The Vishnu Schist

(exposed in Arizona’s Grand Canyon)


Gray and reddish rock face with rough surface adjacent to a river.The Vishnu schist is part of the Vishnu complex in the exposed basement rocks of the Grand Canyon region. This metamorphic layer was formed by the intrusion of plutonic masses from under the crust and the deposit of sediment from an eroded mountain chain.

The oldest rocks in the Vishnu complex are deposits of hornblende and quartz that were laid down around 1.8 billion years ago. These rocks were originally part of a deep ocean trench, and they were subsequently overlain by sediments now known as the Brahma schist, which was laid down 1.75 billion years ago. Within a few million years of the Brahma schist deposit, volcanic activity added the felsic rock of the Rama schist. Together, these layers comprise the Vishnu schist that serves as the basement of the entire Grand Canyon area.

Schist is a metamorphic rock type that is commonly formed by the pressure of overlying sediments over a period of millions of years. The rocks of the Vishnu schist are typical of their type, having elongated minerals that can easily be separated into flakes. Some igneous rock is present in the Vishnu complex, though it represents an intrusion that took place considerably later than the original sediment deposits.

 


The oldest rocks within the Grand Canyon are exposed within Granite Gorge aria are characteristically dark somber gray. They respond to erosion to form a steep-walled V-shaped gorge (Text-fig. 56) through which the Colorado River flows from Mile 77 downstream to beyond Phantom Ranch, These (lark colored rocks are evidence of extensive deformation, during which they were subjected to intense heat and pressure and the effects of fluids and gases. The original sedimentary or volcanic characters have been extensively modified and in some cases obliterated. Early Precambrian rocks are not stratified but possess a planar structure known as foliation, resulting from reorientation of platy minerals, crystals, and grains in response to deformation. Foliation throughout most of Granite Gorge is nearly vertical which contrasts with the horizontal stratification of the overlying younger rocks.

Three major rock bodies are found within the Early Precambrian complex. The first encountered on the river trip consists of metamorphosed sedimentary rocks in which some relict. sedimentary structures are preserved. This body of baked and altered rocks is known as the Vishnu Schist and is exposed downstream beyond Hance Rapids to near Zoroaster Canyon. They represent part of the older rocks of the earths crust. Very little detailed information can be gained about their environment of deposition since the original character of the rock has been nearly completely obliterated.

Downstream from Zoroaster Canyon is a sequence of metamorphic rocks which differ in composition, color, and texture from the Vishnu Schist but superficially appear similar to it because of their degree of metamorphism, These rocks are known as the Brahma Schist and probably represent metamorphosed volcanic rocks. Numerous granitic dikes have intruded into both the Vishnu and Brahma schists. Most of these dikes are nearly vertical and parallel to foliation of the schists and stand out in marked contrast to the metamorphic material. Rocks of the dikes are characteristically pink, light colored, and composed of large interlocking crystals of feldspar and quartz, many of which are over a foot in diameter. These intrusions become very numerous in many areas and in some localities make up more than 50 percent of the rock body. Near Zoroaster Canyon dikes are particularly common and one large massive intrusion is dissected by the canyon.

Large granite bodies of the Inner Gorge are referred to as the Zoroaster Granite and represent a period of igneous activity after deposition and metamorphism of both the Brahma and Vishnu Schists but before deposition of the overlying Grand Canyon Series and Paleozoic formations.

The Vishnu Schist in the upper part of the gorge contains many pink pegmatite dikes. Many units within the schist are well foliated and may appear similar to a gneiss. Locally, relict bedding can be seen indicating a sedimentary origin. Foliation is nearly vertical. The gray-green walls of the Vishnu Schist are crisscrossed by dikes of granite. The ragged, ledgy, V-shaped character of the inner gorge is evidenced downstream.

 


Granite Gorge Metamorphic Suite[edit]

The Granite Gorge Metamorphic Suite consists of lithologic units, the BrahmaRama, and Vishnu schists, that have been mapped within the Upper, Middle, and Lower Granite Gorges of the Grand Canyon. The Vishnu Schist consists of quartzmica schist, pelitic schist, and meta-arenites. They exhibit relict sedimentary structures and textures that demonstrate that they are metamorphosed submarine sedimentary rocks. The Brahma Schist consists of amphibolitehornblendebiotiteplagioclase schist, biotite-plagioclase schist, orthoamphibole-bearing schist and gneiss, and metamorphosed sulfide deposits. As inferred from relict structures and textures, the Brahma Schist is composed of mafic to felsic-composition metavolcanic rocks. The Rama Schist consists of massive, fine-grained quartzofeldspathic schist and gneiss that likely are probable felsic metavolcanic rocks. On the basis of the presence of relict pillow structures, interlayering of metavolcanic strata, and the large volumes of metavolcanic rocks, the Brahma and Rama schists are interpreted to consist of metamorphosed, volcanic island-arc and associated submarine volcanic rocks. These metavolcanic rocks are locally overlain by the metamorphosed submarine sedimentary rocks of the Vishnu Schist that are interpreted to have accumulated in oceanic trenches. These metasedimentary rocks were originally composed of particles of quartz, clay, and volcanic rock fragments that have become metamorphosed into various schists. The Vishnu Schist exhibits relict graded beddingand structures indicative of turbidite deposits that accumulated in oceanic trenches and other relatively deep-marine settings. The Brahma Schist has been dated to about 1.75 billion years ago. The felsic metavolcanic rocks that comprise the Rama Schist have yielded an age of 1.742 billion years ago

