Saturday, December 24, 2011

Erosive Magic at the Devil’s Garden in the Canyons of the Escalante


“And so castles made of sand
fall into the sea
eventually.”
From Castles Made of Sand  by Jimi Hendrix, 1967 


Step back into childhood and let your wildest imaginations take over. Stone goblins. Grotesque gnomes. Fanciful hoodoos. Psychedelic mushrooms. Daliesque arches. Sinister trolls. They’re all there, at Devil’s Garden...part geological excursion and part bedtime story.



WHERE IS THE DEVIL'S GARDEN?
Devil’s Garden, all 10 acres of it, is in the Escalante Canyons section of the Grand Staircase-Escalante National Monument in south-central Utah. It's not to be confused with the garden of the same name in Arches National Park, east of Moab, Utah. Leaving Route 12, drive south from Escalante, Utah, on the Hole-in-the-Rock Road which follows the trend of the Straight Cliffs on the west and the pioneer Mormon's Hole-in-the-Rock trail on the east. After about 13 miles, there’s a turnoff for Devil’s Garden.

HOW DID THE GARDEN GROW?
A few miles to the north of the Garden, along the base of the Straight Cliffs, a small drainage course began a gradual descent to the east as it cut through layers of the Tropic Shale and the Morrison Formation. Turning south to parallel the road, it carved a serpentine track through the undulating plain of the desert downward through the Entrada Sandstone. At Devil’s Garden, erosion sculpted the sandstone into a vast array of whimsical and grotesque shapes.

Simply stated, the Garden represents a region of rock that is more resistant to erosion. Its features all formed from resistant deposits at the top of the slickrock Gunsight Butte Member of the Entrada Sandstone, an orange-brown to red-orange, wind-blown deposit laid down in dunes. To the west toward the cliffs, it is overlain by the brown to red-brown Cannonville Member of the Entrada, a bedded, earthy-weathering, slope-forming sandstone. Above the Cannonville, the Entrada's Escalante Member forms light yellow, are rock-sandstone outcrops at the foot of the Straight Cliffs. The Cannonville Member, containing more clay and silt than the underlying Gunsight Butte and overlying Escalante Members, erodes more readily. Erosion has stripped the Cannonville Member away and exposing the more resistant Gunsight Butte Member.

Interestingly, at the "other" Devil's Garden in Arches National Park, its arches are also formed of Entrada Sandstone, but involve a different stratigraphic member of the Entrada, a contact with an underlying formation, and a different tectonic scenario.



THE FORMATION OF THE ENTRADA SANDSTONE: TECTONICS, ACCOMMODATION SPACE, WIND AND LOTS OF SAND
ERG TECTONO-GEODYNAMICS
The region of the Colorado Plateau during the Mesozoic contains perhaps the best exposed, best documented and highest percentage of desert-sediments in the stratigraphic record of the Earth known as ergs or sand seas. It is believed that the great Jurassic eolian deposits of the Plateau are associated with large-scale tectonic events. Beginning in the Middle Paleozoic to the Early Cenozoic, a continuum of eastward, progressive, and punctuated, yet continuous deformational pulses contributed to the growth of the western margin of Laurentia and the Cordillera. 

During the Jurassic, the generation of shortening events of the Cordillera flexed the continental interior downward. That created a wide topographic depression in direct association with the mountain belt to the west capable of exerting a rain-shadow effect and an environment of prolonged aridity to the east.  Flexural basin subsidence in such a retro-forearc environment provided accommodation space for the preservation of the enormous sand-ergs (and non-eolian deposits) that evolved.

TWO GROUPS AND FOUR ERGS
The Early Jurassic Glen Canyon Group of ergs (Wingate and Navajo, and lateral correlatives of Aztec and Nugget Sandstones) and the Middle Jurassic San Rafael Group of ergs (Page and Entrada) are generally assumed to be associated with the dynamic subsidence generated by the onset of oceanic Farallon slab subduction beneath the North American plate. Note that both groups possess fluviatile and marine components as well.

THE SUNDANCE SEA
The Middle Jurassic Utah-Idaho Trough in Utah has been interpreted as a foreland basin system. It has been speculated to be along the leading edge of the Elko Orogenic belt. Its foredeep was flooded from the north via communication with the Pacific Ocean and facilitated by rising global eustatic sea levels. As sea level fluctuated within the epeiric marine incursion, known as the Sundance Sea (or Zuni Sea for the name of the global transgression), a complex interfingering of marine, marginal marine and non-marine beds formed. These deposits are collectively represented by the San Rafael Group. The Carmel Formation was deposited near the south margin of the sea while the Entrada Sandstone formed from desert dunes to beach and back-beach sands. 

SOURCES OF THE SAND
Geologists (Dickinson and Gehrels, 2011) have found that detrital grains in Jurassic eolianites of the Glen Canyon and San Rafael Groups were derived mostly from Precambrian and Paleozoic granitoid basement provinces in eastern and central Laurentia, and some later (after 285 Ma) from rock assemblages of the nearby Cordilleran orogen. Most age populations reflect derivation from Paleozoic, Neoproterozoic and even Grenvillian sources within the Appalachian orogen or its sedimentary cover. One hypothesis involves the transport of sediment upwind to the north of the Colorado Plateau by a transcontinental Jurassic paleoriver, dispersal by paleowinds confirmed by analyses of eolian cross-bedding, and recyclization by regional depositional systems.

THE BIG PICTURE SUMMARY
The generation of enormous volumes of sand during the arid conditions of the Jurassic, in association with persistant paleowinds, resulted in a massive sand budget that accumulated in a tectonically-formed accommodation space.

THE SAN RAFAEL GROUP AND THE ENTRADA SANDSTONE
This Middle Jurassic (160 Ma) paleographic map (modified from Ron Blakey, NAU Geology) shows the Sundance Sea having invaded Utah from the north. The red dot represents the approximate location of the Devil's Garden in south-central Utah. The sea blanketed the region of the Garden (and at times well beyond through Utah to the Utah-Arizona border and eastward onto the west flank of the Ancestral Rocky Mountain's Uncompahgre Uplift) with sediments of the San Rafael Group. The stratal components of the group vary with the geography across the region as the seaway expanded and contracted. This has greatly  contributed to the complexity of the strata and their inter-relationships, an ongoing source of re-interpretation amongst geologists and stratigraphers.

Very basically, the San Rafael Group is comprised of marine and terrestrial deposits in and around the sea consisting of mudstones, sandstones, limestones and gypsum: the Carmel Formation (siltsone, mudstone, sandstone, limestone and interbedded gypsum deposited near the southern margin of the shallow sea); the Page Sandstone (lies unconformably on the Navajo Sandstone); the Entrada Sandstone (beach and back-beach sands, frequent sabkha); Curtis Formation (marine sandstones and mudstones): Summerville Formation (siltstone, sandstone and gypsum).

 It is the Entrada Sandstone that is integral to the genesis of Devil's Garden. At one time, the Entrada Sandstone covered most of southern Wyoming, Utah, Colorado, northern Arizona and New Mexico, and was almost as widespread as the Navajo erg. 



