Arizona Geologic History
The Coconino Sandstone
by Jan C. Rasmussen, Ph.D
The Coconino -- ARIZONA'S ANCIENT DESERT
by Jan C. Wilt, 1968
Flagstones used in Arizona patios hold clues of a barren landscape within their thin layers. Their gentle ripples., small lizard tracks, raindrop prints, and inclined layers prove shimmering sand dunes drifted across Arizona in the distant past.
Most Arizona flagstones are quarried near Drake and Ashfork from the buff colored sandstone named the Coconino Sandstone. It blankets northern Arizona and peaks out as a light colored band near the top of the most spectacularly scenic canyons in Arizona, South of Flagstaff the Coconino Sandstone towers above Walnut Canyon Sycamore Canyon, and Oak Creek Canyon; it hangs over the Colorado River near the top of the cliffs in Marble Canyon and the Grand Canyon.
The Coconino Sandstone is as much as 900 feet thick at the mouth of the Little Colorado River to 600 feet near Pine on the Mogollon Rim. Although it is thinner northwest-ward and southeastward from these points, the Coconino Sandstone occurs under 35,000 square miles of northern Arizona., That represents the largest known desert ever present in Arizona. It belongs exclusively to Arizona and does not occur in adjacent states.
Fossils above and below the Coconino date it in the Permian period of the geological time scale, 280 to 230 million years ago. The orange red siltstone and sandstone of the underlying Supai Formation accumulated before and during the early part of the
Permian. These orange red sediments were deposited on broad mud-flats and the delta and arid floodplain of the sluggishly meandering, southerly flowing Supai river system. Gradually they were buried under the southerly advancing sand dunes of the Coconino. If you listen carefully to the rustling of the sand grains in your patio flagstones and look with your imagination, you might find yourself among those sand dunes!
The Coconino -- Arizona's Ancient Desert
The black desert sky glitters with the sparkle of billions of star. No haze or clouds insulate the land; the daytime heat radiates unhindered into the blackness of spaces leaving the land bleak and cold. The Milky Way splashes across the sky in a pattern not very different from that of today. But the stars in the Big Dipper are not in their familiar positions and the blue-white stars in the constellation Orion have not yet 'been created on this evening 248 million years ago.
No animals hunt during the cool of this desert night. Amphibians and reptiles, the only land animals living at this time, are cold-blooded and become too sluggish at these cool temperatures. Mammals, the first warm-blooded animals, will not appear until 60 million years later during the Jurassic period of geological time. So the night is truly deserted; no rustlings or scurryings disturb the silence.
As the darkness fades, ghostly dunes hundreds of feet wide and tens of feet high loom in the foreground and diminish into the distance. Their northwestern sides slope gently at 12 degrees up to a U-shaped crest and the southeastern sides slant down steeply at 30 degrees. The east and west sides are bent into south-pointing horns as in a crescent moon and some have joined into long undulating ridges. The gentle north slope is the windward side, scoured by the constant wearing of the wind; the steep south slope is the leeward side where the sand grains tumble and slump down to rest at a high angle of repose,
A closer look at the dunes reveals wet sand grains glistening in the pale morning light. Although no rain has fallen for months, last night, as on many other nights, the temperature fell low enough for a heavy dew to condense on the cold sand, A sudden movement at the base of a sand dunes discloses a small lizard-like reptile similar to Araeoscelisx, a Permian fossil. With a flick of his tail, Dolichopodus vanishes up the lee face of the dune, leaving tiny tracks to prove he was not just a figment of the imagination. His gracefully curved front toes produce clear impressions, whereas the greater weight on his long hind legs leave only toe scuffs.
A little later Laoporus, a slightly larger lizard with short, stubby legs and a wide body, scampers up the lee face of another dune. He makes broad, stumpy footprints about an inch across in a wide trackway with no body or tail drag marks. He pauses at the top of the dune and swivels his flat, ugly head, sniffing scents carried on the faint morning breeze. Then he shambles down other side of the dune, loosening cascades of sand which obliterate his tracks but produce small step-like cracks where the sand is wet. In the distance a short, squat, wide-bodied lumbers around the base of a dune. Agostopus's slow, shuffling gait is recorded in the sand by closely spaced short-toed footprints as much as three inches across.
