The Navajo Sandstone


(EARLY JURASSIC)


SOUTHWEST UTAH

Because of its thickness, massiveness, color, and its decorative carving, the Navajo sandstone is the most conspicuous and best known unit in the Mesozoic sequence in the plateau country. It has been described in many scientific and popular publications and pictured in pamphlets and on postcards issued by tourist bureaus and transportation companies. In Utah it surrounds the Henry Mountains, forms the famous White Cliffs and the walls of Glen Canyon. Complete sections are exposed in Paria Canyon, Kanab Canyon, Parunuweap Canyon, Zion Canyon, and LaVerkin Canyon, and in scores of other deep, narrow gorges that carry water from the Kaiparowits, the Paunsaugunt, and the Markagunt Plateaus. Generally throughout its expanse, the Navajo sandstone lies nearly flat and its sharply truncated edges are unscalable walls of commanding height. In eastern Southwest Utah part of the Navajo retains its normal attitude and is expressed in the topography as vertical cliffs; other parts have been steeply upturned and stand as ridges. The towering escarpment that rims the Kolob Terrace at the heads of Spring and Kanarra Creeks and outlines the lava-capped Square Mountain has been developed by cutting into Navajo sandstone, here fully 1,500 feet thick. Northward across the canyons of Murie, Shurtz, Squaw, and Coal Creeks, where horizontality of bedding is replaced by progressively steep inclination, the edge of the sandstone stands on the skyline as a rugged ridge, here and there broken into pyramids and domes. The Red Wall, prominently in view from Cedar City, is. the base of the uptilted Navajo, from which much of the underlying Chinle formation has been stripped.

Observations at many localities show that in composition and texture the Navajo sandstone in eastern Southwest Utah differs little from that exposed elsewhere. Its salient physical features were long ago outlined by Dutton in his pioneer study on the geology of Markagunt Plateau (1).

"The lithological characters of the Jurassic white sandstone render it a very conspicuous formation. Through a thickness of more than a thousand feet, sometimes of nearly two thousand feet, it is one solid stratum, without a single heterogeneous layer or shaly parting. A few horizontal cracks are seen here and there, but inspection shows that they are merely the seams where two systems of cross-bedding are cemented together. In general, it is one indivisible stratum. This massive character has had its effect upon the cliff-forms that have been sculptured out of it. These forms are bold headlands and gigantic domes, usually without any minor details, but simple in the extreme, and majestic by reason of their simplicity. . . . But of all the features of this rock the most striking is the cross-bedding. It is hard to find a single rock-face which is not lined off with rich tracery produced by the action of weathering upon the cross-lamination. The massive cliff fronts are etched from summit to base with a filagree as intricate and delicate as frost-work."

Supplementing the original description by Dutton, lithic and strati-graphic observations recorded by later students reveal that the dominant cross bedding varies in style from place to place and is locally absent and that the sandstone includes thin, lenticular beds of dolomitic limestone, and in places argillaceous shale and calcareous conglomerate. Detailed examination shows that the Navajo sandstone in Southwest Utah is essentially an aggregate of clear quartz grains of which about 75 percent measure 0.08 to 0.75 millimeters in diameter ; that most of the grains are imperfectly rounded, though many are spherical and some plainly etched; and that, in addition to the dominant quartz, the rock contains fragmentary feldspar, mica, magnetite, more rarely zircon and tourmaline. The cement of the Navajo sandstone consists of loosely compacted lime or dolomite, and iron. The amount and chemical state of the iron oxides are indicated by the color tones: yellow, buff, tan, red, In a few places where leaching has removed the iron, the rock is white, but such great thicknesses of white rock as give character to the White Cliffs in Kanab and Johnson valleys and the Great White Throne in Zion Canyon are absent.

The Navajo sandstone is profusely jointed. Sets of roughly parallel joints with various trends and inclinations cut the sandstones into huge slabs. The major vertical joints are several hundred feet apart and are traceable for as much as a mile. But in places planes of fracture are so closely packed as to form "shatter belts." On flat surfaces the rock joints, open or filled with calcite or iron, appear as surface markings and here and there provide runways for rills. On canyon walls they outline blocks, sheets, and slivers of rock preparatory to their removal by frost and combined with the bedding planes-horizontal, oblique, and curved-determine the shape of talus blocks. The uniformity of grain, the cross bedding, the weak cement, and the joints facilitate the production of the large and small scale erosion features, many times described as characteristic of the flat surfaces, the cliffs, buttresses, and canyon walls developed in the Navajo sandstone.