Eventually, the sedimentary sequences of the foredeep were uplifted and the sea withdrew. During the Late Jurassic, the deposits of the San Rafael Group were covered by those of the Morrison Formation from anastomosing and meandering streams on broad floodplains from the rising highlands to the west.

LET YOUR IMAGINATION GET THE BEST OF YOU

 
This is Metate Arch, perhaps the most photographed feature at Devil's Garden.


Doesn't this look like someone you know?


The erosive magic of Devil's Garden is created by weathering, both mechanical and chemical. Agents of erosion, endless cycles of rain, wind, snow and ice, enter cracks and fissures within the rock. Repeated freezing and thawing breaks the rock at the surface by frost wedging. Rainwater containing absorbed atmospheric carbon dioxide dissolves the calcium carbonate cement. Summer thunderstorms carry away the accumulating debris. Softer rock erodes more readily than resistant rock. In time, isolated fins, ridges and pedestals begin to appear, slowly sculpting the rock into the hoodoos of the Garden.



This thin fin has eroded into a window. Perhaps one day a delicate arch will begin to appear.


An inedible Entrada mushroom



Various stratal horizons within the Entrada are evident and are reflected in its patterns of erosion.




Sunday, December 11, 2011

Memorable Places Here and There on the Colorado Plateau: The “Wide, Open Spaces” of Escalante


For me, there’s nothing like “the wide, open spaces.” Perhaps it comes from growing up in Central New York State. The landscape there is quite striking, but the horizon is usually only as far as the next glacial drumlin.


This expansive vista looks down the long escarpment of the Straight Cliffs, the cliff-face of Fiftymile Mountain in central-south Utah. Follow the cliffs to the horizon through the rugged, wash-punctuated, desert benchland, and you end up at a spectacular drop-off at Lake Powell, the man-made reservoir created by the flooding of Glen Canyon
by the controversial Glen Canyon Dam. That’s exactly the route that the early pioneers of the Church of the Latter Day Saints, better known as Mormons, followed in 1878 called the Hole-in-the-Rock Road.

Their mission was to establish a new settlement in the region of the San Juan River in southeastern Utah. They did so by forging a road through the desert from Escalante, negotiating the sheer cliff at the Hole-in-the-Rock and crossing the untamed Colorado River with 234 men, women and children, 83 wagons, livestock and all their worldly possessions. Their epic “shortcut” was only used for one year before being abandoned. We drove on the Hole-in-the-Rock Road, the contemporary version, that essentially parallels their trail through the region of the Escalante Canyons.

GETTING OUR BEARINGS
The Straight Cliffs rise 1,110 feet or more, and as their name implies, extend for 50 miles to the southeast. The cliffs form the eastern escarpment of Fiftymile Mountain called “mountain lying down” by Native American Paiutes and the “Fifty” by local Mormons. Its rock face is a long, nearly continuous wall from the town of Escalante, Utah, southward to the Colorado River. Largely free from side canyons or protruding spurs, only two canyons throughout its length break its otherwise straight line of cliffs, greened with clumps of juniper, sagebrush and piñon.

At the foot of the cliffs runs Fiftymile Bench, a platform or broad terrace with a line of smaller, lower cliffs of its own, and more well-developed in the southern reaches of the Straight Cliffs. At various intervals, a succession of cusps juts out from the long bench.

And below the bench, lies the sagebrush, blackbrush, rabbitbrush and cacti decorated desert through which we traveled. The area is remote, isolated and majestically beautiful.

A GEOLOGICAL BOUNDARY
The Straight Cliffs and Fiftymile Mountain form the geological boundary between two of three sections of the 1.9 million acre Grand Staircase-Escalante National Monument (GSENM), the central Kaiparowits Basin section, and the easternmost Escalante Canyons section. The desert to the east of the Straight Cliffs, on which we traveled, marks the beginning of the Escalante Canyons region. It includes the winding Escalante River and the canyons it has dissected largely through the Glen Canyon Group’s sandstones.

The uppermost diagram (below) shows the centrally-located, dissected-mesa of the Kaiparowits section of the GSENM, viewed from a northern perspective. Fiftymile Mountain and the Straight Cliffs (circled) form the natural boundary with the easternmost section of the monument, Escalante Canyons (lowermost diagram). The Escalante Canyons section is viewed from a southern perspective.   



THE STRAIGHT CLIFFS ARE GRAY CLIFFS
The Straight Cliffs are one of the boldest expressions of the Gray Cliffs of the Grand Staircase, the GSENM's westernmost section. The Staircase is a series of topographic benches and cliffs, and that, as its name implies,  progressively steps up in elevation from south to north, from northern Arizona into southern Utah. The Straight Cliffs lies to the east and well outside the Grand Staircase section but are composed of the same durable Cretaceous sandstones that form the second highest riser of the Grand Staircase’s Gray Cliffs.



THE REGIONAL GEOLOGY IS A REFLECTION OF THE TECTONIC BIG PICTURE
The relentless drought of the Early Jurassic that dominated the vicinity of the future Colorado Plateau  brought the Wingate and Navajo wind-driven sand seas of the Glen Canyon Group to the region. Likewise in the Middle Jurassic, the Page and Entrada Sandstone eolian ergs inundated the region, but this time in association with the Sundance Sea, a narrow restricted arm of the ocean that entered from Wyoming into the subsidence space of the Utah-Idaho Trough, a foreland basin. The complex and varied deposits of the sea's fluctuating shoreline left the sediments of the San Rafael Group (such as the region's Carmel, Page and Entrada silt and sandstones).

During the Late Jurassic the region experienced the widespread, fluvially-generated Morrison Formation in the wake of the Nevadan Orogeny to the west, a mountain-building event that contributed to the formation of the Cordilleran Arc. Cordilleran Orogenesis in the western United States spanned at least 120 million years from the Middle Jurassic into early Eocene time. It comprised numerous mountain-building events that culminated with the formation of an enormous, elongate mountain range from Alaska to southern Mexico, with a complex and diverse stratigraphy.

Beginning in the latest Jurassic, the subduction of the oceanic Farallon Plate beneath the North American Plate along western North America resulted in the formation of the Sevier fold and thrust belt. Siliciclastic sediments were shed from the west carried by rivers into a seaway that formed in the immense, flooded  foreland basin that developed in the center of the continent. During the Cretaceous, the Western Interior Seaway inundated most of the interior of North America including the GSENM area in southern Utah, leaving a vast array of sediments as its shoreline changed and sealevel rose and fell with deposits such as the region's Dakota Formation deposited in coastal areas ahead of the encroaching sea. The Tropic Shale represents muds deposited at the bottom of the sea. The Straight Cliffs, Wahweap and Kaiparowits Formations represent sediments that were deposited on a piedmont belt between the mountains and the sea, after the sea retreated to the east.

These events left their deposits after which Paleogene uplift and dissection painted the finishing touches on the canvas of the landscape. On the map below at about 85 Ma, note the location of the Sevier Highlands, its associated thrust fault, and the future landform of the Kaiparowits Plateau. The Straight Cliffs-Fiftymile Mountain boundary between it and the Escalante Canyons section to the east formed after Laramide uplift of the Colorado Plateau and the subsequent dissection of the region locally by tributaries of the Colorado River System into the deposits of the Glen Canyon Group.



