Vicinity of the Grand Canyon
of the Colorado River
Sincere gratitude and thanks are due to Dr. John S. Shelton for permitting use of the text from his book, "Geology Illustrated," (p. 248-259) and for supplying photographs of the drawings by his brother, Hal Shelton.
Introduction to the Structural History
The fame of the Grand Canyon of the Colorado River is amply justified by its enormous proportions and colorful scenery alone. Many visitors see only these aspects and go home feeling fully rewarded for their journey to northern Arizona.
But there is a grandeur in the Grand Canyon scene which is less obvious to the casual observer, one that can be appreciated only by those possessing a little geological insight, and which, added to the scenic beauty, makes the Canyon an even more soul-stirring place. This more subtle feature is the long and remarkably eventful history recorded in the rocks so magnificently exposed there.
To the geologist, whose constant challenge is to decipher and understand what has happened beneath the surface, usually all too little is exposed. In a subject inherently three-dimensional he is presented with 50 million square miles of horizontal information (not counting the ocean floors) - and a vertical view limited to a few thousand feet. Anything that reveals rocks in the vertical dimension has special value: which accounts for the geologist's keen interest in mountains, mines, wells, and boreholes. But mountains are usually geologically complex, reflecting the deformation that produced them, and mines, wells, and boreholes provide extremely small samples. The extraordinary value of the Grand Canyon is that it affords such an extensive look, to such a depth, in a relatively uncomplicated part of the crust.
There are three sources of information on the geological history of the Grand Canyon region. The first, the structural relationships of the major rock bodies to each other, is presented in bold terms.
Fig. 1. View of the Grand Canyon, looking downstream (northwest) near the mouth of Shinumo Creek.
Viewing this grand succession of events - each of which deserves pages of elucidation in its own right - is somewhat like reading an outline of a book. The second source is the nature of the rocks themselves, their compositions and textures and all that these tell about where they came from, how they arrived, and what has happened to them since. The third source is the fossils contained in the rocks, the record of some of the life that witnessed and participated in some of these events. Together, the last two supply the details of the story; whatever is not recorded in them is largely lost, so that to obtain a full account of all the time involved is generally impossible.
The scene below Fig. 1, 20 miles downstream from Grand Canyon village, provides a key to the three principal and easily distinguished groups of rocks. These are (1) the horizontal layers, which we will call the Paleozoic strata, (2) the tilted beds, which are known as the Grand Canyon series, and (3) the steeply foliated dark Vishnu schist along the walls of the inner gorge, close to the river. A few faults of small displacement may be seen in the last two groups of rocks but the first is remarkably undisturbed; these conditions prevail throughout the canyon.
The drawings that follow are based on this scene and represent the most obvious of the major geological events recorded and implied in these rocks. They are so arranged that we begin with the familiar scene and reason our way back through time, step-by-step, employing fundamental concepts. In this way we will arrive at each stage aware of the logic by which it is reached and the justification for postulating it. The geologist must always start with what he sees and reconstruct the past from this as a basis, even though he may later tell the story in the conventional manner, beginning with the earliest event.
On the following pages, the text and associated drawings, which deal with this unraveling of the story, should be followed without regard to the legends above the pictures. After Step Ten has been reached, the procedure should be reversed and only the legends should be read, thus recounting the story from the earliest time to the present by examining the scenes in order from Ten to One: this will provide a summary, with emphasis on the earth movements involved, of the steps that led to the present scene.
Remember: To avoid confusion, ignore the legends when first reading the text; they make sense only when read in sequence from Ten to One.
Fig. 2. Step One: After the Colorado River has established itself in a winding course across what is to become the western Colorado Plateaus, further uplift, ultimately totalling more than 8,000 feet, raises all these strata well above sea level. The resulting erosion removes nearly all of the weak upper strata from the vicinity of the Grand Canyon and the entrenched river cuts the Canyon in the more-resistant lower layers.
Step One: Analysis of the Present Scene
First, let us back away from the view on the last page and present the same scene as part of a block diagram that includes a larger area, seen from a higher altitude. The same tilted Grand Canyon series can be recognized to the right of the river, with the hill (marked by a dashed line in the drawing of Fig. 1) produced by resistant beds protruding into the overlying horizontal Paleozoic strata.
On the vertical front of the block the essential relations between the different rocks are shown somewhat diagrammatically. Within the Vishnu schist, represented by a pattern of contorted lines, are a few thin pegmatite dikes marked with xs, a dike of basaltic rock marked with vs, and an irregular body of granite marked by scattered short straight lines. To the right of the river is a tilted wedge of Grand Canyon series sedimentary rocks whose lower boundary is the nonconformity visible on the preceding page (Fig. 1). The strata in this series are cut off at the right by a steep fault along which the relative movement was up on the right and down on the left. The two faults within the Grand Canyon series, visible in Figure 1, are omitted here because they are not essential to the story.
