GEOLOGY OF THE PAINTED DESERT AND THE PETRIFIED FOREST

BY: MATT WALTON,JOSEF MUENCH

GEOLOGY OF THE PAINTED DESERT AND PETRIFIED FOREST

My grandfather visited Arizona in the 1880's and summarized his impressions with the statement, which I doubt was original, that "Arizona has more rivers for less water, more cows for less milk, and you can look farther and see less than any place else in the whole danged world!" How clearly this shows that meaning as well as beauty is in the eyes of the observer, for I would not question my grandfather's judgment on cows and water, but as I return to Arizona to look at this land through the eyes of the geologist, I find that there are few places in this world where you can look farther and see more. The desolation and the very rawness of Arizona's deep-scarred canyons lay bare the rib-rocks of its underlving geologic structure. This land, with its bare bones thus exposed, has a deeper meaning than could be conveyed by mere opulence of superficial detail. For as the eye sweeps across our landscape, even the eye untrained in geology, one cannot escape, at least subconsciously, some sense of an underlying architecture-of a dynamics and a principle of organization that is blurred and lost in the softer outlines of more fertile landscapes. The Painted Desert is a landscape of destruction, a land where the dry dust drifts with the wind, where pelting rains of the infrequent but sometimes violent storms of the plateau country beat relentlessly on crumbling shales and sandstones, where the impact of each raindrop is printed in bare earth, where mud-laden raindrops coalesce to run chocolate-thick in sudden streams, a land carved into a bewildering waste of washes and arroyos separated by knife-sharp ridges and weirdly etched mesas. Such a land is called a "badland," and the Painted Desert is specifically and technically a badland developed by erosion in an area where a stratum of rock known as the Chinle formation crops out. Paradoxically, then, the Painted Desert is constantly being recreated in its own destruction. It is the landscape presented by the Chinle formation in the process of erosion. Its face is always new. No stable soil surface mantles the variegated colors of the raw rock and bleaches beneath the sun to some nondescript hue. The surface and form of the desert is constantly in the process of creation, while the rocks that are the substance of that surface and form are constantly being destroyed. Thus this land, where only form and living process are, stable and substance is transitory, embodies an ancient philosophic concept among its hidden meanings. What, then, is this Chinle formation, the time-worn face of which we call the Painted Desert? In the area of the Painted Desert the Chinle formation consists mainly of shale, a soft, crumbly rock with a tendency to split into small, thin chips and slabs. The shale of the Painted Desert is notable for its strong and subtly varied colors. Interspersed with the shale there are layers of sandstone and conglomerate. Altogether this sequence of shale with interbedded sandstone and conglomerate forms a distinctive layer as much as a thousand feet thick in the complete sequence of rock strata that underlies northern Arizona. It is called the "Chinle formation" simply because this distinctive sequence is well-exposed in the vicinity of the village of Chinle near Canyon De Chelly. In one sense the origin of the Chinle formation is extraordinarily humble and prosaic. Shale is a rock produced by the compaction of mud and silt under pressure, and so the Chinle formation is to begin with nothing more than an accumulation of mud and silt along with a little sand and gravel. But this bald statement of the humble character of the Chinle rocks raises some fascinating questions. How, we must ask, could it come about that layer upon layer of mud and sand approaching a thousand feet in thickness could have been spread out over much of northern Arizona and parts of Utah, Colorado, and New Mexico where now there is a high and arid plateau? How came this deposit to be buried and compacted under thousands of feet of additional sediment, then elevated a mile or so above sea level, and finally resurrected by erosion to crop out, as it now does, in the valley of the Little Colorado? How does it happen to contain the bones and footprints of dinosaurs and other creatures of the distant past and a forest turned to stone? Over what span of time did the complex events suggested by these questions take place? At the very heart of the science of geology is the idea that the present is the key to the past, and that questions like these may be answered by finding out under what conditions extensive deposits of mud and silt are being laid down today in various parts of the world. From an insight into the dynamics of the present, we may recreate in the scientific imagination a picture of the past. Obviously the Chinle formation was not laid down on a high and arid plateau in its present situation, for high plateaus everywhere and by their very nature are in the process of destruction by intensive erosion. In fact, extensive mud and silt deposits are being laid down today in a few inland basins such as Great Salt Lake or the Great Lakes, on coastal plain swamps and continental shelves such as the Gulf Coast region, on the flood plains and deltas of great rivers such as the Mississippi and the Yangtse or in historic Egypt and Mesopotamia, in great embayments of the sea such as the Persian Gulf and the Gulf of Lower California, and on the bottom of the open sea. None of these areas of present deposition is exactly reminiscent of the geography of Northern Arizona, yet to have received this pile of mud and silt we now call the priate to the environment that has been sketched here through the exercise of the scientific imagination. There are various kinds of fish, some of which closely resembled the modern garpike of the Louisiana bayous, and some of which were lungfish, a species adapted to life in streams and ponds that intermittently dry up or become stagnant. There were molluscs resembling the fresh water mussels of the Mississippi valley. There were great, heavily built amphibians with gaping, many-toothed jaws that preyed on fish, and there were reptiles reminiscent of the modern crocodile in form and habitat, though not actually ancestral to the modern crocodile. All of these creatures bespeak the environment of the fresh water swampland traversed by sluggish streams. In addition there are some small, primitive dinosaurs of the types that in the succeeding geologic periods evolved into monsters that ruled the earth-creatures that ran erect on highly developed hind legs and used their disproportionately small front legs merely for seizing and manipulating their food. These creatures we must imagine ranged upon the uplands and in the forests and were only occasionally preserved when they met death in or near streams and swamps, for in order for a record of the life of the past to be preserved in fossil form the animals must die in a situation which permits them to be buried in sediment before they have a chance to decompose. Consequently the preservation of upland faunas in the fossil record is pretty hit or miss.