THE REGIONAL GEOLOGY
THE CLIFFS
The northwest to southeast-trending Straight Cliffs looks somewhat like the east to west-trending Book Cliffs located further to the north, formed under similar depositional conditions and time frames. The former’s trend is roughly parallel and the latter’s trend is roughly perpendicular to the ancient shoreline of the Western Interior Seaway where they were deposited nearly 90 million years ago during the Late Cretaceous. The Straight Cliffs are composed of dark gray, massive marine shales interbedded with tan sandstones. They contain a diverse fluvial and marine architecture of offshore, shoreface, coastal plain, paludal and fluvial facies that reflect the transgressive-regressive whim of the seaway's fluctuations in sealevel. With the mind's eye staring at the Straight Cliffs, you can see layer after layer of ancient beaches and barrier islands formed by the shifting shoreline similar to the Atlantic Coast of today.

The majority of the cliffs are composed of the Straight Cliffs Formation's John Henry Member, a slope and ledge-former, representing thick beach sandstone beds separated by muddy sandstones deposited in a shoreline environment. The John Henry also contains thick, lagoonal coal deposits. Two members form the base of the cliffs, the Smoky Hollow Member (back-beach and lagoonal deposits) and resting on the Tibbett Canyon Member (offshore sandstones).

THE BENCH
Fiftymile Bench is built on the Late Jurassic Morrison Formation, much of which is covered by colluvium and landslide debris derived from the overlying Straight Cliffs Formation. Beneath the bench’s debris, windows of the underlying gray, muddy Tropic Shale and thin Dakota Formation are present. In places, Pleistocene-age, mass wasting deposits have cascaded over the bench’s lower cliffs and ledges in the Morrison Formation's Salt Wash Member to the desert-flats below. In one particular locale, seen from a distance, the erosional process of a flow is evident in the formation of hoodoos (below).



THE FLATS
The Hole-in-the-Rock Road basically follows a strike valley in the Middle Jurassic Carmel Formation. Below Escalante the road is near the top of the Paria River Member of the Carmel but soon enters the Winsor Member. Prominent landforms projecting above the Carmel desert (such as Sooner Rocks where we made camp), are composed of the orange-brown Gunsight Butte Member of the Entrada Sandstone. As the road undulated with the terrain, it weaved on and off of benches of unconsolidated Pleistocene and Holocene mixed eolian and alluvial deposits, and Entrada Sandstone. The overlying, softer, slope-forming Cannonville Member of the Entrada contains more clay and silt. To the east of the road, washes, slots and narrow intricate canyons exhibit the contact with the Windsor Member of the Carmel Formation. These dissections merge into larger corridors that eventually fuse with the Escalante River gorge, which ultimately joins the Colorado River, today drowned by Lake Powell.

This map of the GSENM illustrates the relationship of its three sections and the geological boundary that the Straight Cliffs and Fiftymile Mountain form between the Kaiparowits and Escalante sections. Also notice the Hole-in-the-Rock Road paralleling the strike of the cliffs from Escalante to the southeast towards the Colorado River and Lake Powell. The Kaiparowits Plateau is roofed with marine, fluvial and floodplain deposits; whereas, the Escalante Canyons have been unroofed of such until east of the Waterpocket Fold. 



Here's a bedrock map at the Straight Cliffs-Fiftymile Mountain boundary zone. Note the Hole-in-the-Rock Road running from Escalante to Lake Powell. 
  


Stratigraphic column in the region of the Straight Cliffs (circled)



THE SIGHTS
Having departed from the town of Escalante, we headed south on the Hole-in-the-Rock Road. This view looks back to the north toward Escalante with the Straight Cliffs located to the west. The photo was taken while literally standing on the Mormon's Hole-in-the Rock trail through the desert. Our SUV, off to the right, is parked on the Hole-in-the-Rock Road. After having been constructed over 130 years ago and having been used for only one year by Mormon pioneers, you can still see the swales created by the Conestoga wagon wheels through the red Carmel soil.



Continuing our journey south on the Hole-in-the-Rock Road for fifty miles or so, we turned off towards the cliffs and took a scenic detour on Fiftymile Bench Road. The road uses a landslide over Morrison cliffs to gain access onto Fiftymile Bench. Seen below, we ascended numerous switchbacks on a rugged road that led us to the top of the debris flow that came off the bench. The view of the desert flats far below (at about 4,300 feet above sea level) in the Carmel formation, and the Escalante Canyons and watershed of the Escalante River in the distance was quite spectacular.

Unbeknownst to us at the time, we were to make camp that night at Sooner Rocks, the barely visible, rocky outcrop in the center of the photo. The debris flow (at about 5,600 feet) on which we were ascending is comprised of an unconsolidated rocky mix largely from the Straight Cliffs above. You can spot the array of boulders in the foreground that have cascaded down from the cliffs and the road snaking upward from below. Also notice a cusp of the Fiftymile Bench extending off to the left in the Morrison Formation. About 50 miles away, the Henry Mountain laccolithic complex is faintly discernible at the horizon to the right (north-northeast)  beyond the Waterpocket Fold, while Boulder Mountain capped in Tertiary lava flows is far to the left (north-northwest). 



With sunset rapidly approaching and the wind picking up, we began an earnest search for a good spot for camp. Every desirable campsite with a sheltered wind-break seemed taken. Eventually we settled on Sooner Rocks for camp just off the Hole-in-the-Rock Road, built of smooth, slick, red Entrada Sandstone in the form of a cluster of resistant, domed, bare-rock outcrops. At sundown the temps began to drop and the wind began to gust at 40 mph plus. We set up camp, staked and secured our tents, opened a fine bottle of wine and watched the sun set, while bracing for a storm at night that never really came.


“Goodnight stars, goodnight air, goodnight noises everywhere.”
From Goodnight Moon by Margaret Wise Brown


In the morning, we awakened to a light rain and a spectacular cloud break at sunrise that ignited the Sooner Rocks in brilliant Entrada-orange. Like the Navajo Sandstone, the Entrada Sandstone exhibits large-scale eolian cross-bedding and weathers to smooth surfaces. Notice its swirly, undulating cross-beds which have been selectively etched-out by erosion. It reminded me of graded fields back home in the northeast that had been harvested of corn. Also, notice the criss-crossing, wavy and anastomosing, whitish network of deformation bands on the sandstone-dome. Probably of tectonic origin, they are indicative of accommodation to normal slip-movements, many exhibiting offsets. Their lighter, white color is likely attributable to variable bleaching through interaction with hydrocarbon-bearing solutions or other reducing agents, and indicative of the host sandstone's permeability early in its developmental history. Geochemical modeling implies the removal of some iron by fluids after chemical reduction, further contributing to their color.


As the rising sun illuminated the nearer bench portion of the escarpment to the west, we noticed a dusting of snow along the top of the Straight Cliffs, highly atypical for late May. Exquisite!


After investigating a few slot canyons in the area, we returned to the Hole-in-the-Rock Road and headed north, back to Escalante. This view nicely shows the banded stratigraphy of the Straight Cliffs, the Fiftymile Bench in the multi-colored Morrison Formation, and the heavily vegetated, Carmel desert on which we've been traveling.