 

Early Paleoproterozoic basement

The oldest rocks that are part of the Vishnu Basement Rocks is the Elves Chasm pluton. It consists of metamorphosed mafic (hornblende-biotite tonalite) and intermediate-composition plutonic rocks (quartz diorite). Within it, there are tabular amphibolite bodies that might be dikes, that have been dated at about 1.84 billion years ago. It is regarded to be an older granodioritic pluton that was exposed by erosion prior to being buried by the original volcanic and submarine sedimentary rocks of the Granite Gorge Metamorphic Suite. The Elves Chasm pluton is likely part of the basement rocks on which the original volcanic rocks and sediments of the Granite Gorge Metamorphic Suite were deposited.

The highly tectonized contact between Elves Chasm pluton and the Granite Gorge Metamorphic Suite is exposed near Waltenberg Canyon, in 115-Mile Canyon, near Blacktail Canyon, and in the Middle Granite Gorge. This contact is characterized by a high-grade orthoamphibole-bearing gneiss. This gneiss is interpreted to be a highly metamorphosed and sheared paleosol and associated regolith that originally consisted of several meters of weathered rock debris eroded from older plutonic rocks.

Take Away Lessons from my Experience with the Jerold Williams Search

5 year old Jarold Williams.

5 year old Jerold Williams.

I’m a bit heartbroken as day five in the search for five year old Jerold Williams comes to a close, his body was recovered just hours ago.

I spent a good part of days three and four looking for him, and headed home as storms again moved into the area and made the already slim chances of finding the five year old alive, even slimmer. As I was out alone in the dense forest searching for this child, I gave a lot of thought to what could have been done better in his search (and what I would do if this were my child).  I think the number one take home point was mobilize as many volunteers as quickly as possible, and do not let anyone under 16 be alone anywhere in the deep woods–always use a buddy system. Nine year old David Gonzales who went missing in Big Bear California was a grisly reminder to how predators can silently steal away a child without any sound, less than 50 yards from watching parents. (His remains were finally discovered almost a year later, less than a mile from where he went missing in an assumed mountain lion predation). Twelve year old Garrett Bardsley who went missing the same summer in the Uinta Mountains, likewise teaches us that not even older boys are immune from getting lost and never being found in cold, wet weather.

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A few of the mistakes I consider in retrospect.

Because of legal and bureaucratic considerations, as well as worries that volunteer efforts would interfere with dog searches and air support, volunteers were not called for, and actually turned away in the early stages of this search. Because it rained the evening this boy went missing– this was a huge mistake. All scents were destroyed and air support was stifled by inclement weather. Thick forest cover also made air support & thermal imaging useless in many areas. Volunteer and search mobilization was very slow, and because of the rain, may have been the difference between life and death in this event.

Hundreds of volunteers came from the boys Colorado City community by late day two & three, but nearly all of them congregated at the base camp. It became muddy, overly congested and may have made things more difficult for search agencies. No perimeter camps were set up, and very few ventured more than a mile away from the congested base camp. Really, no-one camped away from base camp.

It was easy to be overly optimistic in the first day or two of the search. Because of an optimistic feeling that he would be found, I believe searching was not as thorough, and volunteers were not properly dispersed or valued.

I saw no visible central command tent. Because of this it was hard to tell who was in charge, or where to get the most up to date and reliable information. There was also no real central media outlet for updates, and no human connection to inspire volunteerism outside of the Colorado City community. Because of this only 200-400 searchers participated, when 2,000-3,000 would have been far more effective.

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Suggestions for possible future searches.

