustal plate were still located near the equator and incorporated in the monolithic supercontinent of Pangea - as yet unseparated into the modern continents through ensuing continental drift.
Above the Kaibab Formation, a mile thick layer consisting of overlying Mesozoic Era formations, partially preserved and exposed in adjacent areas such as the Painted Desert, Four Corners and Zion National Park, were deposited during the next five hours or so while the dinosaurs evolved and flourished during their great reign on Earth, and flowering plants, birds and mammals made their first appearances.
These overlying layers were subsequently eroded and lost from above rim level at the Grand Canyon after a tectonically driven uplift of the Colorado Plateau region during the Laramide Orogeny commenced around 10 pm Saturday evening, approximately contemporaneous with the uplift of the Rocky Mountains and the mass extinction that wiped out the dinosaurs, ammonites and other classes of Mesozoic life.
At left, Gold Hill, about five and a half miles southeast from the confluence of the Little Colorado and the Colorado Rivers, stands before the gorge of the Little Colorado and the Painted Desert at sunset. The Echo Cliffs, eroded from the Echo Cliffs monocline, span the background about 35 miles away at the western border of the Kaibito Plateau. Gold Hill is composed of red mudstones of the Mesozoic Moenkopi Formation preserved under a protective cap of a more erosion resistant member of the Chinle Group. Gold Hill, together with nearby Cedar Mountain, and Red Butte between Tusayan and Valle, are scarce, isolated ramparts of Mesozoic Era strata surviving atop the Kaibab Formation in the immediate vicinity of the Grand Canyon.
The uplift of the surrounding plateaus and erosion and down cutting of the Grand Canyon below its rim rock did not commence until late in the Cenozoic Era, about 11 minutes before midnight, and proceeded at an average rate of approximately 500 feet per minute in the deepest areas. Contemporaneous with the onset of down cutting, the Sea of Cortez began forming as transform faulting started separating Baja from the Mexican mainland and ground sloths and other Cenozoic Era, Pliocene Age mammals roamed the American Southwest. The apes had previously emerged elsewhere, but the Canyon had already reached Grand proportions prior to the appearance of the earliest known upright standing, man-like predecessors of Homo sapiens such as Australopithecus africanus.
Just prior to two minutes until midnight a volcano in the western Grand Canyon region erupted and extruded about 4 cubic miles of lava into the Canyon, forming a dam over 2300 feet high and creating an enormous lake that extended upstream into Utah further than modern Lake Powell. The backed up water overflowed and began down cutting and destroying this dam several milliseconds later. Over the course of the next several minutes similar volcanic eruptions and lava flows create lake forming dams of the Grand Canyon on a dozen occasions, all of which were dispatched relatively quickly by the relentless erosion of the Colorado River.
At approximately one and a third seconds before midnight, the Elephant Hunters, nomadic mammoth hunting Paleo-Indians, crossed over from Asia via an ancient land bridge that crossed the Bering Straight during the last great Ice Age and moved into the Grand Canyon region, becoming its first human residents and appreciators.
During the final instants of this rather eventful six-day work week, former USGS director Major John Wesley Powell's historic 1869 Survey and First Expedition/Float Trip through the Grand Canyon coursed through at approximately fifteen milliseconds prior to midnight.
Then at the end of His geological work week, God surveyed His rocks in the most Grand of all the Canyons in all the Earth that He had created, and all the life that dwelt upon them, and behold, it was very good. Except for the annihilation of several of His species caused by a couple of hydroelectric dams of the Colorado River in the vicinity, and pollution saturated haze emitted by a near by coal burning power plant, that were the works of man. But there wasn't a whole lot God could do about that, as He was still working out some fundamental philosophical issues and problems with the creational ramifications of free will and stupidity. And so on the seventh day God declared geologic Miller time, and He knocked off to enjoy the view.