“Now...Bring me that horizon" (Pirates of the Caribbean)


Highly Suggested Reading: Geology of Utah's Parks and Monuments by the Utah Geological Association and Bryce Canyon Natural History Association, Second Edition, 2003.

Friday, November 25, 2011

Hopi Corn, Kachina Rain and Lessons from the Past



“Over your field of growing corn
All day shall hang the thunder-cloud;
Over your field of growing corn
All day shall come the rushing rain.”

Last stanza of Korosta Katzina Song from the Hopi corn-planting dance

This thousand year old petroglyph at Hopi Clan Rocks in Northern Arizona depicts lightning and clouds with rain falling on a stalk of corn. The rock-carving was created by chipping through a thin veneer of desert varnish into the lighter colored, virginal surface of a displaced block of Wingate Sandstone.

HOPI CORN
The Hopi people or “peaceful ones” are thought to have migrated north out of Mexico around 500 B.C. Primarily living on a 1.5 million acre reservation in northeastern Arizona in the Four Corners area, the Hopi have the longest authenticated history of occupation of a single area by any Native American tribe in the United States.

The Hopi have no religion in the traditional sense. Hopi life IS Hopi religion. There is no separation of a religious life from all other activities of the Hopi. Planting corn is a religious activity, amongst others, that ensures the continuation of life.

For the Hopi, corn is viewed as a metaphor of life. The Hopi say, “Um hapi qaa’oniwti.” “People are corn.” Beginning as seeds, as in a womb, life emerges, blessed by light and nourished by family. A Hopi child is brought from the house on the twentieth day and receives corn as the sun emerges on the eastern horizon. Throughout life, Hopi live with corn as the mainstay of their diet. For Hopi, death is part of the cycle of life. Death does not end a person’s presence in the physical world, but marks a transition from one state of being to another.

KACHINA RAIN
The Hopi believe that it is through respect of nature and spirit essences of the world of the Katsinas that will bring the rains needed to support life. It is both a reciprocity of life and rain that makes the corn grow. It is also the cycle of the corn seed becoming both the food for Hopi and the seeds of the future, and of Hopi life itself. The Hopi emerge and live only to die, and yet continue as ancestral Hopi to support their offspring as the spirit essences that bring rain. At death and their emergence into the Fourth World, Maasawu, the god of death, instructs the people on how to farm the land, to use it only with humility and with good harmonious hearts. Arrogance, disrespect, greed and failure to maintain their obligations to the Creator would bring sparse rains and their labor would be in vain. 

The spirits of important Hopi leaders go to the San Francisco Peaks, north of Flagstaff. Each year, the spirits return to Hopi Land during the Kachina season as bearers of rain, riding within billowy, white clouds. They come in response to Hopi prayers and powers generated by their ceremonies. The rain brought by the Kachinas is essential to crops of the Hopi, as it augments their only other water supply, ground water, a shrinking resource today. The Hopi know that a drought can come at any time. They know that their conduct has a direct bearing on the amount of rain that comes. If the Hopi behave badly, the Kachinas will be displeased and refuse to bring rain. Without rain, nothing will grow, and there will be nothing to harvest in the fall.

Ancestral Puebloans, such as the Hopi, have been cultivating crops adapted to the arid climate of the Colorado Plateau for thousands of years. The Hopi, who have had a long and deep cultural relationship with the Southwest's aridity, use a practice called dry-land or un-irrgated farming by taking advantage of run-off and flood-water from mesas. They farm at the mercy of the spirits to answer their prayers.  

Historically, in the late 1200’s, a massive and prolonged drought forced most of the Hopi villages on the mesas to be abandoned. Perhaps after years of intensive use the land and its resources were depleted. In the face of environmental stress, social and political conflicts are thought to have arisen. For well over a decade, widespread and persistent drought conditions have again plagued the region. Climatologists predict an indeterminate length to these conditions both regionally and globally. Many predict worse. In response, the Hopi Tribe and Navajo Nation’s resource managers are developing a regional climate monitoring network and are discussing long-term climate change adaptation to better prepare for the climate of the future.


LESSONS FROM THE PAST
The lessons of geologic history teach us that western North America has experienced some of the most long-lived arid conditions in Earth’s history. Widespread eolian sandstones in the geologic record bear testimony to this fact. In the Glen Canyon region alone, seven different eolian units are exposed. Drawing the majority of their waters from snow melt in the Rockies, Lake Powell and Lake Mead have achieved record low levels. And in the Southwest, population growth and demands continue to increase. The notion that severe arid conditions are only temporary regionally or can’t be experienced globally should be entertained only with reckless arrogance and abandon. This is true independent of one’s philosophical position on the causes of climate change.

Hopi corn, as with most agricultural crops, can tolerate only a narrow latitude of temperature extremes, drought and flooding, and pathogen and pest resistance. Advances in agricultural knowledge, technology and science are critical to improving crop traits such as tolerance. Many believe agricultural science has gone too far in the use of recombinant DNA techniques to produce transgenic products that could adversely effect the environment and human health. Others believe that advances in genomics will play a critical role in traditional plant breeding as well as in genetically modified crops. Regardless, if the climatologists are correct, time is of the essence. It takes on average a decade and $100,000,000 to breed a new transgenic crop cultivar and for it to become available to farmers.

Many feel that climate change could result in destabilization and the escalation of conflicts as crop yields fall on both a regional and global scale. Southwestern archaeologists have interpreted signs of precisely that having happened with the Ancestral Puebloans in the face of widespread drought.

The world’s population has reached 7 billion. Statisticians tell us there’s a 1 in 7 chance that a person will be born hungry and that nearly 1 billion people go to bed hungry each night. Given predicted climate change scenarios, global food production is unlikely to satisfy future demand without making advances in crop improvement, better use of nutrients, stress tolerance, land management, control of greenhouse gas emissions and crop breeding.

"The corn grows up. The waters of the dark clouds drop, drop.
The rain descends. The waters from the corn leaves drop, drop.
The rain descends. The waters from the plants drop, drop.
The corn grows up. The waters of the dark mists drop, drop."

Fertility song of the Navajo Indians

Saturday, November 19, 2011

Memorable Places Here and There on the Colorado Plateau: Ribbon Falls




About eight miles down the North Kaibab Trail from the Grand Canyon's North Rim, a short detour off to the right beckons sun-parched backpackers to Ribbon Falls. Its irresistible mist is near impossible to forgo on a typically hot and dry day in the canyon, making this side excursion a necessity to visit. But what’s truly fascinating is the geological structure that the falls have produced. The action of ground water, by virtue of its mineral composition, has resulted in the formation of a spectacular travertine dome that's over thirty feet tall.

How did this colossal structure form? Water from the falls makes a 120 foot free-fall landing precisely at the apex of the moss-covered travertine dome. Calcium carbonate is in solution, being made soluble by the absorption of atmospheric carbon dioxide, which makes the water mildly acidic. Its acidity allows the carbonate to be “acquired” from limestone formations at higher elevations such as the Redwall and Muav. Subsequently, carbonate is “released” from the mineral-rich dripping water when it plunges over the falls and releases the carbon dioxide held in solution. The change in water chemistry causes the re-deposition of the carbonate in the form of travertine or tufa (softer and more porous) from the mineral-laden water. Gradually, the mound grows by re-crystallization, molecule by molecule. This landform is called karst, made possible by the dissolution of soluble bedrock. The identical process forms the more familiar stalagmites and stalactites in subterranean limestone-caverns.