-Seek help as soon as possible. But don’t let search agencies completely take over the search. They have legal considerations (especially with liability for searcher they call/control) and bureaucratic considerations which dictate their actions (especially in calling for volunteers and setting up dispersed camps). Go to the media and call for volunteers, and lead the effort which coordinates volunteer efforts with the efforts of the search agencies involved. Law enforcement understandingly often dissuades volunteerism because it distracts from their important efforts. A Father, brother or family friend MUST step up and direct/coordinate all volunteer efforts. He needs to set up a command booth and get volunteers directed & dispersed. There needs to be two heads who work side by side; one for law enforcement and search agencies and one for excess volunteers (those above and beyond what search agencies need or are legally willing to be responsible for). If law enforcement insists that volunteers stay out of the initial search perimeter, they should be directed to search just outside of it. Remember Brennan Hawkins of Bountiful who was found after 4 days by one of nearly 3,000 volunteers in the Uintas. (Garrett Bardsley’s disappearance the year before played a big part in inspiring the huge community outreach– mobilized largely by the Garrett Bardsley Foundation).  At the same time an uncoordinated free-for-all such as the famous Dennis Martin case, needs to be avoided.

-Search and Rescue will typically set up a 2-5 mile radius parameter. But the volunteer effort should focus on manning the outskirts of the perimeter with volunteer campers by the first night. Search parties tend to all congregate at the base camp (usually the place child was last seen).  This creates congestion and complicates the efforts of search agencies. If possible the perimeter should consist of forest roads, cliffs, rivers or fences.  Send 20-50 volunteers to set up camp all along this perimeter and remain as long as needed. Noisy generators and lights should be encouraged. Also (if it can be negotiated with S&R), send volunteers strategically into the search area (as soon as possible) to set up small dispersed backpacking camps (with fires at night if permitted by forest service regulations). Have them set up their tents and lay out sleeping bags before doing any searching. These not only will give the child a greater chance of finding searchers, but will also serve to scare off opportunistic predators such as bears and mountain lions. With any luck they might come back from a search to find the missing child in one of their sleeping bags. Be sure each site is manned by 2-4 people, encourage volunteers to always use the buddy system and always leave some people at the camp area while sending others back to base camp for periodic updates.

-I have yet to read an account of a ‘lost in the forest’ Utah child who was found dead or alive by dogs or thermal imaging equipment. These tools may be useful but they should not preclude the use and placement of volunteers within the search area. Do not allow search agencies to restrict volunteer efforts on account of these tools. Family must press law enforcement to allow them to do this (if not the first night then the second or third after dogs have been through the area). If dogs or trackers can’t find him in the first day, don’t place much faith in them.  I’d love to be proven wrong on this, but I’ve yet to see any solid research showing that search dogs are more than 10% effective, and that thus it’s a fruitful practice to keep volunteers out of the search area for fear they might interfere with the search dogs job.

-Ask for trail runner volunteers the first day. Find very fit teams who can jog the most likely routes from the center point (point of last contact) to the search perimeter. (Make sure they have whistles and bear mace)  These runners could also travel between the dispersed camps to carry news.

-The one who goes for help needs to help create Google Map marked with base camp and a designated search perimeter to be given to the media. They should immediately create a webpage or Facebook page with maps, and accurate up-to-date information. (Use a digital map with offline capability like ArcGis?) The mom or close friend should be encouraged to talk to the media quickly and often–as the more human connection you can make with people the more volunteers you will get. Someone also need to make a few hundred copies of the map with search parameters and bring it back to base camp to distribute to volunteers. Some volunteers can be urged by the media to ride ATV’s on trails outside of the search perimeter, in the unlikely event that the child hiked farther away from base camp than anyone suspects. Supply line volunteers can also be asked for to provide food and water to search personnel. An update should be passed two or three times daily from the field search leads to the home media and website contacts. The more information you can get people, the more volunteers you can get—and the more effective they will be.  Have someone post links to the search website/facebook page on media article and law enforcement pages.

-Helpful details to get from the family and provide to possible volunteers via the webpage media. What direction did they most likely head (where they playing north, south, east or west of camp previous to going missing). Has the child been taught what to do when getting lost and what is their disposition (are they more likely to stay put or try and find their way out)?  How fit is the child (is it common for them to hike several miles or are they more likely to slowly saunter in circles)?

-if no helicopters are available, use drones if possible?

-A printout should be provided for volunteers from the webpage or Facebook page and base camp with some guidelines. It should include 1) Coordinates to base camp and where to go for instruction. 2) A list of oft-overlooked things to bring such as bear spray/mace, whistle or blow horn, flashlight, first-aid kit, compass.

-Once volunteers are mobilized the trick is getting them organized. Search agencies will want two to four large search lines (with 20-30 people each) who will sweep out from the point of last contact and thoroughly comb the designated areas within a mile or two of base camp.  But someone needs to organize smaller search groups (3-6 people) who hike the backcountry and do quick sweeps of the more remote regions sweeping in from the parameter camps.

-Speed is of the essence!  By day two cold conditions and hypothermia can bring loss of consciousness or make lost people do irrational things. Resist the urge to be optimistic to the point where things that could have been done, aren’t done.

-By day three or four volunteers need to be told to look in trees with large branches and under pine bows and other cover where predators are likely to stash or bury their prey.