Not only the bones but the tracks of the land animals of Chinle time are preserved, notably in Dinosaur Canyon about fifteen miles from Cameron. The tracks appear as imprints on the surfaces of shale beds where the surfaces have been uncovered rather gently by erosion. If you have an active imagination it is a little spooky to come upon footprints of great three-toed monsters that walked the earth two hundred million years ago yet look as though made yesterday.

The track tells you of course that a prehistoric creature walked there across soft ground which has now turned to rock, but it also tells you that within a few hours or at most days after the creature walked, before the tracks could be obliterated by the weather, the tracks were filled in and preserved by a layer of drifting, wind-blown sand or dust or by mud deposited by the flooding of the area with turbid water. This is the sort of thing that happens today on tidal flats and flood plains or on the floors of intermittently flooded ponds and playas-another proof that in Chinle time the Painted Desert was a low-lying intermittently flooded plain.

The subtle and varied coloring of the Painted Desert is another manifestation of the infinite variety of humble things, for one of the most important coloring agents is simply iron rust, which even in very small amounts has strong staining power. But there are hidden complexities to this common stuff, for iron oxide exists in at least six different states of oxidation and hydration which depend on rather slight differences in climate and chemical environment, and each of these states has different pigmenting powers and values. The delicate shading and tinting of the Painted Desert rocks highlighted by sudden strong contrasts in rock colors suggest sedimentation under constantly shifting conditions, and this again is typical of an alluviating lowland traversed by shifting streams and dotted by flooded ponds and swamps.

Perhaps the most exotic and exciting coloring agent in the Chinle formation is uranium-an element to conjure with these days. Some of the compounds of uranium form brilliant yellow and orange minerals of which carnotite is perhaps the most common. Here and there in the Chinle formation beds have been found bearing this telltale stain, and in some places, notably near Cameron, there are deposits of commercial grade. The deposits are typically associated with dark gray to almost black layers which are so rich in the carbonized remains of ancient vegetation as to locally resemble very low grade coal. These are clearly deposits formed from vegetation in swamps, but the uranium was not deposited in the swamps at the time the organic sediment was building up. The uranium appears to have been present much later in solution in ground-water circulating through the Chinle formation. Carbonaceous material seems to have the power of precipitating uranium from solution, and so locally these beds were replaced to a small extent by uranium minerals-it doesn't take much, only a fraction of one percent, to make a commercial deposit. It is essentially the same process as that which produced the petrified forest, only the petrifying mineral was carnotite or some other uranium compound and not silica. In fact, entire logs have been found replaced by carnotite worth tens of thousands of dollars for their uranium content.

If our scientific reconstruction of the conditions of deposition of the Chinle formation is convincing, it raises almost as many questions as it answers. What brought this episode of sedimentation about? How did it come to an end? When did it happen? How has the Chinle formation come to its present position? I can only attempt here to briefly sketch the basis for the answers to these questions.

The Chinle formation passes downward without evidence of an interruption in sedimentation into a layer of rock known as the Shinarump conglomerate. The Shinarump in turn rests on another layer of sediment known as the Moenkopi formation, but there appears to be a break between the Shinarump and the Moenkopi. Hills of Moenkopi rocks protrude up through the Shinarump and in places the Shinarump is exceptionally thick, where valleys carved in the Moenkopi formation were filled by the coarse sand and gravel of the Shinarump. We won't attempt to peer back into time beyond the Moenkopi for the story is an endless one which might be followed layer by layer through another mile of sedimentary formations exposed in the walls of the Grand Canyon, and at the bottom we would still be only a fraction of the way through the geologic history of the earth. Suffice it to say that the relationship of the Shinarump to the Moenkopi indicates that for a time the Moenkopi formation made up a land surface which underwent a certain amount of erosion producing hills and valleys of low relief. Then this land surface evidently began to subside, and the valleys to fill with coarse alluvium brought down by streams from the hypothetical highlands I have already described. Ultimately the old Moenkopi surface was inundated by alluvial sediment and aggraded to a rather featureless plain, while the highlands became sufficiently reduced in the process of supplying this sediment so that the streams became more sluggish, lost their power to transport the coarser materials and the mud and silt of the Chinle formation began to accumulate.

The Chinle formation passes upward into the great cliff-forming strata that make bold escarpments to the north and east of the Painted Desert-the Wingate and Navaho sandstones. Much of this sandstone has a peculiar type of irregular, criss-cross bedding characteristic of the sand deposited by wind. So Chinle time came to an end with the drying up of the streams that once watered a plain that harbored abundant life, and over it spread an arid pall of drifting sand dunes.