Ribbon Falls is located in an amphitheater bounded by dark red cliffs of Shinumo Quartzite. The falls plunge over the ledge of a resistant diabase sill. Diabase is the intrusive equivalent of basalt. This sill is part of a system of Cardenas conduits and a massive basaltic outpouring of the same name that fed magma to the Earth’s surface. These rock formations, along with three others, are members of the Unkar Group, which comprise the lower Grand Canyon Supergroup. Beginning 1.2 billion years ago, the formations of the Unkar Group were deposited over a span of 100 million years and appear to have been associated with a continental collision event that culminated in the formation of the supercontinent of Rodinia.



This view is taken from behind the falls, looking out at the top of its verdant, mossy travertine dome. Vegetation such as the moss, and golden columbine, maidenhair fern and scarlet monkeyflower thrives in the oasis of the fall’s unique microclimate. These plants are not indigenous to the hot, arid climate of the Grand Canyon only a few feet away.


Thursday, November 17, 2011

Memorable Places Here and There on the Colorado Plateau: The Solitude of Nankoweap




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Fifty-three miles downriver from Lees Ferry, the put-in for all trips heading into the Grand Canyon, the Colorado River makes a dramatic, sweeping S-turn where its gorge widens into an area called Nankoweap.

A thousand years ago, give or take, a large, flat delta built by numerous debris flows and flash floods, similar to what we see today, was an open invitation for Ancestral Puebloans to grow crops such as corn, one of their staples.

These Native Americans called Anasazi, which is actually a Navajo term meaning "enemy ancestors" or "ancient people who are not us," stored their grain high above the river in granaries etched into the cliffs, where this photo was taken. For scale, notice (above) the hikers descending a trail on the talus slope toward their raft. A few windows of the granary (below) can be seen from the trail.


Why are some regions of the Grand Canyon wide and open with a tranquil river such as Nankoweap and others narrow with towering rock walls and a river that's fast and furious? We know the Grand Canyon was carved by the action of the running water (or more appropriately its carried burden). Perhaps this is an overly simplistic statement, but true nonetheless. But, we must look for other variables to explain the differences in canyon architecture.

As the river downcuts into its bed, it encounters rock layers of variable resistance. Less resistant rock erodes more readily and laterally undercuts more resistant rock. This causes the overlying rock to collapse which widens the canyon. A direct relationship exists between canyon geometry and hardness of the rock strata. Thus, the canyon in the region of Nankoweap widens at the expense of the erodable Bright Angel Shale at its base that undermines and weakens the rock overburden. As the canyon widens, so follows its river bed. That slows the river's rate of flow and encourages the formation of those big deltas as the water releases its sediment. Perfect for farming! Fertile, irrigated and flat. 

Below the shale lies the Tapeats Sandstone which will come into view in another six miles, when the river dissects deeper into its bed. Above lies the Muav Limestone, the cliffs just above river level. These formations comprise the classic, transgressive triad of the Cambrian known as the Tonto Group, formed when the rising Panthalassic Ocean (or ancestral Pacific Ocean) began to lap across the region of the future Grand Canyon around 525 million years ago. The South Rim looms in the distance with the Middle Permian Kaibab Limestone at the top which means we’re viewing the near full extent of the Grand Canyon’s Paleozoic column of deposits.

Suggested Reading: Carving Grand Canyon by Wayne Ranney, 2005. 

Monday, November 7, 2011

Flight Plan: Part III - The Henry Mountains Laccolithic Complex on the Colorado Plateau


This is the third post on my recent aerial investigation of the geology of south-central Utah. For the earlier portion of the flight, please visit my first two posts entitled “Part I – Geology of the San Rafael Swell” and “Part II – Geology of the Circle Cliffs Uplift at Capitol Reef.”


Photo Above: The Henry Mountains
Framed by the Henry Mountains, Factory Butte's badlands are formed in the Blue Gate Shale Member of the Cretaceous Mancos Shale, and its summit at 6,321 feet is in the resistant Muley Canyon Member. Twenty miles to the south, Mt. Ellen at 11,506 feet of the Henrys is clad in late May snow. A faint image of Table Mountain lies directly in front of Ellen, while the snowless peak to the left (east) is Bull Mountain. From their isolated and remote position within the Henry Basin, the Henry Mountains
appear to be “springing abruptly from the desert.” (G.K. Gilbert, 1880)

TAKING TO THE SKIES TO STUDY THE GEOLOGY ON THE GROUND
While traveling through Utah’s backcountry in May, my good friend, geologist and author Wayne Ranney (WayneRanney.com and http://earthly-musings.blogspot.com/) suggested that we take to the air to investigate the geology. From the ground, it can be challenging to fully appreciate the scale and geological relationships of the Colorado Plateau’s massive landforms. From the air, the landscape takes on an unparalleled, big picture-perspective and provides some beautiful photos as well.

OUR FLIGHT PLAN
Taking off from Price, Utah, we flew south over the crests of the San Rafael Swell and the Circle Cliffs Uplift, better known as Capitol Reef, paying special attention to the geology of their monoclines, the San Rafael Reef and the Waterpocket Fold, respectively. On our return to Price, we circled the Henry Mountains just north of Lake Powell. We mapped out a roughly 500-mile, ellipse and lifted off early in the morning to catch the best light on the terrain.
THE HENRY MOUNTAINS
Where are they?
The Henry Mountains are located on the Colorado Plateau in south-central Utah (38°06'36.04" N, 110°49'21.97" W). They project a good 6,000 feet above the contiguous terrane of the blue and red rock desert of the plateau making them a highly recognizable landmark from considerable distances. The range is surrounded by Laramide-age uplifts, while the Henry Mountain complex intruded into the Henry Basin (also Henry Mountains Basin), a synclinal landform of the same age. The basin’s topography varies from steep, rugged terrain in the Henry Mountains in the east to a series of dissected mesas and buttes, and eroded cuestas and hogback ridges along the western margin.

To the west of the basin lies the Circle Cliffs Uplift and its Waterpocket Fold, a portion of which has been set aside as the Capitol Reef National Park. The northern boundary is the badlands and slopes within the Henry Mountains Syncline, and beyond, the uplift of the San Rafael Swell and its monoclinal Reef. To the east and south is the Colorado River, and below it, the Monument Upwarp, Lake Powell and the Glen Canyon National Recreational Area.

The Henry Mountains are located at the far left of center highlighted in yellow. Other regional laccolithic complexes of the Colorado Plateau are highlighted as well. The Laramide-age uplifts, basins and monoclines, the Paradox Basin and the Colorado River system are also labelled. 
(Modified from Tectonics of the Region of the Paradox Basin, Guidebook, Kelley 1958a, 1958b)

Who was Henry?
Beginning with the renown geologist John Wesley Powell in 1869, the remote and unexplored wilderness of southern Utah and northern Arizona along the Colorado River became a source of great scientific and exploratory fascination. Seeing the Henrys, Powell called them the “Unknown Mountains,” and rightly so. They were the last mountain range in the lower 48 states to be explored. Upon his return in 1871, he officially named them the Henry Mountains, after Joseph Henry, a close friend, supporter and secretary of the Smithsonian Institution.