In Dutton's (2) opinion, the sands that compose the Navajo were deposited in the sea: "The Jurassic sandstone appears to have been a littoral or offshore formation thrown down along the coast of the Mesozoic mainland, which occupied the region now forming the Great Basin . . . its red color becomes more common as we recede from the old shore line towards the east." In common with his co-workers of the Wheeler and Powell survey, Dutton treated the Navajo sandstone as basal Jurassic, though recognizing the possibility that it may be "a mere upward continuation of the Vermilion Cliff series" (Chinle formation: Upper Triassic). More recent studies of sedimentation in the plateau country have compiled evidence that the Navajo is a terrestrial deposit, much of it eolian,20 and that its age is probably Middle Jurrassic.

In tracing the Navajo sandstone northwestward from its type locality in the Navajo Reservation, Arizona, it was noted that the part characterized by curved and angular crossbedding, laminae, and lack of division planes decreases in thickness. Particularly in areas where the Kayenta formation and the Wingate sandstone are lacking and the Navajo rests directly on the Chinle the usual single massive stratum is replaced by a massive stratum and below it a sequence of somewhat regular beds. In Southwest Utah fully half, in places nearly all, the Navajo is displayed as thick and thin layers composed chiefly of wedge-shaped groups of oblique crossbeds. These observations suggest that eastern Southwest Utah lies near the edge of an ancient interior basin where sediments deposited by streams were but slightly rearranged by wind.



NOTES

1. Dutton, C. E., Report on the geology of the High Plateaus of Utah; U. S. Geog. Geol. Survey Rocky Mt. region, pp. 152-153, 1880.

2. Dutton, C. E., op. cit., p. 152.


SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory



SOUTHEAST UTAH

In the San Juan country the Navajo sandstone is exposed in the east and west flanks of the broad Monument up warp. The sandstone forms the crest and eastern slope of the "Comb", the prominent ridge that crosses the San Juan and extends northward between Comb Wash and Butler Wash as a steeply dipping monocline, and continues to crop out along the east base of Elk Ridge and across the Causeway into the Indian Creek country." The walls and mosques and alcoves that make the "wonderland" of the Allen Canyon country are chiefly exposures of Navajo sandstone.

In the remote Red Rock Plateau the Navajo is magnificently developed. The plateau is essentially one great sheet of sandstone, cut into huge segments by the San Juan, Castle, Moki, Red, and Colorado Canyons. On this plateau the Navajo shows its characteristic features of erosion. Along canyons and at their boxlike heads it forms vertical or even undercut walls-sheer cliffs 400 to 600 feet high that can be ascended only at fracture zones or on sand dunes that extend from the bottom to the rim. A traverse of miles of canyon floor may reveal no place where the walls can be climbed. Though the Navajo stands first among cliff makers in the plateau province, it does not form platforms or mesa tops. Unlike the Dakota(?) of Sage Plain and the Shinarump of Elk Ridge, which form extensive nearly horizontal plateaus, the Navajo shows very uneven surfaces. Its composition, texture, and structure combine to produce smooth or ribbed mounds on which stream ways are poorly defined. Between the San Juan River and upper Castle Wash and at the junction of the San Juan and the Colorado the surface of Red Rock Plateau is a maze of domes and saucer like depressions. The intricate network of narrow, deep canyons that carry the run-off from bare slopes seems to be arranged with little regard to surface topography.

The published descriptions of the Navajo sandstone in the Navajo country and in the Kaiparowits region apply equally well to the San Juan country and need only be 'generalized here. In fact, the composition, structure, texture, and style of bedding of the Navajo are remarkably alike throughout the Colorado Plateaus: the differences relate chiefly to thickness, color, and degree of massiveness. Essentially the Navajo is a single massive bed of fine-grain ' marvelously cross-bedded sandstone composed of crystal-clear grains of quartz cemented by lime and iron. Cross-bedding is a scrollwork of curves and parallel lines etched on the surface and strengthened here and there by projecting seams of quartz and rows of cylindrical iron concretions. The Navajo sandstone includes lenses of thin regular bedded sandstone and lenses of resistant limestone a few inches to 5 feet thick and a few hundred feet long. On the rim of Lake Canyon dense blue-gray dolomitic limestone near the top of the Navajo caps low mesas and provided building materials for the walls of prehistoric structures. Numerous vertical and oblique joints outline slabs on cliff faces and in conjunction with cross-bedding determine the position and shape of buttresses, recesses, and alcoves on canyon walls and the caves once occupied by Cliff Dwellers.