Fig. 3. Step Two: Submergence of this second peneplain induces deposition of the Paleozoic strata, whose base marks the second unconformity. Some of these layers accumulated on land, which requires fluctuating conditions. The total thickness of the Paleozoic strata today is a little over 4,000 feet, they were deposited in the period from 600 to 200 million years ago. Later, uplift initiates erosion which is here beginning to strip off some of the higher strata.
Derivation of Step Two
The Grand Canyon provides a spectacular example of the meaning of Steno's third observation: sedimentary layers do not accumulate with their edges showing. The perfect match of the strata in the north and south walls of the Canyon throughout its 200-mile length, even to details measured in inches, renders it virtually impossible that they could have accumulated separately. It is safe to conclude that these strata were once continuous across the chasm, and that the canyon must have been cut into these very widespread and remarkably uniform layers.
Some might wish to argue that the river must have had some assistance - perhaps a system of faulting by which a section of the crust was dropped a few thousand feet, after which running water so sculptured the shape as to obscure or remove the evidence. But faulting dislocates rocks, and no amount of erosion can make them match again. Actually, faults in the Paleozoic strata along the Canyon walls are rare, strike across rather than along the Canyon, and at most have displacements of only a few hundred feet.
So, to take the first step back in time, we undo the most recent event by restoring these rocks to their condition before the Canyon was cut. A few layers are added above those of the present rim because scattered remnants of these higher strata can be found in several places near the Canyon today and their general nature is well known from extensive exposures on plateaus in the vicinity of Zion and Bryce canyons to the north. These higher plateaus are shown in the background at the right, extending farther south than they do today because, during the cutting of the Canyon, downslope movements and erosion undoubtedly caused them to retreat northward. We also have backed away again, so that our view of the front of the block can include a second faulted wedge of Grand Canyon series strata (at the left). Its position is hypothetical, but its existence is not; many such are known from different parts of the Canyon.
The fronts of the blocks are strictly comparable in this and the next eight drawings.
Fig. 4. Step Three: A long period of erosion follows, accompanied by enough uplift to bring the base of the Grand Canyon series above sea level in some places. Toward the end of this period the levels of land and sea are stabilized and a second peneplain is developed in which the only remains of the Grand Canyon series are small down-faulted wedges, all the rest having been removed by erosion.
Derivation of Step Three
In the pre-canyon stage just reached, one step back in time, we recognize that the highest layer is the youngest and that its accumulation represents the most recent event recorded in that scene. To take the next step back, therefore, this stratum must be removed, or "undeposited", whereupon the same argument applies to the next lower layer. (This is an application of Steno's first principle, the Law of Superposition). Continuing this process, we remove all the horizontal Paleozoic strata, assuming for the moment that their accumulation was essentially uninterrupted.
Their removal exposes an old erosion surface with very low relief which truncates both Vishnu schist and the upturned edges of the tilted blocks of Grand Canyon series. Only the edge of this erosion surface can be seen today, but at least 95% of the more than 200 miles of such exposures in the Canyon has a relief of less than 150 feet and only a few hills of resistant sandstone rise as much as 600 to 800 feet above their surroundings.
The edge of this surface is well shown at the base of the Paleozoic strata in Figure 1; note the hill several hundred feet high just right of center (dashed line in the drawing) produced by beds of resistant sandstone in the tilted Grand Canyon series. The lowest Paleozoic beds lap out against this hill, which probably, therefore, was an island during their accumulation.
From this evidence we postulate, as shown in Fig. 4 above, a peneplain with a few low ridges of resistant strata and small hills rising as ancient monadnocks or remnants above its otherwise featureless and soil-covered surface.
Fig. 5. Step Four: The Grand Canyon series is uplifted and, probably at the same time, broken along faults between which the blocks are tilted northeastward. The fault-block mountains thus produced are, of course, immediately attacked by erosion.
Derivation of Step Four
Contemplating the pre-Paleozoic peneplain of the preceding scene, we realize that the latest event recorded there is the erosion that produced the peneplain. To take the next step back, therefore, it is necessary to undo this erosion by replacing the eroded rocks. This includes restoring the eroded extensions of the tilted Grand Canyon series strata (Steno's law of concealed stratification). Farther east in the Canyon some of the larger faulted wedges of these strata include many layers above those visible in Fig. 1. Using this information, it is possible to reconstruct these blocks to stratigraphic thicknesses of at least 12,000 feet.