Two groups of high peaks
The Henry Mountains are a 56 mile-long and 19 mile-wide, isolated string of five rugged, high peaks. From north to south, the range is clustered into two main groups, each dome being 6-10 miles in diameter. The larger northern group consists of Mt. Ellen (11,506 feet), Mt. Pennell (11,371 feet) and Mt. Hillers (10,723 feet). The southern group, also called the Little Rockies, includes Mt. Holmes (7,930 feet) and Mt. Ellsworth (8,235 feet). On our flight, we flew between the northern and southern group. That gave us a great view of Mount Hillers.

The two groups of the Henrys lying in the Henry Basin are visible in this NASA image.
They lie on strike with the Waterpocket Fold, ten miles to the west.
The Colorado River can be seen in the south snaking its way to Lake Powell.
(Image Science & Analysis Laboratory, NASA)

BASIC GEOLOGY OF THE HENRY MOUNTAINS COMPLEX AND BASIN
The Henry Mountain Basin
The Laramide Orogeny, a continuation of Cretaceous mountain-building, provided compression on the Colorado Plateau that resulted in numerous high-relief uplifts separated by small intervening basins. The uplifts and monoclines that we flew over on the earlier portion of our flight (my Posts I and II) demonstrated these landforms. One such basin is the Henry Mountains Basin that received localized intrusions of magma into shallow crustal levels, and that "pushed up" the Henry Mountains. More on that later. 

Having been stripped of its Tertiary deposits, the synclinal basin’s surficial bedrock is composed largely of the multi-membered Mancos Shale (which has experienced numerous revisions). These marine mudstone, siltstone, shale and sandstone deposits were deposited during the initial transgression of the Western Interior Seaway during the Early Cretaceous. The sedimentary section in the Henry Mountains is dominated by sandstones and shales ranging in age from Permian to Cretaceous.

In western regions of the basin are found the mesa-capping, fluvial sandstones of the Tarantula Mesa Sandstone that border the Waterpocket Fold. As we shall see, exposures of strata underlying the Cretaceous deposits down to the Permian are dramatically revealed by the formation of the Henry Mountains.

Stratigraphic column for the Henry Mountains region at the time of emplacement (~25 Ma). The approximate structural levels of selected igneous intrusions are indicated in the margin to the right. The Emery Sandstone is now referred to as the Muley Canyon Member.
(From Jackson and Pollard, 1988)

 
How did the Henry’s form?
Powell assigned the geologist G.K. Gilbert the task of studying the Henrys, which he accomplished in two field studies in 1875 and 1876. Gilbert’s 1877 report became the first thorough and classic investigation of the Henry Mountains. In the 1950’s, the geologist Charles B. Hunt further studied them and offered his own interpretation of their formation.

Gilbert reported that the the mountains
“mark a limited system of disturbances, which interrupt a region of geologic calm, and structurally, as well as topographically, stand by themselves.” He was referring to the fact that the peaks that share a common geological genesis. They formed when large igneous bodies intruded the flat-lying stratigraphy of the Colorado Plateau. The emplacement domed the overlying strata into a mushroom-shape which eventually eroded from the summits of what we know as the Henry Mountains of today. Each peak is an intrusive complex consisting of a large central, concordant (forming parallel rather than cutting-across existing strata layers)  "floored" laccolithic intrusion and many smaller satellite intrusions in a manner similar to lava flows emanating from its parent volcano.

In his insightful analysis on the Henrys, Gilbert coined the term “laccolite” (from the Greek word for “cistern” or “pool”) for the igneous structure resulting from the emplacement process, an intrusive (rather than extrusive) phenomenon. It was a "ground-breaking" thought at the time (excuse the pun); however, his hypothesis has been challenged.

Early sketch from the field notebook of G.K. Gilbert in 1875 of his conceptual model of a laccolith
(From Hunt, 1988).

Gilbert eventually devised this “half-stereogram” of a laccolith that intruded between flat-lying rock layers and domed the over-lying strata. Notice the flat-floor of the intrusion. The rear panel shows how erosion has unroofed the sedimentary cover of the dome, thereby exposing its igneous core.

(Report on the Geology of the Henry Mountains, G.K. Gilbert, Department of the Interior,
USGS of the Rocky Mountain Region, 1877)

The geology and geometry of the Henrys
The core of each of five intrusive centers is a separate diorite-porphyry structure that, at the summit, is bordered by an irregular and enigmatic zone of shattered sedimentary rock, appropriately called the “shatter zone.” Rocks within this zone are a complex intermingling of sedimentary and igneous rock. Laccoliths typically arise from relatively viscous magmas such as the diorite found at the Henrys which is texturally designated as largely a sodium-rich plagioclase and hornblende porphyry. Diorite is a gray to dark gray intermediate intrusive igneous rock and results from partial melting of a parent mafic (high iron and magnesium) rock. As we shall see later, that's a significant point of interest regarding the mountain's intra-plate locale on the Colorado Plateau.  

Many of the intrusive centers are surrounded on their periphery by clusters of smaller laccoliths, bysmaliths, dikes and sills. Their partially exposed, eroded pieces and remnants were visible from the air, as we shall see. In varying stages of exposure and surrounding the base of the centers are the basin's exposed sedimentary rocks, ranging in age from Late Permian to Late Cretaceous, which have been uplifted and deflected by the igneous intrusions that have arched their overburden skyward. Erosion has unroofed the cover from the summits of the intrusive centers and differentially exposed the verticalized rocks around their bases.

Emplacement ages for the Henry Mountains intrusions are from about 31 to 23 Ma, which have been radically revised from earlier calculations. A clear pattern in terms of spatial migration of emplacement ages amongst the various intrusive centers does not appear to exist. The entire complex appears to have been assembled in less than one million years.

What is the most likely emplacment scenario?
The Henry's formation appears to have begun with tongue-shaped sills and thin laccoliths fed by vertical dikes  emplaced in successive stacks. With the thickening of a major laccolith, the faulting of bedding planes was induced which began to tilt the overlying sills. Peripheral dikes and faults formed as lateral growth of the laccolith ceased. Formed from multiple intrusions, the major laccolith began to thicken vertically. Vertical growth or domal uplift of the overlying Mesozoic host rocks provided the accommodation space, the space-making mechanism, for the intrusions. The overlying strata, responding to the vertical displacement, developed numerous faults cut by vertical dikes. Subsequent erosion has removed as much as 3-4 km of sedimentary overburden that bared the crystalline cores of the intrusive centers.


Mt. Hillers possesses the best exposures of intrusions and sedimentary rock contacts. Note the diorite-core (red) at the summit and at various sills, satellite laccoliths and bysmaliths on the peak’s periphery (especially at noon to two o’clock). We’ll see these on our fly around. Also notice the large shatter zone (pink) surrounding the summit and the upturned Mesozoic rocks surrounding its base, especially the highly recognizable Navajo Sandstone (yellow).
(Modified from Emplacement and Assembly of Shallow Intrusions, Field Guide, Horsman et al, 2010)


Uplifted and reflected overburden
The following cross-sectional diagram of Mt. Hillers shows its central intrusion having uplifted and deformed the overburden of the basin during its emplacement. The slope of the uplifted layers increases with proximity to the dome and has a “doubly-hinged shape” consisting of a “concave-upward lower hinge” and a “convex-downward upper hinge.” The hinges are connected by a central limb of almost constant dip. This is largely where (A) upturned strata are differentially exposed and eroded, remniscent of the monoclinal erosion we saw earlier in our flight. This is also the zone of peripheral volcanic intrusions (sills that were emplaced horizontally and later tilted) and networks of bedding plane-faults (that have accommodated the strains of bending and stretching).