As most of the Navajo in the San Juan country has been long exposed to erosion, its original thickness has been reduced. At Comb Ridge and in the Allen Canyon country 300 to 600 feet remains. In the south wall of Wilson Mesa, where the Navajo is overlain by younger strata, a complete section measured 880 feet. At most places west of the Colorado River measured thicknesses exceed 1,000 feet; at Zion Canyon, nearly 2,500 feet.


SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory



NORTHERN ARIZONA

STRUCTURE, TEXTURE, AND COMPOSITION.

The Navajo sandstone is nearly everywhere cross-bedded on a scale which for extent and perfection of detail is difficult to exaggerate. Angular cross bedding was observed, but the prevailing type is tangential; curved laminae become tangent to adjoining surfaces. Starting as highly inclined arcs of small radii the cross-bedding laminae gradually decrease in curvature until they merge into contact with the underlying strata. In some places the arcs are tangent to horizontal surfaces or meet them at angles of 1 or 2'; elsewhere arcs of various radii are tangent to one another. (See Pl. XII, A.) Many groups of curved laminae are sharply truncated along horizontal or inclined surfaces. In places the curved laminae have uninterrupted sweeps of 200 to 300 feet; commonly their length is measured in tens of feet, and many cliff faces are decorated by close-set loops and arabesques comparable with the lathe work in steel engraving. In general the cross-bedding laminae are outlined by layers of weakly cemented quartz grains that determine planes of fracture, but in places major joints exert a stronger control and furnish erosion remnants decorated on all sides by intersecting curved lines.

To the tangential cross bedding are due the exceptional erosion features of the Navajo sandstone the innumerable pockets, recesses, and alcoves bounded by curved planes which characterize this formation. Overhanging cliffs are common, and the beautiful arc of the Rainbow Bridge is only an unusually perfect example of the control exerted by curved lamination.

The prevailing color of the rock is light red and is surprisingly constant over large areas. Among the Segi Mesas and on the Rainbow Plateau the red tint is so boldly applied that no other color appears in the view. In places, however, dark reds and maroons are seen, and not uncommonly orange and even tan colors add variety to the landscape, and patches of white are not unusual. In the Echo Cliffs the rich red tints of the Navajo sandstone fade into yellow gray and become nearly white in the vicinity of Bitter Springs but the Wingate and the Todilto retain their dominant tones. It is interesting to note that all parts of the La Plata in Colorado are described as white and that the White Cliff sandstone of east-central Utah has been correlated with the La Plata.

The Navajo sandstone is composed of translucent quartz grains, with small amounts of feldspar, rare fragments of zircon, magnetite, garnet, pyroxene (?), and tourmaline (?). In two thin sections examined the grains are imperfectly rounded but without sharp edges; a third specimen consists of almost perfect spheres. The grains are of two sizes; probably 90 per cent of the rock consists of grains ranging between 0.15 and 0.25 millimeter in diameter; the other grains, formed as an interrupted coat on cross-bedding laminae, average about 0.65 millimeter. Only at a few localities were much larger pebbles of quartz, of shale, and of sandstone noted. In general the cement is calcite, with large or small amounts of iron oxide, which is reflected in the varying color of the rock. Hand specimens from the Echo Cliffs and the Chinle Valley have siliceous cement. In places the cement is stained green by copper, and in the White Mesa copper district the original cement is partly replaced by malachite and chrysocolla. Much of the cement is weak and grains of calcite and of kaolin are disseminated; the rock is consequently friable, and even where continuously swept by the wind crumbles under foot. It was found possible to trail a man who had strayed from camp by the hobnail prints he made on a bare ledge. In many places the joints in the Navajo are lined with quartz, and their position is indicated by a tracery of thin white ridges intersecting at various angles. In Copper Canyon, on Shato and Kaibito plateau and to a less extent elsewhere, some joints are lined with chrysocolla and other copper minerals.