Long shadows from the brightening morning sun accentuate tiny trackways of lines and dots. The trail of three dots on either side of a trail drag ends at the feet of Paleohelcura, the scorpion. That other common desert insect., the millipede Diplopodichnus, traces two or three parallel grooves across the smooth surface of the sand. Spiders, isopods, and worms also inhabit the dune area and supply food for the reptiles and amphibians.
As the sun climbs higher in the sky, the temperature risen even higher. By midmorning the heat is nearly unbearable and the white sand glares with blinding reflections. Because of continental drift and polar wandering, northern Arizona at tbis time is about 10 degrees north of the equator, so the sun's rays are intense. The desert air is hot and dry; no humid haze or filmy clouds screen the land from the sun's scorching rays. The top layer of sand slowly absorbs the heat, until by mid-day it is hotter than 120o F. No reptiles venture out to burn their feet. The few animals living in the dune area are comfortably burrowed in the cooler sand below the parched surface.
By late morning the breeze accelerates and nudges the sand grains over each other and-up the gentle windward slope of the dimes. As the grains reach the crest of the dunes, they slide and tumble down the sheer lee sides forming steeply slanting layers called cross-beds. The wind sweeps up and around the edges of the dune and small eddies and back-currents form gently rounded ripples up and down the lee sides of both horns of the U-shaped barchan dunes.
The wind at first brings relief from the intolerable heat, for it comes from the sea which is in Utah and Nevada to the north and northwest. But the coolness is soon lost and the relentless thrust of the sand-laden wind becomes even more unbearable than the heat. The desert is not silent now; the sand whispers and murmurs as the grains bounce through their veiled leapfrog dance. The sand is churned into knee-high dusky clouds and the whole desert surface is in motion. The wind continues through the day, usually blowing just enough to roll and slide the sand grains across the dunes, but occasionally blowing so strongly that brown clouds move over the dunes.
Sometimes raging sandstorms sweep across the Coconino desert and blot out the sun for days and days. The spasmodic blasts of sand increase in fury until the sun, sky and everything in view is obliterated by smothering clouds full of biting, stinging bullets of sand. The air resounds with roaring, shrieking sand. There is no respite. The dunes cannot stop to rest. The wind drives them ever southward in an undulating, sinuous pattern. But after many days the demoniacal fury finally diminishes and the dunes extend in all directions like waves on a vast sea of sand.
Late in the afternoon the wind begins to dwindle and a few cottonball clouds lift their puffy tops overhead. Thin grey streamers of rain trail from the clouds to nearly touch the ground. A large drop of rain plops on the sand, wetting it and leaving a tiny, cup-shaped depression. Another drop, and another, and then several splatter to the ground. But then the sounds cease. The wispy veil lifts and passes on. Tonight will not witness the fury of a desert cloudburst. Now only a faint pungency of wetted ground and sweet rain linger in the softened air. The desert is wrapped in a peaceful cloak of silence and the western sky is ablaze with a crimson farewell from that monarch of the desert -- the sun.
The shimmering sand dominated Arizona's landscape for several million years. Slowly the land subsided and a shallow sea with blue lagoons encroached southward over the dunes, reworking the sand into beaches and burying them with mud and lime and gypsum. For 200 million years the Coconino Sandstone was hidden, at one time by as much as 4,000 feet of sand and clay in northeastern Arizona. Sometime within the last 25 million years, northern Arizona, along with the surrounding area was uplifted until today it has an elevation over 6,000 feet. As soon an the area was above sea level, rivers began cutting through the topmost rocks.
Now most of the rock pile overlying the Coconino Sandstone has been eroded away, except in Black Mesa, and the Coconino is exposed near the top of the walls of northern Arizona's most breath-taking canyons. In addition to its scenic value, throughout much of northern Arizona the Coconino is important because of ground water held in the spaces between the sand grains. In eastern Arizona these intergranular spaces contain the gas helium where gentle domes provide trapping structures. Although the quarrying of flagstones from the Coconino is also economically significant, for people with imagination, the flagstones have a story to tell about an ancient landscape of drifting sand and blazing sun.
home | classes | earth perspectives | current research | consulting | resume