Are the intrusive centers of the Henry Mountains laccoliths or stocks?
The architecture of the Henry's subterranean volcanic structure underlying the domes is not without controversy. Although they are represented in geology textbooks as classic laccolithic mountains (Gilbert’s concept), contemporary analyses have suggested that they are larger more complex intrusions called stocks (Hunt’s concept).

Structurally, laccoliths may be low in height and range from circular to tongue-shaped in form. Stocks have greater height and are cylindrical in shape. Laccoliths are fed by a dike or stock; whereas, stocks do not have a feeder since they are continuous at depth. Laccoliths grow from a thin sill that thickens, thus are floored and concordant (parallel to rock layers). Stocks grow upward through zone-melting or diapirically, thus are not floored and discordant (cross-cutting rock layers).

Gilbert hypothesized that sill intrusion preceded the inflation of an underlying laccolith. Hunt believed the central intrusions are cylindrical stocks that are sheathed with a zone of shattered sedimentary rocks and that laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. Recent findings have confirmed the presence of a floored laccolithic intrusion but have not ruled out a stock at depth.

Gilbert’s concept of laccoliths in Mt. Hillers: A, Cross section with diorite in black; B, Subsurface structure;
C, Idealized laccolithic intrusion with a narrow feeder at its base.
(Original modified from Gilbert, 1877. From Processes of Laccolithic Emplacement, Jackson)


Hunt’s concept of relationships between the stocks and uplift of beds of Mt. Hillers
            (Modified from Hunt 1953. From Processes of Laccolithic Emplacement, Jackson)

LACCOLITHIC RANGES OF THE COLORADO PLATEAU
The Henrys are not alone
Between the Late Oligocene and Early Miocene on the Colorado Plateau, magmatism in the form of laccoliths occurred on the Colorado Plateau in seven laccolithic ranges, the largest of which are the Henrys (31 to 23 Ma). Their “magma-blister”, laccolithic-architecture is shared by other ranges such as the Abajo (29 to 23 Ma) and La Sals (28 to 25 Ma), and singular Navajo Mountain, about 65 miles due south of the Henrys. Navajo Mountain, still retaining its sedimentary rock cover and its crystalline core not yet exposed by erosion, is believed to represent an early stage in the intrusive process. All these laccolithic complexes possess a coincidence of timing (collectively 32.3 to 22.6 Ma), a style of intrusion, and the same or similar chemical signatures. That has given rise to the conclusion that they share a similar origin.

An enigmatic intra-plate locale
In viewing the aforementioned laccolithic centers as a group, one must also consider potential relationships to coeval igneous activity elsewhere on the Colorado Plateau, namely its east and west margins. The emplacement of the Marysvale volcanics (34 to 21 Ma) on the west and the San Juan volcanic field (32 to 23.1 Ma) on the east fall within the time frame of the laccoliths. All of the aforementioned volcanic centers are found in a rather incongruous location, when one considers that volcanic activity classically occurs at the boundaries of tectonic plates (excluding  intraplate activity at hotspots, not the case here). Furthermore, coeval igneous activity on the west and east margins of the plateau exists in sharp contrast to the locale of our intra-plateau laccolithic centers. I recall my exact thoughts when I saw these landforms for the first time. "What's the big picture? What’s going on tectonically? Is there a relationship of genesis?"

Looking below the surface
One explanation of Oligocene magmatism centers on the crust of the Colorado Plateau, thick (between 45 to 50 km) and largely undeformed, and on its mafic composition, stronger and more resistant to deformation than the more silicic crust to the west. This is in contrast to the thinner, fault-riddled crust of the Basin and Range Province to the west of the Colorado Plateau (~30 km). And to the east at the Rocky Mountains, the crust is thick but also highly faulted. It has been surmised that faults on both sides of the Plateau acted as conduits to facilitate the rise of magma to the surface. So how does that explain mantle-derived, intraplate magmatism at the laccolithic centers and their "sudden" timing during the Oligocene after such a long period of quiescence?

Modern geochronology and geochemistry to the rescue
Revised ages of the intrusions have made it clear that mid-Tertiary magmatism on the Colorado Plateau was part of voluminous regional magmatism in the North American Cordillera. The data suggests the existence of an essentially continuous, thousand-mile plus, intra-continental magmatic zone that extended from western Nevada through southern Utah to southwestern Colorado, and south to west Texas during the Oligocene to Miocene transition. As we shall see, the length of the zone and its perpendicular orientation to the trend of subduction along the western coast, help to explain Mid-Tertiary igneous activity. In addition, the isotopic geochemical signature of the Henry's rocks tells us that the magma was derived from partial melting of subducted oceanic crust in the mantle, the characteristic mark of a magmatic arc.

The Farallon Big Picture
The rapid subduction of the oceanic Farallon Plate beneath the continental North American Plate proceeded at a flatter trajectory in the region that drove the Laramide Orogeny. It was the presence of the Farallon Plate that provided the voluminous and widespread source for arc-related magmatism. But how? Plate convergence presumably slowed by 50 Ma, which drove the dense Farallon deeper into the mantle causing it to founder and break up. That allowed underlying buoyant, hot mantle to rise and heat the base of the crust resulting in its partial melting.

That's not all. As the less-dense melt ascended through the mantle, it pooled at the base of the silicic crust, owing to its greater mafic-density. That facilitated a silicic, crustal melt. The final result was the shifting of the original mafic composition of the mantle-derived melt to a melt with a more intermediate composition (our diorite!). That further retarded its journey of ascension, eventually stalling its rise into the shallow crust in a neutral-buoyancy state. Voila! That produced the magma that fed the Henry Mountains (and the other laccolithic-derived landforms of the Colorado Plateau), identifiable by its mafic, arc-like affinity. Amazing stuff. 

In summary, magmatism at the laccolithic centers is likely a consequence of the subduction of Farallon oceanic lithosphere. That exerted control over the composition, distribution and timing of magmatism after the Laramide Orogeny. The transport of relatively small volumes of magma within the laccoliths to shallow crustal environments indicates suppression by the unique physical properties of the high-strength lithosphere of the Colorado Plateau relative to contemporaneous magmatism in the Great Basin to the west and the San Juan Mountains to the east.


TAKING FLIGHT
Photo Below: The Waterpocket Fold and the Henry Mountains seen from the west
Having crossed the Circle Cliffs Uplift at Capitol Reef National Park from west to east, we emerged at its plunging monocline, the Waterpocket Fold. This curvaceous portion of the monocline, seen below, depicts numerous strike valleys and exposed ridges formed by the variably resistant and susceptible strata to the forces of erosion. About ten miles away looms a portion of the northern group of the Henry Mountains. Table Mountain is located furthest to the north (left), followed by Mt. Ellen and its lesser summits.