When this is done the result is a set of tilted blocks, separated by faults, like leaning books on a shelf, the strata being represented by the printed titles on their backs.
There are many regions west of the Rocky Mountains today with this kind of structure; the resulting mountains are sometimes referred to as fault block ranges. Accordingly, we postulate a landscape similar to those of these western areas. The front face of the drawing shows the relation of the underground structure to the topography.
Fig. 6. Step Five: Submergence of this peneplain induces deposition of the Grand Canyon series sediments. As these are more than 12,000 feet thick the total subsidence must be at least this amount-three times the thickness of the horizontal Paleozoic strata exposed in the Canyon walls today. The base of the Grand Canyon series marks the lowest and oldest recognizable unconformity in the Canyon walls.
Derivation of Step Five
As originally deposited, the strata of the Grand Canyon series were neither tilted nor cut by faults. Applying Steno's second and third laws, we see that the most recent event in the previous scene is the tilting and rupturing of these strata. The next step back, then, is to restore them to horizontality while undoing the faulting - to straighten up the books. This leaves us with the Grand Canyon series strata in the orientation in which they were deposited, but probably above the level at which they accumulated.
The rocks of this series are chiefly shales and fine sandstones showing ripple marks and mud cracks, plus a few beds of limestone and a sill intruded between the layers near the bottom. Near the middle of the full sequence, too high to show in Fig. 1, are lava flows about 800 feet thick. The mud cracks suggest intermittent drying out of the surface, so the water must have been shallow, and the limestones almost certainly mean that some of it was marine. There is no evidence of vegetation.
Thus we postulate a scene (Fig. 6, above) resembling the present head of the Gulf of California, in which arid lands shed sediment into a shallow and fluctuating sea and occasional volcanic eruptions occur near the shore. Under these conditions some beds in the sequence may be marine and others fluvial, as the shoreline shifts with changing levels of land and sea. Possibly the sill near the bottom was injected as part of the same igneous activity that produced the lavas near the middle of the sequence; they are of similar composition.
Fig. 7. Step Six: Uplift, probably amounting to about 10 miles, maintains the mountains for a time. Ultimately erosion wins out and, as the crust becomes more stable, a near-sea-level peneplain is produced on rocks that were once in the roots of the mountains.
Derivation of Step Six
We have now reached a stage that superficially resembles that of Step Two, having reasoned our way back to an earlier time when there was a thick sequence of undisturbed strata. In the last scene (Step Five) we recognize, as before, that the most recent event was the accumulation of the highest stratum. Applying the Law of Superposition to successively lower layers in this sequence, and again assuming that there are no important events hidden within or between layers, we work back through time by removing all of the Grand Canyon series. This exposes a remarkably flat surface cut across the Vishnu schist, a surface that truncates the foliation, the pegmatites and granites-all the structures and variations within it.
Judging by the fragments of this surface that are exposed to view in the present Grand Canyon (Fig. 1), this was an almost perfect peneplain. It can be traced, in remnants, for many miles and is exposed in areas tens of miles apart, yet its relief is everywhere less than 50 feet. Both this and the peneplain of Step Three still retain patches of deeply weathered rock on their surfaces.
Very few, if any scenes quite like this peneplain are known today. To explain the deep chemical weathering we guess that the climate was fairly humid. The drawing above incorporates these considerations but the details of such features as the stream pattern are, of course, conjectural.
Fig. 8. Step Seven: Deformation, arising from movements in the crust, crumples the Vishnu sediments and lavas. Under the pressures and temperatures existing at a depth of around 10 miles this results in metamorphism that converts them to schist. Bodies of granite and pegmatite form in the schist, either as injections from below or as a result of partial melting and recrystallization of the sediments themselves. The deformation almost certainly results in mountains at the surface. These events probably occurred about 1,700 million years ago.
Derivation of Step Seven
Schist, being a rock produced deep in the crust, is visible at the surface only in areas from which vertical miles of rock have been eroded away. In the stage we have now reached (Step Six), the Vishnu schist is not only visible at the surface; it has been leveled to a flat surface known as a peneplain. This implies two earlier stages: first, uplift of something on the order of 10 miles, which was required to raise the schist from the zone where it was produced to one where it could be exposed; and second, a long period of stability during which the peneplain reached near perfection. Erosion, of course, was active throughout both stages.
To reverse these episodes and take the next step back in the story, we must undo this erosion by restoring the vertical miles of rock under which the schist formed. It is highly probable that the 10 miles or so of uplift necessary to bring about this amount of erosion would have produced mountains - perhaps several generations of them. And it is almost certainly in the deeper levels of this mountain-making deformation that the Vishnu schist was produced and that the pegmatites and granites were formed or intruded.