Chinle time was just one episode in a very lengthy span of geologic history during which the area we know as the plateau and canyon country played a rather passive role in the development of the North American continent. This span of time begins with strata on the order of half a billion years old exposed near the bottom of the Grand Canyon and continues until something less than a hundred million years ago. For four or five hundred million years the crust of the earth in this region was relatively stable and for the most part underwent a protracted and gentle subsidence relative to sea level so that at times the surface was below the sea and received marine sediments, and at times at or slightly above sea level and received terrestial sediments similar to the Chinle formation, received no sediment at all, or underwent some erosion.

The total pile of sediments that accumulated is more than two miles thick and extends from a floor of even more ancient rocks at the bottom of the Grand Canyon to the tops of the Hopi mesas looking out over the Painted Desert. Many of these strata are fossiliferous, which permits the geologist to fit them into the geologic time-scale. Approximate absolute ages in terms of millions of years can be given because the geologic time-scale, which is mainly tied to the sequence of fossil life-forms, can be tied in turn to an absolute time-scale based on measurements of radioactive decay in radioactive mineral deposits here and there in the world which can be related closely in time of formation to the formation of associated fossiliferous rocks. In terms of the geologic time scale the Chinle formation was deposited during the Upper Triassic, which means that its deposition began something less than two hundred million years ago and took place over a period of ten or twenty million years.

The story closes with a great series of crustal disturbances which began somewhat less than a hundred million years ago and did much to bring the western part of the continent to its present form. In the Rocky Mountain region the crustal movements were violent and complex and the sedimentary strata which had accumulated in this area were crumpled and distorted by "folding" of the strata on a gigantic scale to produce complicated mountain structures. In the plateau region, however, the crust rose slowly in an arch so broad as to be almost imperceptible in curvature extending across northern Arizona. This uplift is broken here and there by step-like dislocations where adjacent blocks of the crust rose by different amounts. One such structure, known as the East Kaibab monocline, brings the Painted Desert to an end west of the Little Colorado River. As you approach this structure from the southwest on highway 89 you pass the Grey Mountain trading post on a plateau underlain by the pale gray Kaibab limestone. Abruptly, the limestone beds start to slope downward Moenkopi formation and the Shinarump conglomerate as you traverse the long grade down to the Little Colorado at Cameron. At the bottom of the grade the beds again become horizontal and beyond the river the first patches of the Chinle formation appear lying on top of the Shinarump conglomerate. All of the visible formations have been draped over a step-like dislocation in the so-called basement rocks below, like a pile of rugs draped over a step in the floor of a room. On the east side of this abrupt flexure in the rock all the for-mations are from a thousand to two thousand feet lower than on the west side. Consequently formations like the Chinle, which have been stripped away by erosion on the higher platform to the west, are present in the lower platform to the east. Elsewhere the strata, instead of draping in smooth, unbroken folds over these step-like structures in the basement, are abruptly offset by faults. In these places the rift in the ancient basement rocks simply cuts right on up through the layers of sediment and the layers are offset, thousands of feet in some places, along a sharp fracture zone. These so-called monoclines and faults are the principle structural complexities in the otherwise evenly stratified and almost flat-lying uplifted sedimentary formations of the plateau.

The warping of the surface of the earth, manifested in the uplift of sediments that accumulated near sea level, seems very great in the human scale of things, amounting to a maximum near ten thousand feet. Yet on the scale of the earth it was a bulge so slight as to be imperceptible if one were to draw a cross section of the earth to scale on this page. No sooner had this warping begun than the process of its destruction by erosion set in. The canyons, the mesas, the rim-rocks, and the cliffs, all of these dramatic features have been produced by erosion gnawing away at what was once a practically unbroken cover of sediment laying like a pile of slightly warped warped and dislo-cated blankets across northern Arizona. Only one thing has happened to complicate this picture. The rifts in the earth that attended the uplift of the plateau tapped regions deep below where molten rock was generated. This socalled magma rose, red hot, along the fissures to form volcanos and extensive lava flows on the plateau. This volcanic activity broke out here and there at different times even as the erosion of the plateau proceeded, locally filling old canyons with lava and interrupting at times the inexorable destruction of the plateau by the relentless elements.

The story really has no end. It is continuing today and it will continue for tomorrow and tomorrow. Measurement has shown that the surface of the Painted Desert is lowered by erosion an average of about an inch every five years. This sediment finds its way into the muddy waters of the Colorado. At the moment much of it is trapped by Lake Mead behind Hoover Dam, but this is only a momentary interruption in its inexorable progress to the Gulf of Lower California. In a hundred years or so Lake Mead will either fill with sediment or major engineering measures will have to be taken to flush it out, and the wastage of the land will continue on its way. And so in the tidal flats and on the bottom of the Gulf of Lower California a new blanket of sediment is forming from the destruction of our land. Who is to say in some distant geologic age what kind of a land it will make when ours has perhaps subsided again beneath the sea?