Photo Below: Mt. Pennell and the Henry Basin badlands
We then turned to the south and followed the monocline, eventually banking due east toward the Henry Mountains. In this view, we are over the badlands just east of the monocline at the western portion of the Henry Mountain Basin and facing the western flank of Mt. Pennell. Its slope gradually drapes into the foreground until plunging into badlands of the heavily eroded Blue Gate Member of the Mancos Shale and capped by mesas formed in the Muley Canyon Member (formerly the Emery Sandstone Member). Pennell's flanks are covered with Quaternary colluvial deposits consisting of slide material, slumps and talus.



Photo Below: Mt. Pennell from the southwest
This view of Pennell’s southwest slope shows well the gradual drape of the sedimentary rocks out over the basin from uplift of the igneous dome. The slope is comprised of members of the Cretaceous Mancos Shale draped over by Quaternary colluvium shed from the mountains. The rising flank of Mt. Hillers is almost seen to the right.


 
Photo Below: Mt. Hillers from the south
We headed between the northern and southern groups of the Henry Mountains. The photo faces north towards Mt. Hillers’ southern flank. The northern three mountains are more mature intrusive centers than the smaller southern group, each with more component intrusions in a wider range of sizes and geometries. Of the intrusive centers of the northern group, Hillers has the best exposures of intrusions and sedimentary rock contacts. The peak is considered a more mature, later stage in the emplacement process. 

Recall that as the laccolith evolved, it elevated the overlying strata which have since eroded from the dome and flanks of Mt. Hillers, leaving its denuded igneous core exposed. At the southern and southeastern base of Mt. Hillers, Cretaceous and Jurassic strata, and various dikes can be seen to have been uplifted and deflected in a manner analogous to a trap door opening skyward. The crest is composed of diorite, and below it, the shattered zone, not well displayed owing to the distance. The lower flanks are composed of Glen Canyon Group deposits, and closer to the base, the deposits of the San Rafael Group. The slopes surrounding Hillers and all the peaks are marked by radial drainage patterns. It is estimated that almost 5,000 feet of Cretaceous sedimentary rocks that once covered the intrusions and Canyonlands country have been stripped away by erosion, exposing the igneous rocks that cored the intrusions.


Having just left the Waterpocket Fold, its upturned strata bear sharp contrast to the geo-dynamics operating at the upturned strata at the Henrys. At the fold, the uplifted and subsequent exposure of the sedimentary strata is related to its monoclinal drape over a Precambrian fault at depth. On the other hand, the uplift and exposure of the sedimentary strata at the Henry Mountains is related to laccolithic doming. In both circumstances, the forces of erosion have acted upon the strata. The tectonic commonality that the two share is the subduction of the Farallon Plate beneath the North American Plate. In the case of the Waterpocket Fold, the mechanism was Laramide compression; whereas, in the case of the Henrys, the mechanism was post-Laramide magmatism and emplacement related to Farallon foundering.


Photo Below: Mount Hillers’ upturned dikes and sedimentary strata from the southeast
In this view, we have flown around the southern extent of Mt. Hillers to its southeastern flank. Seeing Hillers in profile, the dramatic upturned nature of the Mesozoic strata is readily discernible. Again, notice the sedimentary strata draping away from the base of Hillers. This indicates the areal extent of the intrusion at depth, far greater than what is seen at the surface. Also, note the sharply upturned Early Jurassic Navajo Sandstone. Strata of the San Rafael Group lie circumferentially outside of it, upturned as well, but bending into the subsurface as part of a long, sloping-limb that is buried by colluvium. Vertical dikes can be seen cutting through the buff-colored Navajo Sandstone and running towards the summit. Numerous bedding plane-faults exist in order to accommodate the flexure of the  bedrock. Hillers and its satellite intrusions are thought to have been assembled within no more than one million years.

The steeply-dipping, deflected beds of Jurassic Navajo Sandstone provide photogenic evidence of the primary space-making mechanism for the magma of Mt. Hillers’ central intrusion that of "roof-uplift." The oldest sedimentary unit that is exposed on the southern flank of Mt. Hillers is the Permian Cedar Mesa Member of the Cutler Formation. This provides one with a sense of the depth of the Hillers’ intrusive center!



Photo Below: Mt. Hillers' eastern flank
We’re now facing the eastern flank Mt. Hillers. Amongst the various other peaks of the Henrys, the igneous intrusions exhibit varying stages of development. For example, early centers possess a sedimentary cover that not only dominates the margins but can be traced almost to the summit. In addition, satellite intrusions around main intrusive centers exist in a highly varied spatial variation, but the main intrusive center is consistently a laccolith with numerous dikes and sills above a large central pluton.

 
 

PHOTO BELOW: The Black Mesa bysmalith and Maiden Creek sill of Mt. Hillers
From this vantage point on the eastern slope of Mt. Hillers (just out of view to the left), we again see the peaks of Mt. Pennell (left) and Mt. Ellen (right). As seen on the western flank of the mountains, a moderate inclination of the sedimentary layers continues for several kilometers away from the intrusive centers at each peak, a testimony to the doming that has occured at depth.

Dominating the left center of the photo are a few of Hillers' well studied satellite intrusions, the cliff-forming outcrops of the Black Mesa bysmalith (center), and at bottom center, the Maiden Creek sill. Immediately out of view to the right is the Trachyte Mesa laccolith. Each of these intrusions provides a snapshot of the growth history of a small pluton during its progressive assembly, thought to have occured in multiple pulses, as magma input increased.

The Maiden Creek sill provides evidence of the first episode, the Trachyte Mesa laccolith records the first two stages, and the Black Mesa bysmalith (an overinflated, cylindrical intrusion that cross-cuts adjacent strata) records all three. These satellite structures developed on the margin of the Mt. Hillers complex and are thought to have been emplaced from weeks to years.
             


SW-NE Cross-section of Mt. Hillers
Notice the drape of the sedimentary layers away from the central intrusion, and the dikes and sills bent upward along with the strata by the doming. Also note the depiction of the Black Mesa bysmalith and the Maiden Creek sill fed by a system of dikes and sills.

Schematic cross-sections through three satellite intrusions illustrating their emplacement mechanisms.
(Emplacement and Assembly… Field Guide, Horsman et al, 2010)


Photo Below: Mt. Ellen and Bull Mountain from the east
Having turned the corner on our flight around the Henry Mountains, we’re now heading north. The last of the Henry's peaks, Bull Mountain (9,187 feet), can be seen from the southeast. The intrusive center of Bull Mountain is a bysmalith similar to Black Mesa.



Photo Below: The northern section of the Henrys from the east
Here's our final glance at the northern section of the Henrys from the east while flying over the mesas and benches around the Dirty Devil River. That's Table Mountain, also a bysmalith, at the far right (north) with Mt. Ellen, the South Summit Ridge and finally snowless Ragged Mountain to the south. Notice the omni-present, photogenic clouds hovering over the range "making" their own weather in this arid region of Utah.


I'll continue with my upcoming and final post on our flight, as we head back to Price, Utah.

Highly Recommended Reading: 
Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, 2008.
Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado by Robert Fillmore, 2011.