-It is important for well thought-out checklists of things to do to be created, before an emergency like this.  As illustrated in Atul Gawande’s book “The Checklist Manifesto”, professionals such as airplane pilots and surgeons have been found inevitably to make mistakes in high stress situations unless a checklist exists which can help them remember and practice what they already know. Gawande’s research team has taken this idea, and developed a safe surgery checklist, and applied it around the world, with staggering success.

-I need to create a mockup web page, map and hand-out with step by step instructions that could be used as a template for a search situation. This boy may had lived had he been found earlier. With the skyrocketing increase in tourism of Southwestern Utah’s forests this situations may become more prevalent. Perhaps it would be helpful to create a few brochures, training curriculum or even some training videos to pitch to the dept of public safety or the FCAOG (Five County Association of Governments for Southwest Utah), who work to coordinate resources for local sheriff’s offices.

-When possible, equip even young children with survival items in a back-pack, when hiking in the woods. Including a laser pointer or small LED light, a whistle, a thin poncho or garbage bag with a hole cut in it. Teach them the two essentials, stay warm and stay put. Teach them they should only move if it is needed to stay warm. If they must move to find shelter in order to stay warm (ie. from rain), they need to build arrows to show where they went.

 

example of a map showing base camp, search perimeter,  perimeter camps, and high priority search areas.

example of a map showing base camp, search perimeter, perimeter camps, and high priority search areas.

References
https://www.google.com/maps/d/edit?mid=zKU37Xu8MQ1s.k_W4h_jPgygQ&usp=sharing
http://www.nytimes.com/2005/06/22/us/after-four-days-a-boy-scout-missing-in-utah-is-found-alive.html
http://www.huffingtonpost.com/2011/08/13/jared-ropelato-boy-scout-utah_n_926335.html
http://www.deseretnews.com/article/595086737/Searchers-find-missing-Boy-Scouts-sock.html?pg=all
http://articles.latimes.com/2005/jun/01/local/me-bones1

 

See also What Was I Thinking?! :( Thoughts on Inspiration and Intuition from the ill fated Search for Jerold Williams

Navajo Sandstone

The Navajo Sandstone is a geologic formation in the Glen Canyon Group that is spread across the U.S. states of southern Nevada, northern Arizona, northwestColorado, and Utah; as part of the Colorado Plateau province of the United States.

The Navajo Sandstone formation is particularly prominent in southern Utah, where it forms the main attractions of a number of national parks and monuments including Red Rock Canyon National Conservation Area,[3] Zion National Park, Capitol Reef National Park, Glen Canyon National Recreation Area, Grand Staircase-Escalante National Monument, and Canyonlands National Park.

Navajo Sandstone frequently overlies and interfingers with the Kayenta Formation of the Glen Canyon Group. Together, these formations can result in immense vertical cliffs of up to 2,200 feet (670 m). Atop the cliffs, Navajo Sandstone often appears as massive rounded domes and bluffs that are generally

 

Appearance and provenance

Navajo Sandstone frequently occurs as spectacular cliffs, cuestas, domes, and bluffs rising from the desert floor. It can be distinguished from adjacent Jurassic sandstones by its white to light pink color, meter-scale cross-bedding, and distinctive rounded weathering.

The wide range of colors exhibited by the Navajo Sandstone reflect a long history of alteration by groundwater and other subsurface fluids over the last 190 million years. The different colors, except for white, are caused by the presence of varying mixtures and amounts of hematite, goethite, andlimonite filling the pore space within the quartz sand comprising the Navajo Sandstone. The iron in these strata originally arrived via the erosion of iron-bearing silicate minerals.

Initially, this iron accumulated as iron-oxide coatings, which formed slowly after the sand had been deposited. Later, after having been deeply buried, reducing fluids composed of water and hydrocarbons flowed through the thick red sand which once comprised the Navajo Sandstone. The dissolution of the iron coatings by the reducing fluids bleached large volumes of the Navajo Sandstone a brilliant white. Reducing fluids transported the iron in solution until they mixed with oxidizing groundwater. Where the oxidizing and reducing fluids mixed, the iron precipitated within the Navajo Sandstone.

Depending on local variations within the permeability, porosity, fracturing, and other inherent rock properties of the sandstone, varying mixtures of hematite, goethite, and limonite precipitated within spaces between quartz grains. Variations in the type and proportions of precipitated iron oxides resulted in the different black, brown, crimson, vermillion, orange, salmon, peach, pink, gold, and yellow colors of the Navajo Sandstone.

The precipitation of iron oxides also formed laminea, corrugated layers, columns, and pipes of ironstone within the Navajo Sandstone. Being harder and more resistant to erosion than the surrounding sandstone, the ironstone weathered out as ledges, walls, fins, “flags”, towers, and other minor features, which stick out and above the local landscape in unusual shapes.

 


 

 

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.

 

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.

 

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.

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.

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