So we postulate a mountainous landscape on folded and faulted sedimentary rocks, which grade downward, through tens of thousands of feet, into the metamorphic Vishnu schist.
Fig. 9. Step Eight: Further subsidence results in burial of the Vishnu sediments, shown at the lower right, under several miles of very thick and heavy strata. The earlier Vishnu deposits are now more than 5 miles below sea level.
Derivation of Step Eight
Clues to the nature of the rocks from which the Vishnu schist was derived can be found in its composition and textures. Some layers are clearly derived from impure sandstones; the sand grains can still be distinguished with a microscope. In others, calcareous lumps suggest former concretions. Still others have chemical compositions that resemble those of certain sediments and volcanic rocks more than those of any other plausible ancestors. Some of the banding in the schist resembles sedimentary stratification, even to such details as relict cross-bedding.
From such evidence we conclude that the Vishnu schist (excluding the associated intrusives known as pegmatites and granite) consists largely, if not entirely, of chemically and physically altered sediments and minor volcanics. Before these materials can come up to view as schist they must go down in the crust as sediments and volcanics and there undergo the alteration known as metamorphism.
The next step back, then, is to undo this metamorphism, which takes us back to the stage when the ancestral Vishnu sediments were on their way down to the zone of metamorphism, probably in response to downwarping of the crust. Since subsidence on such a scale cannot take place without inviting sedimentation, we reconstruct a stage in which the Vishnu sediments are subsiding (at the right) and being buried by thousands of feet of overlying material. The character of these overlying rocks is unknown, but their existence is required as a prelude to the metamorphism.
Fig. 10. Step Nine: Subsidence of the crust submerges a large area of this floor and induces deposition of sediments which, with some associated volcanic rocks, are to become the Vishnu schist. Since some of the sediments are types found in shallow or moderately deep water, it is likely that sedimentation more or less kept pace with the subsidence. Some of the volcanic rocks may have originated as submarine eruptions or as intruded sills. The total thickness probably exceeds 5 miles.
Derivation of Step Nine
Before the Vishnu sediments could be buried, they had to be deposited. So the next reconstruction takes us back to the time of accumulation of the sediments that are to become the Vishnu schist.
It is fortunate, for our purpose, that in some of the layers of the schist the recrystallization and contortions have not wholly destroyed the original textures of the deposits. As has already been pointed out, chemical composition and textures show that many layers were once fine sandstones or shales.
Fine sandy and shaly sediments are most abundant today in shallow to moderately deep water, relatively near land - along the Gulf Coast of North America, for example. Accordingly, at this stage our reconstruction somewhat resembles the Gulf of Mexico, to which rivers like the Mississippi and Rio Grande bring abundant supplies of mud and sand to be spread by waves and offshore currents. We add a little volcanic activity to the scene to account for the metavolcanics, not knowing whether the original rocks were erupted on land or on the sea floor, or whether some of them were intruded as horizontal layers known as sills. All we know is that they were of basaltic composition.
It has been estimated that there are more than 25,000 feet of sediments and more than 4,000 feet of volcanics represented in the Vishnu schist; this is more than seven times the thickness of the Paleozoic strata now exposed in the walls of the Canyon.
Fig. 11. Step Ten: The oldest rocks exposed in the Grand Canyon are the Vishnu schists. As they were largely derived from sediments, the earliest event we can infer is the accumulation of these deposits. But on what were they deposited? There must have been a land surface near sea level, which received the first layer of the Vishnu sediments as it was submerged by an advancing sea. This land surface, the floor on which the Vishnu sediments began to accumulate, is the earliest scene we can postulate, although we do not know how it looked.
Derivation of Step Ten
Because in their present metamorphosed condition the original Vishnu strata are wrinkled and folded and standing on end, and because the walls of the inner gorge expose only a small fraction of the total volume of this rock, we know nothing about the character of the top and bottom of the original sedimentary sequence. But of one thing we can be sure: there was a first layer, and this layer was deposited on something.
Consequently, the very earliest event to which the evidence from the Grand Canyon points is the existence of a floor under the original pile of Vishnu sediments. We know absolutely nothing about its character, but as it must have been there a place is provided for the scene without prejudicing the imagination of any who would like to reconstruct it.
Now, knowing why it begins where it does, you should review the history of the Grand Canyon region in the conventional order, Read the legend at the top of this page, then that for Step Nine and the others in sequence back to Step One, studying each accompanying drawing as you do. In so doing you will move forward through time and the order of the events will be the real one. Perhaps the most impressive thing is the great amount of crustal movement that is involved, even without considering changes that may be recorded within, as distinct from between, the major rock units.