The limestone, which is an almost universal feature near the top of the Navajo sandstone, is in lenses. The thin outcrops rarely extend more than a few hundred feet, and most of them are measured in tens of feet. The lenses are usually less than 1 foot thick and break up into shaly beds including sandstone. They are exceedingly resistant, however, and form the caps of low mesas and buttes, irregularly distributed over the otherwise smooth surfaces of Navajo sandstone exposures. The two specimens submitted to analysis proved to be dolomite. Chert and chalcedony are commonly found with the limestone.

CONDITIONS OF DEPOSITION.

The significant features of the Navajo sandstone are uniformity of grain, cross-bedding, and red color. Specimens taken from ledges 200 miles apart are indistinguishable in the laboratory by texture or composition or color; tangential cross bedding is persistent. The structure and composition of the rock suggests aridity and the uninterrupted control of the winds, and the "live dunes" now being formed on the floor of Chinle Valley differ only in color from the "frozen dunes" displayed in the bordering rock walls. There is little doubt that desert conditions prevailed in this region during part of Jurassic time, but the boundaries of this ancient Sahara and its relations to highlands and oceans are unknown. The thin lenses of dolomitic limestone and limestone conglomerate in the upper part of the Navajo sandstone probably represent local bodies of water of ephemeral character. It should be borne in mind, however, that these calcareous beds are in the stratigraphic position of the marine limestone of Kanab, described by Gilbert.

AGE AND CORRELATION.

No fossils have been found in the Navajo sandstone, and its age, like that of other formations of the La Plata group, is determined by stratigraphic position and lithologic similarity. It is the equivalent of the upper La Plata sandstone 2 of the La Plata Mountains, from which it differs in no essential except color and thickness.

On Dutton's geologic map and sections 3 the massive "white sandstone" (Jurassic) is extended to cover the western edge of Kaibito Plateau. He says: "The extension of the Jura south of the Colorado and its exposure in the line of Echo Cliffs has been traced for nearly 60 miles." In my view the sandstones forming the crest and escarpment of Echo Cliffs and the walls of Glen Canyon, mapped by Dutton as Triassic, belong in the La Plata group and Dutton's descriptions and illustrations of the "Jurassic white sandstone" suggest lithologic equivalence with the Navajo and also with the massive phase of the McElmo. The upper limit of the La Plata along the southern base of Kaiparowitz Plateau has not been established. Sections on Warm Creek and Sentinel Creek include more than 100 feet of calcareous and gypsiferous shales and thin sandstones between typical Navajo sandstone and massive strata assigned to the McElmo.

I have been unable to recognize with assurance the Navajo sandstone in the Lower Cretaceous and Jura-Triassic strata along the San Juan described by Newberry, or in the Lower Dakota, Upper Dakota, and Triassic mapped by Holmes in the Carrizo Mountain area.


SOURCE
-from Geology of the Navajo Country, A Reconnaissance of Parts of Arizona, New Mexico and Utah by Herbert E. Gregory





Grand Canyon

One of the most prominent and distinctive formations in the Colorado Plateau is the massive Navajo Sandstone. It weathers into nearly vertical cliffs and dominates the landscape wherever it is exposed. In the vicinity of Lee's Ferry the Navajo Sandstone is approximately 1,400 feet thick and caps the high Vermillion and Echo cliffs behind the boat landing (Text-fig. 3). Exposures of the formation are abundant throughout much of the Navajo country to the northeast. In Utah the Navajo Sandstone forms the prominent White Cliffs north of Kanab and the walls of Zion Canyon. Precipitous canyons controlled by joints (fractures) (Text-fig. 3) are cut into most exposures of the formation and produce some of the most rugged and spectacular scenery of the West.

Large scale cross-bedding characterizes the Navajo Sandstone wherever it is exposed. Many outcrops contain some of the most spectacular development of this structure to be found anywhere in the world.

Navajo Sandstone consists of well-sorted, rounded grains of translucent quartz, many of which are etched and frosted. This, together with the large-scale cross-bedding, indicates that the Navajo sediments accumulated in a vast desert which covered much of Utah, Arizona, and New Mexico during early Jurassic time.

Glen Canyon Dam is built in the vertical gorge cut through the formation by the Colorado River.


SOURCE
Geology Studies Vol. 15- Part 5-1968